Risk Evaluation Methods of Gas Hydrate When Shallow Strata Drilling in Deepwater Area

摘要

Natural gas hydrate often occurred blocking BOP and resulting in other accidents when drilling in deepwater, particularly in shallow strata. So this paper's objective is to establish risk evaluation methods of gas hydrate when shallow strata drilling in deepwater area. With comprehensive consideration of gas hydrate equilibrium conditions and borehole temperature and pressure field, we established method for prediction of gas-hydrate formation region in the shallow strata of deepwater drilling and method for prediction of gas-hydrate formation region when shallow drilling in deepwater area. On this basis, with comprehensive consideration of hydrate formation processes and its factors, we established the qualitative risk evaluation method of gas hydrate based on expert scoring method, meanwhile, established the semi-quantitative risk evaluation method of gas hydrates which takes the influence weight of multiple factors into account and based on Analytic Hierarchy Process. Through the analysis, shallow layer drilling in deepwater area has greater risk in natural gas hydrate than deep drilling. The example shows that: The level of hydrate formation risk near the subsea wellhead and the lower part of conductor is highest; Identification of shallow gas, gas hydrate strata identification, risk level within the predicted gas hydrate formation region as well as the water depth and geothermal gradient, etc. have most influence weight of gas hydrate when shallow strata drilling in deepwater area, we can according to this and combine the actual situation to optimize the design and construction of drilling, thus reduce or avoid the risk.