Recent developments in perforating fluids are helping...
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14 Oilfield Review Optimal Fluid Systems for Perforating Larry Behrmann Ian C. Walton Rosharon, Texas, USA Frank F. Chang Dhahran, Saudi Arabia Alfredo Fayard Houston, Texas Chee Kin Khong Shekou, Shenzhen City, China Bjørn Langseth Stavanger, Norway Stephen Mason Sugar Land, Texas Anne-Mette Mathisen Hydro Bergen, Norway Italo Pizzolante Tian Xiang CACT Operations Group Shekou, Shenzhen City Grete Svanes MI-SWACO Bergen, Norway For help in preparation of this article, thanks to Nils Kågeson-Loe, MI-SWACO, Stavanger; and Charlie Svoboda, MI-SWACO, Houston. PLT (Production Logging Tool), PURE and SPAN (Schlumberger perforating analysis) are marks of Schlumberger. CLEANPERF is a mark of MI-SWACO. Recent developments in perforating fluids are helping operators clean up, both literally and financially. When combined with advances in perforation-gun performance and dynamic underbalanced-perforating technology, these new fluids yield significant improvements in well productivity. Cleaning up after any well operation is critical. In drilling, rock is loosened by the impact of a drill bit and the hydraulic energy of a drilling fluid. Drilling mud carries this rock debris to the surface. Even before the circulating mud removes the loose drilling debris, the formation has been exposed to foreign solids, liquids and chemicals in solution that sometimes damage reservoir rock by reducing near-wellbore permeability. This reduction is often referred to as formation damage, one of the components of skin damage. Similarly, in perforating, a high-energy jet from an explosive shaped charge shoots through casing and cement, and pierces the formation, creating a conductive path deep into the reservoir rock. Immediately after gun detonation, fluid from the borehole fills the perforation tunnel. As in drilling, this initial contact between the wellbore fluid and formation may cause an additional reduction in permeability and a decrease in perforation efficiency. This is particularly true in over- balanced perforating, a condition in which wellbore hydrostatic pressure is greater than formation pressure. A properly designed perfor- ating fluid can help avoid this damage and substantially improve well productivity. Although many technologies are involved in modern perforating, three fundamental elements are critical to maximizing hydrocarbon recovery. Together, they form the basis for an optimized perforation strategy. First, perforations must be properly oriented; second, debris from the perforation tunnels must be effectively removed; and third, formation damage must be minimized during the process. Debris includes not only loose material in the perforation tunnel, but more importantly, crushed sand grains that line the tunnel and constitute what is known as perforation damage. In reservoirs with a potential for sand production, perforation orientation is critical to sustained production. This is particularly true in deviated and horizontal boreholes. Excessive sand production is a common problem that erodes downhole equipment, plugs the wellbore and ultimately chokes off fluid flow. In 2001, BP noted that 60% of its worldwide production, or around 2 million barrels [317,800 m 3 ] of oil equivalent per day, came from fields requiring some level of sand management. 1 Numbers like this reinforce the need for an optimized perforating strategy to ensure that perforations are placed at the proper orientation and phasing to minimize sand flow and maximize hydrocarbon production. 2 After perforating, tunnel debris must be removed. Long perforation tunnels and those in hard, low-permeability formations can be difficult to clean. Underbalanced perforating is sometimes used to help clear these tunnels of debris and minimize perforation damage. 3 However, more recently, engineers have recognized that generating a dynamic underbalance just moments
Transcript of Recent developments in perforating fluids are helping...
14 Oilfield Review
Optimal Fluid Systems for Perforating
Larry BehrmannIan C. WaltonRosharon, Texas, USA
Frank F. ChangDhahran, Saudi Arabia
Alfredo FayardHouston, Texas
Chee Kin Khong Shekou, Shenzhen City, China
Bjørn LangsethStavanger, Norway
Stephen MasonSugar Land, Texas
Anne-Mette MathisenHydroBergen, Norway
Italo Pizzolante Tian Xiang CACT Operations GroupShekou, Shenzhen City
Grete SvanesMI-SWACOBergen, Norway
For help in preparation of this article, thanks to Nils Kågeson-Loe, MI-SWACO, Stavanger; and Charlie Svoboda,MI-SWACO, Houston.PLT (Production Logging Tool), PURE and SPAN (Schlumbergerperforating analysis) are marks of Schlumberger. CLEANPERFis a mark of MI-SWACO.
Recent developments in perforating fluids are helping operators clean up, both literally
and financially. When combined with advances in perforation-gun performance and
dynamic underbalanced-perforating technology, these new fluids yield significant
improvements in well productivity.
Cleaning up after any well operation is critical.In drilling, rock is loosened by the impact of adrill bit and the hydraulic energy of a drillingfluid. Drilling mud carries this rock debris to thesurface. Even before the circulating mudremoves the loose drilling debris, the formationhas been exposed to foreign solids, liquids andchemicals in solution that sometimes damagereservoir rock by reducing near-wellborepermeability. This reduction is often referred toas formation damage, one of the components ofskin damage.
Similarly, in perforating, a high-energy jet froman explosive shaped charge shoots through casingand cement, and pierces the formation, creating aconductive path deep into the reservoir rock.Immediately after gun detonation, fluid from theborehole fills the perforation tunnel. As in drilling,this initial contact between the wellbore fluid andformation may cause an additional reduction inpermeability and a decrease in perforationefficiency. This is particularly true in over -balanced perforating, a condition in whichwellbore hydrostatic pressure is greater thanformation pressure. A properly designed perfor -ating fluid can help avoid this damage andsubstantially improve well productivity.
Although many technologies are involved inmodern perforating, three fundamental elementsare critical to maximizing hydrocarbon recovery.Together, they form the basis for an optimizedperforation strategy. First, perforations must be
properly oriented; second, debris from theperforation tunnels must be effectively removed;and third, formation damage must be minimizedduring the process. Debris includes not onlyloose material in the perforation tunnel, butmore importantly, crushed sand grains that linethe tunnel and constitute what is known asperforation damage.
