A Study of the Conroe Field

Field: Earth and Planetary Sciences (Physical Sciences)

A study of fluid movement in the complex Conroe field indicated that gas was migrating from the Main Conroe sands to the lower-pressure Upper Cockfield sands, causing shrinkage of the Main Conroe gas-caps and loss of oil as residual saturation. Interim measures have been successful in temporarily stopping gas-cap shrinkage, and studies of long-range solutions are under way. Introduction The Conroe field is a 30-sq-mile, highly faulted structure producing from Cockfield sands that are prorated as two producing from Cockfield sands that are prorated as two separate fields. The thin Upper Cockfield sands have produced primarily as a depletion-drive gas reservoir, produced primarily as a depletion-drive gas reservoir, while the thick Main Conroe sands have produced more than 500 million bbl of oil by combination water and gas cap drive. A study of fluid movement indicated that gas was migrating from the Main Conroe sands to the lower-pressure Upper Cockfield sands, causing shrinkage of the Main Conroe gas caps and loss of oil as residual saturation. A computer model of this complex field showed that unless remedial action was taken migration would continue and future shrinkage losses could amount to more than 40 million bbl of oil. As an interim measure to alleviate the shrinkage, the Texas Railroad Commission increased the Main Conroe MER from 50,000 to 60,000 BOPD, which will slightly accelerate pressure decline and expand the remaining gas caps enough to offset the gas migration loss. A joint technical subcommittee is now studying the need for injection projects to stop permanently the migration and the need to form a field-wide permanently the migration and the need to form a field-wide unit for this purpose. Field Description The Conroe field is located 40 miles north of Houston on the upper Texas Gulf Coast. The structure is a large domal uplift, probably underlain by a deep-seated salt dome, and is highly faulted, as shown by the structure map in Fig. 1. The field produces from sands of the Cockfield formation of Eocene age at an average depth of 5,000 ft. Fig. 2 is a type electric log depicting the sandshale sequence, which is remarkably uniform across the structure. The field was discovered in 1931 and was developed rapidly on 20-acre spacing with 1, 100 wells. Essentially all the drilling took place before development of the electric log, and screened open-hole completions were made, generally through several sand members. The field currently consists of 150 leases operated by 27 companies. The Cockfield sands are prorated as two mews, as shown on the right side of Fig. 2. The Conroe field consists of six, thick, high-permeability sands that have provided 95 percent of the cumulative oil production. provided 95 percent of the cumulative oil production. For study purposes, they are designated the First through Sixth Main Conroe sands. Overlying the Main Conroe sands is the Conroe (Upper Cockfield) field. which consists of two thin, shaly sands that are primarily gas productive. productive. Production History and Reservoir Behavior Production History and Reservoir Behavior The Main Conroe sands had a thick (160-ft) original oil column, with uniform gas-oil and oil-water contacts in all sands. The original gas-cap/oil-zone volume ratio was 0.37. Cumulative oil production to Jan. 1, 1971 (the effective date of this study) was 461 million bbl. At that time, the field's assigned reservoir MER was 50,000 BOPD, produced from 600 active wells. JPT P. 813