| Natural gas hydrate(NGH)is a promising new type of clean energy with huge reserves.In-situ exploitation of NGH involves many challenges such as complex phase transition,heat transfer,multiphase seepage,sand production,methane leakage,etc.,compared with sand gas,shale gas,and other natural gas resources,which is considered to be a worldwide problem.The Shenhu area of the South China Sea is the key area of hydrate exploration and development.Based on the existing geophysical exploration data,it is found that the hydrate reservoir has the following accumulation characteristics:firstly,hydrate occurs in clayey-silty sediments with the most difficult to exploit but the highest resource share(more than 90%).This kind of reservoir is characterized by low effective permeability and belongs to a low-permeability gas reservoir;second,there are generally a threephase layer and a free-gas layer below the hydrate layer,belonging to the multi-layer reservoir where free gas and hydrate co-exist.Consequently,the stimulation idea of"multi-layer fracturing and commingled production" is proposed,that is,to improve the production potential of hydrate,hydraulic fractures are used vertically to penetrate multi-layer to achieve commingled production of hydrate decomposition gas and insitu free gas.Thus,fully understanding the hydraulic fracture formation mechanism of non-diagenetic hydrate reservoir,and evaluating multi-layer co-production dynamics and commercial development potential are required.In this study,the shallow-buried,weak/unconsolidated,non-diagenetic,and lowpermeability clayey-silty NGH reservoir is taken as the research object,and integrate research methods of experiments,theoretical deduction,data modeling,and numerical simulation to study the fracture formation mechanism and multi-layer co-production behavior.Firstly,a low-temperature true triaxial fracturing physical simulation system was used to investigate the hydraulic fracturing characteristics of clayey-silty hydrated sediments.The experiment found that due to the uneven distribution of hydrates in sediments,the fracture propagation morphology is irregular and mainly forms a single straight seam.When the horizontal crustal stress is isotropic,double cracks crossed can be generated.The breakdown pressure does not always increase with increasing hydrate saturation,and the turning point is 40%.Moreover,the increase of in-situ stress will lead to a sharp increase in breakdown pressure,making the classical initiation model of elastic formation no longer applicable to the NGH reservoir.Based on the total theory and stress-strain curve,the elastoplastic constitutive equation of hydrate-bearing sediments was established.Considering the in-situ stress and the fluid seepage effect,the elastic-plastic stress field calculation model around the wellbore is derived.Then,combined with the maximum tensile stress criterion and Mohr-Coulomb criterion,the elastic-plastic initiation model is established.The results showed that the yield strength and peak strength increased as increasing hydrate saturation,while the plastic characteristics gradually weaken.The circumaural stress and plastic radius decrease as the hydrate saturation increases,revealing the control mechanism that an NGH reservoir with a higher hydrate saturation is easier for fracture initiation.Furthermore,the fracture initiation mode gradually changes from shear failure to tensile failure as the hydrate saturation increases.By introducing a multi-layer perceptron to determine the importance of multiple parameters,an objective and reasonable fracability evaluation method is constructed.Then,combined with the experimental results,the fracability of clayey-silty hydratebearing sediments was evaluated.The results showed that horizontal stress difference is the primary parameter(weight 0.4)that governs the fracability index(FI),followed by the coefficient of earth pressure at rest(weight 0.31),fracture toughness(weight 0.18),and hydrate saturation(weight 0.05),and vertical in-situ stress(weight 0.05).The higher the FI,the more conducive to fracturing,and the selection of operation parameters has a significant effect on the fracturing effect.Particularly,when (?)>0.33(defined by the normalized displacement and the viscosity of the fracturing fluid)and FI>0.4,a considerable reconstruction area can be obtained.The NGH reservoir with multi-layer in the Shenhu area is taken as the geological background,and based on the extended finite element theory,a 3D multilayer fracturing model considering fluid-structure coupling and damage is established.The results showed that the influence of the in-situ stress gradient and the "shielding" effect of the hydrate-bearing layer(HBL)on fracture penetration should be fully considered when deploying wells.When the propagation resistance of the lower wing fracture is sufficient to offset the shielding resistance of the HBL,the fracture will stop extending downward and turn to the upper formation,where wells can be deployed in the triplephase layer(TPL)to achieve multi-layer fracturing.However,when the hydrate saturation of the HBL is sufficiently high(55%),changing the fracturing parameters can not promote fracture penetration,so it is recommended to deploy wells in the HBL.For the NGH reservoir in the Shenhu area,deploying the wells in the TPL and using the low-displacement and low-viscosity fracturing fluid can effectively reconstruct the multiple layers.Finally,based on the Tough+Hydrae code,a 3D multi-layer co-production model coupled with well-fracture-reservoir was established,and multi-layer co-production dynamics and the commercialized potential were evaluated.The results show that due to the limited reservoir sensible heat and low matrix permeability,the exploitation of HBL has a serious lag compared with the TPL and FGL.Thermal injection in HBL can significantly improve the mining efficiency,help synchronous mining,and shorten the production cycle.Moreover,increasing the number of fractures can increase the hot water swept area,and the improvement of co-production efficiency is more significant,compared with improving the fracture conductivity,For the NGH reservoir in the Shenhu area,more than 70%of natural gas resources can be recovered at an average gas production rate of 50000m3/d(commercialized potential≥Ⅱ)with the following production scheme:injection-production well spacing 30m,horizontal well section≥300m,fracturing cluster spacing≤15m,fracture conductivity of 40D·cm,injection pressure of 16MPa,and water injection temperature of 60℃.In general,this study revealed the elastic-plastic initiation mechanism of the nondiagenetic hydrate reservoir,formed a quantitative and objective fracability evaluation method,gave the main control factors and engineering criteria for the fracability of clayey-silty hydrate-bearing sediments,clarified the control mechanism of the fracture penetration behavior in multi-layer,analyzed the co-production dynamics,and obtained commercial co-production schemes.Therefore,these findings are of great significance in enriching the hydraulic fracturing theory of non-diagenetic strata and promoting the commercial development process of NGH resources. |
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