SPE-172457-MS

Maximising Well Productivity Through Appropriate Design of CleanupRecipe and Proper Execution for New Well Completions

Taiwo.O. Olubamise, Ufuoma.J. Oghene, and Chinwe Uchendu, Shell Nigeria

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the Nigeria Annual International Conference and Exhibition held in Lagos, Nigeria, 0507 August 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

Reduced productivity in newly completed wells could be attributed to impairments caused by drilling andcompletion operation activities which are caused mainly by the drilling and completion fluids and otheradditives that may have been used during these operations on the wells. However, in spite of theseimpairments, proper design and execution of well clean up post completion of the well is critical toensuring that whatever debris left behind at the sand face completion is removed or reduced to improveand optimize the productivity of the wells. Designing the appropriate clean up recipe requires properevaluation of the wells characteristics, conditions, reservoir properties, and the drilling and completionsfluid including additives.

The clean up recipe design usually involves laboratory tests on mud samples from subject well, todetermine the right content and quality of chemical that will properly dissolve the filter cake formed downhole.

Understanding the sand face completion type, well geometry and reservoir characteristics is a criticalsuccess factor to designing the placement of the recipe and achieving a successful well clean up. Effectiveplacement of the designed treatment recipe is mainly done using coiled tubing. Whether to squeeze or jetacross completion interval is subject to the well configuration.

In this paper, the Post clean up productivity of twenty-three (23) wells carried out in six (6) fields werereviewed to establish the relationship between the cleanup design, execution and productivity. Results ofthis shows that in the cases where proper design and execution of clean up were carried out, the wellsproductivity was successfully achieved as planned whereas in some cases of poor design and execution,the wells performance was suboptimal.

IntroductionThe twenty three (23) wells under discussion are located in six (6) different fields, in the Niger Delta areaof Nigeria where the lithology is generally sandstones and clays. The formations in these fields range fromconsolidated to unconsolidated with permeabilities as high as 1.5 Darcies and generally have normalpressure (2000-4800psi) and temperature (150-185deg F).During drilling, weighting materials such asbarites are often added to the drilling mud, Pseudo-oil based muds are often used in sections where clayswelling is imminent and in some cases drill-in fluids are left in the drain section. In some cases where

perforations were carried out, loss circulation materials were used to cure losses and provide a filter cakeagainst the formation.

The wells are grouped into different categories based on type of well (oil or gas), completionphilosophy and the drill-in fluid/material used in the completion intervals.

The focus of this paper is to present the key factors necessary in designing the appropriate clean uprecipe(s) and placement technique(s) to suite individual wells with the aim of highlighting what workseffectively.

StimulationThe assortment of drilling fluid pumped down the well during drilling and completion can often causedamage to the surrounding formation by entering the reservoir rock and blocking the pore throats (thechannels in the rock throughout which the reservoir fluids flow). Similarly, the act of perforating can havea similar effect by jetting debris into the perforation channels. Both these situations reduce the permea-bility in the near well bore area and so reduce the flow of fluids into the well bore.

A simple and safe solution is to pump chemical mixtures from surface into the well to dissolve theoffending material. Once dissolved, permeability should be restored and the reservoir fluids will flow intothe well bore, cleaning up what is left of the damaging material. After initial completion, it is commonto to clean up any mud and skin damage. In this situation, the process is loosely referred to as wellstimulation.

The chemical required to stimulate the new well varies across board.

Treatment DesignThe goal of the treatment design is to achieve an optimal design to dislodge the filter cake formed fromdrilling materials and dissolve them, there improving the well productivity.The design of the treatmentinvolves two parts: the chemical and the mechanical design.

The chemical design involves breaking down polymer chains and dissolving weighting materials thathave been used in the drilling and completions phase. The mechanical involves inducing sufficientpressure to dislodge and erode the filter cake that may have been formed at the sand face.

The chemical design is usually based on the material spotted in the individual completion intervals ofthe wells.Samples are sent to the laboratory to test for the best recipe to completely dissolve mud andimperatively, the volume of treatment to be pumped per well is determined.During the planning, corrosionissues that may be encountered due to treatment fliud compositon are addressed with inclusion ofanti-corrosion substances in the recipes. Nine (9) of the oil wells had LCM pills spotted after experiencinglosses post perforation, the four (4) gas wells had water based drill in fluid that comprise of polymers,spotted in the drain hole prior to installing the EGP, the other ten (10) wells had POBM in the producibleinterval prior to running the completions.

