Cyclic Steam Project in a Virgin Tar Reservoir
Authors: Bott Richard C. 
Published: December 2007

Abstract
This article discusses the production techniques and results obtained by the first two cyclic steam injection wells in the virgin Vaca Tar sand. The two wells drilled by American Petrofina Co. of Texas and partners are located 1,700 ft apart. One well was partially successful before being shut in due to mechanical problems. The second well is now in its fifth cycle and has recovered 43,900 bbl of the 5 deg. API oil in 18 months. This project illustrates a situation where thermal recovery has provided an effective production technique for a reservoir which previously had been considered nonproductive.

Introduction
The Vaca Tar sand is in the Oxnard field, Ventura County, Calif. The field lies just east of the city limits of Oxnard which is located on the Pacific coast 40 miles north- west of Los Angeles. The reservoir. defined by deeper wells in the Oxnard field, covers approximately 1,700 acres. Several early completion attempts in the Vaca Tar sand were unsuccessful due to sand influx resulting from the combination of a viscous crude and an unconsolidated sand. A cyclic steam project was initiated to determine if the viscous crude could be produced by thermal techniques. When the viscosity of the crude around the wellbore was lowered by steam injection, the wells could produce the crude through gravel-packed completions without having sand entry. Production results show that the reservoir has prolific production capacity under cyclic steam operations.

History
The Vaca Tar sand was discovered in Jan., 1937, by Vaca Oil Exploration Co.'s Well 1 located in the south-west corner of what is now American Petrofina's Vacca Transamerica lease. The well established a production rate of 50 B/D but was abandoned because production could not be maintained due to sand influx. In May, 1937, a second well in the area discovered low-gravity oil in a fractured shale reservoir 500 ft below the base of the Vaca Tar sand. Subsequent drilling was to reach the low-gravity oil in the fractured shale, and the Vaca Tar sand was not exploited. Attempts were made to recomplete three such wells in the sand as salvage projects before abandonment. The zone was gun perforated, but sand influx resulted. In 1953, oil was discovered in the Sespe zone at 6,000 ft, and wells drilled to this formation provided additional geologic control for the Vaca Tar sand. The Vaca Tar sand remained in a virgin state until Dec., 1964, when American Petrofina initiated the thermal recovery project by drilling and steaming Vacca Transamerica Well 203. In Feb., 1965, three core holes were drilled on the Hunsucker tract, a northeast offset to the Vacca TransAmerica lease, to gain more reservoir data and establish the continuity of the sand in this area. In May, 1965, Well 203 was shut in due to down-hole mechanical problems. Vacca Transamerica Well 702 was completed in June, 1965, and has been produced by cyclic steam injection since that time. In partnership on the project with American Petrofina are Helmerich and Payne, Inc, and Kewanee Oil Co.

Geology
The Vaca Tar sand is Pliocene in age and overlies a Miocene unconformity in most areas.* The sand is fine- to medium-grained with scattered coarse grains and pebbles. Cores show the sand to be totally unconsolidated with sand grains held together by the viscous tar. The structure is a northeast-southwest trending homocline, dipping about 151 to the northwest. The Vaca Tar sand has a productive area of approximately 1,700 acres.* Average depth to the top of the pay in American Petrofina's project, located in the northeast portion of the reservoir, is 1,870 ft. Of the five wells drilled by American Petrofina, one did not penetrate the entire pay section and the other four had gross pay thicknesses ranging from 103 to 480 ft with corresponding net pay thicknesses ranging from 76 to 435 ft. Fig. 1 is a map of the net pay. Fresh water lenses exist in the upper portion of the sand in some wells, the most prominent of which is in the Vacca Transamerica Well 203 (Fig. 2).