Performance of a Miscible Flood with Alternate Gas-Water Displacement
Authors: Holloway H.D. , Fitch R.A. 
Published: December 2007

A miscible displacement project was begun in 1960 in the Midland Farms (Wolfcamp) field. Initially, enriched gas was injected which was equivalent to 2 per cent of the project-area hydrocarbon pore volume. This solvent volume, which is small compared to volumes proposed in some of the literature, was followed by a residue gas buffer and is being displaced by alternate gas and water slugs. Favorable response, such as increased reservoir pressure, increased producing capacity and apparent miscible-bank movement indicates predicted recoveries may be achieved. Additional performance may prove many field applications now considered impractical can be economically justified.

Introduction

A full-scale miscible-drive project has been in operation in the Midland Farms (Wolfcamp) field since May, 1960. The project utilized a propane volume equivalent to 1 per cent of the hydrocarbon pore volume in the project area. It began with the injection of 13.9 million gal of residue gas which, with the propane, formed the enriched gas phase of about 2 per cent hydrocarbon pore volume. The enriched gas was followed with a dry-gas buffer, and now alternate slugs of water and residue gas are being utilized as the displacing media.

Geological and Reservoir Data

The Midland Farms (Wolfcamp) field, one the many fields in the huge Midland Farms complex, is located approximately 14 miles southeast of Andrews, Tex. The location of the field with relation to other major fields in the general area is shown in Fig. 1. The field lies on the western edge of the Midland basin in a transition zone between the Central Basin Platform and the Midland basin. Fig. 2 illustrates the Wolfcamp structure contoured on the top of the limestone section which is considered the top of the reef build-up in the area.

The pay section, ranging in depth from 8,350 ft to 8,500 ft, is composed of fine- to medium-crystalline organic limestone. Some oolitic deposits are present which are associated with the better-quality pay. In some wells, Wolfcamp reefing is absent. The degree of reef build-up at the various wells determines the structure. Fig. 3 is an illustration of the typical pay characteristics of the field. Table 1 lists the reservoir rock and fluid properties.

Primary Performance and Secondary Recovery Planning

Until secondary recovery was started, the reservoir had operated since discovery in 1953 under a solution gas- drive mechanism. Through May, 1960, the cumulative production for the field amounted to 1.97 million STB. During the primary production phase the average reservoir pressure had declined to approximately 1,600 psi. As a result, the producing wells in many cases had also reached a stage requiring artificial lift equipment. Fig. 4 graphically illustrates the field performance since discovery.

The field was unitized for the purpose of conducting secondary recovery operations. Although initial plans were to water flood the reservoir, laboratory experiments were conducted which indicated that a miscible drive could be established with moderate propane requirements. Subsequently, the enriched gas-drive project was selected, based on favorable incremental economics over conventional water flooding.

JPT

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