Plugging Thief Zones in Water Injection Wells

Abstract

Abstract Standard Oil Co. of California is improving performance of its California water floods by improving vertical coverage and reducing water cycling. This program is being achieved by selectively plugging thief zones in water injection wells with a mixture of silicic acid gel and either gilsonite or silica flour. Previous field trials over a period of 10 years with many other materials and techniques had not achieved the desired results. This article discusses the theory and placement techniques of silicic acid gel and presents the results of successful plugging operations in unsegregated, multilayered reservoirs consisting of interbedded soft shales and unconsolidated sands. The operations have been successfully conducted in slotted liner completions, gravel-packed liner completions and cemented gun- or jet-perforated liner completions. The silicic acid gel is prepared in the field, and the entire operation can be conducted with a production hoist and cement truck. Results of this program show conclusively that silicic acid gel in conjunction with proper placement techniques can be utilized to effectively plug thief intervals in water injection wells. This is expected to become a major factor in improving waterflooding efficiency with a resultant decrease in operating costs and increase in net profit. Introduction The economic success of a waterflood in thick, multilayered sands is dependent upon the efficiency of the displacement process. Control of vertical injection coverage is necessary for the maximum oil recovery and the reduction of excessive water cycling. Effective vertical coverage can be achieved by plugging thief intervals or with subsurface controls. The Vickers zone, Inglewood oil field, Calif., is an Unsegregated, 1,500-ft thick, multilayered reservoir consisting of unconsolidated sands interbedded with soft shales (Fig. 1). The average net sand thickness in this interval is 520 ft. The porosity and permeability values range from 28 to 36 percent and 500 to 2,500 md, respectively. Most injection wells are completed with either gravel-packed or slotted liners. In unsegregated zones this type of completion precludes the use of packers and subsurface beans to control or exclude injection into thief intervals. Within this zone, thief intervals are not confined to particular stratigraphic intervals; therefore, they cannot be excluded upon completion. General Theory Selective Plugging Selective plugging in injection wells is not new to the petroleum industry. Cement, colloidal clays, inert solids, paraffins, waxes, organic resins and gels have long been utilized. Opinions differ as to the benefits obtained from using a selective plugging technique since apparently only the area adjacent to the wellbore is affected. JPT P. 999ˆ