Numerical investigation of the factors affecting the cement sheath integrity in hydraulically fractured wells

Abstract

The current industrial practice calls for wells to be hydraulically fractured in as many as 50 stages, depending on the formation. The integrity of the cemented wellbore sections around the perforations has been a major concern in most hydraulically fractured wells. In this paper, we have conducted a comprehensive numerical modelling study to understand the possible impact of hydraulic fracturing operation on the integrity of the cemented wellbore sections, especially around the perforations where the fracture fluid is injected. A numerical model allowing full coupling of solid/liquid flow has been adopted to simulate the fracturing fluid injection and the resultant deformation of cemented casing sections around the perforations. Field data from a hydraulically fractured deep shale gas well in China were used to demonstrate the impact of the operational variables on the integrity of the cemented wellbore section. We also studied the influence of fracture fluid injection pressure, mechanical properties of the cement, casing and the reservoir, perforation azimuth, and fracture fluid viscosity on the cement sheath failure. Simulation results have shown that the cement sheath failure at the cement-casing interface may occur in the form of debonding in the radial direction. The cement debonding occurring at the casing-cement interface due to high frac fluid injection pressure, would be one of the main reasons for the poor hydraulic fracturing job performance commonly observed in shale gas wells. Following conclusions can also be drawn from simulation results: 1) Injection pressure has an important impact on the cement debonding. For the specific reservoir conditions investigated in this paper, to avoid cement debonding, the injection pressure should be no more than 75 MPa; 2) Use of a casing with a larger elasticity modulus and cement with lower elasticity modulus and lower permeability are conducive for improved cement bonding quality; 3) For a shale gas reservoir with lower permeability, these requirements for the casing and cement should be even stricter. Resultsof this study can be useful for optimum selection of the limiting pump pressure, cement and casing properties, and the perforation azimuth to be used for the design of a successful hydraulic fracturing job.