Development of Degradable Seal Elements for Fully Degradable Frac Plugs Takeo Takahashi, Shinya Takahashi, and Masayuki Okura, KUREHA CORPORATION

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1 OTC MS Development of Degradable Seal Elements for Fully Degradable Frac Plugs Takeo Takahashi, Shinya Takahashi, and Masayuki Okura, KUREHA CORPORATION Copyright 2016, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference Asia held in Kuala Lumpur, Malaysia, March This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright. Abstract Demand for hydrocarbons is continuing to grow, leading to an increase in the stage count of hydraulic fracturing in unconventional reservoirs. The plug-and-perf system is a typical stimulation method that uses frac plugs for zone isolation. Conventionally, the plugs must be milled out to recover the flow path for hydrocarbons after fracturing is complete. Fully degradable frac plugs would eliminate the need for milling out, and this in turn allows for longer wellbores in the lateral direction, lower costs, and reduced risk in well completion. A key material for fully degradable frac plugs is a sufficiently degradable rubber with good sealing properties for the seal elements of the plugs. This paper presents the fundamental properties of degradable rubbers developed in a comprehensive materials study. The degradation behaviors of the degradable rubbers were investigated under various temperature and fluid media conditions. The mechanical strength was compared with that of conventional nitrile rubber. Nonlinear, three-dimensional multi-body contact analysis and qualification testing were integrated to estimate the sealing performance of seal elements made of the degradable rubbers. Introduction Tremendous technical advances in multistage hydraulic fracturing have recently enabled efficient recovery of unconventional oil and gas resources. The stage count in plug-and-perf systems, which are a typical technology for multistage fracturing, has steadily increased over the last decade (Griffin and Nicols 2012). The plug-and-perf method uses isolation plugs called frac plugs for zone isolation and diverting fracturing fluids into the perforations at the desired location in a wellbore. The frac plugs generally consist of a mandrel, a seal element composed of one or more rubber parts, cones, and slips. After the first stage of fracturing, a frac plug is inserted into the front side adjacent to the fracked zone in the cemented wellbore, and then set with a setting tool that can generate longitudinal tensile or compressive force. The force expands the rubber seal element radially outward, sealing the annulus between the casing and the mandrel. The force also expands the slips radially outward, creating an anchor to fix the plug at a predetermined place in the wellbore and to hold the rubber seal element blocking the annulus. A ball is dropped into the wellbore, isolating the zone to seal the through hole of the mandrel. Second fracturing is then performed. These steps are repeated from the toe to the heel section of the wellbore to achieve multistage fracture. The plug-and-perf system is more mature than other techniques for multistage fracturing, such as the sleeve system, and has long been used in the field. Moreover, frac

2 2 OTC MS plugs can be set in a wellbore without limitations as needed, so the fracturing location can be set flexibly (Burton 2013). Most current plug-and-perf methods require post-fracturing processes to remove the frac plugs and to recover the flow path for production fluids after the fracturing operations are complete. Milling out with coiled tubing (CT) has been the preferred method for removing the plugs. However, CT operations often face problems related to wellbore and reservoir conditions. Milling out with CT can be time-consuming, increasing operational costs (Wozniak 2010; Tompkins et al. 2013) and resulting in complications (Mallalieu and Burgos 2013). In addition, milling out with CT interferes with extending the length of the horizontal wellbore because of the limited maximum depth and extended-reach capability of CT. Intense focus has recently been placed on the use of degradable downhole tools for multistage fracturing. The degradable tools disappear after use, meaning that such an approach provides an alternative to milling out with CT. Degradable ball sealers for sleeve systems are already used commercially (Aviles et al. 2013; Xu et al. 2013; Okura et al. 2015). Polyglycolic acid (PGA) is widely used degradable polymer in ball sealers in North America. Compared with polylactic acid, polybutylene succinate, and other degradable polymers, PGA is mechanically strong at the temperatures typical of shale formations in North America. PGA has also been modified to improve mechanical strength and widen the pressure range of PGA ball sealers (Takahashi et al. 2015a). Moreover, PGA degrades in water, whereas the degradation of other materials often requires acids or salts as a trigger. Degradable materials, including PGA, can also be used for the body members of frac plugs, such as mandrels, cones, or slips, but it has been difficult to replace the seal elements with degradable materials. Even if the body members degrade, nondegradable seal elements may get stuck in the wellbore, decreasing the effectiveness of the production fluids. Sufficiently degradable seal elements with sealing performance equivalent to the rubber materials that are used in conventional frac plugs are highly desirable. We have developed degradable rubber materials through a comprehensive material study. The rubbers degrade in the presence of water with no degradation triggers such as acids, alkalis or salts. Several grades of rubbers for different temperature ranges can be obtained. The durometer hardness of the rubbers can be customized between A60 and A100. In this paper, we describe the properties of degradable rubbers that can be used in a temperature range of F. The mechanical strength and degradability under various conditions are discussed and compared with conventional nitrile rubber (NBR) to evaluate the suitability of the degradable rubber as seal elements for frac plugs. The sealing performance of the degradable rubber obtained from tests with a specially designed high-pressure vessel is also reported. Material Properties of Degradable Rubbers Degradability Table 1 shows the changes in the appearance and the durometer hardness of the degradable rubber that has an initial durometer hardness of A82 as a function of immersion time in deionized water (DI) at 250 F. The hardness of the degradable rubber decreases with time and finally reaches A0 after 48 h immersion. The mechanical properties of the degradable rubber, such as tensile elongation at failure, tensile strength, and compressive strength, also deteriorate with immersion time, and the rubber easily collapses under a load when the hardness of the rubbers reaches A30. After further immersion of the rubber in water, for example, for 72 h at 250 F, the rubber degrades to a gel. The degradation rate does not depend on the sample thickness of the rubbers, suggesting that the degradation proceeds via a bulk degradation mechanism. Rubber that degrades to A30 or the gel state can easily fragment into small pieces in water. The results suggest that sufficiently degraded rubber can be recovered at the surface of the wellbore during the flowback process in well completion. Hence, we expect that the degradable rubbers will not decrease the production efficiency of oil and gas from wells by clogging the flow path.

