Evaluation of Gas Productivity from Layered Heterogeneity Methane Hydrate Reservoirs by Depressurisation

Abstract

Marine gas hydrate drilling explorations at the Eastern Nankai Trough of Japan revealed the variable distribution of hydrate reservoirs, which are composed of alternating beds of sand, silt, and clay in sediments, with vertically varying porosity, permeability, and hydrate saturation. The main purpose of this work is to evaluate gas productivity from the layered methane hydrate reservoir by depressurization through a conventional vertical well, and to investigate the effects of reservoir heterogeneity on production performance. We first established a history matching model by incorporating the available geological data at the offshore production test site in the Eastern Nankai Trough. The reservoir model was validated by matching the fluid flow rates at production well and temperature changes at monitoring well during field test. 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 cumulative volume of gas produced at well is 1.18 × 107 ST m3 over one-year period. The homogeneous reservoir with uniform value of permeability and hydrate saturation leads to significant underestimation of gas productivity when compared with the layered hydrate reservoir.