Numerical Simulation And Experimental Study On Hydraulic Fracturing Of Clayey Silt Hydrate Reservoir

The economic exploitation of marine natural gas hydrates(NGHs)resources will help to improve the present energy structure of China,which is dominated by oil,coal and other non clean energy.At present,the daily gas production of marine NGHs production test is low,which is still far away from commercial production.Improving gas production rate of NGHs reservoir through reservoir simulation is regarded as an important technical measure to realize commercial production.However,compared with shale gas reservoirs and coalbed methane reservoirs,clayey silt NGHs reservoirs have special mechanical properties,which leads to its unique fracturing process.To achieve high-efficiency production,it is necessary to study the fracture initiation and propagation law and stimulation characteristics of clayey silt hydrate reservoir.First,a small-scale three-dimensional hydraulic fracturing numerical model based on cohesive element was established,and the fracture initiation pressure of laboratory experiments in the previous work was successfully fitted,which verified the reliability of this method applied to the hydraulic fracturing simulation of NGHs reservoir.Then,a large-scale two-dimensional hydraulic fracturing numerical model was established to systematically study the effects of hydrate saturation,reservoir intrinstic permeability,horizontal in-situ stress,horizontal in-situ stress difference,fracturing fluid injection rate and fracturing fluid viscosity on injection pressure and fracturing initiation pressure,thus revealing the influence of various factors on fracture morphology.It was shown that high hydrate saturation will increase the strength and elastic modulus of hydratebearing sediments,thereby increasing the injection pressure during the fracturing process.Meanwhile,it was easy to form narrow and long fractures in the NGH reservoir with high hydrate saturation.The higher intrinsic permeability of NGH reservoir would increase the fluid loss of the fracturing fluid,resulting in a significant drop in the injection pressure.Low horizontal stress would reduce the strength of hydrate-bearing sediments and the injection pressure of fracturing fluid required to offset horizontal stress,thus reducing the difficulty of hydraulic fracturing,which would easily form long fractures in NGH reservoir.Increasing the injection rate of fracturing fluid was an important measure to realize the formation of wide and long fractures in the reservoir.Meanwhile,an increase in the injection rate would also lead to a significant increase in the injection pressure.In the case of low injection rate,the increase in fracturing fluid viscosity would not have a significant impact on the injection pressure and fracture morphology.When the injection rate increased to a certain extent,the impact of fracturing fluid viscosity on the fracturing process increased.Secondly,according to the geological conditions of NGH reservoir at SH2 site in the Shenhu area of South China Sea,a large-scale three-dimensional hydraulic fracturing numerical model based on the cohesive element was established,and fracture initiation,propagation and fracture morphology of single-cluster and multi-clusters hydraulic fracturing were deeply studied.The simulation results showed that,due to the small effective stress and pore pressure in the upper part of the reservoir,the resistance of upward propagation was less than that of downward propagation,so the fractures were mainly distributed in the middle and upper part of the reservoir.In the simultaneous multi-clusters hydraulic fracturing with horizontal well,a high stress concentration area would be formed between the fractures,which obviously squeezed the middle fractures,resulting in the width of the middle fractures becoming smaller or even closed.In the sequential multi-clusters hydraulic fracturing with horizontal well,the right fracture could form a high stress concentration area in the middle and upper part of the reservoir,thus increasing the resistance of the middle fracture to propagate upward and promoting the middle fracture to propagate downward,while the middle fracture could form a high stress concentration area in the middle and lower part of the reservoir and promoting the left fracture to propagate upward;when the cluster spacing increased to 15 m,the interference between fractures became smaller.Then,a self-developed multifunctional experimental simulation system was used to conduct hydraulic fracturing experiments on clayey silt frozen sand and clayey silt hydrate.The effects of ice saturation,fracturing fluid injection rate,fracturing fluid viscosity and axial pressure on the hydraulic fracturing of clayey silt frozen sand were studied.Meanwhile,the effects of fracturing fluid viscosity and hydrate saturation on hydraulic fracturing of clayey silt hydrate were studied.The experimental results showed that the increase of ice saturation,fracturing fluid injection rate,and fracturing fluid viscosity would significantly increase the fracture initiation pressure of clayey silt frozen sand.The difference between the axial pressure and the confining pressure was an important influencing factor to control the fracture morphology in clayey silt frozen sand.When axial pressure was higher than confining pressure by more than 2 MPa,vertical fracture was likely to occur in the sediment samples.Increasing the viscosity of fracturing fluid would significantly increase the initiation pressure of clayey silt hydrate.The gap in initiation pressure between 20.50% and 32.14% hydrate saturation conditions was very small,however,during the fracture propagation process,the injection pressure under the 32.14% hydrate saturation condition was significantly greater than that under the 20.50% hydrate saturation condition,which meant that the increase in hydrate saturation increased the resistance to fracture propagation.Finally,the Tough+Hydrate software was used to simulate gas production from the NGH reservoir at the SH2 site in the Shenhu area of the South China Sea,and the influence of fracture number,fracture permeability and fracture morphology on the production was studied.The simulation results showed that the existence of fractures could provide high-conductivity channels for depressurization production of NGH reservoir,thereby promoting fluid outflow in the reservoir,and increasing the amount of hydrate decomposition and gas production rate.Increasing fracture number made the decomposition gas of hydrates in a larger range flow into the fractures rapidly,which further increased the gas production rate.With the increase of fracture permeability,the flow resistance of fluid in fracture became smaller,which made it easier for fluid to flow into production well;this further increased the range of depressurization and promoted hydrate decomposition,resulting in a significant increase in gas production.The increase of fracture area could make the hydrates far away from the production well decomposed more quickly,thus increasing the gas production rate.