Fracability Evaluation And Fracture Propagation Mechanism Of Hydraulic Fracturing In Hydrate Reservoir

At present,natural gas hydrate has not been developed efficiently.The main reason is that the sweep range of bottom hole pressure drop in traditional depressurization production is limited,and the reservoir with high gas hydrate saturation has low permeability,which leads to low hydrate decomposition efficiency.The artificial fracture formed by hydraulic fracturing can effectively expand the bottom hole pressure sweep range,and the artificial fracture can also be used as an effective seepage channel.It is a hot spot that whether the natural gas hydrate can be developed by hydraulic fracturing technology and the fracture propagation mechanism of hydrate reservoir.In this study,the physical and mechanical parameters of natural gas hydrate-bearing sediment were tested to clarify the evolution law of hydrate occurrence on the mechanical properties.Based on the experimental results,the shear strength formula of hydrate-bearing sediment was established,which consisted of hydrate saturation,effective confining pressure,cohesion and internal friction angle.The modified Mohr-Coulomb criterion was proposed to provide the damage evolution basis for the subsequent hydraulic fracturing numerical simulation of hydrate reservoir.Through hydraulic fracturing tests,the influence of different factors on the fracability of hydrate sediments was analyzed.Based on analytic hierarchy process(AHP)and entropy method,the fracability evaluation model of hydrate reservoir was established.Combined with the experimental results of hydraulic fracturing,the applicable conditions of hydraulic fracturing for hydrate reservoirs were proposed based on the evaluation criteria of fracability index,fracturing fluid viscosity and injection rate,which provided a theoretical basis for the selection of target blocks for hydraulic fracturing of hydrate reservoirs.For the hydrate reservoir that hydraulic fracturing can be used,a coupled thermo-hydrologic-mechanical-damage(THMD)model was established to analyze the phase transformation mechanism of hydrate during hydraulic fracturing,and the influence of different factors on the hydraulic fracture propagation of hydrate reservoir.The mechanism of initiation and propagation of hydraulic fracture in hydrate reservoir was clarified.The results show that the hydrate saturation and clay content have significant effects on the cohesion of hydrate-bearing sediments,but have little effect on the internal friction angle.This is mainly due to the clay filling in the pores of sandstone particles,which increases the contact area of particles and thus increases the friction resistance in the shear process.In addition,due to the cementation of hydrate,some clays play the role of connecting adjacent sandstone particles,which changes the hard contact between sandstone particles into the connection of clay cemented by hydrate,thus improving the cohesion of hydrate-bearing sediments.The modified Mohr-Coulomb criterion can reflect the strengthening effect of hydrate cementation on shear strength.On the whole,the hydrate reservoir with fracability index less than 0.48 can not form fractures during hydraulic fracturing,even with high viscosity and large injection rate of fracturing fluid.Hydraulic fracturing is not suitable for this kind of hydrate reservoir.The hydrate reservoir with fracability index between 0.48 and0.6 has good hydraulic fracturing conditions.However,it is necessary to increase the injection rate and viscosity of fracturing fluid to reduce the filtration of fracturing fluid.Hydrate reservoir with fracability index higher than 0.6 are basically suitable for hydraulic fracturing,which is the preferred reservoir for reservoir stimulation by hydraulic fracturing technology.For the hydrate reservoir suitable for hydraulic fracturing,the fracture initiation and propagation is along the direction of the maximum horizontal principal stress.Fracturing fluid injection rate,fracturing fluid viscosity,initial hydrate saturation and reservoir permeability are the main factors affecting fracture propagation.Increasing the injection rate and viscosity of fracturing fluid can increase pore pressure,inhibit hydrate decomposition and increase fracture length.The higher the hydrate saturation is,the lower the formation permeability is,and the more favorable it is to form long fractures during hydraulic fracturing.