Prediction and risk assessment of natural gas hydrate formation in deepwater wellbores

Abstract

Hydrate formed in the wellbore during deepwater shallow drilling will affect the operation safety, so it is necessary to predict and analyze the hydrate formation range and degree in the wellbore annulus. One development well in Lingshui Gas Field in South China Sea was taken as the research object, and the numerical simulation method for hydrate formation in deepwater drilling was built based on CSMHyK hydrate dynamics model and OLGA multiphase flow equation. The temperature-pressure field of the wellbore under various working conditions was calculated, the subcooling density was introduced to present the possibility of hydrate formation, the range and degree of hydrate formation in the wellbore annulus were predicted, and the parameter sensitivity of drilling fluid displacement, inlet temperature, density and shallow gas storage pressure was analyzed. The results show that under the assumed working conditions, the riser annulus near the mudline has the risk of hydrate formation, with the largest amount of hydrate formation in the annulus with a water depth of 800-1 000 m. The smaller the displacement of drilling fluid is, the lower the inlet temperature is, the greater the risk of hydrate formation will be, and the amount of hydrate formed in hazardous pipe section may exceed 40 kg/m3. The subcooling density, length of interval where hydrate is formed, total amount of hydrate formed and peak rate were selected as the characteristic values for assessing hydrate formation, and the quantitative analysis method for wellbore hydrate risk level was built. The research results are expected to provide theoretical guidance for hydrate control and optimization design of drilling schemes during deep water shallow drilling.

Keywords

• deepwater drilling
• wellbore annulus
• olga
• hydrate prediction
• subcooling density