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1 SPE 1344 The Successful Evolution of Anton Irish Conformance Efforts D.D. Smith, M.J. Giraud, C.C. Kemp, M. McBee, J.A. Taitano, and M.S. Winfield, Occidental Petroleum; J.T. Portwood, Tiorco Inc.; and D.M. Everett, Halliburton Energy Services Copyright 26, Society of Petroleum Engineers This paper was prepared for presentation at the 26 SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, U.S.A., September 26. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 3 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box , Richardson, TX , U.S.A., fax Abstract The Anton Irish field was discovered in 1945, unitized in 195 for a produced gas pressure maintenance project and converted to a waterflood in In 1997 CO 2 flooding began and currently accounts for about 85% of the unit production. Presently, the entire field produces around 6,5 BOPD; 36.5 MMCFPD of recycled CO 2, and 69,2 BWPD. Over the years of flooding, various conformance problems have been identified and many attempts have been made to address these problems with limited to no success. In 23 a new program was initiated to re-evaluate the problems and design better solutions. This paper will outline the diagnostic efforts that were undertaken, discuss the basic findings of that effort, review the resulting solutions that were designed to resolve these problems, and show the results of this work. Basic Problem / Justification For many years the rapid breakthrough of the displacing fluid whether it was water or CO 2 has been recognized as a severe problem in the Anton Irish Field. This rapid breakthrough and excess cycling of CO 2 in the Anton Irish Clearfork flood has led to numerous wells being shut in due to limited processing capacity at the gas plant. In addition wells that had previously produced at relatively high oil rates became uncompetitive due to this excess gas production. Thus, in order to recover reserves from these areas, a solution needed to be found to address these excess cycling problems. A key complicating factor to deal with at Anton, is that almost all wells have open hole completions. This creates additional diagnostic problems as it is difficult to get to bottom with wireline on most injectors which prevents good injection profile data. Conformance Project Progression: Early in 23 plans were developed to expand on past experiences, identify key knowledge gaps, and refocus efforts that would lead to a better understanding of the conformance problems. This effort included: identifying key assumptions and understandings; researching historical data; determining additional data needs; running diagnostics; and evaluating this data in order to modify and improve our problem model. In this process we included the ability to learn from any solutions designed and executed, so that reformulated solutions would be more effective as our problem knowledge increased. Early Conformance Efforts: Prior to 23 the Anton conformance problems were perceived to be limited to relatively low volume near wellbore features or controlled by high permeability streaks. Efforts to control these features lead to the solution designs outlined in Table 1. Three things characterized this effort, first, all the treatments were relatively small in volume, second, all the solutions were perceived as near wellbore solutions, and third they were all unsuccessful in improving the control over the Anton Irish conformance issues. Reviewing additional reservoir monitoring data including, production and injection pattern rate analysis, interwell tracers, re-evaluation of core samples, ultrasonic image logs, Hall plot analysis, step rate tests, and downhole camera inspections, led to a new perception of the problem. Examples of this data are included in Figures 1 thru 6. As we continued to improve our understanding of the problem, all data pointed to large void space features, originally generated as induced fractures and enhanced over time by rock dissolution and/or erosion caused by the flooding process. Step rate tests which showed bottom hole pressure increases of < 2 psi for a 4,5 BWPD rate increase, and fast connections to wells ~1,2 away allowed us to calculate an equivalent of approximately 4.5 pipeline flow. Camera inspections also showed > 3 wide openings on the injectors and ½ wide openings on the producers, even though we were only able to effectively inspect small portions of the wellbores. Careful inspection of all this information led us to conclude that we have created massive void space flow conduits with relatively direct communication between many injectors and specific producers. Obtaining this diagnostic data, and the resulting drastic change that this made to our problem model, was critical to identifying and characterizing the root cause and allowing us to design a new and unique solution to control the inter-well communication for this field.