In reservoirs with a potential for sandproduction, perforation orientation is critical tosustained production. This is particularly true indeviated and horizontal boreholes. Excessivesand production is a common problem thaterodes downhole equipment, plugs the wellboreand ultimately chokes off fluid flow. In 2001, BPnoted that 60% of its worldwide production, oraround 2 million barrels [317,800 m3] of oilequivalent per day, came from fields requiringsome level of sand management.1 Numbers likethis reinforce the need for an optimizedperforating strategy to ensure that perforationsare placed at the proper orientation and phasingto minimize sand flow and maximizehydrocarbon production.2
After perforating, tunnel debris must beremoved. Long perforation tunnels and those inhard, low-permeability formations can be difficultto clean. Underbalanced perforating is sometimesused to help clear these tunnels of debris andminimize perforation damage.3 However, morerecently, engineers have recognized thatgenerating a dynamic underbalance just moments
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after perforating-gun detonation may actuallypromote better perforation cleanup than under -balanced perforating, and in some cases, is bettersuited to the completion design and wellconditions.4 A dynamic underbalance cangenerally be created from an initial state that iseither under- or overbalanced.
1. Morton N: “Screening Out Sand,” BP Frontiers, issue 2(December 2001): 18–22.
2. For more on perforation orientation: Bersås K,Stenhaug M, Doornbosch F, Langseth B, Fimreite H andParrott B: “Perforations on Target,” Oilfield Review 16,no. 1 (Spring 2004): 28–37.Acock A, ORourke T, Shirmboh D, Alexander J,Andersen G, Kaneko T, Venkitaraman A, López-de-Cárdenas J, Nishi M, Numasawa M, Yoshioka K, Roy A,Wilson A and Twynam A: “Practical Approaches to SandManagement,” Oilfield Review 16, no. 1 (Spring 2004):10–27.
3. For more on underbalanced perforating: Bakker E,Veeken K, Behrmann L, Milton P, Stirton G, Salsman A,Walton I, Stutz L and Underdown D: “The New Dynamicsof Underbalanced Perforating,” Oilfield Review 15, no. 4(Winter 2003/2004): 54–67.
4. Chang FF, Kågeson-Loe NM, Walton IC, Mathisen AMand Svanes GS: “Perforating in Overbalance—Is ItReally Sinful?,” paper SPE 82203, SPE Drilling &Completion 19, no. 3 (September 2004): 173–180.
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The PURE perforating system for cleanperforations generates a dynamic, or transient,underbalance pressure immediately after thecreation of the perforation tunnel.5 Thisinstantaneous decompression of reservoir fluidsaround a perforation assists in removal of thecrushed material from the perforation tunnelwhile the rest of the well may be in a staticoverbalanced condition (left). In most cases, the PURE technique produces a lower skin than that observed after conventional under-balanced perforating.
Once the guns have detonated at the requiredorientation and a dynamic underbalance hashelped clean the perforation tunnels, thehydrostatic pressure in the perforations returns tothat of the wellbore. If the initial wellbore state isunderbalanced, then there is little opportunity forwellbore fluids to infiltrate the formation throughthe perforation tunnel. However, depending on thewell configuration and formation characteristics,when perforating overbalanced, fluid from thewellbore may rush to fill the perforation tunnels,providing an increased potential for furtherdamage to the formation.
Engineers recognize that perforating with aninitial overbalance is potentially damaging andsometimes unavoidable. However, overbalancedperforating is often the most economical andefficient process, particularly when the operatorneeds to remove the gun assembly from thewellbore after perforating. The operatoressentially has three options:• Drop the guns immediately after perforating.
This requires a special connector called a dropsub, sufficient wellbore depth below the com-pletion, a wellbore deviation less than about60 degrees and prior installation of the uppercompletion. In these circumstances, the wellcan be perforated with an initial underbalance,the guns dropped and the well immediatelyplaced on production. This is the least damag-ing of the three choices.
• Perforate with an initial underbalance andthen retrieve the guns through a wellheadadapter that allows tools to be pulled throughthe wellhead while under pressure. Thismethod causes little damage to the formation,but the use of these specialized tools is notalways a practical or cost-effective option.
• Perforate overbalanced so that the guns canbe safely retrieved and the upper completioninstalled with the well under control. In thiscase, the perforating fluid, often a solids-ladenkill pill, is generally circulated out of the well-bore before the well is placed on production.
16 Oilfield Review
> PURE pressure dynamics. Within 0.1 s of perforating gun detonation,pressure (blue) in and around the perforation decreases dramatically. In awellbore open to the surface, pressure recovers to that of the hydrostaticload at around 0.15 s. This action helps clear the perforation of shatteredformation debris and improves production efficiency. To minimize perforationdamage as hydrostatic pressure recovers, the perforating fluid must quicklygenerate a competent filtercake, or seal, over the newly exposed formation.
2,500
2,000
1,500
1,000
500
0
–500
–1,000
–1,500
–2,000
–2,5000 0.1 0.2 0.3 0.4
Time, s
0.5 0.6 0.7 0.8 0.9 1.0
Over
bala
nce,
psi
Unde
rbal
ance
, psi
> Filtration at the formation face. In an overbalanced condition, when thewellbore hydrostatic pressure is greater than formation pressure, theformation face within the perforation acts as a filter. As the fluid in thewellbore is pushed into the formation by the pressure differential, solids arefiltered out at the rock face leaving only the liquid and fine particulates tomigrate back into the permeable rock (inset). The size of the particles allowedpast the initial filtration zone is, for the most part, a function of the rock’spore-throat size and the dimensions and characteristics of the solid-phasematerials contained within the fluid. Typically, solid materials are depositedjust inside the formation and across the surface forming an internal andexternal filtercake. The depth, thickness, elasticity and other mechanicalcharacteristics of the cake determine its ease of removal during production.
