Summary. in 1980-81, Tenneco Oil Co. conducted an in-situ combustion pilot test on the Lower Hospah formation of the South Hospah field, McKinley County, NM. Although the reservoir appeared to be an excellent candidate for in-situ combustion, the pilot project recovered only a very small amount of incremental oil and was terminated as a consequence. To evaluate the pilot test and to determine the reasons for its performance behavior, reservoir model of the test site was developed from the historical performance of secondary recovery operations in conjunction with available performance of secondary recovery operations in conjunction with available log and core data. The result was a three-layer, four-quadrant model of the test site. The volumetric sweep efficiency of the combustion front was estimated from two interior core holes drilled after the project was terminated. This resulted in a postcombustion model of the test site depicting the vertical sweep of the combustion front.

Stoichiometric relationships were used to evaluate the combustion performance of each layer of the model. The stoichiometric evaluation performance of each layer of the model. The stoichiometric evaluation provided a means to compare quantitatively the postcombustion reservoir provided a means to compare quantitatively the postcombustion reservoir model with actual test performance, thus verifying the model. The project failed because the combustion front migrated beneath the oil zone, processing an interval containing low oil saturation and resulting in very processing an interval containing low oil saturation and resulting in very large air/oil ratios.

Introduction

In 1980, the Lower Hospah sandstone formation was in the declining stages of secondary recovery and was producing at very high water cuts. Combined primary and secondary recovery was about 34% of the original oil in place (OOIP), leaving about 6.2 MMSTB [986 x 10 3 stock-tank M 3 ] as a tertiary recovery target.

A number of EOR alternatives were studied, and a conventional in-situ combustion process pilot was proposed. The Lower Hospah appeared to exhibit a combination of reservoir and fluid properties amenable to the combustion process.

A small, 0.592-acre [2396-m2], inverted five-spot combustion pilot was initiated in Nov. 1980. By June 198 1, following 215 days pilot was initiated in Nov. 1980. By June 198 1, following 215 days of continuous air injection, the pilot had recovered less than 45% of the tertiary target, yet injected air volumes were 35% greater than the estimated total pilot requirement. As a result, the pilot was terminated and plans for full-field expansion were abandoned.

Tenneco released the data to the authors for the purpose of evaluating the Lower Hospah combustion pilot and to determine who incremental recovery was low. At the time the project failed, its poor performance was generally attributed to one (or a combination) poor performance was generally attributed to one (or a combination) of the following factors: (1) waterflood interference (the pilot was surrounded by an active waterflood); (2) combustion of the coal seam overlying the Lower Hospah formation; or (3) excessive heat loss to the underlying aquifer preventing efficient combustion.

Although these factors would appear to be detrimental to a combustion process, the results of this evaluation indicate that none of the above were responsible for the low recovery observed in the Lower Hospah combustion pilot.

The methodology used to evaluate the combustion pilot was first to review the historical performance of the Lower Hospah reservoir, placing particular emphasis on the production response to various placing particular emphasis on the production response to various development programs and secondary recovery techniques.

Then, the incremental tertiary recovering was determined from pilot combustion data and the historical performance of the reservoir, Actual incremental tertiary recovery was compared with projected tertiary recovery, and the magnitude of the production shortfall was realized. The third phase of the study was a detailed evaluation of the Lower Hospah waterflood. Log and core data from the pilot wells were used to determine the volumetric sweep efficiency of the injected water, and a precombustion reservoir model was developed. The fourth phase of the study was the evaluation of the combustion front sweep efficiency by analysis of data obtained from two interior core holes drilled after the project was terminated. The result was a postcombustion reservoir model depicting the vertical sweep of the combustion front,

The final phase of the study was a stoichiometric evaluation of the Lower Hospah combustion process. This provided a quantitative means of comparing the postcombustion reservoir model to actual incremental combustion recovery and verified the reservoir model. The model illustrated the reasons for the production shortfall in the Lower Hospah combustion pilot.

Reservoir Description and History

The South Hospah field is in the Hospah dome on the Chaco slope of the San Juan basin. The field comprises two producing sand-stone reservoirs, the Upper Hospah and the Lower Hospah.

The Lower Hospah sand was deposited by a regressive Cretaceous sea sequence and is part of the massive Gallup formation. As the sea regressed, a layer of plant sediment was deposited on the Lower Hospah sand by the swampy, back-beach environment that followed the shoreline. This organic layer was subsequently buried by continental sand deposits and formed a thin coat seam separating the two Hospah sands.

The Lower Hospah formation is a clean, blanket sand deposit about 100 ft [30 m] thick. Productive limits are defined by a fault on the northwest flank and the original oil/water contact (OWC), as shown on the structure map, Fig. 1. Fig. 2 shows the original net pay isopach of the Lower Hospah formation. The Lower Hospah reservoir is sealed at its top by a 2- to 3-ft [0.6- to 0.9-m] -thick coal seam and bounded below by a low-permeability bioturbated sandstone.

The structure dips approximately I @ [0.02 rad] to the southeast. The thickest portion of the pay zone averages +/- 40 ft [ +/- 12 m] along the fault, and the thinnest portion, +/-20 ft [+/-6 m]. lies near the eastern edge of the field. The original pay section averages 28 ft [8.5 m] over the areal extent of the field and is continuous with no shale barriers. Table I lists reservoir and fluid properties.

The structure map and the net pay isopach map (Figs. 1 and 2) represent the original reservoir conditions. The position of the OWC was subsequently altered as a result of water encroachment or water-flood underrunning. A high degree of water coning also occurred.

The original aquifer below the Lower Hospah formation is active and apparently tilted approximately 0.86deg. [0.015 rad] to the east.

SPERE

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