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Abstract

One of Mobil's principal heavy-oil reservoirs, the Miocene Monarch, in the South Midway-Sunset Field, Kern County, California, is a thick sequence (400 ft.) of unconsolidated deep-sea turbidite gravels, sands, silts, and muds. Disappointing steam injection rates and poor production responses over the anticline prompted a laboratory and simulation investigation of the causes of the shortfalls. High-temperature coreflood experiments were conducted on selected facies, backed by liquid and solids analyses. The experiments showed that fines migration and smectite clay swelling are the primary causes of the restricted injectivity and quantified the severe damage.

Numerical thermal simulators lack the ability to model dynamic formation damage, but it was necessary to capture this progressive loss of permeability to match the field performance. Using a commercial simulator, we represented our laboratory data through the artificial regulation of the temperature dependence of the relative permeabilities. With this, and the recognition of some secondary effects, a very good history match of wellhead rate and pressure responses was achieved. The work shows that the use of standard nitrogen permeabilities gravely over-estimates field performance in this type of reservoir.

Introduction

The MOCO 35 Fee Property in the South-Midway Sunset Field, Kern County, California, is one of Mobil's principal heavy-oil reservoirs, producing from the Miocene Monarch sands. Limited steam and combustion operations had been conducted for decades, and a major expansion of steamflooding was initiated in 1989. By 1991, many injectors were restricted to rates as low as 100 bbl of steam per day (BSPD). This value was an order of magnitude lower than predicted by simulations, based on standard nitrogen permeabilities, and much lower than needed for economic recovery of the resource. The severity of the problem was not uniform: the anticlinal area was worse than the down-dip flank and the syncline. Our objective was to identify the damage mechanisms, seek remedial measures, and offer recommendations for future development.

GEOLOGIC CHARACTERIZATION

The Monarch C reservoir depositional environment is a deep-sea submarine fan complex consisting of unconsolidated gravels, sands, silts and muds of arkosic composition. The C reservoir (Figures 1 and 2) is comprised of at least 100 episodes of turbidite deposition (fining-upward sequences). Bed thickness averages approximately 2 ft., creating complex vertical and horizontal facies heterogeneities. Anticlinal and synclinal sediments (distribution of lithofacies) were strongly controlled by sea-floor paleotopography. The system is retrogradational, in response to the northward movement of the granitic source material along the San Andreas fault away from the depositional site at MOCO. The general structural configuration of the anticline and syncline in Section 35 was present at the time of deposition. The steep-dip area was further uplifted post-deposition.

Reservoir quality is controlled by the lithofacies and their distribution. Sediment that was deposited on the anticline (depositional high) is of an overall "finer-grained" regime as compared to the "coarser" sediment that accumulated in the syncline (depositional low).

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