Method for Selection of Cement Recipes To Control Fluid Invasion After Cementing

Abstract

Summary A method to scale down well parameters to laboratory conditions for more realistic testing of cement recipes to be used to control wellbore invasion/migration of formation fluids after cementing is introduced. The scale procedure proposed here addresses, a worst-case scenario. It assumes that the offending gas zone (source of the invading gas) has enough permeability, thickness, and gas volume to invade fully and to pressure- permeability, thickness, and gas volume to invade fully and to pressure- charge the cemented annulus (cement column), if conditions allow. The method is demonstrated in conjunction with the use of a certain laboratory gas-flow apparatus. This general scale-down procedure can be used with other test equipment. An imaginary set of well conditions is used to illustrate the mechanics of the use of the technique. Tests run with the procedure and the computer-monitored gas-migration test cell are described. A preliminary so of criteria for selecting a cement recipe for a given well is presented. A field case frustrates the use of the procedure. Introduction The oil industry wrestled with the problem of gas migration after cementing since the early 1960's. An extensive review of papers on the problem of gas invasion/migration of wellbores after cementing was given earlier, and Ref. 2 lists additional related papers. It is generally recognized that gas invasion/migration after cementing can occur through three different mechanisms : contaminated cement and mud channels, microannuli along the cement/pipe interface and along channels at the cement/formation interface, and the cement itself There are no short cuts in minimizing gas invasion/migration. Without good mud-displacement efficiency, gas movement in the wellbore cannot be prevented. Invasion/migration through the cement itself can be with proper slurry design. Some operators have had success with the use of cement slurries that prevent gas migration by forming an "impermeable" matrix to gas and with expanding cements. Although some gas-migration-control cement recipes (e.g., the impermeable type) are generally quite effective, they are expensive. For certain well conditions, using high-cost formulations can be an overkill when using less expensive recipes could be just as successful. The question then is, how does one select the recipe to be used in a given well? We believe that the best way is to test the proposed cement recipes in the laboratory with a realistic test proposed cement recipes in the laboratory with a realistic test procedure. When gas invasion/migration is a concern, laboratory procedure. When gas invasion/migration is a concern, laboratory testing of a cement formulation for gas-migration control should be considered as important as thickening-time and compressive-strength tests. Several laboratory devices and test procedures have been developed to test the gas-flow-control properties of a cement recipe. One common problem exists with all the test devices and techniques: a standard, satisfactory procedure has not been developed to test a cement recipe under conditions that closely simulate downhole conditions. Therefore, a method to scale down the well environment to Laboratory conditions was needed because the available laboratory equipment and procedures cannot realistically duplicate certain parameters (hydrostatic head, gas formation pressures, and pressure parameters (hydrostatic head, gas formation pressures, and pressure gradients potentially driving the gas through the cement column) that are critical in tests to design cement slurries for this application.