Evaluation and analysis of nitrogen gas injected into deepwater wells to mitigate annular pressure caused by thermal expansion

Abstract

Deepwater wells have the basic conditions for the generation of trapped annular pressure caused by thermal expansion. Nitrogen is eligible medium to mitigate annular pressure for its high compressibility. Therefore, this paper studies the action mechanism, effect evaluation and applicability analysis of the mitigation by injecting nitrogen. To this end, an equation is established to calculate the annular pressure when nitrogen exists in annulus according to PVT property and volume relationship. And then a method is proposed to solve the equation based on volume segmentation and wellbore radial thermal resistance distribution. The casing strength is checked by safety efficient to verify the mitigation effect. The results indicate that nitrogen mitigates trapped annular pressure mainly by volume compensation. The mitigation is strong and effective. Considering mitigation effect and cost-effectiveness, the optimal injection volume should be between 5% and 15%. The nitrogen is suitable for mitigation of in deep/ultra-deep water wells although the increase of water depth undermines the mitigation effect. The mitigation is still effective even the annular liquid is of high expansibility and low compressibility when adequate nitrogen is injected. Strong mitigation effect can be obtained in high temperature reservoir and wells of high production rate. In conclusion, nitrogen has good applicability and strong effect in the mitigation of trapped annular pressure.