Investigation of the hydraulic integrity of cement plug: Oilwell cementitious materials

Abstract

The loss of hydraulic integrity in oil and gas wells due to the loss of cement sheath sealability can lead to environmental contamination, annular pressure build-up, and safety threats. In this study, we examine the hydraulic integrity of geopolymers an alternative to cement to be used in well cementing. The hydraulic integrity of geopolymer was compared to conventional API class G and Industrial expansive cement. Down-scaled test specimens representing cement-plug in casing were prepared and tested using an in-house experimental set-up that allows continuous curing and testing of the cementitious materials under undisturbed pressure and temperature conditions. The samples were cured at 90 °C and 172 bar for 7 days after which the hydraulic sealability of the specimens was examined by applying a pressure differential to one end of the specimen and observing the resulting fluid leakage rates on the other end. The leakage rates were then expressed in terms of permeability and microannuli aperture. By injecting nitrogen and water, it was possible to compare the effects of fluid type on the hydraulic sealability of cementitious materials. Lastly, we examined the hypothesis of a linear relationship between plug length and its hydraulic sealability. The results indicate that geopolymer and Industrial expansive cement have higher hydraulic sealability compared to API class G. Geopolymers also have sufficient hydraulic bond strength to perform as much as Industrial expansive cement. The fluid type used in testing does not play a critical role in the loss of hydraulic sealability of cementitious materials. The influence of cement plug length showed varying trends on the hydraulic sealability of the cementitious materials. The results presented in this work help us understand the sealing potential of cementitious materials and the need for standards for performing laboratory-scale hydraulic sealability tests. This can benefit the improvement of cement integrity tests and well abandonment operations.