The shale reservoirs in our country are rich in clay minerals and have types of porosity for storage and flow,including nano-scale pores,micro-fractures and large hydraulic fractures.In the development of shale reservoirs,clay minerals can produce hydration swelling when contacting with the water in drilling fluid or fracturing fluid,which can lead to the borehole instability,the low flowback of fracturing fluid and the decrease of oil production.While most of the current theories for flow and methods for numerical simulation do not take the effects of the hydration swelling of clay minerals in shale on oil production into consideration.Aiming at this problem,a hydro-mechanical coupled numerical simulation method for shale reservoirs considering hydration swelling reaction is established in this article,and the main innovations and conclusions are as follows.Firstly,based on the geological characteristics and hydration and swelling mechanism of shale and the elastic deformation of matrix,we proposed a stress and strain model,in which hydration stress is taken into consideration.Combined with the flow equations and the mechanical equations obtained from the hydration stress and saturation experiment,the constitutive equations considering the hydration swelling effect is established.Secondly,according to the volume stimulation techniques,hydraulic fractures are modeled explicitly by using the embedded discrete fracture model,and numerous micro-fractures are modeled by using the continuum models,and a coupling rock micro and hydraulic fractures model based on regular grids is established,in which the embedded discrete fracture model incorporates the effect of each fracture explicitly based on the dimensionality reduction.The flow and mechanical equations are discretized by the finite volume method and finite element method,respectively.Then the fixed-stress splitting method is used to iteratively solve the coupled flow and geomechanics problem.And the corresponding numerical simulator is compiled using Fortran language and the correctness of the model is verified according to experimental results and the data in related literatures.Lastly,based on the numerical simulator,the sensitivity analysis of fracturing construction,reservoir geology and hydration swelling parameters was carried out for productivity.Then we studied the development dynamics of typical shale reservoirs under different influencing factors.The results are summarized as follows.Within the parameter setting range of this article,the half-length,spacing,density of hydraulic fracture and microfracture density are all positively correlated with productivity,while the initial water saturation is negatively related to productivity.When considering micro-fractures and the whole stress sensitivity,the cumulative oil production and oil production rate decreased significantly.When considering hydration and swelling effect,the cumulative oil production is significantly reduced,while the oil production rate is less affected.The results show that the longer off production time,the smaller the cumulative oil production in the same time.Based on the history matching and productivity prediction according to the actual production data of a shale reservoir in some oil field,this article provides certain theoretical and technical support for the efficient development of shale reservoirs. |