In order to determine the technical and economic feasibility of carbon dioxide (CO2) flooding the naturally fractured Spraberry Trend Area reservoirs in west Texas, a team with multiple disciplines was formed A thorough geological study has been performed based on analyses of thin sections, core samples, and open- and cased-hole logs. A rock-log model has been developed for identification of pay zones in the reservoirs. The natural fracture system has been characterized based on horizontal coring. Fracture intensities and orientations have been determined. Permeability anisotropy was estimated based on pressure transient data from well testing. The anisotropic permeabilities have been used in numerical simulation of a waterflood pilot in the field. Successful history match has been achieved Laboratory water imbibitions tests have been conducted to understand the waterflooding performance in the reservoirs. An analytical model has been developed for scaling-up the imbibitions data to field scale water flood. Good agreement has been observed between up-scaled imbibitions data and oil production decline in the oil field The result indicates that the weakly-wet Spraberry sands are partially responsible for the low performance of water flooding in the Spraberry Trend Area reservoirs. Possible effects of stress-sensitive fracture conductivity and formation damage on oil productivity of wells have been analyzed using a newly developed analytical model. The result indicated that the stress-sensitive fracture conductivity was partially responsible for the fast productivity decline during the primary production. The efficiency of CO2 flooding the reservoirs have been investigated in the laboratory using whole cores, reservoir oil, synthetic reservoir brine, and CO2 under reservoir conditions. Results of oil recovery from low permeability Spraberty cores during CO2 injection experiments are promising. A mathematical model has been developed for scaling-up the laboratory CO2 ternary oil recovery to pilot scale to assist pilot design. The result indicates a good potential of CO2 flooding in the reservoirs. After two years of team work, we have gained significant understanding of the naturally fractured reservoirs. The CO2 flooding pilot is being carried out. This paper presents our integrated geology-reservoir description and modeling of the naturally fractured Spraberry Trend Area reservoirs for CO2 flooding.
Carbon dioxide (CO2) injection is a proven technology to enhance oil recovery. Every CO2 flood reviewed in the literature reported improved oil recovery.1 Because of the success of CO2, reservoirs which were previously believed to be poor candidates for CO2 injection are now being considered as potential targets. This is especially true in the case of naturally fractured reservoirs. Historically, the perception of rapid channeling of injected CO2 to production wells precluded the concept as too risky. However, laboratory evidence2 and a pilot test in the naturally fractured Midale reservoir3 has opened up the possibility that naturally fractured reservoirs may be better candidates for CO2 injection than previously believed.
Naturally fractured reservoirs are, by nature, characterized by low permeability matrix. Permeabilities in tight gas sands or coal bed methane have been recorded in the microdarcy range. Naturally fractured oil reservoirs are often characterized by matrix perrneabilities in the submilidarcy range.
