Prospects of gas production from the sedimentary complex hydrate reservoir using conventional technology

Methane hydrate is expected to be an alternative energy source with extensive distribution in nature in permafrost and in marine sediments. As shown in Fig. 1, a few short-term production tests focusing on reservoir depressurization have been conducted in recent years. As more field data from logging-while-drilling, seismic interpretation, and core sample analysis became available to characterize the actual hydrate deposits, more realistic reservoir models can be developed. In this work, we extensively used the well-logging data and core samples analyses results at the Eastern Nankai Trough of Japan. A more realistic reservoir model was constructed, which considers the vertically varying lithofacies, porosity, permeability, and hydrate saturation. The reservoir model was validated by comparing the fluid flow rates at production well and temperature changes at a monitoring well. Based on the validated reservoir model, we investigate (1) the multiphase flow behavior, and (2) the long-term gas production potential from the sedimentary complex hydrate reservoir. Additionally, the systematic comparison of gas production between heterogeneous reservoir and homogeneous one was performed to clarify the essential mechanism that results in the difference caused by the vertical heterogeneity. This is very important to guarantee the effective productivity forecast in the future. The modeling results indicate that the hydrate dissociation zone is strongly affected by the reservoir heterogeneity and shows a unique dissociation front. Gas production rate is expected to increase with time and reaches the considerable value of 3.6 × 10⁴ ST m³/d, owing to the significant expansion of the dissociation zone. The numerical model using a simplified description of porosity, permeability, and hydrate saturation leads to significant underestimation of gas productivity from the sedimentary complex hydrate reservoir. The most advanced dissociation front exceeds 500 m from the production well after one year production. This emphasizes the field faults system should be carefully considered when conducting the long-term field production test in the future. The results also suggest that the interbedded hydrate occurrence systems might be the better candidate for methane gas extraction than the massive hydrate reservoirs.