SPE-171015-MS

Eco-Friendly Biodegradable Materials For Zonal Isolation Of MultiplePerforation Clusters During Refracturing Of a Horizontal Well: Case HistoryFrom Marcellus Shale Hydraulic Fracturing

David Arnold, Anastasios Boulis, and Francisco Fragachan, Weatherford International

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Eastern Regional Meeting held in Charleston, WV, USA, 2123 October 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

For some time now the industry has been concentrating on horizontal wells in shale formations whichrequire multi-stage fracturing operations to tap their production potential. Several different techniqueshave been developed for performing these operations each with their own advantages and drawbacks;however, when it comes to performing re-fracs the isolation of zones to be treated presents some majorproblems in logistics and operating costs no matter the initial fracturing operation method used.

A key characteristic in unconventional reservoirs development is that they need massive hydraulicfracturing to create high permeability conduits to connect the reservoir to the wellbore and assureappropriate flow rates to make the development economical. In general, production performance for thistype of reservoirs show an early high inflow followed by a steep decline. Refracture jobs have been acommon practice to mitigate the flow rate decline and revitalize wells productivity. In the cases whereinfill refracturing is used, the isolation of the existing perforations is imperative, to mitigate the loss offluids during the refracturing process.

One approach involves the use of mechanical diverters, such as perforation balls, rock salts etc., whichmay be useful for one stage; however, cessation of pumping operations results in them falling out of theperforations or dissolving. In this paper, a new approach is presented using degradable materials. Thesematerials can be tailored to plug perforations on a temporary basis but degrade over time, depending onformation temperature, to re-open them. This method has applications in initial fracturing operations asan alternative to the often used plug and perf. method and can be extremely advantageous in refracturingoperations in plugging off the existing, productive perforations and exposing the unproductive ones fortreatment. It also provides similar advantages in wells with existing perforations, where additionalperforations are added and need to be treated.

Moreover, a case study is used to show the application of these degradable materials in a refracturingjob in a shale gas well in the Marcellus Shale. Based on the results, the sealing agents successfully bridgedoff the existing perforations, diverting the refracturing fluid to the new clusters. In this paper the authorwill discuss the evolution of fracturing methods and describe the development of degradable materialswhich can degrade over time for a range of bottom hole temperatures. He will continue to present a case

study of its use in a refracturing treatment in a well with 26 sets of existing perforations and 24 sets ofrecently perforated clusters, with the intent to seal the older clusters and begin accessing the new clusters.

IntroductionIn the past decade the most significant technological breakthrough in the Oil & Gas industry has been theshift towards horizontal drilling. This has led to single wellbores access more viable rock downhole, asmaller footprint on surface, allowing more wells for increased drainage and more efficient workflowthrough sliding-sleeve systems and plug and perf zipper fracturing operations. This has also shiftedfocus from traditional reservoir rock or conventional wells to the tighter source rock with lowerpermeability commonly referred to as unconventional wells, and most often to referred to as shale plays.

With this focus trending towards operational efficiency and time savings, there is less emphasis on astage by stage design approach, often leaving unstimulated reservoir that has the potential to produce morehydrocarbons. Many shales exhibit steep productivity declines within a short amount of time (some shalesshow a 60-80% decline within a year, Jayakumar et al. 2010) due to proppant crushing, embedment, lowreservoir permeability, ineffective initial fracturing jobs, etc. This leads to the potential of manyre-fracturing candidates in the major shale basins.

That leads to several important questions: which wells should be refractured? What type of treatmentand by which operational method? This paper will focus on proper candidate selection, current completionmethods and a case history relating to the Marcellus Shale.

There are several operational approaches to re-fracturing well, some of which require sealing-offexisting zones and losing the previous fracturing treatment production, while others require installingsophisticated downhole tools. The following section presents some of these tools:

Sliding Sleeve SystemsThese are completion tool systems that can be custom tailored to allow access ports to open directly at thereservoir area of interest. A sequence of varying sized, or more recently same sized balls can be droppedat designated time to open the ports on the sleeve, allowing the fracturing treatment to have access to theformation. These systems allow for increased efficiency and zonal accuracy, deployment of a slidingsleeve system on an existing well could be costly and time consuming. Also, there are rate and frictionpressure considerations.

