FLC2000 and Wellbore Strengthening

7/30/2019 FLC2000 and Wellbore Strengthening

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FLC2000and Wellbore Strengthening 1

FLC2000

And

Wellbore Strengthening

Note : No warranty is given (including warranty as to fitness for a particular purpose or use or

application and any user of this report agrees to absolve and hold Impact Fluid Solutions harmless

against any consequences resulting or liabilities from the use thereof.


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TABLE OF CONTENTS

Page

1. Introduction 3

1.1 Wellbore Strengthening 3

1.2 Technical Approaches 3

1.2.1 Chemical Consolidation of Weak Rocks 31.2.2 Stress Caging 4

1.2.3 Formation of an Ultra-Low Permeability Seal 4

2. Proof of Performance that FLC2000 Provides Wellbore Strengthening 5

3. The Effect of Shear on the Sealing Ability of FLC2000 versus Calcium Carbonate 6

4. How Does FLC2000 Work 7

4.1 Formation of Small Deformable Aggregates 7

4.2 Broad Particle Size Distribution 7

4.3 Formation of a Low Permeability Barrier 7

4.4 Preventing Fracture Propogation 8

5. FLC2000 Proof of Performance Case History Summary 10


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1. Introduction

FLC2000

is Impact Fluid Solutions key additive to engineer ultra-low invasion drilling fluids,completion fluids and work-over fluids. To date, FLC2000 has been used successfully in over 1,000 wells

world-wide. FLC2000 greatly reduces fluid invasion into matrix permeability and micro-fractures. By

forming a very low permeability barrier over pores and micro-fractures, FLC2000 limits transmission of

the well-bore pressure to the pore fluid. This is instrumental in preventing formation fracturing by

preventing fracture propagation and effectively raises the fracture initiation pressure. For reservoir

applications, laboratory studies have shown that properly formulated FLC2000 fluids typically give

return permeabilitys in excess of 90% and show low flow initiation pressures. Excellent results have been

obtained for solids-free fluids as well as weighted systems.

1.1 Wellbore StrengtheningWellbore strengthening is currently of considerable interest to the drilling community. This is driven

mainly by the increasing challenges of drilling in environments such as deepwater and depleted zones

where the fracture gradient is low and the mud weight window is very narrow. The problem can become

even more extreme in deviated and high angle wells. Given that in most situations it is still very

desirable to keep the hydrostatic mud pressure higher than the pore pressure, this means that the ECD

will approach and frequently exceed the fracture initiation pressure. The result is induced lost circulation.

1.2 Technical Approaches

Several approaches have been considered with regard to well-bore strengthening. These include:

1.2.1 Chemical consolidation of weak rocks

In principle, the rock can be made stronger by using chemicals that form strong inter-granular

cements. However, in practice, most candidate treatments are difficult to apply successfully because of

one or more of the following problems:

The chemical reaction may be hard to control, leading to early setup in the pipe or well-bore The reaction may require unacceptably long soak times, particularly in low permeability rocks where

penetration of the treatment into the matrix will be slow

The chemicals may be incompatible with the drilling and completion fluids If used in the reservoir, the strengthening reaction may irreversibly damage the permeability and

hence well productivity

Many of the products considered carry health and safety concerns.In addition, while consolidation may significantly increase the compressive strength of some rocks,

any increase in tensile strength is usually very small. It is the tensile strength that needs to be increased if

the wellbore strengthening is to increase the fracture gradient.


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1.2.2 Stress Caging

The concept of stress caging involves forming fractures around the well bore and propping these

open with a hard material such as a coarse marble. Propping open these fractures (resisting the fracture

closure stress) alters the hoop stress around the well-bore and, in theory, increases rock strength. An

additive such as graphite is also frequently added to reduce the permeability of the proppant and hence

reduce fluid and pressure transmission into the fracture. Several SPE papers have been published

describing the success of this technique and programs exist to calculate the amount and size of proppant

required for a particular rock type and stress regime. Information supplied by several users of this

technique indicates that it works best in permeable formations.

