A Novel Cement Additive to Prevent Gas Migration in Producing and Abandoned Oil and Gas Wells

Abstract

Abstract Gas leakages from producing and abandoned oil and gas wells are considered a threat to the environment and increase greenhouse gas emissions. They are also a cause of sustained casing pressure and other wellbore integrity problems, which are significant operational and safety issues. Cement integrity in oil and gas wells is crucial to ensure excellent zonal isolation and prevent gas migration to the surface over a long production time. Therefore, cement slurry should be carefully designed to yield better performance in sealing the annulus between the casing and drilled formations. This study introduces a novel additive to improve the cement slurry properties and mitigate the gas migration problem. The new cement formulation consists of water, dispersant, retarder, cement (class G), and a novel polymeric anti-gas migration additive. To evaluate the performance of the new additive, several formulations were prepared by varying the additive concentrations from 0 to 6.0% by weight of cement (BWOC). The mixing process was first optimized to fulfill better slurry performance. The effect of the new additive on static gel strength (SGS), gas migration, slurry rheology, fluid loss, consistency, thickening time, and unconfined compressive strength (UCS) was studied at 70°C and 90°C. Static gel strength was the primary indicator of cement slurry's ability to resist gas migration; therefore, it was initially used to optimize the additive concentration. Eventually, the optimized formulation was tested on a new experimental pipe setup to simulate the actual well conditions. The SGS results showed that the new additive could effectively reduce the gas transition time to 15-25 minutes for the optimum concentration, compared to around 50 min for the base cement. The optimum concentration was found in the range of 1.5-3% BWOC. Adding more than this concentration would increase the cementing operation cost without further improving the performance. Consistency and rheological measurements confirmed the excellent stability and pumpability of the new cement slurry with this concentration range. The new additive performed better with increasing temperature up to the maximum testing temperature, 90°C. A high reduction in the fluid loss was also observed with the introduced additive compared to base cement slurry and other commercial additives. Pipe test results showed that the new additive completely stopped gas migration, and no gas leakage was observed for more than 24 hrs. In contrast, a high leakage rate was observed with the base cement after only 4-6 hrs. The findings of this study are promising. Adding 1.5-3.0% BWOC of the new additive was adequate to maintain cement slurry expansion and develop enough static gel strength in a short time. Using the introduced cement formulation with more optimization to the cementing operation would significantly improve wellbore integrity for short-term and long-term operations.