Integrated Optimization Process Advances Steamflood Performance
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Duri Field in Indonesia is the largest active steamflood project in the world. The field produces 73,000 BOPD, and 10,000 optimization jobs are executed annually to support base production. The operator identified an opportunity to implement a new process to improve the quality of reservoir analyses, define short- and long-term reservoir depletion strategies, and increase the ability to develop a prioritized queue of executable opportunities to increase production.
Duri Field was discovered in 1941 in the central Sumatra Basin in Riau Province. The first production-optimization effort, in 1967, used cyclic steam stimulation. That led to a successful steamflood pilot trial in 1975 and implementation of a full steamflood areal expansion in 1985. Thirteen production areas currently exist in Duri Field as a result of steamflooding.
The geological setting of the field involves an anticline structure 20 km long and 14 km wide that covers more than 21,000 acres. The historical tectonic process results in shallow reservoir depths.
The field is composed of multiple sand deposits of a tidally influenced estuarine depositional system. The reservoirs are characterized by stacked sand lobes with a total thickness of approximately 240 ft, porosity ranges from 25 to 35%, and relatively high permeability of 600 md to 2 darcy. As a giant field, Duri had an original oil in place of approximately 5 billion bbl. The oil is characterized as heavy, with gravity from 17 to 22 °API and high oil viscosity (300 cp) at 100°F.
The depletion-review process discussed in the complete paper aims to optimize the drainage process and address current challenges in Duri steamflood operations, some of which include the following:
- Steam distributions were based on work flows that were not consistent across areas.
- The field is operated by a wide range of experienced practitioners, resulting in inconsistent optimization approaches, variations in evaluation quality, overlooked opportunities, and a less-structured depletion strategy.
- The variation of reservoir depletion stages requires different depletion methods and optimization strategies.
- The field production area is comprised of several variations in steamflood design: varying pattern design and size, a completion design for producers, and a completion design for injectors.
- The volume of surveillance data requires a comprehensive and systematic data-integration process.
Steamflood-Depletion Optimization Process
Methodology. Steamflood Depletion Stages Definition. The stages in the process are as follows:
- Early/Primary Depleted represents initial production (cold production) and early response to steamflood injection.
- Partial Depleted 1 represents steamflood development in the reservoir.
- Partial Depleted 2 represents the maximum phase of steamflood response in the reservoir.
- Late Depleted represents the late phase of steamflood response and lower recovery.
- Complete Depleted represents the last phase of steamflood operation in the reservoir.
Reservoir Quality. The basis of the reservoir-quality grouping is stratigraphy. The reservoir decreases laterally in grain size, thickness, and permeability but has increasing clay content and sand/shale lamination moving from the axis channel into margin tidal channel deposits. The reservoir thickness, number of lobes, and total thickness of the reservoir are used to define the reservoir-quality group.
This approach is applied to three reservoir intervals across the entirety of the field. The reservoir-quality grouping is presented on a pattern basis as one integrated map that enables practitioners to make a consistent comparison among production areas.
Reservoir Depletion Progress and Temperature Response to Steamflood. Characterization of the depletion process defines the overall field-production strategy. This process provides a standard definition for interpretation of the depletion condition, either stalled or normal depletion.
Depletion is defined primarily from well-log data and is supplemented by the production profile and recovery. Normal depletion is shown by a continuous reduction of oil pay in the reservoir, continuous growth of oil recovery, and a normal production profile. Stalled depletion shows no reduction in oil pay through time, no increment in recovery factor, and rapid production decline or zero production despite reservoir temperatures remaining sufficiently high for oil to be mobile.
Drainage Maps as a Standard for Pattern-Based Depletion Status. Drainage-map construction is started from a fieldwide reservoir-quality map, original net-pay map, and remaining oil and production data to define the depletion/drainage stage at the pattern level. The depletion-progress status from observation-well data is used to identify stalled depletion. The example drainage map in Fig. 1 presents the first-pass integration of static and dynamic data. The depletion/drainage state of the patterns without observation-well data is determined using an interpolation method that combines the original condition, production profile behavior, pattern configuration, well-completion similarity, steamflood history, and current recovery factors.
Steamflood-Depletion Review Work Flow. The work flow is an evergreen process that ensures that all production areas are reviewed and maintain high performance. The work flow is built in a sequential approach, from a high-level total area performance review to the reservoir and then well levels.
Static Geological Review. The geological static properties are generated at the reservoir level in the form of a 3D geologic model that incorporates well-log, core, structural, and stratigraphic interpretation.
The dynamic geological review is performed to understand the current state of the steamflood-depletion process. The dynamic evaluation focuses on several aspects such as current remaining oil or recovery factor, heating response in the reservoir, and effect upon the depletion process. The recovery and remaining oil calculation from log data provide the total and the reservoir level remaining potential. In addition to remaining oil information, the observation well provides a temperature log that is used to generate the average temperature map at the reservoir level. These maps present the current stated potential and heating condition across the production area.
The combination of reservoir quality, remaining oil or depletion stages, temperature, and depletion-process data is used to create drainage/depletion maps across the production area.
Reservoir Engineering Review. Reservoir engineering evaluation of steamflood performance is performed after the geological evaluation. This review is focused on the reservoir behavior by evaluating a combination of dynamic reservoir and production data, including production-injection data and observation-well and production-well surveillance data. The comprehensive evaluation reveals the story of heating progression and reservoir depletion. A sequential analysis of the depletion-optimization process, including production-performance review, comprehensive reservoir-depletion review, and heating and drainage strategy alignment, is provided in the complete paper.
Depletion Plan Validation Through Benchmarking. The final depletion strategy is represented by a recommendation effort that is tied to well workovers, producer stimulation, and injection adjustment that targets patterns or groups of patterns. The recommended plan is validated against the performance of similar types of workovers and stimulation from the previous year’s activities. The benchmarking review is focused on the production response and economic metrics of the result.
Opportunities identified with a success flag are then reviewed together with geological, reservoir, and surveillance information. The review provides a revision of the existing screening process to identify job-opportunity candidates and the appropriate workover method. Most of these proactive jobs delivered a good result and exceeded their economic expectations. Fluid and oil production improved after the job and the total success rate was maintained at approximately 70 to 80% each month.
Surface-Facility Constraint Review. The final stage of the optimization-depletion review is verification of the plan against surface-facility conditions and capabilities. Surface limitations are mitigated by facility improvements that should consider time, cost, feasibility, and economics. Final plan selection is based on screening with these factors, an adjustment for schedule, and the execution plan.
Final Depletion Plan. The final depletion plan is the result of a series of standard heating and drainage review processes that are validated by a review of previous proactive jobs in consideration of surface-facility limitations.
The typical final recommendation for a depletion plan is divided into drainage and heating strategies. Heating recommendations are related to the steam-injection strategy. Heating recommendations are proposed on a pattern, or group-of-patterns, basis. The review result would be a decision to increase, decrease, or maintain steam injection. Drainage recommendations are focused on the producer side (production site) on a well-level basis. The optimization target is to ensure that the producer operates smoothly and that all identified problems and anomalies are mitigated proactively.
Integrated Optimization Process Advances Steamflood Performance
01 June 2020
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