南海北部天然气水合物藏垂直井网降压开采数值模拟

为了提高南海北部低渗透率、泥质粉砂型天然气水合物（以下简称水合物）储层降压开采的气产量和采收率，基于我国2017年水合物试采W17站位水合物层含有少量游离气且下伏泥层的条件，根据实际试采数据，针对单垂直井和垂直井网两种布井方式，利用TOUGH+HYDRATE软件进行了水合物层降压开采数值模拟，研究了开采井产气/产水特征及开采区温度场、压力场、水合物饱和度场的变化特征，进而分析了渗透率、井间干扰对压力场、温度场及流场变化的影响机制。研究结果表明：①低渗透率泥质粉砂型水合物层在降压开采过程中，水合物的分解使水合物沉积层渗透率增大，从而使气、水产量增加；②在降压开采初始阶段，开采井的气、水产量短时达到峰值后急剧减小，水合物迅速分解、吸热及游离气的涌入使得井筒附近温度降低，而后随着开采时间的延续，气、水渗流阻力增加，压降传播速率降低，水合物分解气产量和井口气产量不断降低，水产量则缓慢上升；③水合物的分解由压降和周边流体渗流、传热联合控制，井筒附近及水合物层上下界面处的水合物优先分解，井口产出的天然气有较大部分来自于周边水合物层中的游离气和孔隙水溶解气；④采用垂直井网进行水合物开采，每口井的控制面积减少，单井的产气/产水速率及累计产气/产水量均明显低于单垂直井，但垂直井网开采总的气产量更大、水合物采收率更高；⑤井距决定了每口井的控制面积和最终累计产气量，井间压降叠加效应加速了水合物的分解，井间区域的压力及温度显著低于单井，但井间对称流场的干扰会阻碍气液流动，在井间中心区域将形成“静止区”。结论认为，多井联合开采可以提高井场总的气产量，但需要根据钻井成本、水合物层渗透率、预计生产周期、井场总气产量和水合物采收率等指标来综合确定合理井距。

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, this paper adopted the TOUGH+HYDRATE software 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. Then, 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 investigated. Finally, the influence mechanisms of permeability and inter-well interference on the change of pressure field, temperature field and flow field were analyzed. And the following research results were obtained. First, during the depressurization production of low-permeability muddy silt NGH reservoir, the permeability of NGH sediment layer increases due to NGH dissociation, which leads to the increase of gas/water production rate. Second, in the initial stage of depressurization production, gas/water production rate of production well decreases sharply after reaching a peak value within a short time, and the temperature around the borehole drops 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 decreases. As a result, the production rates of NGH dissociated gas and wellhead gas decrease continuously, while the water production rate increases slowly. Third, 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 dissociates preferentially. Most wellhead gas comes from the free gas in the peripheral NGH layers and the dissolved gas in the pore water. Fourth, when the vertical well pattern is used for NGH production, the control area of each well is reduced and its single-well gas/water production rate and cumulative water/gas production are much lower than those of single vertical well, but its total gas production and NGH recovery factor are higher. Fifth, well spacing dominates the control area and the ultimate cumulative gas production of each well. The superposition effect of inter-well pressure drop speeds up NGH dissociation, so temperature and pressure between wells are much lower than those in a single well, but the interference of inter-well symmetric flow field hinders the gas/water flow, which results in the formation of “dead liquid area” in the central area between wells. In conclusion, multi-well joint production can be used to increase the total gas production of the well site, but a reasonable well spacing shall be determined comprehensively according to drilling cost, NGH reservoir permeability, expected production cycle, total gas production rate of the well site and NGH recovery factor.