Gas recovery enhancement from fine-grained hydrate reservoirs through positive inter-branch interference and optimized spiral multilateral well network

A spiral multilateral well network is a promising production method to enhance long-term gas recovery from prevalent fine-grained hydrate reservoirs. However, practical application is greatly restricted before the optimal well network parameters are determined and the mechanism behind a unique phenomenon in multilateral wells, namely inter-branch interference, is clear. In this study, we numerically optimized the well configuration and spacing when spiral multilateral wells were deployed in two typical fine-grained hydrate reservoirs, i.e., ultra-low permeability hydrate reservoirs (ULPHR, <1 mD) and low-permeability hydrate reservoirs (LPHR, >1 mD). The mechanism behind inter-branch interference was innovatively revealed. The results indicated that the number of spiral branches should be increased, and equidistant branches should be deployed uniformly in the lower ULPHR or throughout LPHR to enhance production efficiency. A wide spacing of spiral multilateral wells with long branches contributed to long-term productivity in fine-grained hydrate reservoirs with any permeability; however, narrow spacing was more favorable for short branches or short-term production. Our study found three inter-branch interference stages during gas production, namely, “no effect” stage, “positive” stage, and “negative” stage; all the three stages are controlled by reservoir permeability, production distance, and production time. Owing to the “positive” interference effect, longer equal-length branches resulted in superior long-term production enhancement in ULPHR, particularly for lengths greater than 30 m. Gas production from LPHR using only two optimal spiral multilateral wells exhibited high production performance similar to that of the sandy hydrate deposits in Japan, suggesting that the optimal spiral multilateral well network is promisingly suitable for commercial production in the future.