Sawtelle Field, California: Simulation of a Complex and Unusually Shaped Reservoir
Authors: Engineer Roy , Quinn J.S. , Holst P.H. 
Published: December 2007

The Sawtelle field is located in a heavily populated residential area of west Los Angeles, CA. Two separate hydrocarbon traps occur in the field, and both result from the Santa Monica fault. Pool I occurs in an asymmetrical fold, and Pool II in beds upturned in the drag zone of the footwall block of the fault. The Pool 11 reservoir contains about 65 % of the field's reserves. Reservoir engineering studies indicate primary depletion will permit the recovery of 18.8 % of original oil in place (OOIP). This paper describes a two-dimensional (2D), three-phase, black-oil simulation of the Pool II reservoir in the field. The purpose of the study was to investigate, through numerical simulation, the feasibility of implementing a pressure maintenance scheme by gas injection or water pressure maintenance scheme by gas injection or water injection in this unusually shaped reservoir.

Introduction

The Sawtelle field in west Los Angeles (Fig. 1) was discovered in Aug. 1965. The field is one of a series of oil accumulations in the "urban trend" at the extreme north end of the Los Angeles basin. Fig. 1 shows that the Sawtelle field is the westernmost of the 12 fields in the trend. Most of these fields were developed in the early 1960's. Ultimate recovery from the trend is expected to surpass 1 billion bbl [0.16 × 109 m3] . Production from the field is derived from two separate hydrocarbon traps (Pools 1 and 11). The producing interval in both pools is the Rancho sands at an average depth of 10,000 ft [3048 m]. The productive area under the field encompasses 300 acres [121 × 10 4 m2 ] and the net pay averages 400 ft [122 m] in an average gross interval of 800 ft [244 m). The two reservoirs appear to have communication through a common water table but have no similarities in structure, reserves, or fluid properties. Of particular interest is the difference in oil gravity between particular interest is the difference in oil gravity between the two: the southerly Pool I is 26 deg. API [0.898 g/cm3) and the northerly Pool 11 20 deg. API [0.934 g/cm3). Pool I had 23.83 million STB [3.79 × 10 6 stock-tank m 3] OOIP compared with 36.47 million STB [5.80 × 10 6 stock-tank m3] in Pool II. This paper presents the results of a 2D, three-phase black-oil simulation study carried out to predict Pool 11's response to pressure maintenance by gas and water injection. Of the two secondary exploitation possibilities, gas injection was of primary interest because of a unique source of the fluid. It was possible to secure the supply of a low-Btu gas with a composition of +55% methane and + 45 % CO2 from a nearby landfill site. The maximum deliverable volume would have been about 3.0 million scf/D [0.08 × 10 6 std m3/d]. It was believed that a CO2/methane mixed-gas-injection project in the steeply dipping Pool II could result in higher oil recovery. A production history match of 17.5 years was made. It is our production history match of 17.5 years was made. It is our contention that, by including a large performance history, we obtained an improved reservoir characterization. The simulator prediction runs indicate that the final recovery efficiency for this unusually shaped reservoir would be about 18.9% OOIP for primary recovery, 27.4% OOIP for crestal gas injection, and 23.6% OOIP for bottomwater injection. Of the preceding, the gas injection alternative is expected to exhibit the highest risk.

Development

The Sawtelle field was discovered with the completion of Well DF-1, RD-3 in Pool 1. The well initially produced 1,000 B/D 1159 m3/d]. A total of 14 wells had been drilled in the field during 1965–69. Pool 11 has five producing wells and one plugged and abandoned well at this producing wells and one plugged and abandoned well at this time. All wells in the field (except DF-1, DF-7 and DF-13) are cased with 7-in. [18-cm] casings to the top of the sand, cemented, and then drilled to total depth (TD) and lined with 5-in. [13-cm] slotted liners. The other three wells are drilled with 5-in. [13-cm] liners, cemented, and selectively jet-perforated at the producing intervals. All wells were drilled from a drilling pad and, therefore, are deviated holes. The wells are producing with fixed insert pumps and open power fluid. Most of these wells use pumps and open power fluid. Most of these wells use 27/8-in. [7–3-cm] main tubing with 1 1/2-in. [3.8-cm] inner tubing strings. The power oil goes down the inner string and returns, combined with produced fluid, up the tubing annulus. Currently, the average bottomhole flowing pressure (BHFP) in individual wells is in the range of 600 pressure (BHFP) in individual wells is in the range of 600 to 800 psi [4.1 to 5.6 MPa].

Geology

Pool I and Pool II reservoirs under the Sawtelle field result Pool I and Pool II reservoirs under the Sawtelle field result from movement in the footwall block of the Santa Monica fault. Pool I occurs in an asymmetrical fold and Pool II in beds upturned by the drag zone of the fault. Fig. 2. is a structure map of the Sawtelle field showing the pronounced anticlinal fold immediately in front of the pronounced anticlinal fold immediately in front of the footwall drag zone.

JPT

P. 488