Ten Years of Miscible Displacement in Block 31 Field
Authors: Herbeck Eugene F. , Blanton Jay R. 
Published: December 2007

Abstract
A decade ago The Atlantic Refining Co. introduced to the oil industry a new concept of secondary recovery--miscible displacement by the high- pressure gas-injection process. The initial application of this process was made in the Block 31 Devonian field in Crane County, Tex. Since the first injection of gas, a cumulative of 127 billion cu ft has been injected. Total field production has been 55 million bbl of oil, or nearly 80 per cent of the estimated production by primary. Current producing rate for the field remains at 15,350 BOPD. Performance to date indicates that the process is operating essentially as predicted and that miscible sweep of the reservoir is being achieved. To achieve optimum sweep of the reservoir and maximum oil recovery, considerable reservoir engineering effort is directed toward maintaining pattern balance. To insure miscible sweep, annual pressure surveys are taken to ascertain that the pressure at each of the displacing gas fronts is at least 3,500 psig, the pressure needed for miscible displacement. Several problems which threatened the success of the program have arisen but are now under control. The most serious of these was a siege of injection-well plugging which occurred in 1958. Most of the wells were affected before the problem could be remedied. Breakthrough of injectedgas has been detected in six wells to date. In three of these, the cumulative oil produced at breakthrough was essentially as predicted. The other three wells have experienced breakthrough earlier than expected, but these wells are balanced by three others that should have broken through but have not. The results average out so that an ultimate recovery as predicted for miscible sweep can be expected.

Introduction
The first miscible project ever undertaken is that in the Devonian reservoir of Block 31 field in West Texas. Here miscible sweep is maintained by the high-pressure gas-injection process. The process entails the injection of lean gas into the reservoir at high pressure. The Block 31 Devonian crude properties and reservoir conditions are such that, by maintaining 3,500 psig at the injected gas front, miscible displacement is achieved. The entire oil-producing industry has shown a high degree of interest in this program since its initiation. A decade ago the concepts of miscible displacement were in their infancy and the program in Block 31, being the first field application of the process, was experimental in nature. A progress report was made early in its life. However, the program now has been operating long enough for significant results to be reported. Several producing wells have experienced gas breakthrough, making it possible to render at least a limited interpretation as to the success and ultimate outcome of the program. The program of high-pressure gas injection was initiated in June, 1949. Initially, the gas produced from the Devonian and Ellenburger formations was injected into the crest of the Devonian reservoir to partially maintain the reservoir pressure. At the same time, laboratory research was being conducted on the application of the high-pressure gas process to the field. The research showed that high additional recovery could be expected from its application. The plans for Block 31 field were revised accordingly, and the decision was made to fully maintain pressure in the field. On Oct. 1, 1952, the program was expanded to full pressure maintenance. This entailed the purchase and injection of gas in addition to that being produced. Currently, 57 MMcf/D of gas are being injected into 15 injection wells on a nine-spot pattern. Production from the field is 15,350 BOPD.

Geology and Reservoir Properties
The Block 31 field is located in the east-central part of Crane County, on the eastern side of the central basin platform. The Devonian accumulation is situated on an anticline lying in a northeast-southwest direction. The top of Devonian is encountered at about 7,900 ft and has a gross thickness of approximately 1,000 ft. As shown in Fig. 1, the Devonian has three pay horizons referred to as the Upper, Middle and Lower Devonian horizons. The main oil-producing section is the Middle Devonian horizon, which covers the interval from 450 to 800 ft below the top of the Devonian. This 350-ft interval contains about 85 per cent of the oil in place and is the only portion of the reservoir which is being miscibly swept. The middle horizon itself is further stratified with dense and permeable streaks. The permeable zones can be correlated from well to well over most of the field. Up to 185 ft of this horizon is net pay in some of the structurally higher wells.
JPT
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