CT ToolsAdvanced tools can be deployed on coiled tubing that can be used to isolate zones of interest in thewellbore for a more precise treatment. This approach can provide the over the zone of interest accuracymuch like the sliding sleeves systems but also has similar drawbacks. Coiled tubing has a smaller innerdiameter than traditional casing used during the fracturing treatment, which allows for less rate andproppant concentrations. Additionally, there are costs associated with using coiled tubing and there maybe availability issues and wellsite size limitations.

Perforation SqueezeSealing the existing perforations with cement (perf squeeze) can be an alternative to using a downholecompletion system or coiled tubing. A cement slurry is pumped into the perforations and the existingfractures in order to seal and allow for new perforations to be shot during refracturing operations. Thetreatment would start at the toe or end of the casing, and work stage by stage up the well; similar to howthe initial fracturing operations were conducted. The application of this method will eliminate all existingperforations and might potentially damage the original producing fractures.

Mechanical & Chemical DivertersA more simplistic approach, considered a tried and true method, of sealing perforations and zones arethe mechanical diverters. Traditionally this is accomplished by pumping ball sealers or degradable ballsin the fracturing fluid to seat on the perforations and diverting fluid away from the sealed perforations.

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Operationally the perforation ball method is very easily deployed; needing only a ball gun on the surfaceand they can be pumped at designated times during the treatment. The main drawback is due to themulti-zonal nature of wells. Constant pressure to maintain the balls seated on the perfs is needed, and ifat any point the treatment needs to be shut-down (i.e. equipment maintenance), these balls will unseat.French et. al. (2013) presented the results of refracturing treatments that utilized perf balls and biode-gradable ball sealers as a means of temporary perf cluster isolation. While operationally successful therewere challenges and concerns related to their use; In a horizontal well, achieving efficient seating actioncan be especially challenging, given that the ball sealers may settle to the bottom of the casing during thepumping of the treatment, this is especially true when the stimulation treatment suffers any form ofinterruption or drop in the velocity below critical flow . . .Furthermore, if the hydraulic fracturingoperation is temporarily halted (for any reason) due to operational or mechanical issues, then much of theprevious ball diversion effect that was achieved could be subsequently lost. There are other types ofmechanical diversion that are commonly used, such as rock salt, benzoic acid flakes and other solidsubstance that will actually enter into the perforations and near wellbore fractures, allowing a bridgingeffect. The drawback of these materials is they are short-lived and will degrade, quite possibly before allthe fracturing treatments have been completed.

Ideally, a method of achieving zonal isolation and diversion by incorporating the simplistic nature ofmechanical diverters, that will enter into the perforations and will allow for shut-downs when necessary,with the life-span that can last the time frame needed to complete the entire well should be applied.

Degradable DivertersRevisiting the concept of mechanical diverters has led in recent times to consideration of using degradablematerials to plug off perforations, providing zonal isolation for a temporary basis. The use of suchmaterials is commonplace in the medical profession (degradable sutures which dissolve over time) and inthe food industry where biodegradable material is being used for food packaging that will degrade overtime in landfills and address some of todays ecological concerns. However, this type of material degradesat temperatures lower than the temperatures associated with downhole conditions in oil and gas reservoirs.In order to set standards for the development of a biodegradable material, a set of criteria was developedfor its use at elevated temperatures (Erbstoesser et. al.):

The need to be robust enough to survive the placement process associated with fracturingoperations.

Its shelf life should be such as to enable use in worldwide operations, to survive long term storagein all climates and be unaffected by compaction.

It should require no secondary treatment to enable degradation, i.e. self-starting. Particle size should be such that it is compatible with pumping equipment and can pass throughthe fluid end of a pump.

It should be cost effective compared to existing methods. It should be capable of temporary blockage in unknown geometries and borehole configurations.

Biocompatible and biodegradable polymers have recently attracted a lot of attention from advocates ofsustainable development of green chemistry. These materials are made from renewable sources, havea small carbon footprint and are environmentally friendly with properties that can compete withnon-biodegradable plastics. These materials are attractive because they are naturally occurring compoundsand chemists have been working hard of late to tailor their properties to make it practical for a wide varietyof applications. After experimental testing, these polymers have been developed that can degrade in abroad temperature range, over an expansive time range and meet all the desired field application criteria.Using a particle diverter system and temperature ranges to be able to address the widest possible rangeof downhole conditions.