The stress cage method has been less successful in shales and it is difficult to see how it can be

effective in soft formations where the rock will deform around the propped fracture. The stress cage

mechanism is challenged by some groups who suggest that rather than creating fractures, invasion offluid into the pores is more likely to be a key factor. Solids added to the fluid form a protective filter cake

at the well-bore wall rather than act as a proppant in an induced fracture. Since the method, irrespective

of which of the above mechanisms might operate, generally uses coarse solid additives, it is difficult to

maintain the materials in the mud system. Hence it is often best suited to short sections of open hole or to

treating prior to, for example, casing running operations.

1.2.3 Formation of an ultra-low permeability seal

If an impermeable barrier can be formed on the wellbore wall such that the mud pressure (and

accompanying fluid) is not transmitted to the formation pore fluid then rock mechanics considerations

tell us that the rock will stay strong. If pressure invasion is allowed to take place, this will increase thepore pressure, which reduces the effective stress and thus reduces the rock fracture pressure.

While theoretically attractive, it is difficult to explain how a totally impermeable barrier can be set up

immediately fresh rock is exposed to the drilling fluid. All fluids in overbalance will invade to some

degree. In actual practice, there is emerging evidence that fluids that form an ultra-low permeability

barrier very quickly across matrix permeability or micro-fracture openings can restrict pressure and fluid

invasion enough to give an appreciable measure of wellbore strengthening. Impacts FLC2000 mud

additive falls into this category of material.


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1. Proof of Performance For FLC2000 Providing Wellbore Strengthening?There are many examples where FLC2000 has increased the mud weight above that used in offset

wells without inducing lost circulation. One example (from SPE 101329), is taken from the Gulf of Mexico

and typifies the results obtained there and offshore Brazil, Middle East, North Africa and elsewhere:

An offset well had used oil mud to drill from 9,304 to 15,800 feet in 117 days. The upper part of the interval

was drilled with continuous partial losses and then an influx of formation water was detected. The drill string

showed snatches of high torque suggesting stuck pipe was imminent. As a result of these problems a cement plug

was set and the well sidetracked. While at 11,309 feet in the sidetrack, simultaneous losses and a water flow were

detected. An LCM pill was pumped but total losses were experienced and attempts to cure them were unsuccessful.

In all, some 25,000 barrels of oil mud were lost to the formation and it required 4 trips from shore by the supply boat

to make enough mud available to keep the well full.

On the subsequent well, FLC2000 was added at the start of the 12 section (8,425 feet) and was maintainedin the system until TD at 16,027 feet. A leak off test (LOT) was made at the 13 3/8 shoe with old water based mud

before switching to the FLC2000 oil mud: this LOT value was 15.7ppg. Drilling commenced with the FLC2000

mud at a density of 15.0ppg but this was increased to 15.3ppg when a pressure transition was encountered at 9,196

feet. In the next 6 days, drilling progressed to 10,157 feet, with the mud continuing to be weighted up as the well

went further into the high pressure zone. The static mud weight finally reached 16.6ppg. Even though this gave a

calculated ECD of 18.53ppg below the 13 3/8 shoe that had previously shown a LOT of 15.7ppg, no losses were

observed despite the high mud weight. Using the FLC2000, the operator had drilled more than 650 feet deeper

into the high pressure zone than on any previous well.

Unfortunately, having made a bit trip, the string was run back in the hole to just below the 13 3/8 shoe and

circulation was established by bringing the mud pumps up to speed rapidly, rather than by first rotating the pipeand then bringing the pumps in slowly. The resulting pressure surge broke down the 13 3/8 shoe and about 3,700

barrels of mud were lost before the loss zone was sealed with a dilatant LCM pill. Drilling recommenced once the

losses were controlled but the hole was unintentionally sidetracked. This sidetrack was drilled without problems to

9,678 feet using the FLC2000 fluid. 9 5/8 casing was set at 11,302 feet and, continuing with the same mud system,

the hole was deepened to 16,027 feet and 7 5/8 casing run and cemented without problems.

