Numerical simulation of the depressurization production of natural gas hydrate reservoirs by vertical well patterns in the northern South China Sea

In order to improve gas production rate and recovery factor of low-permeability muddy silt natural gas hydrate (NGH) reservoir by depressurization production in the northern South China Sea, the TOUGH+HYDRATE software was adopted to numerically simulate the depressurization production of NGH reservoir in two well deployment modes (i.e., single vertical well and vertical well pattern) according to the actual production test data, based on the condition that the NGH reservoir at W17 NGH production test site contained a little free gas and was underlain by mud layer in 2017. The gas/water production behaviors of production wells and the change characteristics of temperature field, pressure field and NGH saturation field in the production area were studied. The influence mechanisms of permeability and inter-well interference on the change of pressure field, temperature field and flow field were investigated. During the depressurization production of low-permeability muddy silt NGH reservoir, the permeability of NGH sediment layer increased due to NGH dissociation, which led to the increase of gas/ water production rate. In the initial stage of depressurization production, gas/water production rate of production well decreased sharply after reaching a peak value within a short time, and the temperature around the borehole dropped due to the rapid dissociation and heat absorption of NGH and the influx of free gas. Then, as the production continues, gas/water flow resistance increases and the propagation rate of pressure drop decreased. As a result, the production rates of NGH dissociated gas and wellhead gas decreased continuously, while the water production rate increased slowly. NGH dissociation is under the joint control of pressure drop and peripheral fluid seepage and heat transfer. NGH around the wellbore and near the upper and lower interface of NGH layer dissociated preferentially. Most wellhead gas came from the free gas in the peripheral NGH layers and the dissolved gas in the pore water. When the vertical well pattern is used for NGH production, the control area of each well was reduced and its single-well gas/water production rate and cumulative water/gas production were much lower than those of single vertical well, but its total gas production and NGH recovery factor are higher. Well spacing dominated the control area and the ultimate cumulative gas production of each well.