Abstract

Devonian strata in Dollarhide Field are expected to produce three times as much oil during secondary and tertiary recovery as was recovered during primary production. The purposes of this paper are to show geological features which contributed to the prolific secondary and tertiary recovery, and to explain the implementation of the tertiary CO₂ flood. Devonian strata of the Thirtyone Formation at Dollarhide Field originally contained 144 million barrels of oil (MMBO) in place with 18.6 MMBO produced by primary recovery, 39.3 MMBO projected to be produced by secondary recovery (waterflood), and 27.4 MMBO projected by tertiary recovery (CO₂ flood). Dollarhide Devonian Field occurs in a faulted anticline. Although most porosity is secondary (diagenetic), porosity within Devonian strata follows depositional facies because most diagenesis is facies-selective. Devonian strata contain five main lithologies (from top to bottom): (1) upper dolomite (50-90 feet thick), (2) middle bioclastic limestone (47-92 feet), (3) laminated tripolitic chert (0-65 feet), (4) burrowed chert/dolomite (10-35 feet), and (5) interbedded microcrystalline chert and dolomite. These were deposited as a shallowing-upward sequence approximately 180 feet thick.

Porosity occurs in two stratigraphic zones separated by a tight limestone. The upper porosity zone in the upper dolomite is 0-90 feet thick and heterogeneous with stratified porosity which bifurcates, coalesces, and pinches out in several different intervals. The lower porosity zone (laminated tripolitic chert and part of the burrowed chert-dolomite; 0-100 feet thick) is generally unstratified and homogeneous with high porosity (commonly 25-35 %), moderate permeability (5-20 md), and good continuity. The homogeneity, moderate permeability, and good continuity of the lower reservoir allowed excellent secondary recovery and are ideal for uniform sweep during CO₂ flood, whereas thinner, more heterogeneous, less-continuous, and more stratified pay in the upper reservoir should cause less uniform CO₂ sweep. Reservoir continuity is interrupted by vertical faults which act as impediments to lateral flow, so CO₂ flood patterns were arranged accordingly. CO₂ injection began in the first flood area in mid-1985. Increased oil production associated with the CO₂ flood started in early 1987 and has been increasing to the present. No channeling or premature CO₂ breakthrough have been observed.