Assessment of the potential for gas production from marine methane hydrate reservoirs by numerical simulation

In this work the potential for gas production from two selected methane hydrate deposits which are situated offshore from Uruguay is assessed along with the validity of numerical simulations as a tool for analysis in this environment. Gas hydrates are crystalline solids formed by gas and water, in which gas molecules are accommodated within a solid water lattice in a cage-like structure. They form in thermobaric conditions of relatively high pressure and low temperature which in nature occur in permafrost and deep water sediment environments. Marine methane hydrates represent a huge potential as an unconventional gas resource and production tests have already been perfomed offshore Japan and China confirming the validity of depressuration as a method of production. Available 3D seismic data was utilized for the identification of interesting areas for gas hydrate studies focused on resource exploitation allowing the acquisition of the corresponding architectural parameters. Due to the lack of well data at the selected locations, geological models and reservoir properties were defined based on published data from studies on analogue situations including data from the first production test performed offshore of Japan. Reservoir simulations were carried out to assess the response of selected prospects to depressurization induced dissociation. Two prospects, interpreted as turbidite type deposits and located at 1850 m and 788 m of water depth, were selected for the modelling studies. The simulation of short term production tests of 60 days indicates average gas release rate values from 34 100 std m³/d to 6700 std m³/d for the deeper and shallower prospect respectively. The simulations were greatly affected by geometrical non-geological parameters like the proximity of model boundaries as well as type and level of discretization. We found that for finer discretization cases, the use of logarithmically distributed radial grid cells led to the existence of artifacts at early time on the gas release rate curves while the use of uniformly distributed radial cells results in more stable behaviour of the gas release rate. Several realizations of the geological models were used and sensitivity analysis was carried out regarding permeability and hydrate saturation. A longer term production regime (10 years) for a heterogeneous layered case was also simulated for the deeper prospect resulting in very useful average gas release rates of approximately 70 000 std m³/d, essential to satisfy gas requirements of Uruguay. We predict that only a few wells would be needed. For the first time, reservoir simulation was applied for prospects in Uruguay and the gas release potential for marine methane hydrate deposits in the Southern Atlantic margin was assessed. Simulation results are encouraging. Additionally the results of this work at the identified prospects may be useful for site selection for any future campaign for gas hydrate exploration offshore Uruguay.