Combustion as a Primary Recovery Process-Midway Sunset Field
Authors: Gates C.F. , Sklar I. 
Published: December 2007

Despite the complicated structure of this reservoir, in-situ combustion has been an effective primary recovery process essentially since the beginning of production more than a decade ago. To date, the recovery is 9 million barrels, or 24 percent of the oil originally in placesubstantially higher than the estimated ultimate recovery of 17 percent.

Introduction
The Moco zone reservoir, located in Sections 34 and 35-T12N-R24W of the Midway Sunset field, Kern County, Calif., was discovered in 1957 and partially developed. However, because there was no demand for the heavy crude, it was essentially shut in until Nov., 1959. It was expected that without fluid injection of some kind there would be a rapid decline in oil production rate. Although there were multiple sands present, the reservoir properties of the Moco zone appeared favorable for in-situ combustion. The structure appeared to be entirely on Mobil Oil Co. property. Predictions of performance under combustion property. Predictions of performance under combustion were based on the results of the South Belridge Thermal Recovery Experiment and on prior supporting work.
Evaluation of several methods of operation indicated that the most desirable economics would be obtained if the combustion process were applied immediately. Accordingly, combustion operations were undertaken as soon as practicable after the reservoir was returned to production and development was resumed (this took place in Nov., 1959). Air injection was started in Jan., 1960, at which time the cumulative oil production from the reservoir was 160,000 bbl, or 0.4 percent of the oil in place, and the reservoir pressure was only slightly below its initial value of 1,000 psi. Case histories of other combustion projects have been reported in the literature. projects have been reported in the literature. Reservoir Characteristics
The reservoir is a small anticline with six major sands. Structural contours, a type log, and the areal extent of the six sands are shown on Fig, 1. An east-west cross-section (X-X') along the longitudinal axis is shown in Fig. 2. The sands generally are separated by interbedded shales and to the east become thin and disappear. To the west some of the sands merge. The sands were separately identified, correlated, isopached, and designated M1 through M6. This permitted an appropriate design of air distribution permitted an appropriate design of air distribution in the injection wells, which have various combinations of sands open.
There are productive zones above and below the Moco reservoir, and the Moco zone itself merges with the Obispo-Pacific fractured shale zone to the east. The presence of these other zones added to isolation problems and costs during development drilling and problems and costs during development drilling and well repair.
A north-south cross-section (E-E') across the structure is shown in Fig. 3. The dip is up to 45 degrees to the north and 20 degrees to the south. General reservoir characteristics are as follows:
Productive area, acres 150 Average depth, ft 2,100 to 2,700 Gross formation thickness ft 500 Average net sand thickness, ft 129 Porosity, percent 36 Oil saturation, percent 75 Water saturation, percent 25 Formation volume factor 1.06 Initial oil in place, bbl/acre ft 1,980 Initial oil in place, total bbl (millions) 38
JPT
P. 981