A Full-Field Reservoir Simulation Model of the Eastern Shallow Oil Zone in the Elk Hills Field, California
Authors: Mayer D.F. , Weyland H.V. 
Published: December 2007

Abstract
A full-field reservoir simulation model of the Eastern Shallow Oil Zone (ESOZ) was constructed to assist in determing ultimate recovery from these sands. The simulation model grid was constucted using corner-point geometry to represent the complex fault pattern made up of sloping faults. Non-neighbor connections resulting from the juxtaposition of sands across faults were required to model inter-sand communication. Variations in oil gravity with depth and the corresponding fluid properties variations were represented in the model using the API tracking feature in the simulation program. Over 75 years of production history were matched with the model. Model predictions indicated the ESOZ contains reserves of over 100 million STBO and will produce 44 percent of the original oil in place.

Introduction
The Elk Hills Field, Naval Petroleum Reserve No. 1 (NPR- 1), is located 30 miles southwest of Bakersfield, California. The Shallow Oil Zone (SOZ), which has produced over 450 million STBO from the Elk Hills Field, is divided into two groups of sand reservoirs, the Western SOZ comprised of older sands and the younger Eastern SOZ sands which were the focus of this study.

Background.
The first significant production from the Shallow Oil Zone began in 1919 with a completion in Section 36R. This was followed in 1920 by a well drilled in Section 36S, outside the NPR-1 boundary at that time, which produced at a rate of over 5000 BOPD. This led to rapid development along the eastern boundary of NPR-1. In order to prevent drainage from the undeveloped lands within NPR-1, the government issued strip leases to private oil companies offsetting the development outside the reserve. Development in these two parts of the field progressed rapidly with most of the production coming from the ESOZ sands.
In October 1942 the limits of NPR-1 were extended to include all productive areas which had been identified at that time and in June 1944 the unit agreement was signed. Prior to unitization there had been significant amounts of production from the two developed areas identified above while the remainder of the field was undeveloped.
In June 1944 Congress authorized the development of NPR-1 to increase production to 65,000 BOPD because of World War II. Development was continued until August 1945, the end of World War II. Practically all of the 15 million barrels produced from June 1944 until August 1945 was from the SOZ.
Drilling during this period in the undeveloped portions of the ESOZ sands indicated significant pressure depletion had occurred throughout several of the sands. Other ESOZ sands from which minor amounts of production had been removed were found at pressures below their estimated initial pressures indicating communication had occurred across faults with the more severely depleted sands.
Immediately after World War II production was reduced to 15,000 BOPD and in 1946 a ceiling of approximately 8,000 BOPD was set with the intent of maintaining the field as a reserve.
In 1951 the third major development drilling program in the ESOZ began. This development further defined the productive limits of the ESOZ sands and increased the production capability from the field.
The most recent development of the ESOZ occurred after open up of the field in 1976. In the last few years, excellent results from gas injection into the ESOZ have resulted in further development drilling.

Producing Mechanisms
Early in the producing life of the ESOZ, pressure dropped rapidly below the bubble point and gas was liberated from solution, some of which migrated to the crestal portions of the reservoir, but much of the produced gas was vented to the atmosphere. The result was a generally rapid drop in reservoir pressure. Some water influx occurred primarily along the north and north-east edges of the field, but the primary drive mechanism was and still is a combination of low pressure gas cap expansion and the associated gravity drainage.