Acid treatmentsIn cases of removing filter cake induced by drilling materials, acids are typically effective in dissolvingcarbonates, saturated salts. There are several types of acids: Hydrochloric (HCl), Hydrofluoric acid (HF),Acetic acid, Formic acid and Sulfamic acid.In the case of the wells in focus, Hydrochloric acid is the mostsuitable candidate for dissolution of the filter cake induced by the LCM pills that have been spotted in thewells.

It is imperative to note that acid can create a number of well problems which include;

Realease of fines which may further plug the formation Formation of emulsions Sludge creation

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Corrosion

Additives are therefore included in designing the acid treatments.These include surfactants, Seques-tering agents, mutual solvents and corrosion inhibitors.The functions of these additives;

Surfactants: are used to reduce surfaces and interfacial tension, to prevent emulsions and to waterwet the formation.

Sequestering agents: Also known as Iron control, inhibits precipitation of iron as HCl is spent Mutual solvents: they are routinely used in a range of applications, such as removing heavy

hydrocarbon deposits, controlling the wettability of contact surfaces before, during or after atreatment, and preventing or breaking emulsions.

Corossion inhibitors: temporarily slows down the reaction of acid on metal.

Typically, HCl concentration ranges from 15 35% concentration and duration of acid in the wellbore(usually not for long) is determined while conducting laboratory tests.A number of factors affect acidreaction rate which include; acid concentration, area per unit volume of acid, formation temperature andpressure which are all factored into the design of the acid treatment.For example; the higher the bottomhole temperature, the quicker the acid gets spent.

Solvent Soak TreatmentsSolvents treatments can be used either as a cleanout fluid to remove mud from the wellbore and formationor as a formation breakdown fluid.For cases where parafins, olefins, ashphaltenes are present; solvents arerequired to completely dissolve them. For the POBM used to drill the reservoir section of the wells, thebase oil is the linear paraffin, light in nature as the Carbon-Carbon bonding is not very strong and low inaromatic content.Because parafins do not chemically react with either concentrated alkalines or acids, toeffectively dissolve/decompose paraffin, xylene, diesel or kerosene based solvents are used.Additive areadded to the xylene, diesel or kerosene based solvent treatment, as in the acid treatment;

Surfactants: are used to reduce surfaces and interfacial tension, to prevent emulsions and to waterwet the formation

Mutual solvents: ensures that treating fluids remain compatible in the formation, reduces surfactantadsorption onto the formation and helps water-wet sandstone formations.

There are also cases where an acid treatement is used ahead of solvent treatment.This is usually a casewhere acid soluble materials have been used in the mud system.

Delayed (Organic) Acid based Filter cake breaker systemDrill-in fluid (DIF) filter cake deposits are one of the major causes of restricted flow from the producingformation after a new well has been completed. Filter cake breaker is usually dissolved in the carrier brineof choice and spotted over the production interval of interest. Because the acid liberation action is slow,the solution contains little or no free acid when placed across the zone to be treated. Acid will be generatedover the subsequent number of hours. The acid released reacts with and dissolves the calcium carbonateand polysaccharides in the filtercake, thus destroying the filtercake and removing mud damage.

They are more effective in:

Long treatment intervals of horizontal wells Gravel packed wells Water-based drill-in fluid cleanup Synthetic- or oil-based drill-in fluid cleanup Stimulation of dolomite and limestone formations in new and mature wells

Additive added to the filter cake breaker system includes;

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Surfactants: are used to reduce surfaces and interfacial tension Corrosion inhibitors: To prevent acid corrosion. Corossion inhibitor added is different from what

can be added to HCl acid system, as organic acids aregenerally weaker than HCl.

Tools DescriptionThe fluid placement is carried out using Coiled tubing.At the bottom of the coiled tubing BHA, is the washnozzle where the placement fluid exits.The wash nozzles are offered in different configurations, depend-ing on what is to be achieved from the fluid placement.For newly completed wells, especially wherescreens are present, washing across the screens is very critical to create a force strong enough to dislodgethe debris inside/behind the screen slots. In most of the wells, the coiled tubing was run to the bottom ofthe completion interval and the treatment fluid is pumped through the jet nozzle across the screens inseveral passes to induce filter cake pop off.

More recently introduced, the rotary jetting nozzle which was used in the case of these wells, isdesigned to create an impact force and permits 360-degree coverage using two jets.The rotary jet tool isalso designed to accommodate the most aggressive fluids and accommodates bottom hole temperatures ashigh as 392F (200C), which is still within range of all the wells in this case.

