SPE-215325-MS Yudi Rahman, Hendro Tjahjono, Faizal Al Marawi, and Yayu Anggreiny, PT. Pertamina Hulu Rokan Copyright 2023, Society of Petroleum Engineers DOI 10.2118/215325-MS This paper was prepared for presentation at the SPE/IATMI Asia Pacific Oil & Gas Conference and Exhibition held in Jakarta, Indonesia on 10 – 12 October, 2023. 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 have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper 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 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract KILAU Field is a mature field that produced through primary recovery. Managing mature field to keep performance in existing wells has many challenges, especially in performing workover activity. With limited data to knowing current subsurface condition, it has a high level of uncertainty in achieving a success rate in workover activity. In 2021, 10 workover programs were executed but success rate was only 30%. Five jobs were failed with no oil gain and others only delivered average 19 BOPD/well. Part of improvement process, it conducted lookback to find the root cause. The result observed that if the well was produced from A-1 sand, it would have failed. The type of well completion, both even produced with commingled completion or single completion were failed since producing the A-1 sand. Detailed subsurface review as part of lookback process was carried out. It turns out that the A-1 sand, based on log data from new wells (which were drilled in Q3-2021 until Q1-2022), shown that the level of OWC (Oil water contact) has clearly raised, or the oil already drained. So, if the well was perforated or produced from A-1 sand, then the production of the well will experience high water (high WC). Another analysis result obtained from information of new wells was a high oil opportunity in sand of A-3, A-5, & C-1. So, the new wells log data told the mystery of the current condition of the subsurface. This very meaningful information enabled revised strategy to optimize existing old wells. This paper discusses how to utilize new wells data to improve success rate of doing workover job by integrating updated subsurface information and production data though well log correlation and mimicking the completion of high production wells. The improvement process has been carried out in 7 wells through workover programs, which targeted reservoir A-3, A-5, & C-1 and to avoid producing sand A-1. It was obtaining a 100% success ratio with total oil gain of 966 BOPD. Keyword(s): Utilizing Data, New Drilled Well, Improved, Workover Performance Introduction KILAU field was discovered in 1972 and POP in February 1974. The peak production was 20,4 MBOPD in 1979 from 20 wells. It is located at Sumatera Island, Indonesia that managed by PT. PERTAMINA HULU Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 Best Practice Utilising New Drilled Well Data to Improve Workover Performance in Existing Old Wells 2 SPE-215325-MS ROKAN (PHR). This field is separated by a major fault into 2 (two) compartments, NW closure and SE closure. The NW closure has higher potential reserves and good reservoir continuity compared to SE closure, which is more faulted. (Fig-1) The field has 12 (twelve) oil-bearing reservoirs in 3 prolific formation the A, B and C (Fig-2). Those reservoirs are classified in two categories such as high-quality reservoir (HQR), which is characterized by clean sand with high porosity and high permeability and lower quality reservoir (LQR), which has lower porosity and lower permeability. Figure 2—KILAU Field Reservoirs The KILAU field is operated through primary depletion using electrical submersible pump (ESP) completion. The strategy area is to maximize production through drilling new wells and workover existing wells. A total of 59 wells have been drilled, drilling activities had stopped in 2017 because the previous operator was in the final stage of the concession. Then just started drilling additional new wells by PT. PHR in Q3-2021 until early 2022 as many as 12 wells. Currently, daily field rate production is about 1600 BOPD that consist of 39 active producing wells, 14 idle wells, and 6 P&A wells with. To date, this field was reaching recovery factor with ~42%. Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 Figure 1—KILAU Field Location & Map SPE-215325-MS 3 Background Workover job is the backbone activity to maintain field performance. Various types of job performed such as re-perforation, water shut off, perforation of new zone, reactivation of idle well and pump rate optimization. All kinds of jobs require detailed subsurface review (and) evaluation. Availability of updated surveillance data such as new wells information and its production data was very critical to deliver successful workover or production enhancement jobs. Workover performance in 2021 showed a 30% success rate only from 10 workover programs executed. The success criteria is based on (oil gain) that delivered impact to economic parameter such DPI rate. The DPI less than 1.2 will be categorized as failed job. (fig.4) Figure 4—List of well workover in 2021 Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 Figure 3—Production History of KILAU Field 4 SPE-215325-MS Some failed jobs caused oil loss from previous production and others had low oil gain less than 10 BOPD. The average oil gain from total wells was only 8 BOPD. As part of the improvement process, the team conducted lookback from those programs to find the root causes. Starting Q-3 2021, there were 6 wells drilled in NW closure area to recover bypassed oil to reducing well spacing from 300 m to 150 m. The new wells log data delivered very meaningful information to tell the mystery of current subsurface condition. By utilizing the new data to support evaluation and validate the lookback process. Well Completion Evaluation The lookback process started from reviewing the well completion type of each well. A perforated casing completion is completion technique in Kilau Field. There were single sand completion and commingled sand completion(fig.5). The commingle sand completion is producing more than one interval together through one tubing string. Regardless the workover success of failure in this completion, it is relatively difficult to determine the reservoir contribution for production. Also, which sand would contribute more water or more oil. That is why the single completion more accurately describes the sand condition either wet or oil. Figure 5—Completion type From Ten workovers in 2021, oil loss or low oil gain was obtained from 50% single completion candidate that produced from A-1 sand. When A-1 sand is produced commingle with other sand, the well also delivered low oil gain. During program execution, another surveillance performed swab job to find out the potential either water or oil from the reservoir. This is part of surveillance to confirm the potential from a subsurface review standpoint. By reviewing the well completion history, it was found that during 2012 – 2018, massive commingle production comes from A-1 sand in attic NW closure. Then campaign to shut off interval A-1 sand was conducted in 2019 and let reservoir idle until 2020. Well Correlation The new wells log data delivered very meaningful information to break the mystery of current subsurface condition. Well-logs provide various information about geological layers, including their boundary depths. Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 Methodology SPE-215325-MS 5 Figure 6—Well Correlation on top structure NW closure Determine Current Fluid Contact Analysis (OWC) Fluid contact analysis is needed to visualize the remaining oil potential opportunities. Current Oil Water Contact (COWC) will help the understanding of regional and full-field description of the current reservoirs’ potential. Fluid contact analysis is determined based on new well log analysis that includes gamma ray log, resistivity, porosity, saturation, side wall core, and supported with swab data, and production data. Contact limit fluid is carried out by taking several wells to create cross sections. As known that A-1 sand is one of High-Quality Reservoirs, due to massive production during the period of 2012 – 2018. However, unusual condition was found as attic wells were watered out while flank wells still showed good oil performance. This condition happened due to the water conning issue at A-1 sand in attic area. This is confirmed from well log of new wells that has resistivity depleted at attic position and higher resistivity at flank position (Fig.7). Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 The phenomena of conning effect shown clearly from well-to-well correlation between old well with new wells. By flattening A-1 sand level, comparing old well such; KL#08 which is the highest well position structurally shown A-1 was LKO originally. Currently after 44 years production through new wells (KL#51, KL#48 &KL#49) which are on top structure shown current OWC (Oil water contact) already move up to top sand (Fig.6) This made a change strategy to optimize existing old wells though wells to wells log correlation of new wells and mimicking the completion of high production wells by avoiding produce A-1 sand. 6 SPE-215325-MS Current Net Pay Map Reservoir properties from new wells are mapped to promote optimal well intervention(and) development. Subsurface maps dictate remaining potential and enable engineers to calculate reserves and monitor trends in reservoir performance. Geologists play a key role in subsurface mapping by using interpretations of depositional environments and diagenetic events to project reservoir data away from relatively few well control points. Net pay (see Effective pay determination) implies that some formation thickness has been excluded from consideration by either (1) occurring below an oil-water contact (or above a gaswater contact), or (2) having porosity and/or permeability values below a "cutoff" limit for productivity. Discontinuous productive horizons between wells might be described, for example, by the concept of net pay to net connected pay ratio. The reservoirs that still have thick current pay with good location is the top priority to be the workover candidates. Stratigraphic oil trap was also visualized well from current pay map that previously none. This map is also very useful for future development and optimization strategy for this mature field (Fig.8) Figure 8—Net Pay Map of Kilau Field Result In existing recent subsurface data from new wells deliver meaningful information to reduce risk and uncertainty. Unsatisfactory result from 2021 workover could be revealed the root cause and got solution to improve in 2022 workover performance. New log data show the sand that has already watered out and the one that still has potential bypassed oil. The A-1 sand for example, which is a part of HQR and has strong water drive mechanism. It shown that the current OWC clearly move up because historically most producers on top structure had been produced massively from A-1 sand for long time and just shut off in 2019. Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 Figure 7—An Example of Current Fluid Contact Analysis of DR_1 Sand SPE-215325-MS 7 Table 1—List workover after improvement Figure 9—Workover Performance result Conclusion Lesson Learn: • • New well log data is valuable information that become reference to optimize existing well. Importance to understand the root cause then develop strategy to optimize wells. Best Practices: • • Ensuring proper way to tackle out the root cause of wet zone (A-1) by cementing job. Requiring detailed subsurface evaluation of new wells data through focus on low sand quality which sometime show lazy resistivity. Downloaded from http://onepetro.org/SPEAPOG/proceedings-pdf/23APOG/3-23APOG/D032S008R009/3284402/spe-215325-ms.pdf by Universidad Industrial De Santander user on 31 January 2024 The recommendation to re-produce A-1 sand in 2021 had assumption that the conning would reverse after idling for two years, but the fact was different than the prediction. Every well that had produced from A-1 sand were failed, proven through single completion or commingle completion. So, if the well had produced from sand A-1, the production had high water (high WC). Others remaining potential shown at A-4, A-5, B-3, B-5 and C-1. This made change strategy to optimize the existing old wells though wells to wells log correlation of new wells then mimicking the completion to high production wells. Total improvement performed on 7 wells that targeting of sand A-4, A-5, B-3, B-5, C-1 and avoid producing A-1 sand. 100% success ratio was achieved through delivering total oil gain 966 BOPD (table.1) 8 SPE-215325-MS • Identifying workover potential and executing the job by campaign mode to achieve excellent Reservoir Management (RM). Challenges: • Limited alternative for isolation method to shut off A-1 due to highest sand position. ACKNOWLEDGMENT The authors would like to special thanks to all stakeholders with excellent collaboration internal and external party; especially engagement with SKK Migas to secure GOI approval on share this topic to all practitioner oil & gas Industry in this event. Excellent teamwork between AOT, D&C and others. Then to management of PT. Pertamina Hulu Rokan – WK Rokan and Ditjen Migas for their permissions to publish this work. REFERENCES 1. 2. 3. 4. Amir Mohammad Karimi, Saeid Sadeghnejad and Mansoor Rezghi. 2021. 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Mimicking wells completion by referring to highest production wells.
