Zonal Isolation Assurance: Relating Cement Mechanical Properties to Mechanical Durability

Abstract

Sustained casing pressure (SCP) is a common problem in US shale reservoirs. Wells drilled in US shale plays present many issues that can affect complete zonal isolation, such as long horizontal sections and complicated post cementing operations like continued drilling or hydraulic fracturing. Incomplete zonal isolation can create a path for gas migration. Short term gas migration occurs when the cement slurry is in transition from liquid to solid while long term gas migration, the focus of this paper, occurs after the cement has set. Cementing methods to alleviate gas flow via one flow path initiation mechanism will not solve gas flow resulting via the other mechanism. A previous investigation was performed in response to field data showing a significant difference in SCP incidence with one cement system compared to another, specifically on the intermediate by surface annulus. This first investigation identified the flow initiation mechanism as long-term gas migration and linked the improved seal performance to improved mechanical properties and overall durability. This paper reports continued investigation of mechanical properties of different cement compositions and their effects on long term seal durability. Testing methods developed for this project are designed to determine the cumulative energy applied by cyclic stresses that cement can withstand as well as to correlate this to the intrinsic properties of cement systems. This method determines the amount of cyclic stress cement can withstand in a field situation before compromising the annular seal. This paper describes development of the method to evaluate the ability of cement systems to withstand energy applied laboratory scale. Cement systems evaluated include differing API classes, gypsum concentrations, and fluid loss additives as well as two systems used in the Marcellus field. A preliminary quantitative relationship between laboratory-measured cement seal failure and a dimensionless ratio of cement mechanical properties and well geometry has been developed. This relationship applied to field data from the previous study corroborates the relationship’s potential as a cement system screening tool for prevention of gas leakage and SCP. Successful outcome of this ongoing investigation provides a laboratory design method to improve annular seal durability thereby alleviating SCP in the Marcellus shale. By reducing SCP, the costs and risks associated with gas migration will be reduced drastically. Other potential improvement areas include improvement in hydraulic fracturing efficiency and ultimately in production efficiency.