Formationdamage
from drillingPerfo
ratio
n tu
nnel
Undamaged formation
Low-permeability zone and perforation debrisexpelled by surge of formation fluid
CementCasing
Filtrationsurface
Internal filtercake
Liquid-invasion zone
External filtercake
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In this article, we focus on the third elementof an optimized perforating strategy, the perfora -ting fluid. We describe extensive laboratory tests that form the foundation for development of a new perforating-fluid system. Then, we show how one operator in the South China Seautilized these theoretical concepts to improveproduction efficiency.
Evaluating Fluids for Overbalanced PerforatingAs fluid leaks off into a formation afterperforating, it may cause permeability damageradially away from the perforation. The extent ofradial permeability damage is determined bynumerous factors including the initial formationpermeability, the pressure differential betweenthe wellbore and reservoir, the amount and typeof clay and other debris present within theformation pore throats, the liquid-phase chemicalcomponents, and the solid-phase chemical andphysical characteristics.
The most commonly used wellbore fluid forperforating is completion brine. When losses ofcompletion brine are significant, based either onfluid volume or cost of the fluid being lost, asecondary fluid system typically referred to as afluid-loss control pill (FLCP), or kill pill, isplaced across the perforated interval to seal theperforations against further losses. Most often,these postperforation FLCPs contain a mix ofliquids and solids, the solids being polymers andparticulates such as calcium carbonate [CaCO3]sized to minimize fluid loss to the formation.
As leakoff occurs within the perforation tunnel,the solid and liquid phases of these fluidsseparate as they are filtered across the formationface (previous page, bottom).
Fluid leakoff into the formation can reducepermeability through several mechanisms. Thesubstances contained in the leakoff fluid mayreact with clays in the formation-pore throatscausing them to swell or mobilize, thus reducingeffective permeability. Compounds such assurfactants and polymers migrating into thereservoir can change pore-throat wettability andeffective diameter, thus altering frictionalpressures and possibly limiting hydrocarbon flow.
As the liquid phase leaks off into theformation, solids and polymers in the perforatingfluid are deposited within the perforation tunneland formation, forming a low-permeability filter -cake, or seal, between the tunnel wall and theformation. In permeable rock, the speed withwhich this seal builds, along with the charac -teristics of the sealing materials, deter mines theleakoff rate, the total fluid volume lost into thereservoir rock, and inevitably, the level ofpostperforation formation damage.
Realizing the importance of minimizingformation damage created during leakoff, Hydro,Schlumberger and MI-SWACO engineers beganresearch in 2001 aimed at developing anoptimized perforating fluid to help minimizepostperforation formation damage in over -balanced environments.6 To establish a baselinefor perforation-fluid damage, engineers firstevaluated water-base and oil-base completion
fluids typically used for overbalanced perfor -ating. Initial fluid formulations were designed inclose collaboration between Hydro Oil & Energyand MI-SWACO at Hydro’s laboratory in Bergen,Norway. The test fluids were blended andshipped to the MI-SWACO laboratory in Houstonfor verification of the fluid properties. Then,samples were taken to the SchlumbergerReservoir Completions Technology Center (SRC)in Rosharon, Texas, where the perforation testswere conducted.
At the Rosharon facility, six fluid types wereevaluated in a test cell using various configura -tions (below). Since zinc-cased shaped chargeshave been shown to be incompati ble with certainwater-base completion fluids, several of the testfluids were evaluated with both zinc and steelcasing materials.7 The first round of tests wasconducted using Castlegate sandstone cores withpermeabilities that ranged from 600 to 1,000 mD.
In the laboratory, engineers dried the testcores at 300°F [149°C] for 16 hours. These coreswere evacuated and saturated with kerosene, and
5. For more on PURE technology: Bruyere F, Clark D,Stirton G, Kusumadjaja A, Manalu D, Sobirin M, Martin A,Robertson DI and Stenhouse A: “New Practices toEnhance Perforating Results,” Oilfield Review 18, no. 3(Autumn 2006): 18–35.
6. Chang et al, reference 4.7. Javora PH, Ali SA and Miller R: “Controlled Debris
Perforating Systems: Prevention of an UnexpectedSource of Formation Damage,” paper SPE 58758,presented at the SPE International Symposium onFormation Damage Control, Lafayette, Louisiana, USA,February 23–24, 2000.
> Testing perforating-fluid types. Fluids in the first test series included oil-base fluids and perforating fluids built fromcompletion brines. The density of each was nearly the same, with most being weighted with calcium carbonate [CaCO3].
Fluid Base fluid Weighting agentSpecific
gravity, g/cm3 Solids
Oil-base mud
Cesium formate, low-solids, oil-base mud
Calcium bromide, low-solids, oil-base mud
Potassium formatekill pill
Potassium-cesiumformate kill pill
Calcium bromidekill pill
Oil-externalemulsion
Oil-externalemulsion
Oil-externalemulsion
Potassium formate
Potassium-cesiumformate
Calcium bromide
1.65
1.67
1.34
1.63
1.63
1.65
Barite
Cesium formate/calcium carbonate
Calcium bromide/calcium carbonate
Potassium formate/calcium carbonate
Potassium formate/cesiumformate/calcium carbonate
Calcium bromide/calcium carbonate
Barite
Calciumcarbonate
Calciumcarbonate
Calciumcarbonate
Calciumcarbonate
Calciumcarbonate
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an initial porosity was measured. Techniciansestablished permeability in both axial- anddiametral-flow geometries under ambienttemperature and pressure to simulate overburdenpressure. The core was then loaded into theperforating vessel with casing and a cement plateattached to the face of the core (left).Overburden stress was applied to the core, thegun assembly installed and the simulatedwellbore filled with the test fluid. Most of thetests involved rotating the test cell so that theguns fired vertically to simulate orientedperforating in a horizontal well. Once the testcell reached the desired reservoir temperature,pore pressure, overburden stress and wellborepressure were applied to create an overbalanceof 450 psi [3.1 MPa]. Once all pressures hadstabilized, engineers fired the guns, and allowedwellbore- and pore-pressure readings to restabilize.Technicians shut in the system and maintainedan overbalanced condition for three days.
