Phased Pilot Approach Reduces Uncertainty in Carbonate Steamflood Development
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The First Eocene is a multibillion-barrel heavy-oil carbonate reservoir in the Wafra field, located in the Partitioned Zone (PZ) between Saudi Arabia and Kuwait. After more than 60 years of primary production, expected recovery is low and provides a good target for enhanced-oil-recovery (EOR) processes. A phased piloting approach has been used to reduce the uncertainties (subsurface and surface) related to application of thermal EOR processes in this field.
Wafra is one of four major fields located in the PZ (Fig. 1). Because of the low primary oil recovery and large original oil in place of the Wafra Eocene reservoirs, a significant EOR opportunity exists. A detailed geological description of the reservoir is provided in the complete paper.
Because of key surface and subsurface uncertainties, along with the high cost of implementing a full-field steamflood-development (FFSFD) project, a phased, sequential approach, from piloting to FFSFD, was implemented. This approach was required so that critical uncertainties could be evaluated efficiently and sequentially in a timeframe consistent with overall reservoir-development objectives while managing capital exposure.
Pilot Planning and Design
Steamflood Pilots. A specific set of objectives, primary and secondary, was developed for each of the continuous-steam-injection (CSI) pilots. Primary objectives were identified as being critical for progression to the next phase (piloting/FFSFD); secondary objectives were regarded as important in understanding identified uncertainties, but not necessarily project roadblocks. Success measures for the pilots were derived from the key metrics that were prerequisites for the next phase. These success measures were formulated to ensure that learnings gained will be used to support the design and operational planning of the next phase.
A five-spot pattern on a 1.25-acre steamflood-pilot, small-scale test (SST) was a continuation of the phased approach in managing and mitigating technical uncertainties related to implementing steamflooding. The primary objectives of the SST were to test whether Eocene-produced water could be treated to produce suitable feedwater for steam generation and to test steam injectivity in the First Eocene reservoir. The SST was conducted in First Eocene C Zone/Steam Zone 3 and successfully proved sustained injectivity using steam generated from the produced water. The SST was a prelude to a multipattern steamflood pilot known as the large-scale pilot (LSP).
Following the achievement of the primary SST objectives, a 16-pattern inverted five-spot pattern on a 2.5-acre LSP was initiated in 2009. The LSP was designed to evaluate the technical and operational feasibility of CSI and further reduce
- Subsurface uncertainties such as recovery, vertical/horizontal (high-permeability zones) heterogeneity, steam/rock and steam/water interactions
- Operational uncertainties associated with scaling and corrosion. with primary objectives focused on gaining reliable recovery and operational information. Learnings from the secondary SST objectives (anhydrite-scale and corrosion mitigation) were implemented into the LSP.
Well Design. Wellbore integrity during the steam-injection period was critical to project success and well construction was a key focus area during pilot design. Well design was based on a worst-case scenario assuming no cement behind pipe and a maximum steam temperature of 500°F.
Zonal-completion decisions were driven by the consideration that evaluation of one zone at a time was necessary to establish and understand individual zone performances to help design an FFSFD plan. The design of the pilot well completion was based on stratigraphic descriptions developed previously in the literature.
Pressure profiles were used to understand the presence of possible barriers to steam and guided the design of completion intervals of producers and injectors. Surveys conducted on the five wells in the LSP pilot area during drilling of the wells, when correlated with stratigraphic tops, showed significant differences in the degree of pressure depletion in the different steamflood zones (A, B, and C).
For the LSP, initial completion was in the C Zone. The initial plan was to test the three potential steamflood-development zones sequentially, with C Zone (or Steam Zone 3) first, followed by B Zone (or Steam Zone 2), and ending with A Zone (or Steam Zone 1). Producers and injectors were initially completed into C Zone. After establishing a positive response to CSI, steam injection was terminated earlier than planned. The wells were recompleted in B Zone to ensure that sufficient steamflood-response data could be obtained from B Zone, a key development target for the FFSFD project.
