Miscible Drive Field Applications in the Block 31 Field

Introduction

Pressure maintenance by gas injection has been in operation for the past eight years in the Devonian formation of the University Block 31 field. The object of this operation is to achieve miscible displacement of the oil and consequently increase the ultimate field recovery. It is estimated that miscible high pressure gas displacement will yield three times as much recovery as by natural depletion (solution gas drive) mechanism.

Earlier papers have dealt with the mechanics of miscible displacement and a description of this operation. We will not attempt to expand upon this subject further here, but merely state that by laboratory experiment it has been found that in order to achieve miscibility between injected gas and this Devonian reservoir fluid, it is necessary to maintain 3,500 psi pressure at the displacing gas front. Although the average reservoir pressure has declined below 3,500 psi, the injection pressure has been sufficiently high to maintain at least 4,000 psi pressure at the gas front in all injection wells.

After two years of injecting gas into the G–2 well it was noted that this area was not increasing in pressure as expected. It was suspected that injected gas was leaking to other formations because there was less repressuring than predicted by material balance. This paper deals with one approach to the problem of gas losses and the corrective and control measures involved.

Reservoir Description and Injection Operation

The Block 31 Devonian field covers about 7,000 acres. The Devonian formation is on an anticlinal structure at a depth of 7,900-ft subsurface at the crest. The gross thickness is 1,000 ft. The field boundary is defined by water closure at 9,000-ft subsurface. The main producing section is located 450 to 800 ft below the top of the structure. This interval contains 85 per cent of the total in-place oil.

Seventy-three wells are presently completed in the Devonian on about an 80-acre spacing. Fifteen of these wells are currently being used for gas injection, basically in nine-spot injection patterns.

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