The Fracture Initiation-propagation Behavior And The Influence Of Fracture Morphology On Development Stimulation Potential Of Shenhu Hydrate Reservoir

The hydrate reservoir in Shenhu sea area of the South China Sea is the main exploration and development target area in China.So far,two trial production projects have been implemented in this area.However,Shenhu hydrate reservoir is a typical clayey-silt reservoir,the low permeability seriously limits the CH₄ production capacity.Hydraulic fracturing technology has made great progress with the development of unconventional gas reservoirs such as shale gas and tight gas.However,the mechanical behavior of Shenhu hydrate reservoir fracturing and the stimulation behavior of hydraulic fractures on hydrate reservoir development need to be clarified.Focusing on this,the fracture initiation-propagation behavior and the influence of fracture morphology on stimulation behavior in Shenhu hydrate reservoir were studied.The in-situ stresses of Shenhu hydrate reservoir in the conditions of pore water connected and disconnected were calculated based on core analysis,logging data and related theories.Then,the hydrate sediment fracturing experiments were carried out using the self-designed low-temperature true triaxial hydraulic fracturing equipment.The effects of hydrate saturation and clay kinds on the fracture initiation and propagation behaviours of hydrate sediment under different stress conditions were studied.Through the calculations of fracture propagation area and fractal dimension of fracture contour,the morphological characteristics of fracture propagation were quantitatively analyzed.Based on the elastic theory,the stresses distribution of borehole wall rock were solved,and the rupture criterions of hydrate sediment was established based on the classical rock rupture criterions.The results showed that under the conditions of pore water connected(low in-situ stress)and unconnected(high in-situ stress),the fracture propagation area increased gradually with the increase of hydrate saturation.While the initiation pressure increased and then decreased with the increase of hydrate saturation,the critical hydrate saturation of low in-situ stress was 70%and that of high in-situ stress was 40%,respectively.Under low in-situ stress,clay increased the propagation area of fractures,improved the occurrence probability of complex fractures,and significantly reduced the initiation pressure of hydrate sediment.The corresponding influence degree:montmorillonite>kaolinite>illite>chlorite.Under high in-situ stress,clay greatly reduced the initiation and propagation pressures of hydrate sediment,and the influences degree of montmorillonite and kaolinite were similar,while illite and chlorite were similar.Compared with sand-sample,the initiation pressures of samples containing montmorillonite and kaolinite were reduced by about60%,the expansion pressure were reduced by about 50%;and,the initiation and expansion pressures of samples containing illite and chlorite were reduced by 30%.According to the relevant theories of shale gas development,the multi-phase flow model in the fracture was established,and the geological model of Shenhu hydrate reservoir was established using the logging data.Then,the stimulation behavior of single fracture and fracture network on hydrate reservoir development mode of depressurization combined with thermal stimulation was numerically studied using Tough+Hydrate software.The results of the stimulation behavior of single fracture showed that the fracture has significant stimulation effect on hydrate development.The CH₄ recovery ratio and energy efficiency were significantly improved by the fracture.The results of the stimulation behavior of fracture network showed that fracture conductivity(C_F)was a key factor affecting the fracture network stimulation effect on hydrate development.With increased C_F,the cumulative hydrate decomposition and CH₄ production amount increased significantly.Before the water breakthrough,the hydrate dissociation rate(Q_R)and CH₄ production rate(QP)increased significantly with increasing CF;however,after the water breakthrough,with increased C_F,Q_R and Q_P decreased.A sufficient pressure gradient along the fracture strike is necessary for the fracture network'stimulation of hydrate development.Fracture speacing(SF)variations can cause a hydrate decomposition mode transformation.Under a large SF,hydrate decomposition occurs in piston-like decomposition mode and the effect of SF on hydrate decomposition and CH₄ production is minimal;however,when the SF is small enough to transform the hydrate decomposition mode into a non-piston-like decomposition mode,Rd and QP increase sharply.Referring to the concept of multiple-well production systems proposed by Japan accoding to the summary of two oceanic hydrate reservoir trial productions,the commercial productivity of depressurisation and thermal fluid injection(D+T_i),through an injection-production well pattern was studied.By exploring the influence of production parameters and well patterns on hydrate development,the challenges faced in the development of D+T_i and its commercial production potential were explored.Furthermore,the stimulation potential of the fracture network on D+T_i was investigated.The results showed that the interlayer contradiction caused by the effective permeability difference between the reservoir and caprock and the low methane flow capacity between the injection-production well led to a poor CH₄ production ability and the insensitivity of CH₄productivity to production parameters for D+T_i.In addition,the permeable caprocks limited the well pattern infilling potential of D+T_i.These defects made it impossible for D+T_i to obtain commercial production in the Shenhu hydrate reservoir without stimulation technology.However,the fracture network significantly improved the productivity and well pattern infilling potential of D+T_i.After network fracturing,the gas production rate and production degree of D+T_iwere promising to reach the commercial production standard.The finding of the hydraulic fracturing characteristics of Shenhu hydrate reservoir in this study can provide theoretical and technical supports for Shenhu hydrate reservoir fracturing.The clarification of the stimulation behavior of fracture on hydrate development can provide a meaning reference for the selection and optimization of development scheme in the future...