Steamflood-Pilots Lessons Learned
SST. The SST’s primary objectives were to
- Assess steam injectivity into dolomite reservoirs containing gypsum or anhydrite
- Test feasibility of a to treat produced water from Eocene reservoirs for steam generation.
Secondary objectives were to evaluate well productivity, changes in composition of produced fluids, downhole scaling and corrosion tendencies, well casing and cement integrity, and reliability and operability of well-testing equipment and vapor-collection facilities.
SST success measures included achieving steam breakthrough in at least two pilot producers, and completing 6 to 12 months of total evaporator continuous runtime in treating produced water from Eocene reservoirs.
The pilot was successful and met the objectives of steam generation from produced water, demonstrating long-term steam injectivity into a dolomitic reservoir. Using automated steam generators, high-quality steam was delivered to the reservoir. CSI began on February 2006 and steam breakthrough occurred by November 2006 in three producers, pointing to a successful distribution of injected steam across the 1.25-acre pilot pattern. Following identification of steam breakthrough, injection rates were reduced.
The feasibility of treating Eocene water with a mechanical seeded slurry evaporator was demonstrated. The key learning was the optimization of metallurgical components of the evaporator.
LSP (C Zone). The LSP was designed to allow significant reduction in the following uncertainties:
- Reservoir response to steamflooding
- Steamflood recovery
- Reservoir variability by pattern and by steam zone (assuming multiple steam zones are present)
- Operational uncertainties, including those directly related to scale and corrosion
Work on the LSP facilities began in 2007, with the first producer online in May 2007. All but two producers were online by August 2008. Steam injection began in June 2009 after a cold production baseline for C Zone (Steam Zone 3) was established. Positive incremental response to steam injection was observed with significant increase in peak oil rate relative to the primary production baseline trend. After establishing positive incremental production response to CSI and identifying C Zone as a viable steamflood candidate, a decision was made in 2010 to recomplete in B Zone (called LSP-B), a primary target for the FFSFD project. Steam injection into C Zone ceased in January 2011.
Key findings of this LSP include the following
- Twenty-four out of the 25 producers in the pilot showed a significant response to steamflooding.
- Although of limited duration, C Zone’s observed cumulative production was significantly higher than the expected ultimate primary-only recovery and was achieved with a cumulative incremental steam/oil ratio that pointed to the technical success of the steamflood process in C Zone.
- At the time injection was halted, a greater than sixfold increase in oil production relative to the baseline was observed, whereas only a threefold increase to total fluid produced occurred.
- C Zone pattern variability observed during cold production was diminished as a result of steam injection.
LSP (B Zone). This LSP's primary objectives were to
- Reduce technical and economic risks and uncertainties and determine whether First Eocene FFSFD is feasible
- Determine recovery efficiency and production response
- Monitor variability of response by patterns
A number of success measures were also developed for the pilot, all of which were met. LSP-B showed excellent response to CSI with high cumulative oil production, peak oil rates, and oil recovery. A steamflood life-cycle profile similar to that observed in sandstone steamfloods was developed in a carbonate reservoir. Heat-management injection-rate cuts were taken without harming production response. Significant improvements in performance of 25 producers during steamflood were observed and CSI significantly reduced the production response variability across the pilot producers. Well performances were affected by both the reservoir quality and the boundary effect (fluid influx).
As expected, four central patterns were less influenced by fluid influx from outside the project area. Oil recovery from this central-patterns area, calculated from the validated reservoir simulation model, was greater than 50%. It was demonstrated that carbonate steamflood in the First Eocene reservoir can yield recoveries similar to that measured in sandstone steamfloods. A key success measure and project milestone was achieved when the buildout of a steam zone over 90% of the pilot project area was validated.
High well reliability was achieved in the reservoir with high scaling tendencies and corrosive fluids; producer runtimes averaged 96% during the cold production and 93% during the steamflood period. No injector downtime occurred because of subsurface failures, and surface-related issues were minimal. Extensive monitoring and mitigation efforts and applying the leading concepts for scale and corrosion mitigation from the SST pilot learnings were the main factors for the high well run-times.
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Phased Pilot Approach Reduces Uncertainty in Carbonate Steamflood Development
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