3 OTC MS 3 Table 1. Appearance and durometer hardness of degradable rubber at 250 F. Immersion time (h) Appearance Hardness A82 A25 A0 A0 Figure 1 shows that the time for the rubber to reach a durometer hardness of A0 decreases with increasing temperature. The linear relationship indicates that the degradation rate of the degradable rubbers has an Arrhenius-type temperature dependence. Although the degradation rate of the rubber at 270 F is too fast for use in the wells in this temperature range, the rubber could be used if fluids were injected prior to inserting the plugs to decrease the well temperature temporarily. Moreover, for use in higher or lower temperature ranges, the degradation rate of the rubber can be adjusted by changing its chemical structure and additives. Figure 1. Temperature dependence of the time to A0 of the degradable rubber in DI water. Table 2 shows the changes in the appearance and the durometer hardness of the degradable rubber that has an initial hardness of A82 during immersion in DI water and in aqueous 3% HCl at 176 F. Although the hardness of the degradable rubbers decreases with immersion time in DI water at this temperature, the rate is slow. In 3% HCl, the degradation proceeds faster than in DI water; fine cracks initially appear on the surface of the rubber, and subsequently, the material on the surface disintegrates into small pieces. Fragmentation begins faster in 3% HCl than in DI water. The core of the rubber maintains a durometer hardness of more than A30, at which the degradable rubber easily collapses. This result suggests that acids affect the surface more than the core of the material. In contrast, alkaline and salt solutions do not affect the degradation rate and mechanism (data not shown).

4 4 OTC MS Solution Deionized water Table 2. Appearance and hardness of degradable rubber at 176 F. Appearance Immersion time (days) Hardness A82 A81 A81 A75 3% HCl aq Appearance Hardness A82 A77 A69 A0 Mechanical Properties Figure 2 shows the tensile and compressive stress-strain curves of the degradable rubbers that have a durometer hardness of A80 and A90. The stress-strain curves of A80 NBR, which is widely used for seal elements in conventional composite frac plugs, are also shown in Figure 2. The tensile elongation at failure for the degradable rubbers is over 400%, whereas that of NBR is around 300%. These data indicate that seal elements made of the degradable rubbers may have a lower risk of failure during actuation or fracturing operations compared with NBR. The stress-strain curves of the degradable rubbers in tensile and compressive mode are similar to those of NBR. This fact suggests that the deformation behavior of the degradable rubbers is close to NBR; therefore, the degradable rubbers can replace the conventional NBR seal elements without significant design change. (a) (b) Figure 2. Stress-strain curves of degradable rubbers and NBR in tensile mode (a) and compressive mode (b) at 150 F. Application of Degradable Rubbers for Seal Elements in Degradable Frac Plugs The degradable rubbers could be used as seal elements in degradable frac plugs. Seal elements are an essential part of frac plugs that maintains pressure during hydraulic fracturing. Typically, a seal element consisting of single or multiple parts made of an elastomeric material is placed between two cones that are disposed around a mandrel. When the setting force is hydraulically, mechanically, or electrically applied to the mandrel via a setting tool, the rubber seal element is axially compressed by the cones, and expands radially, blocking the annular space between the plug and the well casing, providing a tight seal

