Scab Cementing-An Economical Workover Technique For Effective Production Control

The Main Gatchell zone of the Coalinga Nose field, Fresno County, Calif., is one of the major oil pools in the state. One very significant phase of over-all operations in this field is scab cementing. Scab cementing used in the Coalinga Nose field as a method of well workover, establishes cement bonds between the perforated liner and the formation, separating a single long productive interval into several short intervals. Cup-type tubing packers and choke nipples are set opposite these cement bonds to allow selective production of oil, gas and water using the control obtained from bottom-hole chokes. The cemented perforated sections are called "scabs".Detailed descriptions of the various phases of scab cementing are given, as well as an analysis of the costs involved. Nearly all of the wells recompleted by scab cementing in the Coalinga Nose field are still flowing. Some or all of the techniques used in scab cementing for production control have been applied to other workover operations where a cement bond is needed between perforated or blank pipe and the formation. Successful application of scab cementing in other areas would depend on a careful examination of reservoir conditions, producing mechanisms and methods of lift. Where conditions are favorable for its use, scab cementing is an economical and effective workover technique that allows selective production control through designed tubing strings. Introduction The Main Gatchell zone of the Coalinga Nose field, Fresno County, Calif., is one of the major oil pools in the state. Discovered in 1938, it has produced over 300-million bbl of 31 degrees API oil primarily by a gravity-drainage mechanism. More than 94 per cent of the productive acreage was included when the Coalinga Nose Unit was formed on March 1, 1950.The Main Gatchell zone is a stratigraphic trap situated on a plunging anticline. The reservoir is a thick, massive quartzose sand of Eocene age. The air permeability averages about 750 md in the up-structure portion of the reservoir which is very nearly isotropic. Dips range from 100 to 300 at the sand and underlying shale contact, depending upon regional location on the structure. Drill depth to the top of the Main Gatchell zone averages about 7,500 ft. There was no primary gas cap in the Main Gatchell zone. Early in the productive life as the reservoir pressure decreased, gas accumulated at the top of the structure and expanded into the up-structure producing wells as a well-defined secondary gas cap, indicating that an effective gravity segregation of oil and gas was taking place. The attendant increases in gas-oil ratios of up-structure wells were controlled in many instances by decreasing the oil rates. When this was no longer effective (or when the ratios became excessive) the wells were shut in or, if feasible, deepened. Fig. 1 shows the area affected by the growth of the secondary gas cap over the years. History of Cementing Workovers It was recognized early that remedial operations would be necessary to control gas production from the encroaching secondary gas cap. The current scab-cementing method of production control, with minor variations, was initiated in 1950 and is the only method described in detail in this paper. Scab cementing is used in the Coalinga Nose field as a method of well workover, or repair. This establishes cement bonds between the perforated liner and the formation, separating a single long productive interval into several short intervals. Cup-type tubing packers and choke nipples are set opposite these cement bonds to allow selective production of oil, gas and water using the control obtained from bottom-hole chokes. The cemented perforated sections are called "scabs". JPT P. 349^