Natural gas hydrates - issues for gas production and geomechanical stability

Natural gas hydrates are solid crystalline substances found in the subsurface. Since gas hydrates are stable at low temperatures and moderate pressures, gas hydrates are found either near the surface in arctic regions or in deep water marine environments where the ambient seafloor temperature is less than 10?C. This work addresses the important issue of geomechanical stability in hydrate bearing sediments during different perturbations. I analyzed extensive data collected from the literature on the types of sediments where hydrates have been found during various offshore expeditions. To better understand the hydrate bearing sediments in offshore environments, I divided these data into different sections. The data included water depths, pore water salinity, gas compositions, geothermal gradients, and sedimentary properties such as sediment type, sediment mineralogy, and sediment physical properties. I used the database to determine the types of sediments that should be evaluated in laboratory tests at the Lawrence Berkeley National Laboratory. The TOUGH+Hydrate reservoir simulator was used to simulate the gas production behavior from hydrate bearing sediments. To address some important gas production issues from gas hydrates, I first simulated the production performance from the Messsoyakha Gas Field in Siberia. The field has been described as a free gas reservoir overlain by a gas hydrate layer and underlain by an aquifer of unknown strength. From a parametric study conducted to delineate important parameters that affect gas production at the Messoyakha, I found effective gas permeability in the hydrate layer, the location of perforations and the gas hydrate saturation to be important parameters for gas production at the Messoyakha. Second, I simulated the gas production using a hydraulic fracture in hydrate bearing sediments. The simulation results showed that the hydraulic fracture gets plugged by the formation of secondary hydrates during gas production. I used the coupled fluid flow and geomechanical model "TOUGH+Hydrate- FLAC3D" to model geomechanical performance during gas production from hydrates in an offshore hydrate deposit. I modeled geomechanical failures associated with gas production using a horizontal well and a vertical well for two different types of sediments, sand and clay. The simulation results showed that the sediment and failures can be a serious issue during the gas production from weaker sediments such as clays.