The Conroe Field Unit - A Challenge In Reservoir Control
Authors: Dalton Benjamin A. , Lovejoy Joe B. , Brinkman Fred P. , Simpson William R. 
Published: December 2007

The Conroe field is a 30-square-mile (7800-hectare), highly faulted structure producing from Cockfield sands. The thick main Conroe sands have produced over 600 million bbl (95.4 Mm3) of oil, produced over 600 million bbl (95.4 Mm3) of oil, primarily by water drive. Cumulative gas production primarily by water drive. Cumulative gas production exceeds 550 Bcf (15.6 Gm3). The field has experienced severe gas cap shrinkage, resulting in loss of recoverable oil as residual saturation in the original gas caps. A field-wide unit was formed on January 1, 1978, following over five years of effort.

Tie Unit Operating Plan adopted includes (1) restriction of net gas withdrawals by reduced gas production combined with injection of excess produced production combined with injection of excess produced gas and (2) redistribution of liquid withdrawals for control of reservoir fluid movement. Alterations to established liquid withdrawal patterns were required to reduce the large pressure differentials between individual fault blocks and sand members which had developed under competitive operations. Detailed analysis of recent field performance was necessary to establish a basis performance was necessary to establish a basis for redistribution. This analysis involved defining oil-water- and gas-oil contact movement for 131 separate study areas over a three-year period. Water influx, fluid migration, and gas cap shrinkage during the period were then calculated by volumetric balance. From this information, liquid production rates required to stop shrinkage were established as targets for near-term operations.

Since unitization, net gas withdrawals have been reduced to an acceptable level, while the redistribution of liquid withdrawals is still in the implementation phase. Initial results of the program to match liquid production to water influx program to match liquid production to water influx rates have confirmed that gas cap shrinkage can be arrested in this manner. Stabilization of contacts in this complex field will require a carefully integrated program of limited gas withdrawals, balanced liquid production, intensive surveillance, and timely response to changing conditions.

FIELD DESCRIPTION

The Conroe field is located 40 miles (64 km) north of Houston on the upper Texas Gulf Coast. The field was discovered in 1931 and developed on 20-acre (8-hectare) spacing with some 1100 wells. Production occurs at an average depth of 5000 feet Production occurs at an average depth of 5000 feet (1500 m) from Cockfield sands of Eocene age. The structure is a large domal uplift, probably underlain by a deep-seated salt dome. Faulting is highly complex, as shown in Fig. 1, with throws ranging from less than 25 feet (7.6 m) to over 300 feet (91.4 m). These faults are generally non-sealing and provide natural paths for inter-reservoir communication between juxtaposed sands.

The Cockfield is treated as two separate fields for regulatory purposes and has been further subdivided for reservoir analysis. These designations are shown on the type electric log, Fig. 2. The Conroe (Cockfield, Upper) field is made up of two thin, shaly sands that are primarily gas productive. The Conroe field is composed of high quality sands, with permeability averaging 1250 md. Original gas-oil and water-oil contacts were uniform in all Conroe field sands; the ratio of original gas cap to oil zone volume was 0.37:1. Currently, the Second Main Conroe (2MC) sand contains the bulk of remaining oil reserves.

PRODUCTION HISTORY AND RESERVOIR BEHAVIOR PRODUCTION HISTORY AND RESERVOIR BEHAVIOR Conroe's pre-unitization history may be separated into two phases. In the first, through 1971, production rates from the Main Conroe sands varied production rates from the Main Conroe sands varied widely, from 60, 000 B/D (9540 m3/d) in 1944 to less than 14,000 B/D (2230 m3/d) in 1962 (Fig. 3). The field's assigned MER was reduced from the World War II level of 60,000 B/D (9540 m3/d) to 50,000 B/D (7950 m3/d) in 1948; the decline which followed was a function of market demand proration in Texas. Reservoir pressure at the end of 1971 was still at 79 percent of the original 2190 psig (15.1 MPa), and average water production was 74 percent, confirming the presence of a strong water drive.