On some tests, leakoff continued during theshut-in period, causing the wellbore pressure todecrease and approach reservoir pressure (nextpage, top left). If the pressure dropped to apredetermined level, technicians increased thepressure to maintain a 450-psi overbalance. Thisprocedure simulates field operations in whichthe hydrostatic column in the wellbore is toppedoff periodically to maintain hydrostatic pressure.In some of the tests, this pump-up and leakoffcycle occurred several times throughout theshut-in period as a function of the perforatingfluid’s ability to control fluid loss.
After three days, the system was allowed tocool and pressure was reduced to atmosphericlevels. Postperforating productivity was measuredat ambient temperature by flowing kerosenethrough the core in the axial direction. Startingfrom a low flow rate, production continued untilsteady-state flow was established. Then, the flowrate was increased to measure the incrementalcleanup as a function of flow rate.
To compare the loss-control characteristics ofthe various fluids tested, engineers determinedthe rate at which the filtercake builds, which canalso be interpreted as a leakoff rate (next page,top right). Technicians also captured data fromconventional high-pressure, high-temperature,(HPHT) fluid-loss tests. The volume of filtratecaptured during the first minute of the test, orspurt loss, also helped in comparing thefiltercake-building characteristics of thedifferent fluids (next page, bottom).
18 Oilfield Review
> Full-scale perforating test instrumentation. The test cell (top left) is shown with the core enclosedin an elastomer sleeve. Once the instrument is sealed, pressure and temperatures are controlled atsimulated downhole conditions. Small and large accumulators provide far-field, or hydrostatic,pressures (bottom diagram). During tests, the perforating gun (red) is fired through a steel platebacked by cement into the formation core, thus simulating wellbore conditions (inset).
Simulated reservoir core samples
Shootingleads
Wellbore-pore
Wellbore pressure
Micrometer valve
Confining chamber
30-gallon accumulator
Shooting plate simulatingcasing and cement
5-gallon accumulatorconnected to wellbore
Simulated wellbore
Gun with shaped charge
Core sample
Conf
inin
g pr
essu
re d
ata
Wel
lbor
e pr
essu
re d
ata
pressure differential
Perforation
Perforating fluid
Steel to simulatecasing
Cement
Core
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> Typical shut-in pressure profile. A wellbore-pressure profile wasacquired during a 72-hour shut-in period. The pressure spikesoccurred when technicians increased the simulated hydrostaticpressure to account for fluid leaking off into the core. The rate offluid leakoff is derived from the slope of the leakoff curve (inset).
100
80
60
40
20
00 5 10 15
Square root of time, s1/2
Pres
sure
diff
eren
tial (
P wel
lbor
e –
P por
e), p
si
20
5,000
6,000
3,000
2,000
0 12 24 36 48 60 72
1,000
0
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5,000
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Pres
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, psi
Time, h
> Fluid leakoff rate. The pressure differential between wellbore-hydrostatic pressure and pore pressure just after wellbore pressurehas stabilized is plotted against the square root of time. Normalizedfor variations in the surface area of the perforation-tunnel wall, theslope of the line indicates the rate at which the filtercake builds. This value can also be interpreted as the leakoff rate, indicating thevolume of fluid leaking into, or through, the core over time.
> Initial results from the first series of 10 tests (top). Tests 1 and 2 compared perforations shot using oil-base perforating fluids with perforations shot inthe horizontal and vertical directions. A significant improvement in core-flow efficiency (CFE) was seen with the guns oriented vertically. In Tests 5 and 6,fluids built from calcium bromide [CaBr2] were tested with steel- and zinc-cased charges, confirming the negative impact of bromine and zinc in solution(bottom left). The normalized perforation/permeability ratio (NPPR) was improved with steel-cased charges. Also of note is the CFE comparison betweenwater- and oil-base fluids. With the exception of Test 4, perforating in oil-base fluids produced the least damage (bottom right). Engineers suspect that thelow-solids, oil-base mud used for Test 4 suffered a broken emulsion and therefore produced poor CFE values relative to the other oil-base fluids tested.The high CFE produced in Test 7 with water-base fluid is not completely understood. Because water-base CFE values this high are inconsistent with allother water-base tests, engineers considered this test an anomaly.
Testnumber
Initial permeability, mD Coreporosity, %
Wellbore fluid,specific gravity, g/cm3Axial Diametral
Perforatingdirection (charge)
HPHT leakoffat 1 min, mL
Leakoff rate,psi/s1/2/in.2 CFE NPPR
0.0
0.2
0.4
0.6
0.8
1.0
0.1
0.3
0.5
0.7
0.9
Wat
er Oil
Wat
er
Wat
er
Wat
er
CFE
Wat
er Oil
Wat
er Oil
Oil
Test
9
Test
4
Test
8
Test
10
Test
5
Test
6 Test
1 Test
7 Test
3 Test
2
0.0
0.5
1.0
1.5
2.0
2.5
Spurt Leakoff CFE NPPR
Zinc
Steel
24.8
25.7
24.9
24.9
24.6
25.0
24.4
24.7
24.1
24.6
1
2
3
4
5
6
7
8
9
10
550
768
750
575
1,030
1,040
600
990
940
920
450
510
500
675
715
720
530
680
670
720
Oil-base mud (1.65)
Oil-base mud (1.65)
Calcium bromide, low-solids oil-base mud (1.34)
Cesium formate, low-solids oil-base mud (1.67)
Calcium bromide kill pill (1.65)
Calcium bromide kill pill (1.65)
Cesium formate kill pill (1.63)
Cesium formate kill pill (1.63)
Potassium-cesium formate kill pill (1.63)
Potassium-cesium formate kill pill (1.63)
Horizontal (zinc)
Up (zinc)
Up (zinc)
Up (zinc)
Up (zinc)
Up (steel)
Up (zinc)
Up (steel)
Up (zinc)
Horizontal (zinc)
0.2
0.2
4.0
3.5
2.1
2.1
0.7
0.7
1.3
1.3
0.32
0.22
0.38
0.62
0.65
0.46
0.39
0.09
0.25
0.28
0.67
0.90
0.82
0.52
0.55
0.57
0.74
0.53
0.47
0.54
51.70
1,150.00
169.00
0.67
1.10
1.90
22.00
7.85
4.42
7.40
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Data from the test series indicated that most of the fluids slowed the egress of filtrateinto the core. However, calcium bromide [CaBr2]brine and low-solids oil-base mud (LSOBM)formulated with cesium formate [CsCOOH] brinewere exceptions.