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The combination of these two polymer materials which are degradable and environmentally acceptablecan be tailored for use in two temperature ranges, the LT version is for temperatures in the range 130 -180F (54 - 82C) and the standard version for temperatures in the range 180-300F (82 - 149C). Whenmixed with a viscous water-based carrying fluid, suitably gelled to allow them to remain in suspension,they can be pumped into the well and bridge off the existing perforations. It can be used in severalapplications:

During the flush following the fracturing operation to seal off & provide zonal isolation to theinterval just treated a plug and perf type operation.

In vertical and/or deviated wells treated through tubing where use of bridge plugs is not possible. Sealing off existing perforations to prepare for refract rather than squeeze cementing them. For zonal isolation in horizontal wells before treatment to seal off existing perforation cluster thisis particularly applicable to refract candidates.

This material combination will effectively block existing perforations, frac channels and sliding sleeveports with typical 300 1,000 psi pressure increase and when in place is stable under large pressureincreases upwards of 4,000 psi.

Case StudyA unique case is presented for the use of degradable diverter technology. A horizontal shale gas well thathad been previously fractured was chosen to be re-stimulated and all the existing perforation clusters werescreened-off with high concentrations of sand. Afterwards, additional perforation clusters were shot inpreviously bypassed formation. These perforations were interspersed between the older clusters. Afterfurther consideration the operator decided to seek out more permanent sealing and isolation of the olderclusters that had already be stimulated in order to divert the new fracturing treatments solely towards thenewly perforated clusters. This would be an interesting challenge and a way to showcase the ability ofin-situ mechanical diverters to seal existing clusters while there were already new perforation clustersdownhole.

This candidate re-fracturing well is located in the southwestern Marcellus Shale in Pennsylvania. Theinitial fracturing treatments were conducted using traditional plug and perf type completion, since there-fracturing treatment was to be conducted through the existing production casing, and there were nodown-hole tools deployed. The treatment was designed around typical Marcellus treatments for desiredrate, water volumes and sand mesh sizes and concentrations for that area.

The re-stimulation candidate well was drilled to a True Vertical Depth (TVD) approximately 6,500=deep and had a Measured Depth (MD) approximately 9,400= in length, and 5-1/2= 20lbs/ft. P-110 casingstring was cemented in place. The initial treatment had 26 clusters of 12 perforations each, for a total of312 perforations, the new sets of perforations were in 24 clusters with 8 perforations per cluster for a totalof 192 virgin perforations.

Certain equipment considerations need to be taken into account when pumping these diverters. Thelarge nature of some of the particles can remain in the valves & seats of the frac pump fluid ends. To avoidthese complications and maintain the material in a concentrated batch, the material is deployed onsecondary equipment isolated from the main frac fleet.

Before the first treatment stage could be placed, the diverter needed to be run until there was confidencethat all, or the majority of the existing perforation clusters had been properly diverted. It was also knownthat the typical breakdown pressure of new perforations for this area ranged from 6,500 psi to 7,000 psiand this would be important for determining amount of the sealing material. The process involved the mixof a batch of the diverter, which was pumped at a rate of 25 bpm and flushed until the diverter would reacha perforation cluster, indicated by an increase in treating pressure. This was repeated for a total of six

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times, and on that sixth deployment a sharp pressure increase occurred with several breakages, at whichpoint the first fracture stage began (Figure 1 and Figure 2).

The treatment stages were treated in typical fashion: perform a low rate breakdown, acid spearhead,PAD stage, typical proppant concentrations and amounts for that region and followed by a flush.

Figure 1Deployment of the degradable diverter in six (6) separate batches - Pressure was gradually increased from 4,800 psi to above 6,500 psi,which is the typical breakdown pressure for new perforations in this region.

