Coupled thermal–hydrodynamic–mechanical numerical simulation of natural gas hydrate horizontal well depressurization production: Method and application in the South China Sea

In order to produce natural gas hydrate safely and efficiently, it is quite crucial to research the stability of hydrate reservoir in the process of its exploitation. The coupled thermal-hydrodynamic-mechanical (THM) numerical simulation can clearly present the mechanical response laws in the process of hydrate exploitation by horizontal well, but the model is often simplified as a profile vertical to the production well, so the evolution characteristics of real formation deformation space cannot be described sufficiently. To this end, this paper develops a coupled THM simulator applicable to the large-scale hydrate exploitation by horizontal well by introducing interactive interface to the FLAC mechanics program based on the hydrate exploitation program TOUGH + HYDRATE. Then, based on the field data of second hydrate production test in South China Sea, the gas and water production and formation deformation laws under the middle- and long-term development conditions of horizontal wells are studied by taking the actual gas production of horizontal well during the production test as the constraint of coupled THM model. And the following research results are obtained. First, when the production pressure is 7 MPa and the horizontal well length is 300 m, the gas production of horizontal well can reach 1190 × 104 m3, the pressure drop has a larger influence range in the free gas layer, the produced gas mainly comes from the free gas layer and the hydrate layer and the hydrate dissociation range is not large. Second, it is predicted that the seafloor settlement is about 0.16 m after 60 days' production test and 0.52 m after osne year's production. In conclusion, hydrate exploitation by horizontal well can cause a large range of seafloor settlement. The seafloor surface settles linearly and continuously and the spatial settlement distribute is in the shape of ellipse. What's more, the greatest deformation (vertical displacement) occurs at the horizontal well and above it, where formation damage happens the easiest. It is recommended to study seafloor deformation and landslip risk under the conditions of middle and long-term production based on seafloor topography, valley distribution and nonuniform spatial thickness of mineral ores.