Testing and Evaluation Method Investigates Cement Expansion in Cased-Hole Wells

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208679, “Cement Expansion in Cased-Hole Environments: A Novel Laboratory Testing and Evaluation Method With Successful Field Implementation,” by Vasilii Sukhachev, Amir Salehpour, SPE, and Ilshat Akhmetzianov, SPE, Schlumberger, et al. The paper has not been peer reviewed. Preventing bulk shrinkage of cement in cased-hole wells is important when dealing with the risk of sustained casing pressure (SCP). Current studies based on membrane tests suggest that, even with a high-percentage content of expanding agents, cement systems show little or no expansion in dry media. An alternative laboratory method for measuring cement expansion, followed by a successful field application, was used to evaluate the total bulk volume change of the cement based on curing in a dry environment and in water. Optimal cement-slurry properties were obtained. The developed cement system, which was successfully implemented in the field, provided long-term zonal isolation with no indication of SCP to date. Introduction For a major operator in the Caspian region, drilling development wells required passing through shallow overpressured permeable zones containing water and biogenic gas. The optimal casing design used to minimize the likelihood and severity of SCP consisted of isolating the shallow-soil zone by use of a cemented 24-in. liner followed by a 20-in. surface casing cemented using a two-stage method (Fig. 1). During the first stage, the cement was pumped inside of the 24-in. liner. Then, an external casing packer placed in the 20-in. casing was inflated against the 24-in. liner before performing the second-stage cement job. The second-stage cementing operation brought the cement above the top of 24-in. liner and provided sufficient length to isolate against the shallowest distinct permeable zone. Cement is considered a primary wellbore barrier element. However, the cement placed between the 24-in. liner and the 20-in. casing and in the annulus of the 30-in. conductor and 20-in. casing is subject to shrinkage. This shrinkage leaves a potential leak path all the way to the wellhead on an offshore platform. The cement systems chosen consisted of the 1.92-SG (16-lbm/gal) slurry for the second stage cementing of the 20-in. casing because this stage was the system most susceptible to shrinkage. This slurry was placed in the annular space of the 20-in. casing and both the 24-in. liner and the 30-in. conductor. The curing temperatures and pressures in the field are 32°C and 6.74 MPa at the bottom of the well and 28°C and 2.48 MPa at the top of the cement. The top of the cement will be in direct contact with the spacer, a fluid with 89% water content; however, the remainder of the cement column will not have access to excess water during curing. If acceptable results could be achieved with the 1.92-SG cement system, then the same blend could be used for the 24-in. liner tail and the first-stage tail slurry of the 20-in. casing. This method is a possible solution to obtain a more-robust system that prevents the risk of SCP.