In-Depth Rheological Evaluation of Gas Tight Cement for Shallow Gas Application

Abstract

Short-term gas migration during the cementing process has been a challenge in the oil and gas industry. During the curing process, the cement slurry will slowly lose its hydrostatic pressure allowing gas from the formation to enter the annulus. The ideal cement should generate gel structure rapidly after the pumping is finished to hold gas from entering the slurry. Therefore, the rheological properties of cement play a key role in designing reliable gas tight cement. Sets of rheological evaluations were performed on four different zonal isolation materials (neat G cement, geopolymer, gas tight cement, and rapid hardening cement). The cement samples used in the evaluation are industrially used for cementing jobs in the North Sea. The flow rotation and oscillation test were executed at 25°C by using a rheometer. The flow rotation test was performed initially and followed by the oscillation test. In the flow rotation test, the slurry was pre-sheared at 100 1/s for 60 sec, continued ramp-up (0.01-511 1/s) and ramp-down (511-0.01 1/s). The hysteresis area between ramp-up and ramp-down shows the degree of thixotropy of each material. Afterward, an amplitude sweep test was performed at a constant frequency of 10 rad/sec under oscillation movement. The test showed the behavior of materials under a deformation phase. The set of tests measured the storage modulus (G′) and loss modulus (G″) at elevated strain. It also shows the elasticity of the material and its internal strength which could be important parameters for handling the gas invasion. Of four different types of cementitious material, the commonly used gas tight cement had the highest thixotropy and good elasticity compared to other cementitious materials which could explain how it has been widely used for shallow gas cementing. This paper provides an in-depth rheological evaluation of gas tight cementitious material to better understand the importance of rheological parameters for shallow gas and water bearing zones.