A New Method Based on Simulation Experiment to Evaluate Subsea Wellhead Stability of Drilling from Gas Hydrate Sediments in Deep Water

摘要

The gas hydrate dissociation may cause subsidence of the subsea wellhead and conductor during the drilling behavior in gas hydrate sediments. This paper proposed a new method based on simulation experiment to evaluate subsea wellhead stability of drilling in gas hydrate sediments in deep-water. The experiment was investigated in a high-pressure gas hydrate autoclave at 4°C and 15Mpa to simulate submarine environment. The process involved generating gas hydrate in the autoclave, inserting a double-layer assembled pipe and circulating fluid in it to simulate the drilling process. The influence of temperature variation to the bearing capacity and subsea wellhead subsidence were researched. There are 81 groups of experiments were conducted in both strongly and weakly consolidated gas hydrate sediments with the saturation of 18.2%, 26.5%, 38%. The results show that wellhead subsidence in the simulation experiment consists of 3 stages. Starting stage: the radius of the hydrate dissociation increases with the rise of the formation temperature; the wellhead starts to subside. Accelerating stage: the wellhead subsidence accelerates when the formation temperature rises to an inflexion value. The higher saturation and weaker consolidation of the hydrate lead to the more subsidence of the subsea wellhead. Stabilization stage: the wellhead subsidence tends to be stable after the temperature exceeds the acceleration range. This suggests that there is a drilling fluid temperature window to keep the wellhead stable that can be determined by the wellhead load physical and mechanics parameters of the hydrate. For instance: the safe drilling fluid temperature window is from 4°C to 8.15°C during drilling in the weakly consolidated hydrate sediments with the saturation of 38%. The experiment was carried out to verify that comparing with the normal temperature drilling fluid, the wellhead subsidence decreases 92% if using the drilling fluid temperature in the safety drilling fluid temperature window. The novelty of this paper is to propose the safe drilling fluid temperature window based on a series of drilling simulation experiments to keep the subsea wellhead stable during drilling in the gas hydrate sediments in deep water. The drilling fluid temperature can be controlled in this window, which can reduce the risks of the wellhead subsidence, improve the wellhead stability and ensure the safety of the drilling operation.