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The Paleocene/Eocene age First Eocene dolomite reservoir is estimated to contain than 10 billion barrels of oil of which only a small percentage will be produced during primary development. Consequently, steam flooding is being investigated as an appropriate EOR option. A 1.25-acre, single pattern pilot (SST) and a 40-acre, 16 pattern pilot (LSP) are in progress. The detailed pilot area log, core, and seismic data provide a unique opportunity to assess reservoir heterogeneity. Analysis of temperature and petrophysical logs obtained in a temperature observation well located 35 feet from the SST injector show that a vertical barrier to steam migration exists. Two, relatively thick, very low porosity and very low permeability nodular evaporite-rich zones that were predicted to be the most likely barriers do not appear to be a vertical barrier. Instead, an interval characterized by numerous thin, cycle caps, characterized by muddy, finely crystalline dolomites interpreted to be tidal flat facies may be the vertical barrier. Each of these cycle caps also exhibit signs of subaerial exposure which may also contribute to the generally low porosity and very low permeability of the cycle caps. Detailed studies, including micro-permeameter measurements, quantitative mineralogical studies, and micro-CT scans were used to further characterize this interval. The geological assessments of heterogeneity are supplemented by a history-matched simulation model that suggests the evaporite-rich zones may have acted as short term baffles but that the vertical barrier to steam migration is coincident with the interval with abundant tidal flat cycle caps and exposure surfaces. Geological and other reservoir data obtained from the LSP suggest that similar vertical barriers may exist in the pilot area. Early steamflooding results show a very positive response to steam injection as well as multiple thermal "events" (most likely baffles rather than barriers) in the lowermost flooded zones at the LSP. The LSP data allows inferences to be made regarding the occurrence and distribution of lateral high permeability "connections" between injectors and producers as well as the overall reservoir response to steam injection. While the rapid temperature response observed in a few wells may reflect localized fractures or karst-like zones, numerical simulation using very fine grids (1.25 m cell size) shows that some of the LSP wells may experience very short breakthrough times without the need for fracture or karst-like zones.

The concept of steamflooding the Wafra Eocene dolomite reservoir originated in various studies conducted in the 1980's. In 1999, a comprehensive EOR study and Eocene huff-n-puff pilot suggested that steamflooding could be a viable recovery process for the reservoir. As a result of these studies, a staged development approach was incorporated to test the viability of pattern steamflooding the Eocene reservoir. The objective was to assess key technical challenges associated with steamflooding an anhydrite and gypsum rich carbonate reservoir. Additional challenges were the lack of fresh water available for steam generation, high concentrations of hydrogen sulfide gas, and higher reservoir pressures compared to most active steamfloods. The staged approach called for a single pattern steamflood test followed by a larger multi-pattern pilot. As a result of this strategy, a single pattern steamflood test was implemented in 2006. The design and initial performance of the small scale test (SST) single pattern steamflood pilot in the Wafra 1st Eocene reservoir are described in this paper. The pilot is comprised of one, 1.25 acre inverted five-spot pattern, consisting of four producing wells, a single injector and a single observation well. Continuous steam injection began in February 2006 at a rate of approximately 500 barrels per day cold water equivalent, 600 psig and a temperature of 489 ºF. The primary goals of the single pattern test were to test application of a mechanical seeded slurry evaporator to process produced water for steam generation and to assess steam injectivity into dolomite reservoirs containing gypsum and/or anhydrite. Injectivity assessment included evaluating reservoir response to steamflooding and investigating the variation over time due to rock/fluid interactions. Secondary objectives included analyzing well productivity and evaluating well testing equipment, facilities, and well construction. The SST has a comprehensive data collection and surveillance plan to support evaluation of these goals and objectives. The surveillance plan includes the collection of pre-flood and post-flood core data, frequent well testing for rates and fluid compositions, daily temperature recordings and periodic logging. After two years of operation, primary goals have been tested and exceeded expectations. A continuous thermal zone was developed in the 1st Eocene reservoir and steam breakthrough occurred at several of the producers. Generator feed quality water was produced at maximum throughput rate of 1,200 bwpd via mechanical seeded slurry evaporator equipment. Secondary objectives are currently being assessed with focus on current challenges of corrosion and scaling of producing wells.

The 2nd Eocene dolomite reservoir in the Wafra field is an attractive target for enhanced oil recovery (EOR) because of its large resource base and low primary oil recovery. Primary oil recovery is low because of heterogeneity, low oil gravity, and lack of significant aquifer support. Screening studies and numerical simulations indicate that steamflooding is the most attractive EOR method in terms of reserves and economics. Key technical challenges in steamflooding the 2nd Eocene reservoir include reservoir heterogeneity, relatively high reservoir pressure, scale deposition and corrosion due to rock-steam interactions, and uncertainty regarding the location and impact of barriers to vertical steam migration.A pilot steamflood was designed to test the viability of steamflooding the 2nd Eocene. The pilot location was selected based on a number of criteria including reservoir quality and proximity to existing steam generation facilities. Several pattern configurations were evaluated based on their ability to depressurize the reservoir and to yield timely information, taking into account steam availability considerations. Based on these considerations, a configuration of seven 2.5-acre inverted 7-spot patterns was chosen for the pilot. Steam injection rates were designed to yield steam breakthrough after approximately one year of steam injection.A robust surveillance plan was developed to collect the data necessary to satisfy the pilot objectives and to understand the critical reservoir uncertainties. The surveillance plan was carefully considered in the well and surface facilities designs to ensure that all required measurements could be made with minimal impact on operations. Reservoir simulations indicate that a significant amount of fluid influx will occur from the reservoir surrounding the pilot after steam breakthrough, thereby obscuring the steamflood production response. For this reason a dense array of observation wells was designed to monitor reservoir temperatures and saturations and to provide enough information to understand fluid movement, steam chest development, and steamflood recovery factors in the presence of this influx. A thorough understanding of these mechanisms will be crucial for making future steamflood expansion decisions.