Effect of expandable cement on increasing sealing ability of cement sheath in shale gas wells

Abstract

The sustained casing pressure (SCP) problems in shale gas wells have not been effectively controlled by now. In field, the expandable cement sheath used in wells has decreased the SCP problems and in this paper, an analytical model is established to study how the expandable cement increases the sealing ability of cement sheath and give suggestions on optimized choosing of expandable cement. Meanwhile, the stress equilibrium state in well before cementing work which has not been considered in previous and it is considered in the analytical model here. During the curing process, the radial stress on the outer interface of cement sheath changes with time and increases much faster than it on the inner interface. In wells with single layer of casing, the radial stress on interfaces does not increase linearly with the cement elastic modulus (Et) and it will be affected by the elastic modulus of formation rock. In wells with double layers of casing, the expandable cement can significantly increase radial stress on interfaces during the production process and alters circumferential tensile stress into compressive stress which is useful for increasing the sealing ability of cement sheath. How the expandable cement increase the sealing ability of cement sheath in field has been analyzed and an optimized expand ratio of cement is proposed to control the SCP problems in shale gas wells.