A Two-Spot Combustion Recovery Project
Authors: Bowman C.H. 
Published: December 2007

Recovery of 400-cp oil from a 4,500-ft deep reservoir by forward combustion was successfully demonstrated. Of the 226,000 bbl produced during the project, at least 132,000 bbl represent additional recovery over primary. Vertical conformance was estimated at 75 per cent. Sixty percent of the oil was produced after completion of air injection. Reservoir and performance data are given.

Introduction

This in situ combustion pilot test was undertaken to investigate the possibility of recovering heavy (9 to 12 deg. API) oil from deep (4,500 ft) reservoirs. Mene Grande Oil Co. holds or operates concessions in Eastern Venezuela with considerable reserves of these heavy crudes. Primary recovery from these very flat reservoirs is generally low (2 to 7 per cent), and viscosities range upward from 400 cp. Conventional water and gas injection have in most cases been economically impractical. Only combustion holds hope of sweeping a sufficient fraction of these reservoirs to make secondary recovery projects feasible. As a modest initial venture, but to yield maximum information in the shortest possible time. an isolated two-spot pattern with 100-meter spacing was selected.

Test Site Description

The sand selected for this project is highly unconsolidated, with a porosity of 35 per cent and permeabilities ranging from 2 to 5 darcies. Initial oil saturation was estimated to be 94 per cent, with connate water occupying the remaining 6 per cent. The initial reservoir pressure was 1,600 psi, but had dropped to about 1,360 psi by the time air injection commenced.

Although many wells had penetrated this sand, virtually all are completed in other reservoirs. However, one existing well was found which could serve as an injectors Some 23 ft of net oil sand was present in two tenses, an upper of 10 ft and a lower of 13 ft. Although a 2-ft shale section separated the lenses, the entire interval was perforated. Prior to inception of the air-injection project, the injection well had produced 89,000 bbl of 9.5 deg. API crude. Its initial rate was 225 BOPD, and a decline- curve analysis yields an estimated cumulative recovery of 183,000 bbl of oil. Following selection of the location and spacing for the proposed two-spot, a second well was drilled 100 m northeast of the injector. It encountered only a disappointing 10-ft sand. considered inadequate for a good producing well. Another well was then drilled 100 m to the southwest. A single 15-ft sand was found, and the well was cased and completed. An isopach map of the sand had previously been prepared from logs of all wells penetrating the section. Spacing is 600-m hexagonal. Most nearby wells are completed in other sands. In view of the rapid changes in thickness found over 100-m distances, this map was rendered meaningless. Thus the effective size of the reservoir could be obtained only from a material balance. An effective volume of 3.56 million bbl of oil in place was computed. The producing mechanism was pure fluid expansion.

Equipment and Completions

The producing well was equipped with 9 5/8-in. casing, gravel packed, and completed with a 5 1/2 -in. OD slotted liner (see Fig. 2). Two parallel tubing strings were run-a 3 1/2-in. OD production string with a pump holddown nipple, and a 2 7/8-in. OD observation string which swaged to 2 3/8-in. OD to enter the liner. This latter string was plugged on bottom, and a thermocouple line was run inside of it. Three thermocouple junctions were spaced at 5-ft intervals oppositive the perforations.

JPT

P. 994ˆ