One very important aspect of the rotary jetting tool is that the rotational speed is not a function of flowrate or bottom hole temperature.This means that the required critical speed can be achieved irrespectiveof the pump rate.Jet selection is based on the fluid pump rate and completion geometry.

These selections are done using software for simulations to analyze the performance of various jetprofiles and determine the flow rates.

Case studiesThe twenty three (23) wells in this case study will be grouped according to the DIF/LCM pill, asapplicable, contained in the producible interval, completion type and configuration.They are grouped intothree (3) as follows:

Four (4) gas wells (Wells K-1, K-2, K-3, K-4)These four (4) gas wells are deviated gas development wells completed with EGP and seven (7) inchchrome tubulars. The reservoir section was drilled with a water based drill-in fluid which containedpolymers.The mud samples (all similar) from the reservoir section was tested in the laboratory for testingwith the reservoirs BHT ranging between 150 -190 deg F.The most suitable selection to dissolve the filtercake formed by the DIF used was the delayed (Organic) Acid based Filter cake breaker system.

RecipeDESCRIPTION CONCENTRATION/Mgal

KCl 9.4ppg

pH Adjuster 24 lbs.

Enzyme 0.20%

Surfactant 0.50%

Corrosion Inhibitor 0.20%

Filter cake Breaker 12%

The clean-up operation involved the deployment of 1-1/2 (CT) fitted with the jet nozzle to the bottomof the completion intervals, and the treatment fluid jetted across from bottom to top of the interval inseveral passes and was left to soak.

The time duration for the treatment soak was six (hours) based on laboratory tests carried out at thedesign stage.The total volumes are calculated based on the individual lengths of completion.

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The wells at the point of clean up, are not yet hooked up to the production facility.The main parameterused to evaluate the proper clean up of these wells is primarily the FTHP on a fixed choke.The expectedFTHP on a fixed choke is predicted by simulation with /10% margin.

Fig. 1 below shows the FTHP performance based on variance from expected.

Ten (10) oil wells (Wells C-1, C-2, C-3, C-4, C-5, C-6, C-7, C-8, C-9, C-10);These ten (10) oil wells had POBM in the reservoir section as drill-in fluid prior to running thecompletions.Nine (9) of the wells are horizontal and four (4) out of them are completed with SAS for sandcontrol and the remaining five (5) have pre-drilled liners installed for borehole support.Only one (1) ofthese wells is deviated and it was completed with SAS for sand control. Wells C-1, C-2, C-4, C-5 & C-6were completed on the same reservoir and have the same completion philosophy, pre-drilled liner in thehorizontal section for borehole support.Also C-7, C-8 & C-9 were completed on the same reservoir andhave the same completions installed, SAS for sand control.

In these ten (10) wells, the BHT ranges from 150 180 degF and BHP ranging from 2500 4500 psi.To properply dissolve the paraffin in the POBM used as DIF, a solvent soak treatment was applied in

all these wells. Primarily the base content being Diesel and Xylene including other additives.

Solvent recipeDESCRIPTION CONCENTRATION

Xylene 50%

Diesel Oil 50%

Mutual Solvent 5% of XyleneDiesel

Surfactant 5% of XyleneDiesel

The treatment fluid was spotted using coiled tubing.The coiled tubing was run to the bottom of thecompletion interval and the treatment fluid was jetted across the screens in several passes, and some ofthe fluid was squeezed and left to soak with a treatment time of 12hours.

Nine (9) oil wells (Wells A-1, A-2, A-3, A-4, A-5, A-6, A-7, A-8, A-9)These nine (9) oil wells were cased and perforated completions.In all cases, perforation was done inoverbalanced completion fluid, and hence losses were experienced after the perforation.LCM pills were

Figure 1Wells K-1 to K-4 % variance of actual FTHP to expected

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spotted to cure the losses.These LCM pills, typically CaCO3 or other saturated salts, form a thin filter caketo prevent further seepage of the overbalanced completion fluid into the formation.

Acid treatment was used for these nine (9) wellsclean up.

Acid treatment recipeDESCRIPTION

Raw acid (HCl)

Corrosion inhibitor

Surfactant

Iron Control

Mutual Solvent

Figure 2Wells C-1 to C-10 % variance of actual FTHP to expected

Figure 3Wells A-1 to A-9 % variance of actual FTHP to expected

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Figure 4Typical completion schematic for wells K-1 to K-4

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ResultsThe wells at the point of clean up, are not yet hooked up to the production facility.The main parameterused to evaluate the proper clean up of these wells is primarily the FTHP on a fixed choke.The expectedFTHP on a fixed choke is predicted by simulation with error /-10% margin.