Figure 2Deployment of the diverter in six batches, followed by the first fracturing treatment

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The initial treatment of the well called for eight stages of equal design to be performed, and the finalresult was the successful placement of seven and half of the eight total stages, finishing in a screen-out.The last few stages had treated at a higher pressure than initial stages and that was likely due to fewerperforations clusters taking good fluid after so many diversion runs. The early diversion design called forhigh concentrations of diverter material per perforation, and after a few very high diversion pressureresponses, that amount was reduced by 75%. The fourth stage alone saw a sharp 3,000 psi increase (Figure3) when the diverter sealed perforation clusters, the material remained intact after that pressure spike, andit was at that point it was decided to reduce the concentration of the diverter.

ConclusionsThere are many new shale plays coming to the forefront in the coming years, but the re-fracturing ofexisting wells will also play an important role for restoring declined production.

An array of downhole tools (sliding sleeves, coiled tubing etc.) is available to perform this type oftreatment. However, some of these methods while effective can be costly, time consuming and may evenlimit the treatment design due to rate restrictions and increased friction pressure.

In the case where infill refracturing is decided, and the existing perforations have to sealed, a series ofalternative methods have developed. Mechanical diverters are commonly used due to their operationalsimplicity and cost effectiveness. However, this type of diverter exhibits some operational challengeswhich have to do with the maintenance of the pressure on the materials to constantly divert the fluid. Othermethods, such as cement squeeze have been applied. This method requires less intervention; however, theexisting perforations are permanently sealed and can potentially damage the zones that are still contrib-uting to the flow.

A new temporary sealing agent, which involves chemical and mechanical diverters is introduced in thispaper. These diverting agents will enter the perforations and bridge-off, giving that long seal that allowsfor the multiple shut-downs that occur during a refracturing treatment. This method offers a viable

Figure 3Prior to treating the fourth stage, the diverter batch had a 3,000 psi pressure spike when entering the perforation clusters

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alternative to using completion tools as well as traditional mechanical diverting agents. Another advantageof using this system is there would be no intervention needed post-treatment (mill plugs, retrieve slidingsleeves etc.). These materials degrade over a period of depending on the downhole temperature and themajor or all of the material would return as a liquid, in some instances there may still be some semi-solidmaterials that can be collected in a sand separator. The results of the application of these diverters in ashale gas well in the Marcellus Shale were presented. Based on these, the diverting agents successfullysealed the existing perforations (pressure increase until it reaches the breakdown pressure of the rock) anddiverted the fracture fluid to the new clusters.

In general, this type of diverters can replace several current fracturing or refracturing methods sincethey are more time and cost effective. Nonetheless, careful planning is needed in the concentration ofdiverter in each stage, the surface equipment | layout, and the criteria for what determines a successfulpressure increase and breakage for each zone.

AcknowledgementsThe authors wish to thank Weatherford for permission to publish this paper and their encouragement todo so.

ReferencesJayakumar, R., Sahai, V. and Boulis, A. 2010. A Better Understanding of Finite Element Simulation

for Shale Gas Reservoirs through a Series of Different Case Histories.Brocco, M., Polylactic Acid: The next generation of plastics, Organic Molecules, Project Chemistry

623.French, S., Rodgerson, J., Feik, C.: Re-Fracturing Horizontal Shale Wells: Case History of a

Woodford Shale Pilot Project, SPE 168607 paper presented at the SPE Hydraulic Fracturing TechnologyConference, The Woodlands, TX 4-6 February 2014.

Allison, D., Curry, S., Todd, B., CSUG/SPE, Halliburton, Restimulation of Wells Using Biodegrad-able Particulates as Temporary Diverting Agents, SPE 149221 prepared for presentation at the CanadianUnconventional Resources Conference on 15-17 November in Calgary, Alberta, Canada.

Erbstoessner, S., Cooke, Jr., C., Sinclair, R., Epstein, M. 1998. Use of Degradable Ball Sealers to SealCasing Perforations in Well Treatment Fluid Diversion. US 4,716,964.

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Eco-Friendly Biodegradable Materials For Zonal Isolation Of Multiple Perforation Clusters During ...IntroductionSliding Sleeve SystemsCT ToolsPerforation SqueezeMechanical & Chemical Diverters

Degradable DivertersCase StudyConclusions

AcknowledgementsReferences