Optimization of Well Layout for Gas Production from Hydrate Dissociation by Using Dual Horizontal Wells

Natural gas hydrate (NGH) has received increasing attention for being an abundant source of clean methane production and enhancing its productivity is one of the most important issues to be solved for commercialization exploitation. In this work, the commercial package HYDRATE V1.5 is used to simulate hydrate dissociation by depressurization method from an NGH reservoir by using dual horizontally parallel wells (DHPWs). The effect of a few factors, such as well layout and different well separations (WSs) on the production performance (i.e., rate and mass of the total gas released in the reservoir, gas and water production from the well, gas and hydrate saturations, and pressure distribution), is investigated. It is found that, compared to single-well case, the dual horizontal wells with vertical layout do not increase the hydrate dissociation rate and thus are not recommended for well layout. The arrival time for reaching the first and second peaks of the gas release rate in the reservoir is mainly related to the distance between the well and NGH layer boundary, d_wb (i.e., the larger d_wb, the larger arrival time). Compared to the single well, the dual wells with horizontal layout could increases the gas production by one time when the well works individually. In addition, the optimal WS exists for the dual wells with horizontal layout. If the WS is too small, the two wells work like a single well after some time in terms of working efficiency. Overall, the larger the WS, the larger the rate and mass of gas released in the reservoir. Among the cases studied in which the thickness of the NGH layer L_H ranges from 22 to 120 m, horizontal layout with WS between 60 and 120 m is a potentially good option for dual-well production. This is due to the following two reasons. First, given the number of wells is fixed, when the WS is larger (say 120 m or above), the mass of gas released is larger than that in the case with smaller WS (say 60, 80, or 100 m), but the difference is very small. Second, for a given NGH reservoir with finite width, smaller WS means that more wells are allowed to be drilled, thus potentially enhancing the gas productivity.