Previous tests had shown that a chemicalreaction occurs between the zinc debris andcalcium-containing brine during perforating withzinc-cased charges. This typically causes theCaBr2 perforating fluid to lose its fluid-losscontrol capability as illustrated by the immediateequalization between wellbore and porepressures (right).8 However, fluid-loss control ismaintained when steel-cased charges are used.
The CsCOOH-base LSOBM demonstrated lessfluid-loss control capability. A high initial fluidloss was observed, and more fluid entered theformation, particularly during the initial spurt-loss phase.
When examining the cores after the tests, theresearch team noted that the perforation tunnelswere filled with material, and in some cases,tightly packed with solids from the perforatingfluid and formation sand grains. This material inthe tunnel may have acted as a porous mediumwithin a tunnel of otherwise nearly infiniteconductivity. To further understand the cleanuppotential of the various fluids, engineerscalculated a perforation permeability that takesinto account the packing of filtercake materialwithin the perforation tunnel.
The team used a numerical simulator tocalculate perforation permeability based onmeasured productivity and perforation-tunneldimensions. Once perforation permeability wasobtained, a normalized perforation/permeabilityratio (NPPR) was defined by dividing theperforation permeability by the root-mean-square of core axial and diametral permeability.9
The NPPR provides a measure of howpermeable the perforation is in comparison withthe original rock permeability. The measurementis independent of the length and diameter of theperforation tunnel. Data from the NPPRcalculations confirmed that using oil-baseperforating fluids results in cleaner perforations(next page). It also provided a tool to helpevaluate the cleanup efficiency of the water-base perforating fluids not otherwise defined bycore-flow efficiency (CFE) calculations.10 Thedata further demonstrated the directrelationship between fluid-loss control andproductivity impairment. The less effectively aperforating fluid builds filtercake, the more
damage it will create; this is particularly true ofwater-base fluids.
In general, the greater the volume of fluid lostto the formation, the more concentrated anddehydrated the internal and external filtercakebecomes. Thus, the filtercake is more difficult toremove during production and causes moredamage to the perforation tunnels. When water-base perforating fluids are used, the NPPRdeclines log-linearly with the leakoff rate,demonstrating the inefficiency of filtercakeremoval as well as the adverse relativepermeability effect caused by water-base fluids.
The LSOBM fluid tests showed higher leakoffvolumes that could be expected to impairproductivity. However, despite their higherleakoff rate and higher HPHT values, the LSOBMfluids tested do not significantly impair
permeability, as long as the oil-base fluids arestable and maintain their oil-external emulsionsthroughout the perforating process.
The ability to measure simulated wellboreand formation pressures helped engineersunderstand the fluid dynamics of leakoff, and thepotential damage caused by perforating. Resultsof this first series of tests indicate that, witheither water- or oil-base perforating fluids, thekey to minimizing permeability damage is rapidlybuilding a high-quality filtercake across theperforation-tunnel formation face. Although oil-base perforating fluids demonstrate superiorityto water-base fluids in reducing formationdamage, minimizing fluid loss should still helpreduce productivity impairment.
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> Charge-casing interference with fluid-loss additives. Engineers suspectthat powderized zinc from the zinc-cased charges reacts with salts in brine-base perforating fluids. These reaction products negatively affect polymersused for fluid-loss control in perforating and kill fluids. Leakoff-pressure datademonstrate the lack of fluid-loss control with zinc-shaped charges (top);wellbore (green), near- (blue) and far-pore (orange) pressures are equal,indicating the absence of a filtercake and fluid-loss control. With steel-casedcharges (bottom), the fluid is able to build a filtercake; wellbore (green) andpore (orange and blue) pressures are easily differentiated.
6,000
5,000
4,000
3,000
2,000
1,000
00 12 24 36
Time, h
Steel-Cased Charge
Pres
sure
, psi
48 60 72
6,000
5,000
4,000
3,000
2,000
1,000
00 12 24 36
Time, h
Zinc-Cased Charge
Pres
sure
, psi
48 60 72
Wellbore pressureFar-pore pressureNear-pore pressure
Wellbore pressureFar-pore pressureNear-pore pressure
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Simulating Field ConditionsAlthough the first test series clarified theefficacy of various fluids in the laboratory, therewere unanswered questions about perforatingstrategies in the field. Since perforatingprocedures vary from one project to the next, issimply using a low fluid-loss perforating fluid allthat is necessary to ensure minimal permeabilitydamage? Should wells be perforated in clearbrine or a fit-for-purpose perforating fluid?Should isolation packers be used to helpoptimize dynamic underbalanced conditions? Toanswer these questions, researchers designed asecond series of tests to evaluate theperformance of water- and oil-base perforatingfluids under varying simulated field conditions.
Several simulation scenarios were designedto replicate conditions that might exist in thefield—for example, perforating in an open well,
in an isolated wellbore, and with a clear fluid.11
In the first, the “quick-kill” process, a wellboreopen to the surface was perforated while filledwith a specially designed solids-laden perfor -ating fluid. Simulated hydrostatic pressure fromthe far-field wellbore fluid provided the energysource to quickly increase downhole pressure tothe desired overbalanced state.