Despite the lost circulation incident and the unintentional sidetrack, this section was drilled in 69 days with the

FLC2000, compared with 117 days for the offset well using conventional invert OBM. Although there were induced

downhole losses totaling 5,700 barrels of mud, this volume was significantly lower than the 25,000 barrels lost in

the offset well. It is estimated that, compared with the offset well, around US$3.2 million was saved.

Laboratory tests have shown that an FLC2000 filter cake built on 100 Darcy 20/40 frac sand will

maintain its integrity to at least 3500psi overbalance in both WBM and OBM. The laboratory tests have

also shown that an FLC2000 mud will seal the mouth of a 0.25mm wide fracture in Ohio Sandstone

(


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3. The Effect of Shear on the Sealing Ability of FLC2000 versus Calcium Carbonate

Calcium carbonate is often used as a bridging agent to protect permeable formations from theexcessive invasion that can contribute to formation damage and differential sticking. To be most effective,

it should be sized for the specific pore throat sizes of the rocks being drilled. However, these pore sizes

are not always known and, even when they are, pore sizes commonly vary both horizontally and

vertically in a formation. Even if the particle size distribution (PSD) of the carbonate can be optimized at

the start of drilling, the particles are ground down as drilling proceeds, so the size distribution shifts from

the ideal case and increased formation invasion can result. An alternative is to replace the calcium

carbonate with particles that are much more resistant to grinding. The flexible particles formed by

FLC2000 meet this requirement.

Laboratory tests were carried out to compare how sized calcium carbonate and FLC2000 fluids

respond to shear. The carbonate chosen for the study was a commercially available material commonlyused in drilling fluids. The carbonate had a PSD with a d90 of about 120 microns, a d50 of about 50 microns

and a d10 of about 10 microns. The materials were mixed into a 20 ppb bentonite mud and sheared at two

different shear rates on a Silverson mixer for varying lengths of time. The depth of penetration of each

fluid into a bed of 20/40 frac sand in the Impact sand bed test (SBT) was measured.

The figure below shows that the base bentonite mud cannot seal the sand and the fluid

completely penetrates the 15 cm bed. Addition of 20 ppb CaCO3 or 10 ppb FLC2000 into a 20 ppb

bentonite mud produces a good seal in the SBT with penetrations of less than 3 cm. However after only 5

minutes of shearing on the Silverson mixer at 6000 rpm, the CaCO3 mud completely penetrates the sand

bed. 10 minutes of shearing at the slower mixer speed of 3000 rpm also produces a CaCO3 fluid that

cannot seal the sand bed. In comparison, after 30 minutes of shear at 6000 rpm, the fluid containing

FLC2000 still seals with fluid penetration still less than 3 cm.

Figure 3.1 Sand Bed Test Results


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4. How Does FLC2000 Work?

FLC2000 is a blend of modified cellulosic polymers and surface functionalized organic solids. Themolecular weights of the polymer components are low, which allows easy mixing and does not

contribute significantly to fluid viscosity. The technology has been presented in several SPE papers and

other acknowledged industry publications, including SPE 77189, SPE 85304, SPE 85326, SPE101329,

AADE HO-DF-04-33 and AADE 06-DF-HO15.

4.1 Formation of Small Deformable Aggregates

The high-grade polymers and organic solids in FLC2000 have been modified to exhibit a range of

water and oil solubilitys and wettabilitys, covering a wide range of HLB values. In water-based fluids,

some components dissolve and disperse to provide fluid loss control similar to many conventional

additives, while other components only partially solvate because of their oil-loving properties. Theseother components assemble into small deformable aggregates that give an FLC2000 fluid its ultra-low

invasion, low damage characteristics. A similar mechanism operates in oil-based fluids except that here it

is the more water-loving components now produce the aggregates.

Particle analysis shows the aggregates are present in the fluid in a very wide range of sizes, from a

few microns to several hundred microns in diameter. The d50 is around 60m, d10 9m and the d90 340m.