Fig. 1 below shows the FTHP performance based on variance from expected for the first group of wells(K-1 to K-4)

The actual recorded FTHP after the cleanup was in all the wells higher than predicted, an indicator thatthe well was properly cleaned up and impairments removed.For these set of wells, the design andexecution of the well clean up was properly done.

Fig. 2 below shows the FTHP performance based on variance from expected for the second group ofwells (C-1 to C-10)

Eight (8) of the wells in this group had % variance in FTHP from prediction within 10% whichindicates the well was properly cleaned up as predicted.

C-3 had a -20% variance.In the case of this well, the formation was exposed to drilling fluids invasionfor a longer time than planned due to wellbore instability issues.It is expected that the damage will bemore severe than expected from regular esposure time.

C-7 had a -47% variance.In horizontal completions, especially in cases where Screens are installed ascompletions as in the case of this well, it is very critical that the coiled tubing is run to the bottom of thescreen and the treatment fluid jetted across the screens to properly wash and induce filter cake pop off.Ifthis cannot be achieved, the productivity of the well cannot be fully restored.

Figure 5Typical completion schematic for wells C-1 to C-10

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Figure 6Typical completion schematic for wells A-1 to A-9

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Fig. 3 below shows the FTHP performance based on variance from expected for the second group ofwells (A-1 to A-9)

Five (5) of the wells in this group had % variance in FTHP from prediction within 10% whichindicates the well was properly cleaned up as predicted.

Well A-9 did not flow after treatment.The treatment was not effective to dissolve the filter cake in theperforations.Post-clean up, the mud samples taken from the well was tested in the laboratory.It was foundthat solvent treatment would properly dissolve it.

ConclusionsDesigning the appropriate recipe a new well involves understanding most importantly the contents of theDIF in the completed interval, the type and length of the completion.From the case studies on the twentythree (23) wells, the following conclusions can be drawn;

1. It is important to carry out laboratory tests to determine the most suitable treatment to use; Insome cases, analogues can be used to select the treatment fliud based on wells that werecompleted in the same reservoirs and have similar completions.

2. The well geometry is critical to deploying the coiled tubing to bottom especially for horizontalwells.

3. Where proper chemical design/execution is not done, the well productivity is jeopardized.4. In cases where proper clean up was achieved, the performance was between 90 120%; the cases

where the clean up objective was not achived, the performance ranged between 0-80%.

AcknowledgementsThe authors would like to thank the Mangement of Shell Petroleum Development Company of NigeriaAsset Development team, Completions and Well Interventions team, Drilling/Mud Engineering team forpermission to present and publish this paper.

Nomenclature

BHA - Bottom Hole AssemblyBHT - Bottom Hole TemperatureCT - Coiled TubingDIF - Drill in FluidEGP - External Gravel PackFTHP - Flowing Tubing Head PressureLbs - poundsLCM - Lost Circulation MaterialPOBM - Pseudo Oil Based MudPpg - Pounds per gallonSAS - Stand Alone Screen

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paper IPTC 11652 presented at the 2007 International Petroleum Technology Conference, Dubai,December 4-6

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5. Beall, B.B., Brannon, H.D., TjonJoePin, R.M., ODriscoll, K., BJ Services Company.: Evalu-ation of a New Technique For Removing Horizontal Wellbore Damage Attributable To Drill-inFilter Cake paper SPE 36429 presented at the 1996 SPE Annual Technical Conference &Exhibiton Denver, U.S.A October 6-9

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Maximising Well Productivity Through Appropriate Design of Cleanup Recipe and Proper Execution f ...IntroductionStimulationTreatment DesignAcid treatmentsSolvent Soak TreatmentsDelayed (Organic) Acid based Filter cake breaker systemTools DescriptionCase studiesFour (4) gas wells (Wells K-1, K-2, K-3, K-4)RecipeTen (10) oil wells (Wells C-1, C-2, C-3, C-4, C-5, C-6, C-7, C-8, C-9, C-10);Solvent recipeNine (9) oil wells (Wells A-1, A-2, A-3, A-4, A-5, A-6, A-7, A-8, A-9)Acid treatment recipeResultsConclusions

AcknowledgementsReferences