In the second scenario, the “slow-kill”process, a wellbore was perforated overbalanced,but isolated below packers. After perforating, thedynamic pressure effect of gun firingimmediately reduced the wellbore pressure aftercharge penetration. Without access to the fullhydrostatic column, however, the isolated sectionof the wellbore cannot return to an overbalancedstate until the packers are manually unset. Last,in a variation called “kill later,” the test cell wasconfigured to simulate overbalanced perforating
in a clear completion fluid. After perforating, theclear fluid was displaced by a kill fluid similar tothe perforating fluids used in the previous tests.The weight of the kill fluid effectively bull -headed, or forced under pressure, the clear fluidinto the formation until the kill fluid reached theperforation to build a filtercake.
Engineers simulated these processes in thelaboratory. Fluid accumulators with gas capsacted as the hydrostatic column that providedfar-field pressure effects above the perforatedzone. A large accumulator volume representedperforating in an open well so that sufficientwellbore fluid and energy were available toreplenish the pressure deficit around theperforated section after the guns were fired.Conversely, a small accumulator volumerepresented perforating in an isolated wellbore;it extended the dynamic underbalance periodbecause there was insufficient energy toreplenish the pressure deficit immediately aftercharge penetration.
In the “kill-later” test, a clear completionfluid was added to the test cell. A pistonaccumulator filled with the kill fluid wasconnected to the test cell, but isolated by a valvein the closed position. The accumulator pressurewas increased to the wellbore pressure so thatthere was no pressure loss in the test cell whenthe valve was opened; this procedure immedi -ately established an overbalanced condition.
Once the guns were fired, the wellborepressure and pore pressure reached equilibrium.The valve between the kill-fluid accumulator andthe test cell was opened, and an overbalance wasapplied to displace the clear fluid through thecore sample. Once a filtercake built up, theleakoff ceased and a stable overbalance wasmaintained. During the kill process, a largevolume of clear perforating fluid was pushed intothe core, causing the pore pressure to increase. Ableed-off valve on the back side of the coreallowed technicians to maintain a relativelyconstant pore pressure.
10. Core-flow efficiency (CFE) is defined by the ratio of themeasured productivity index (PI) (after the core ispenetrated by a shaped charge) to a theoretical ideal PI(as if the perforation tunnel and surrounding formationwere free of any perforation damage).
11. Chang FF, Mathisen AM, Kågeson-Loe N, Walton IC,Svane G, Midtbø RE, Bakken I, Rykkje J and Nedrebø O:“Recommended Practice for Overbalanced Perforatingin Long Horizontal Wells,” paper SPE 94596, presented at the SPE European Formation Damage Conference,Scheveningen, The Netherlands, May 25–27, 2005.
8. Chang et al, reference 4.
9. NPPR = where k = permeability.
> Calculation of perforation permeability. The normalized perforation/permeability ratio (NPPR) helpsengineers compare true formation permeability with that of the rock after perforating. It is also useful indifferentiating the various fluids tested. The goal is to achieve a high NPPR. Oil-base fluids (blue diamond,red square and brown triangle) show a distinct advantage over the water-base fluids evaluated.
10,000
1,000
100
10
1
0.10 0.1 0.2 0.3
Leakoff rate, psi/s1/2/in.2
Trend line for water-base fluids
NPP
R
0.4 0.60.5 0.7
OBM
CaBr2 kill pill (KP)
KCOOH KP
(K/Cs)COOH KP
CaBr2 LSOBM
CsCOOH LSOBM
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CFE calculations for each perforatingstrategy indicated that the lower the fluid lossduring the shut-in period as shown by the shut-inpressure behaviors, the higher the CFE (left andabove). An appropriately designed perforation ina well open to hydrostatic pressure would beexpected to cause less formation damage than aperforation with the same fluid in a wellboreisolated from hydrostatic pressure. Further,perforating with clear brine and then displacingto a heavier fluid capable of killing the well, oftencalled a kill pill, appears to cause the mostdamage, probably due to excessive brine loss intothe formation.
Engineers conducted similar tests usingCastlegate sandstone to evaluate the perfor -mance of oil-base perforating fluids. Whencompared with the water-base fluids, the oil-basefluids showed generally the same trend but withhigher CFEs, indicating less formation damage.The data show that perforating with an oil-basefluid in a well open to the surface produces theleast damage of all the fluids and methodologiestested. Engineers noted that perforating in oilthen later killing the well caused more damage,again demonstrating that rapid filtercakedevelopment is necessary to minimize invasion ofdamaging solids and fluids into the formation.
For all tests, there was a consistent trendshowing that perforating pressure dynamicsinfluence fluid-loss control behavior for all typesof kill fluids. The shut-in pressure profile forperforating an open well showed good filtercakecompetency. The shut-in pressure profile in anisolated wellbore showed less filtercakecompetency, as indicated by the need to replenishpressure more often during shut-in. When thesimulated wellbore was perforated with a clearfluid and then killed later, poor fluid-loss controlwas observed. Finally, during tests of perforatingwithout dynamic underbalance, the fluid-loss
22 Oilfield Review
> Core-flow efficiencies. Castlegate sandstone was perforated using thequick-kill, slow-kill and kill-later processes. For quick- and slow-kill tests, thegun was fired with oil-base perforating fluid in the test cell. For the kill-laterprocess, firing occurred with clear kerosene in the cell that was laterdisplaced with oil-base kill fluid. The pressure (green) for quick-kill shows alow leakoff rate and a minimum number of pump-up cycles (top left). Theslow-kill process required more frequent pressure adjustments (middle), whilein the kill-later process, fluid-loss control could not be achieved until the clearkerosene fluid was displaced by an oil-base kill pill (bottom). Core-flowefficiency (CFE) calculations show that the quick-kill process using oil-baseperforating fluid causes the least amount of permeability damage (top right).
