Design of a Major CO2 Flood—North Ward Estes Field, Ward County, Texas
R. Winzinger, J. L. Brink, K. S. Patel, C. B. Davenport, Y. R. Patel, G. C. Thakur, 1991. "Design of a Major CO2 Flood—North Ward Estes Field, Ward County, Texas", The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management, Robert Sneider, Wulf Massell, Rob Mathis, Dennis Loren, Paul Wichmann
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The reservoir engineering aspects of the design of a major West Texas CO2 flood are presented.
A detailed fieldwide geologic study was conducted to characterize the principal Yates reservoirs which consist of very fine grained sandstones to siltstones separated by dense dolomite beds.
A CO2 injectivity test was conducted to obtain CO2 injection rates and to identify any reduction in injectivity either during or following CO2 injection. Laboratory work included oil-CO2 phase behavior, slim tube tests with pure and contaminated CO2, and corefloods to determine recovery of waterflood residual oil by CO2 flooding for various WAG ratios.
A comprehensive waterflood evaluation proceeded the selection of average pattern models for reservoir simulation. These three-dimensional models, which have up to eleven layers each, were history matched over the 35-year waterflood period. Predictions were made for continuation of the waterflood and for CO2 flooding. Scale-up procedures were developed to predict from the average patterns the incremental oil production for the entire project area. Additional reservoir simulation was conducted to study the differences in recovery efficiency between line drives and five spots, the optimum economic slug size and the changes in hydrocarbon gas composition and GOR due to the stripping effect of CO2.
A comparison of the predicted performance with the actual performance since start-up in early 1989 will also be presented.
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The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management
Bima Field, offshore northwest Java, is a sizeable reservoir containing reserves of approximately 700 MM bbls OOIP with a 50 BCF gas cap. At present only the northern 1/3 of the field is developed, with 7 platforms and 54 producing wells, of which 20 are horizontal. The field has multiple drive mechanisms and high viscosity oil (21 cp), resulting in rapid GOR and water-cut increase after 3 years of production. The high stakes (both reserves and facility investments) and the reservoir's complexities, make an effective reservoir management scheme critical. For this reason an integrated geological, geophysical and engineering description was carried out to provide a 3-D Reservoir Simulation Model to evaluate development options. Geologically, the Oligo-Miocene age Batu Raja Limestone was deposited on the Seribu Platform, a basement-controlled, fault- bounded structure. The Upper Batu Raja carbonate build-up is thickest on the structurally highest parts of the platform where the rock comprises a series of "cleaning upwards" cycles (muddy deposits overlain by progressively more grain-rich sediments). A Lower Miocene drop in sea-level caused subaerial exposure of much of the platform and leaching by meteoric fluids. This diagenetic event resulted in contrasts in the reservoir quality (porosity, permeability, fluid saturations) at various intervals of the Upper Batu Raja. Based on these dissimilarities, the reservoir was zoned into 6 model layers. Once zonation was established, well logs could be calibrated to whole and sidewall core. A dense grid of seismic data were used to map the Batu Raja structure. From these data, color seismic inversion sections were produced and calibrated to the well logs. The calibrated seismic data were then used to map the top of structure, the carbonate build-up's edges, the total thickness of the Upper Batu Raja (needed to control aquifer size in the model) and the thickness of the main pay zone (layers 1-3). Engineering reservoir description began with a detailed compilation of capillary pressure, relative permeability, production and DST data. The 3-D simulation model required special treatments, including varying the GOC depths to honor separate gas cap closures; making permeability pressure dependent in poorly-consolidated zones; and setting up horizontal well completion treatments. Results suggest that water injection into the oil rim and gas cap is an effective approach toward maximizing recoveries and minimizing gas cap resaturation. However, waterflood reserves are sensitive to injection timing. The synergistic approach of geological, engineering and geophysical input into the Bima reservoir study has had impact by delivering a reservoir management tool that can evaluate future development expansion and possible gas sales. The simulation model can also track fluid migration during the field's producing life. The geological/geophysical model led to an enhanced understanding of Batu Raja depositional and diagenetic processes that has potential in regional exploration strategies.