Engineering and Geologic Characterization of Giant East Texas Oil Field: Northern and Southern Pilot Studies

Abstract

Abstract East Texas oil field (ETOF), discovered on September 3, 1930, is the second-largest oil field discovered in the United States. It has produced 5.42 BSTB of 39o API oil from >31,000 wells from lower Woodbine sandstones. Estimates of original oil in place (OOIP) have ranged from 6.8 to 7.5 BSTB. Based on a midrange OOIP value of 7.0 BTSB, current recovery efficiency is 77%, which is the highest of any giant oil field in the world. Two areas of the field, one in the north and the other in the south, were selected in 2007 for study of the primary engineering, geological and operational factors controlling recovery; for describing the detailed reservoir architecture; and for developing strategies for future exploitation. The Upper Cretaceous lower Woodbine Group in ETOF, ~150 ft thick in the western downdip area, is truncated by a pre- Austin Chalk unconformity to the east. The Woodbine is composed of stacked, NE-SW-trending fluvial-dominated deltaic deposits, with dip-elongate distributary-channel sandstones pinching out over short distances into delta-plain and interdistributary-bay siltstones and mudstones. High-quality fluvial- and distributary-channel sandstones with average porosity and permeability of 25.2% and 2,098 mD, respectively, are the major producing lithofacies. The fluvial-channel sandstones that dominate the northern pilot area are laterally and vertically more continuous than the delta-dominated facies of the southern pilot area. ETOF, with an average of 4.2-acre well spacing, is one of the most densely drilled fields. Production is supported by a active downdip aquifer. Major operating strategies have been downdip water injection, plugback, deepening for untapped intervals, and waterflooding in areas with poor reservoir quality as well as aquifer support. Its high-recovery efficiency stems from its high reservoir quality, low residual oil, favorable stratigraphic dip, effective aquifer support, stable water movement controlled by production rates. Of the estimated 1.58 BSTB of remaining oil, only 70 MMSTB is likely to be produced under current operating techniques. However, a fraction of 410 MMSTB of remaining mobile oil may be produced by depositional-trend-guided deepening and water injection. The remaining approximately 1.1 BSTB of residual oil will only be produced by enhanced oil recovery methods. Because the reservoir is shallow and reservoir pressure low, CO2 flooding would most likely be immiscible. In addition, with most wells >60 years old, well-bore leakage is an environmental factor that must be considered prior to CO2 injection.