6,000
5,000
4,000
3,000
2,000
1,000
00 12 24 36
Time, h
Kill Later
Pres
sure
, psi
48 60 72
6,000
5,000
4,000
3,000
2,000
1,000
00 12 24 36
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Quick KillPr
essu
re, p
si
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Quick Kill Slow Kill Kill Later
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CFE
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control was achieved by debris plugging.Although adequate fluid-loss control wasobtained without dynamic underbalance, thereturn permeability suffered from the debris leftin the perforation tunnel (above).
Once all tests were complete, engineers andscientists at the Hydro Oil & Energy ResearchCenter in Bergen performed petrographicstudies on thin-section samples of the cores andobserved the changes in grain and porestructures between the crushed zone near theperforation tunnel and the undisturbedsandstone matrix away from the perforationtunnel. In addition, they studied polished epoxy-impregnated samples by scanning electronmicroscope (SEM) and analyzed micrographs ofbackscattered images.
The thin-section images from the rockadjacent to the perforation-tunnel wall revealedthe effect of pressure dynamics on perforationcleanup (right). It was apparent that the shapedcharges created a crushed zone near theperforation-tunnel wall. For both the quick- and
> The importance of underbalance. In this test, Castlegate sandstonewas perforated without achieving a dynamic underbalance(top left). Although the leakoff-pressure profile (bottom left) showswellbore pressure (green) well above pore pressure (orange),indicating good fluid-loss control, the low leakoff rate is attributedto failure to clear perforation debris from the tunnel afterperforating. Computerized axial tomography (CAT) scan images(above) show a solid, high-density mass (white) in the perforationtunnel. For comparison, a similar core was perforated using thesame fluid design, but dynamic underbalance was achieved witha higher core-flow efficiency (above right). The gray coloring inthe perforation tunnel indicates that significantly less debris is leftin the tunnel.
2,500
2,000
1,500
1,000
500
0
–500
–1,000
–1,500
–2,000
–2,5000 0.2 0.4 0.80.6
Time, s
No Underbalance
Over
bala
nce
and
unde
rbal
ance
pre
ssur
e, p
si
1.0
6,000
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4,000
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00 12 24 36
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Pres
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, psi
48 60 72
No Underbalance Shot with Underbalance
> Changes in porosity. Image sets A, B and C (bottom) are thin-section samples cut from low-permeability Berea sandstone cores (top). The cores were perforated using oil-base fluids. Images Aand B show a low content of fine material compared with Image C, indicating that the crushed zonewas removed by the dynamic underbalance achieved in the perforation process. Sample A showshigher damage due to higher fluid loss resulting from the slow-kill process. Sample B shows slightlymore fine material in the thin section; however the quick-kill process helped clear the perforationtunnel. Image C shows a high content of fine material and no removal of the crushed zone from theperforation tunnel because dynamic underbalance was not achieved. Results of these tests wereconsistent with those performed on higher permeability Castlegate sandstone.
ORSPR07_Don_ThinSec_1
A. Slow kill B. Quick kill C. No underbalance achieved
BR7-25-11 BR7-18-12 BR7-22-12
BR7-18-7
BR7-18-2BR7-33-8
BR7-22-2BR7-25-4
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slow-kill cases, dynamic under balance wasachieved and the crushed zone was removed.Laboratory studies showed little difference ingrain sizes in the two cases. However, in the slow-kill case, the higher fluid-loss levels may havecaused the increased level of observed damage.In the case with no dynamic underbalance, the crushed zone was not removed. The resultwas retention of a significant amount of fine-grained material in the perforation tunnel, thusreducing CFE.
Based on the collected data and petrographicobservations, the research team concluded thatthere is a delicate balance between the degree ofcleanup in the perforation and the susceptibilityof the perforation to perforating-fluid invasion.Creating a perforation tunnel that is sufficientlyclean to allow effective filtercake to build may bemore beneficial to overall damage prevention
than trying to create the cleanest perforation,possibly resulting in more filtrate loss to theformation. Examination of polished epoxy-impregnated SEM samples provided furtherevidence that achieving a dynamic underbalanceduring overbalanced perforating is necessary tominimize permeability damage.
Data from these extensive studies show thatduring overbalance, the characteristics of theperforating fluid, the method used to kill andisolate the perforation zone and success inachieving dynamic underbalance during theperforation process strongly influence final wellproductivity (above). An optimized strategy foroverbalanced perforating must include anappropriate perforating fluid capable of rapidlybuilding a filtercake, while achieving dynamicunderbalance during the process.
In the Field with a Quick-Kill Perforating FluidChina National Offshore Oil Corporation(CNOOC), Chevron and Eni, the field operator,are partners in the development of the HZ oil andgas fields, operating as the CACT OperatorsGroup in the South China Sea. The HZ fieldsprimarily consist of stacked, thin sandstones inwhich sufficient single-well productivity can beachieved by commingling production frommultiple sandstones, by drilling horizontal wells,or both.
Traditionally, tubing-conveyed perforation(TCP) has been preferred for thick productionzones. However, CACT engineers found thatwireline-conveyed casing guns are an economicalternative for thinner production zones that arespread over a large interval.12 In these wells,multiple wireline-conveyed casing-gun operationsare usually performed slightly overbalancedbecause it is operationally easier and safer.
Previous perforating operations using tubing-conveyed underbalanced-perforation methodsand static underbalanced wellbore pressureshave required additional rig time, and operationshave been complicated. In many instances, staticunderbalanced perforating has deliveredunderperforming wells, probably because theperforation-induced skin has not beenadequately removed. Further, when TCP is used,unless sufficient rathole is drilled to allowdropping the guns to the bottom of the wellbore,the well must be killed to retrieve them, creatingthe risk of postperforation completion-fluidinvasion damage. To minimize cost, simplifyoperations and minimize perforation damage,CACT elected to perforate most new wells andreperforate existing wells overbalanced usingwireline casing guns.
After studying candidate wells, the CACTreservoir and production department, workingwith Schlumberger and MI-SWACO engineers,elected to test two new completion technologiesfor overbalanced perforating: the PURE systemand the CLEANPERF fluid, a noninvasiveperforating fluid. These technologies wereexpected to improve well-completion efficiency.
