A geological model of typical shallow water flow (SWF) sediment in deepwater basins is constructed based on its basic characteristics, and a reservoir simulation model is then established that incorporates two-phase flow as the basis for the simulation of water and sand blowout process during penetrating SWF formations. The model assumes that sands can start flowing along with water at a critical pressure difference when drilling through SWF formations. The cumulative water and sand blowout as SWF formations are penetrated are calculated and the influencing factors, such as SWF zone size, overpressure factor of SWF formations and penetration rate, are incorporated into the model to quantitatively analyze their impacts on the water and sand blowout from SWF zones. The data are useful for the assessment of the severity of SWF hazards. The results show that the cumulative water blowout and sand blowout will be as high as 100m3 and 1m3, respectively, when a typical SWF sediment is totally penetrated. SWF formation overpressure factor has a remarkable effect on SWF blowout, while SWF zone size has more moderate effects. However, the effects of SWF sediment size are expected to become increasingly significant as time proceeds. 10% increase of overpressure factor of SWF formation will result in around 90% increase of cumulative water blowout and 150% increase of cumulative sand blowout after the SWF zone is totally penetrated, while 10% increase of SWF zone size will lead to about 20% elevation of cumulative water blowout and 25% ascending of cumulative sand blowout. The water and sand blowout can be effectively reduced while increasing the penetration rate, which indicates that elevating the penetration rate can be a useful measure for the control of SWF hazards during deepwater drilling operations.
Comprehensively understanding the gas hydrate accumulation mechanism is significant for the investigation of subsea gas hydrate reservoir which can provide further guidance for the hydrae exploration and development as well as the safe deep-water drilling. In this paper, a preliminary conceptual model is established to study the characteristics of gas hydrate accumulation in the typical shallow formation under the sea using a reservoir simulation method. A partial equilibrium reaction model based on the phase equilibrium of gas hydrate is used to describe the trigger mechanism of hydrate formation when methane from deep formation migrates into the upper hydrate stability zone under the seabed. Two cases are simulated for comparison, one considering the barrier effect of cap rock at seabed while the other assuming a cold spring at seabed. The simulation results indicate that in the cap rock case, a thick hydrate layer tends to be formed in the upper subsea formation but with a relatively smaller hydrate saturation, while in the case of cold spring, nearly 90% of methane from deep reservoir would leak into the sea water, nevertheless the long-term slowly gas driving water process is favorable for generating high hydrate saturation. Generally, low flux of methane gas, cap rock barrier, deep water depth, and small geothermal gradient below mud line are beneficial to forming valuable hydrate reservoirs with larger thickness and high abundance. This study has proven that the reservoir simulation method can be an effective tool to simulate the process of gas hydrate formation and accumulation in the shallow formation under the sea, which deserves for further study.
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