Annular Gas Flow After Cementing: A Look At Practical Solutions

Abstract

Abstract This paper discusses field application of techniques for preventing annular gas flow after cementing. Included are sections outlining a theory explaining the annular gas phenomena, a graphical productive technique, theoretical preventive methods, productive technique, theoretical preventive methods, and case histories of actual field applications of these preventive techniques. The theory discussed in this paper is based upon analysis of laboratory research conducted by the Texas A and M University Research Foundation and Exxon Company, U.S.A. The results of this research indicate that annular gas flow after cementing is associated with a reduction in the effective hydrostatic head exerted by a cement column during its initial curing period. A graphical technique which predicts the potential for annular gas flow after cementing has potential for annular gas flow after cementing has been developed. This technique consists of a plot of depth versus hydrostatic pressure of the annular fluid columns and assumes that the cement slurry reverts to a fluid gradient equal to that of its mix water density. The estimated formation pore pressure is then plotted on the same graph. Any pressure is then plotted on the same graph. Any differences between the pressure values expressed by these plots provide an indication of the degree of overbalance or underbalance existing between the annular fluid column and the formation. If the potential for gas migration through the lightened cement column is indicated by the graphical plot, several preventive techniques may be used for field application. Some of these techniques include: minimizing height of the cement column, imposing surface pressure on the annulus, increasing annular mud density, adjusting slurry thickening times, conventional multistage cementing, increasing cement mix water density, or modified cement slurries. Each of these procedures has been successfully utilized in the Gulf Coast area for both tubing less and conventional type completions. Introduction Annular and interzone gas flow shortly after the placement of cement continues to be a major problem placement of cement continues to be a major problem associated with cementing casing and liners. These problems are of particular significance in abnormal problems are of particular significance in abnormal pressure gas wells and remedial measures to control pressure gas wells and remedial measures to control such flow often require a major expenditure. Annular gas flow has occurred behind protective casing, production casing, and liners on both inshore and production casing, and liners on both inshore and offshore wells. It is significant to note that these occurrences are not unique to any single operator but have been experienced by the industry worldwide. While the exact failure mechanism may be very complex, there is substantial evidence indicating that gas flow occurs due to a reduction in the hydrostatic pressure exerted by a cement column during its initial pressure exerted by a cement column during its initial hydration period. LABORATORY INVESTIGATION Some previous studies of the annular gas flow problem have shown that the phenomena can be caused problem have shown that the phenomena can be caused by such factors as excessive cement dehydration, cement shrinkage, and nonuniform cement hydration. Recent laboratory studies indicate that as a cement slurry begins to thicken, the hydrostatic gradient provided by the cement decreases from an initial value provided by the cement decreases from an initial value equal to that of the slurry density. After the cement obtains its initial set, it provides little or no hydrostatic gradient since it is not a mobile fluid. The apparatus used in one of these recent laboratory investigations consisted of a 1" tube centered inside an instrumented 4-1/2" casing. The instrumentation, consisting of pressure transducers and thermocouples, was used to investigate pressure behavior while circulating and curing the cement. The entire apparatus was enclosed inside a temperature bath to ensure uniform temperature throughout the cement column (Figure 1). The total height of the casing model was 47 feet; a 12-foot cement column was used during each series of tests.