2 2 SPE 1344 Solution Design This new problem perspective brought with it a better defined focus to design an effective solution. However, we still faced one significant problem: just how big were these features and what would it take to control them. Analysis of the tracer breakthrough data and consultation with several conformance industry experts led to some initial volume estimates for the void system. However, the ultimate design came down to simply filling as much of the void as we felt we could achieve economically and logistically. In addition, a second tracer study including an additional row of injectors showed 7 injectors connected to 2 producers (Tracer breakthrough in Figure 7). This caused us to choose a non-standard conformance solution of addressing the flow in the producers rather than the injectors. The initial treatment designs focused on two factors. First, as stated earlier fill as much of the void space conduit as possible; and second, design a material to fill the system that will balance both strength and economics. The critical part of this was to design a system that was custom fit with enough strength to match or at least overcome the drawdown stress yet not be to expensive for the volume needed. The resulting design used 8, Bbls of chrome cross-linked gel, and 2, Bbls of foamed cement. The gel was designed with a lower concentration (weaker) to be pumped first and thus farthest away from the wellbore. This was followed by a thicker gel and foamed cement that graded from a weaker blend in the initial 1,4 Bbls to successively stronger systems as we graded back toward the wellbore. This added strength near the wellbore was very important due to the high drawdown environment (ΔP= 1,5 psi), and the large void features that were present in the near wellbore region. First two Treatments (Wells 251 & 252): The first treatments were executed during December of 23, and drilled out in early 24. Since we did not fully comprehend the true extent of the void features we had to design a placement technique that would allow us both the flexibility to stop the treatments at any point in the process. Wellbore pressures could build up beyond our expectations, or may continue to track to full displacement. The decision was made to pump through a cement retainer to enable us to unsting at relatively any time in the treating process. This would then allow the system to develop full strength relatively undisturbed. These initial designs were the first successful conformance solutions to be placed in the Anton Irish field. However, post treatment evaluation showed that the features were even more prolific than we had anticipated. The jobs pumped easier and with basically no formation resistance. The cement strength design however was inadequate, once exposed to the drawdown environment of the producers. This stress was greater than the set foamed cement was able to withstand, as evidenced by powdered cement produced into the test separators. The low tensile strength of the cement and the expansive nature of the nitrogen combined to create a much weaker system than we had expected. Improved Solution Design: With this additional improvement in our problem understanding and the strength of our system, further improvements to our treatment designs were possible. The next three treatments were executed in 25, and a comparison of the original design to the current design is shown in Tables 3 and 4. The design changes using this improved knowledge included increasing both the molecular weight and concentration of the chrome cross-linked gel and increasing the strength of cement. Reducing the Poz and N 2 concentrations throughout the cement stages, and adding fiber to the final cement stage to improve tensile strength, eliminated the cement production problem. Since the volume of the void space features clearly exceed the 1, bbls of filler being pumped, the need for a cement retainer was removed. By eliminating the cement retainer, pre-job preparation simply involves removing the current completion equipment, and installing a retrievable packer and temporary workstring that is set approximately 3 above the openhole interval. The job is then pumped through the workstring and under displaced to leave the cement top in the bottom of the production liner above the open hole. This alteration to the placement process provides several advantages; first, the rig work is minimized and does not have to be coordinated with the availability and timing with the gel and cement pumping. Likewise, once the initial rig work is completed, scheduling and execution of the gel and cement is easier to coordinate. Second, although more cement is left under the liner to drill out, the time to complete this task is slightly reduced because there is no retainer to drill out. In addition, the cost of the retainer is eliminated and when coupled with the reduced rig time resulted in lowering the overall average treatment cost by approximately $5M/treatment. Results The combined initial rate impact from all five wells was approximately 49 BOPD with a reduction in CO 2 gas rate of > 3 MMCFPD. The combined rate for this work as of March 26 was approximately 32 BOPD with a combined CO 2 rate of only ~5 MMCFPD. Table 4 shows the rate data for the 5 treated wells. Understanding just how long these solutions will assist in controlling the breakthrough of CO 2 is a key unknown. This and any potential ability to extend these results with subsequent conformance work must be evaluated. So far the first two treatments have lasted for more than 2 years, and the more recent treatments continue to perform as expected with only small changes since original placement. Ultimately we expect the CO 2 and water to somehow break around the cement and gel place in the void conduit. At that time consideration will be given to retreating these producers with smaller staged volumes in an attempt to stay ahead of the full feature development.

3 SPE Future Plans & Extension of Results Future plans for additional work at Anton are being designed based on our improved understanding but are now primarily focused toward treating injector-dominated systems, (i.e. injectors which clearly tie to more than one producer based on our tracer work). In addition we will continue to identify and treat more producers when appropriate. Results of past treatments will be monitored and evaluated for additional insight, so that we can continue to optimize the size and strength of these solutions. Knowledge gained from this effort has influenced diagnostic efforts in several OXY-operated Permian Basin fields. At this time two successful solution treatments (one injector and one producer) have been designed and executed in the Cogdell and North Cowden fields. These solutions although still unique for their situation did utilize several elements of the knowledge gained from this conformance effort. Additional potential to utilize this approach and similar solutions within Oxy Permian are already taking shape. CONCLUSIONS Efforts to improve our understanding of conformance issues at Anton successfully identified important additional complexities which led to a very new and unique perspective of the problem. This new understanding and some extensive efforts to better define the complete nature of the problem led to a novel and bold solution design. Continued focus on integrating knowledge learned from the initial successful efforts led to improved strength and reduced cost on subsequent treatments. Lessons learned from this work are providing additional insight and potential conformance solutions in other areas. These insights are being utilized across Oxy Permian and shared across Occidental Petroleum through our conformance engineering efforts. Acknowledgement The authors want to thank Occidental Petroleum and Devon Energy for approval to complete and permission to publish this work.