5 OTC MS 5 to maintain the hydraulic pressure. The sealing performance is especially important in this application; that is, the rubber seal element must withstand high pressures of over 10,000 psi without failure. Nonlinear finite element analysis (FEA) is the primary tool for design verification and optimization of the seal element (Al-Kharusi et al. 2011; Doane et al. 2012; Deng et al. 2013). We have previously reported the design capability and sealing performance of degradable rubber seal elements, as obtained by FEA and qualification tests (Takahashi et al. 2015b). Nonlinear FEA considering three-dimensional multi-body contact was performed to investigate the integrity of the degradable rubber seal elements for frac plugs. The computational models used a three-piece, multi-hardness seal element, which was axially compressed between the PGA cones in a 4.5 in, 9.50 lb/ft casing. The tensile and compressive stressstrain curves of the rubbers and PGA were modeled by a poly-linear approximation. Figure 3 shows the stress contours of the seal element after applying a 100 kn setting force. The red area in the figure shows that the stress is above the allowed level. There is no red area in the stress contours; thus, there is no risk of failure in this assembly on the setting force. The 4.5 in., 9.50 lb/ft casing has the largest internal diameter in a 4.5 in. casing in the API standards; therefore, a large deformation will be applied to the seal element during setting. The low risk of the failure for the degradable rubber seal element suggests that the good mechanical properties of the degradable rubber make it suitable for seal elements. Figure 3. Stress contours after applying a setting force of 100 kn. A specially designed high-pressure vessel was used to examine the sealing performance of the seal element made from the degradable rubbers. Figure 4 shows the cross-sectional view of the high-pressure vessel. The vessel with an inner diameter of 4.09 in. is equipped with a stainless steel mandrel in its center. In experiments, two PGA cones and a seal element in between the cones are placed around the mandrel, and a female screw part is attached to the mandrel. Tightening the female screw part exerts axial compression on the seal element and the cones, resulting in radial expansion of the seal that seals the annular space between the mandrel and the vessel. Hydraulic pressure is subsequently applied by the pressure generator to the seal element, and the pressure history is recorded. A three-piece, multi-hardness seal element made from the degradable rubbers with a 3.54 in. outer diameter was tested with the high-pressure vessel at a temperature of 150 F. The seal element consists of two backup elements and a center element in between the backup elements. The durometer hardnesses of the backup elements and the center elements were A90 and A82, respectively.

6 6 OTC MS The tests were carried out in the following two steps. 1) Maximum pressure test. The cones and the rubber seal element were axially compressed to a predetermined position. Subsequently, pressure was hydraulically applied until the seal failed. The pressure at which water leaked was defined as the maximum pressure. 2) Pressure holding test. The rubber seal element was axially compressed to the predetermined displacement at which the maximum pressure was 103 MPa (15,000 psi). The pressure was held for 10 min, and then released. Figure 5 shows the result of the maximum pressure test. With increasing displacement, namely the compression of the seal element, the maximum pressure increased. In this system, the maximum pressure reached was 103 MPa at a displacement 50 mm. Figure 4. Cross-sectional view of the high-pressure vessel. Figure 5. Relationship between maximum pressure and the displacement. Figure 6 shows the results of the pressure holding test for the assembly consisting of the degradable rubber seal element and PGA cones. The assembly held 103 MPa for 10 min with no sign of failure or leaking. The results indicate that the degradable rubber is suitable as a material for seal elements of frac plugs and has a sufficient safety margin for use in hydraulic fracturing of typical shale formations.