To test the new perforating system design,engineers planned to compare the recompletionresults on reference Well 1 with those obtainedfrom the newly completed Well 6. Pressure-buildup data were not available from referenceWell 1, so the productivity index (PI) wasanalyzed to estimate its completion skin factor. APLT Production Logging Tool was run in the wellafter completion to determine the flow rates of
24 Oilfield Review
> Choosing a perforating fluid. Core-flow efficiency for each fluid tested isshown by process and sorted by core-flow efficiency (CFE). In Castlegatesandstone (top) and Berea sandstone (bottom), an oil-base perforating fluidcombined with the quick-kill process (purple) and dynamic underbalanceproduced the most favorable results; higher CFE values indicate the leastamount of perforation damage. The kill-later process (blue) and tests in whichdynamic underbalance was not achieved (yellow) were the most damaging.
1.0
1.2
0.8
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0.4
0.2
0
CFE,
PI m
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red/
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eal
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OBM OBM OBM OBM SizedSalt
SizedSalt
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Berea Sandstone
Castlegate Sandstone
Sized Salt Sized Salt
Quick kill with dynamic underbalanceSlow kill with dynamic underbalanceKill later with dynamic underbalanceNo dynamic underbalance
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Spring 2007 25
all the layers when water cut was lowest. PLTevaluation of the reference well for the fourlayers perforated using wireline-conveyed gunsand the PURE system at an overbalance of about1.3 MPa [188 psi] indicated permeabilities from9.4 to 1,605 mD, and skin factors from 0 to –0.97.
Although reference Well 1 and Well 6 wereperforated using the PURE system, each was shot using a different perforating fluid. Inreference Well 1, engineers used CLEANPERFfluid system, while Well 6 was perforated using a typical polymer kill pill.
M-I SWACO designed the CLEANPERF fluidsystem for use with the Schlumberger PUREperforating system, primarily in overbalancedperforating situations. The perforating fluidprovides a low-permeability barrier that limitsdeep invasion of solids and fluids into thereservoir along the perforation tunnelimmediately after perforating. To further helpminimize postperforating damage, the systemreadily flows back without a remedial treatmentduring production.
CLEANPERF fluids are designed for eachspecific application based on several criteriaincluding formation characteristics andexpected pressure differentials. Critical to eachdesign are providing adequate density for therequired overbalance; quickly establishing athin, low-permeability seal across the formationface; allowing development of minimal adhesiveand cohesive forces within the seal to promoteuniform release from the formation and removalduring flowback; maintaining thermal stabilityfor the period of time in which the system is inthe wellbore prior to production; and beingchemically compatible with perforation charges.
Data from thin-section analyses of corematerial provided by CACT helped engineersdesign an appropriate blend of bridging agents toeffectively seal the full range of pores present inthe formation. Engineers elected to use a water-base CLEANPERF system for Well 1. The perfor -ating fluid was formulated using 4.21% by volumeof sized bridging agents and two different clay-inhibition additives. To assist with filtercakecleanup, chemicals were added to reduce theadhesion of the filtercake to the wall of theperforation tunnel. The fluid also contained abiopolymer viscosifier, a starch-based filtration-control additive, and stabilizers for pH andmicrobial activity.
In the field, engineers perforated Well 1using the PURE system and the CLEANPERFfluid, and then compared perforation efficiencyresults with those in Well 6. Measuredcompletion skins were evaluated againstcompletion skin values modeled using the SPANSchlumberger perforating analysis software. Thebenchmark was set at the technical limit forperforation efficiency defined by modeling theperforation-completion skin of a fully cleanedcrushed zone. Engineers compared themeasured completion skin for reference Well 1from the PI equation with that of Well 6 frommultilayered reservoir testing. The modeledideal completion skin value for reference Well 1was approximately –1.38, while the simulatedcompletion skin based on field results was –1.37; this was close to ideal. The modeled ideal completion skin for Well 6 (Layer A – 40) was approximately –1.85, while the simulatedcompletion skin based on field results was –0.97,indicating that the completion skin was 48%below the modeled optimal results (above).
Data from both wells were carefully sortedand analyzed. Taking into account thesignificance of laboratory data (discussed earlierin this article), engineers concluded that sinceboth wells were perforated using the PUREsystem and all other parameters were relativelyequal, there was a strong likelihood that theimproved completion efficiency of Well 1 was due to use of the noninvasive CLEANPERFperforating fluid.
The Third Element of Perforation Design Although field data are still somewhat limited,the research presented in this article suggeststhat engineers now have the necessary tools toformulate an optimized perforation strategy. Aswith many E&P activities, the man-made fluidspresent in the borehole during completionoperations have a direct effect on ultimateefficiency and productivity.
Properly designed, fit-for-purpose perfora -ting fluids show great promise in helpingoperators improve the return on theirperforating investments. There is little doubtthat the elements of an optimized perforatingstrategy—optimal perforation-gun orientation,dynamic underbalanced perforating and newperforating fluids—will grow in time. But fornow, the addition of fit-for-purpose perforationfluids provides an easily adoptable step-changein the design and execution of modernperforating techniques. —DW
12. Pizzolante I, Grinham S, Xiang T, Lian J, Khong CK,Behrmann LA and Mason S: “Overbalanced PerforatingYields Negative Skin Values in Layered Reservoir,” paperSPE 104099, presented at the SPE International Oil & GasConference and Exhibition in China, Beijing,December 5–7, 2006.
> Improving skin with a fit-for-purpose perforating fluid. Data from field test and PI calculations show that for reference Well 1 perforated with CLEANPERF fluid,the actual results (purple) matched planning estimates (green). By comparison,results from Well 6, perforated with a conventional kill fluid, were 48% belowmodeled optimal results.
0
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
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plet
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val
ue
Reference Well 1 Well 6
Modeled optimal results
Actual field results
48%
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