Zone 1 Steam Project, Coalinga Field
Authors: Taschman Barry M. 
Published: December 2007

Summary
This paper discusses thermal development, both steam soak and steam drive, of the Temblor Zone 1 reservoir in Coalinga field, Fresno County, CA. Zone 1 is a thick, multilayered; steeply dipping reservoir. It contains a 12 degrees API (973 kg/m3) oil and its viscosity averages 3,000 cp (3 Pa.S) at reservoir conditions. Zone 1 has been producing on primary since the early 1930's, but cumulative production has been very small as a result of low reservoir pressure and high crude viscosity. Attempts at thermal production were made in the 1960's and early 1970's, but they were unsuccessful. Results of recent activity indicate that both steam soak and steam drive are applicable to the Zone 1 reservoir at Coalinga; however, it appears that a combination of steam drive and steam soak is the most desirable process. The reservoir now is being developed as a full-scale steam drive project.

Introduction
The Coalinga field is midway between Los Angeles and San Francisco. It has been producing since the 1890's and to date almost 700 MMbbl (111 × 10 to the 6th m3) of comparatively light [20 degrees API (934-kg/m3)] oil have been recovered from its Zone 2 reservoir. Large volumes of heavy oil in the Zone 1 and Etchegoin reservoirs have been produced only recently. This paper refers to the Zone 1 formation underlying a productive area of approximately 300 acres (121 ha) in Sections 29, 30, and 32 as shown in Fig. 1. Zone 1 is a shallow marine deposit. It is productive at various locations in the Coalinga field. It is about 300 ft (91 m) thick; however, the lowermost third of the section is tightly cemented and considered nonproductive. The reservoir rock averages about 200 ft (61 m) in thickness with net pay up to 150 ft (46 m). The reservoir is a layered system containing up to five distinct members separated by silts or diatomites. Fig. 2 shows a composite type log from a typical well in Section 29. The productive reservoir ranges in depth from 800 to 1,700 ft (244 to 518 m) and dips at 21 degrees (0.37 rad) to the southeast. It is limited by a depleted zone updip and an oil/water contact downdip. The accumulation is separated from the overlying and underlying formations by shales. It outcrops to the northeast, but is thought to be sealed completely by crushing, folding, and inspissation. Zone 1 subcrops to the northwest and appears to be continuous into Section 31. It is estimated that there are 50 MMbbl (8 × 10 to the 6th m3) of oil currently in place on Sections 29, 30, and 32. Fig. 3 shows a net pay isopach of Zone 1 and the wells completed in it. Table 1 lists some of the pertinent reservoir rock and fluid properties.

Previous Recovery Attempts
Primary development began during the 1930's but with very poor results caused by the highly viscous nature of the crude. Cumulative primary production has amounted to 1.7 MMbbl (0.27 × 10 to the 6th m3) from 52 wells.
Attempts at both steam soaking and steam driving Zone 1 were made during the 1960's and early 1970's. During early 1961, a 1 1/2-acre (0.61 -ha) inverted five spot steam drive pilot consisting of one injector, three producers, and four observers was begun. The pilot responded in the dip direction with both increased temperature and increased oil production. No production or temperature increases were seen in the strike direction. Cumulative production as of Jan. 1, 1980, amounted to 65 Mbbl (10 × 10 to the 3rd m3) at an oil/steam ratio of 0.34. On the basis of pilot results. it was decided to try a full scale pilot, but with a line drive pattern. Unfortunately, this second pilot was placed in a then unknown depleted zone and was a complete failure, producing no additional oil. It was run during 1963 and 1964. During the early 1970's, four steam soak wells were completed in Zone 1. They were completed in the lowermost 50 ft (15 m) of the productive Zone 1 interval and given 10,000-bbl (1600-m3) soaks.
JPT
P. 511^