7 OTC MS 7 Figure 6. Experimental holding pressure for the sealing system with degradable rubber. Conclusion Degradable rubbers with appropriate degradation and mechanical properties were developed for use in seal elements for degradable frac plugs. Initial mechanical strength of the degradable rubbers is equivalent to or above conventional NBR, indicating that the degradable rubbers have higher reliability than NBR and can replace the NBR seal element without significant deign change. The durometer hardness and other mechanical properties of the degradable rubbers decreased as the immersion time in water increased, and the rubbers eventually deteriorated into small fragments or a gel. The immersion in 3% HCl accelerates the degradation. Nonlinear FEA and the qualification tests indicated that the seal elements made of the degradable rubbers functioned at pressures up to 100 MPa (15,000 psi). Acknowledgements The authors would like to thank the management of KUREHA CORPORATION for permission to present and publish this paper. References Al-Kharusi, M.S.M., Qamar, S.Z., Perez, T., and Akhtar, M Non-Linear Model for Evaluation of Elastomer Seals Subjected to Differential Pressure. Presented at the SPE/DGS Saudi Arabia Section Technical Symposium and Exhibition, Alkhobar, Saudi Arabia, May. SPE Aviles, I., Marya, M., Reyes Hernandez, T., Dunne, T., Dardis, M., and Baihly, J Application and Benefits of Degradable Technology in Open-hole Fracturing. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September 2 October. SPE Burton, A Unconventional Completions: Which one is Right for Your Application? Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September 2 October. SPE Deng, G., Doane, J., Ruffo, A., Shyu, G.D., and Collins, S Design Verification, Optimization and Validation of Ultra-HPHT Completion and Production Tools. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September 2 October. SPE Doane J., Deng, G., and Collins, S A Completion Technology Milestone - The First 25,000 psi 500 F Packer Seal System. Presented at the Annual Technical Conference and Exhibition, San Antonio, Texas, 8 10 October. SPE Griffin, J.N., and Nicols, C.M Optimization of Coiled Tubing Extended Reach Practices in the Bakken Region: A Case Study. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8 10 October. SPE MS. Mallalieu, R. and Burgos, R Stuck Coiled Tubing: Addressing the Risks in a Complex Operating Environment. Presented at the SPE/ICoTA Coiled Tubing & Well Intervention Conference & Exhibition, The Woodlands, Texas, March. SPE MS. Okura, M., Takahashi, S., Kobayashi, T., Saijo, H., and Takahashi, T Improvement of Impact Strength of Polyglycolic Acid for Self- Degradable Tools for Low-Temperature Wells. Presented at the SPE Middle East Unconventional Resources Conference and Exhibition, Muscat, Oman, January. SPE MS. Takahashi, S., Okura, M., Kobayashi, T., Saijo, H., and Takahashi, T Filler-Reinforced Poly(Glycolic Acid) for Degradable Frac Balls Under High-Pressure Operation. In Mechanics of Composite and Multi-functional Materials, Volume 7, Eds. Ralph, C., Silberstein, M., Thake, P. R., and Singh, R. pp , Springer.

8 8 OTC MS Takahashi, S., Okura, M., Kobayashi, T., and Takahashi, T Development and Verification of Degradable Sealing Elements for Fully Degradable Frac Plugs. Presented at the SPE Asia Pacific Unconventional resources Conference and Exhibition, Brisbane, Australia, 9 11 November. SPE MS. Tompkins, R., Smith, N. Wellhofer, B., Yuyi, S. Rhodes, B., and Stivers, P Factors Affecting Effective Millout of Multistage Fracturing Sleeves in Horizontal Wellbores. Presented at the SPE/ICoTA Coiled Tubing & Well Intervention Conference & Exhibition, The Woodlands, Texas, March. SPE Wozniak, G Frac Sleeves: Is Milling Them Out Worth the Trouble? Presented at the SPE Tight Gas Completions Conference, San Antonio, 2 3 November. SPE Xu, Z., Richard, B., and Solfronk, M Nanostructured Material Based Completion Tools Enhance Well Productivity. Presented at the International Petroleum Technology Conference, Beijing, China, March. IPTC MS.

9 tensile stress, MPa compressive stress, MPa Kureha Degradable Rubber (KDR) Key Features Higher mechanical strength than conventional nitrile rubbers. Degrades in water without injecting acid/alkali solution. Different grades / shapes are available in accordance with temperature range of wellbore and application. Grades Different grades are available in accordance with temperature range of wellbore. Mechanical properties (strength and hardness) are adjustable in accordance with application. Grade KDR01H KDR01M Temperature range High: 250 degf or higher Mid: degf Mechanical Strength Deformation behavior is similar to conventional NBR. Lower risk of failure: elongation at fracture is greater than NBR. 30 compliant with ISO KDR01H KDR01M NBR hardness KDR01H: A80 KDR01M: A90 NBR: A KDR01H KDR01M NBR hardness KDR01H: A80 KDR01M: A90 NBR: A strain, % strain, % Applications Degradable sealing elements Temporary protector/barrier for downhole tools Kureha can provide comprehensive technical services according to customer's demand. ver /10/21 The values shown above are typical values, not guaranteed values.