4 4 SPE 1344 TABLE 1 Past Conformance Effort Review. Time Frame Well # and Treatment Type - Results Late 7 s Large fly ash treatments in injectors Micro Matrix cement squeezes in producers No Improvement 1991 #43, 41, 225, 292: 4-9 bbls polymer No Improvement 1993 #21 37 lbs 1 mesh, 2/4, & 8/16 sand #32 25 bbls monomer (insitu polymerized gel) #88 Cement and 21,5 lbs of silica flour, 1 mesh and walnut hulls 2 #59-6 lbs sized dried crystalline - swelling polyacrylamide (15 bbls in formation) No Improvement No Improvement 21 #59 43 bbls of Foamed Cement No Improvement TABLE 2 Original and Improved Gel Design Original Gel Design Pumped ~ 6 Bbls o 3 PPM o Medium MW (13 MM) Pumped ~ 2 Bbls o 5 PPM o Medium MW (13 MM) Current Gel Design Pump ~ 6 Bbls o 4 PPM o High MW (16-18 MM) Pump ~ 2 Bbls 6 PPM High MW (16-18 MM) Overflushed w/ ~ 2 Bbls Overflushed w/ ~ 7 Bbls Pumped through Cement Retainer Pump down Tbg with minimal displacement. TABLE 3 Original and Improved Cement System Original Gel Design Current Gel Design Pumped ~ 14 Bbls o 1 % Poz Cement o 3% N2 Pumped ~ 4 Bbls o 8/2 Poz o 3% N2 Pumped ~ 2 Bbls o 8/2 Poz o 2% N2 Pumped last 5 Bbls o No N2 Pump ~ 1 Bbls o 6/4 % Poz Cement o 2 % N2 Pump ~ 5 Bbls o 6/4 Poz o 1% N2 Pump ~ 3 Bbls o 6/4 Poz o 5 % N2 Pump ~2 Bbls no N2 o With 1% fiber Pumped through Cement Retainer Pumped down Tbg with Wiper Ball Under displace in liner

5 SPE TABLE 4: Rate Impacts Well Rate Prior to SI Date Shut-in /371/2*- O/W/G 5/22 * One test at 5+ mscfpd /28/8* - O/W/G 5/22 * One test ~ 13 mscfpd 292 5/25/58* - O/W/G 2/22 *Unable to Test / Limit Est Gas > 8 mscfpd Average Rate After Treatment 87/135/44 - O/W/G 4/24 165/173/13 - O/W/G 4/24 2+*/3/2 - O/W/G 5/25 * Oil cut estimated from shakeout. Peak Rate (PR) Current Rate (CR) PR: 18/14/1 - O/W/G 4/25 CR: 5/14/1 - O/W/G 3/26 PR: 35/18/17 - O/W/G 6/25 CR: 12/196/145 - O/W/G 2/26 PR: 15/37/95 - O/W/G 6/25 CR: 11/55/35 - O/W/G 3/ /57/2* - O/W/G 12/22 *Unable to Test / Limit Est Gas > 1 mscfpd 6/31/1 - O/W/G 8/25 PR: 79/37/119 - O/W/G 8/25 CR: 6/2/15 - O/W/G 3/ /2/22* - O/W/G 5/25 Flowing gas often higher. 162/555/156 - O/W/G 1/26 PR: 21/46/24 - O/W/G 1/26 CR: 76/28/17 - O/W/G 3/26

6 6 SPE 1344 FIGURE 1 (Pattern Rate Analysis) 16 AICU # AICU # Injection Pressure (psi) bwipd / mcfipd Injection Pressure (psi) bwipd / mcfipd 2 2 Jan- Apr- Jul- Oct- Jan-1 Apr-1 Jul-1 Oct-1 Jan-2 Apr-2 Jul-2 Oct-2 Jan-3 Jan- Apr- Jul- Oct- Jan-1 Apr-1 Jul-1 Oct-1 Jan-2 Apr-2 Jul-2 Oct-2 Jan-3 5 AICU #252 9 bopd mcfd / bwpd.3.8. Jan- Apr- Jul- Oct- Jan-1 Apr-1 Jul-1 Oct-1 Jan-2 Apr-2 Jul-2 Oct-2 Jan-3 35 AICU # AICU # Injection Pressure (psi) bwipd / mcfipd Injection Pressure (psi) bwipd / mcfipd Jan- Apr- Jul- Oct- Jan-1 Apr-1 Jul-1 Oct-1 Jan-2 Apr-2 Jul-2 Oct-2 Jan-3 Jan- Apr- Jul- Oct- Jan-1 Apr-1 Jul-1 Oct-1 Jan-2 Apr-2 Jul-2 Oct-2 Jan-3 FIGURE 2: Interwell Tracer study # days

7 SPE FIGURE 3. Step Rate Tests 35 STEP RATE TEST ANTON IRISH CLEARFORK UNIT B.H. PRESSURE AICU #22 AICU #37 AICU #38 AICU # RATE (BPD) FIGURE 4. Hall Plot for Injector 21

8 8 SPE 1344 FIGURE 5:Cast V Logs (36* sonic imaging logs) pre- & post-hydraulic frac completion Before After Before After FIGURE 6: Downhole Camera Inspections

9 SPE FIGURE 7: Tracer Study #2.