These aggregates provide the excellent invasion control seen with FLC2000. As well-bore fluid tries to

enter rock pores or micro-fractures because of the overbalance pressure, an ultra-low permeability layer

of aggregates quickly forms and greatly reduces any further invasion of solids or fluid. The aggregates

making up the layer are deformable so, as the pressure is raised, they are increasingly compressed and

the barrier permeability is reduced even further. In laboratory tests and field measurements FLC2000

gives much lower fluid invasion than conventional fluids, including sized calcium carbonate muds.

4.2 Broad Particle Size Distribution

The broad size distribution and compressibility of the aggregates means that the one grade additive

can seal a wide range of pore sizes and micro-fractures. There is no need to change the size distribution of

the product as drilling moves from formations of one permeability to another, as is the case with calcium

carbonate. In some respects, the aggregates in an FLC2000 fluid act like the water droplets in invert

emulsion oil muds. The water droplets in OBM are known to concentrate in the filter cake where they

make a major contribution to the good fluid loss control seen with oil muds. The major difference, and

benefit, of the FLC2000 aggregates is that they are more deformable and cover a much wider size range.

Hence they are better sealing agents and work over a much wider range of pore sizes and permeabilities.

4.3 Formation of a Low Permeability Barrier

A key benefit is that, by forming a very low permeability barrier, the FLC2000 additive also has the

ability to protect weak formations against pressure transmission and fracturing. The very low

permeability barrier formed by the FLC2000 additive is much more effective at preventing fluid invasion


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than conventional mud additives and as such greatly reduces formation damage, the risk of differential

sticking and (by controlling leak-off into micro-fractures in shales) certain types of wellbore instability.

The barrier also effectively increases the fracture initiation pressure and widens the safe drilling window

by allowing the wellbore fluid density to be raised without inducing losses.

Effective concentrations of FLC2000 in a drilling, completion or work-over fluid range between 3 and

8ppb. This concentration is much lower than more conventional additives such as sized calcium

carbonates where 30 to 40ppb may be required and, even then, sealing is not as efficient as with FLC2000.

The optimum FLC2000 concentration within the above range will depend on the base fluid properties

determined both in the laboratory and in the field by using a simple sand bed invasion test. The

effectiveness of FLC2000 in giving wellbore strengthening in permeable formations is undoubtedly due to

the very low permeability of the filter cake formed. It is too soft a material and is present in too low a

concentration to work by a stress cage mechanism (see discussion of the stress cage method in Section 3).

4.4 Preventing Fracture Propagation

While it is feasible that the product can form a very low permeability seal that is complete enough,

and formed quickly enough, to prevent the initial pressure penetration into a permeable formation, and

hence stop a fracture initiating, it is more likely that the additive mainly functions by plugging a fracture

with a very low permeability membrane once it starts to grow. The formation of this seal will stop the

fracture propagating, much in the way a tip screen-out will stop a hydraulic fracture from growing. In

low permeability formations such as shales, FLC2000 works by one of 2 mechanisms:

In fracture blocking experiments on Ohio Sandstone, the product can seal the mouth of a 250 micronfracture. Since in the timescale of the experiment the Ohio Sandstone can be treated as an

impermeable rock, we can envision a similar seal will form on a low permeability shale. While a base

mud without FLC2000could not seal the fracture, mud with FLC2000 sealed and held a differential

pressure of almost 1600psi before failing.


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There is growing support for the concept that, when shale breaks down, the failure is at an interfacebetween the low permeability shale and a higher permeability streak of sand, silt or marl.

Alternatively the failure can occur within a thin streak of more permeable rock enclosed in the shale.

If this is the case, then the invading fluid can leak off and a filter cake forms to stop fracture

propagation in the same way as in a more massive permeable rock.


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No warranty is given (including warranty as to fitness for a particular purpose or use or application) and any user of this

report agrees to absolve and hold Impact Fluid Solutions harmless against any consequences resulting or liabilities fromthe use thereof.

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FLC2000 and Wellbore Strengthening

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