Stress Field And Fracability Anomalies In Shale Gas Reservoir

As a kind of unconventional oil and gas resources,shale gas has become the focus in the world.Shale gas reservoir is different from conventional tight gas reservoir.Influenced by the ultra-low permeability and ultra-low porosity in shale formation,shale gas reservoir always do not exist the natural productivity and must rely on some hydraulic stimulation treatments to achieve productivity.We should choose the shale reservoir to fracture,which has the characteristics of natural fracture development,high brittleness index and low horizontal stress difference.And the natural fractures would be extended and activated by the continuous propagating hydraulic fractures,which results in large-scale and asymmetric fracture pattern.Finally,as further propagation of fractures,the connection and extension will lead to network fractures and increase the SRV(Stimulated Reservoir Volume)in shale gas reservoirs.In this paper,firstly,we establishes the reflection coefficient approximate equation which is based on Young’s modulus eflectivity and Poisson ratio eflectivity,and Young’s modulus and Poisson ratio are considered to be the indicator of rock brittleness.Secondly,by analysing the typical horizontal bedding-fracture model and shale rock physics characteristics,we derived the theoretical expression for calculating the horizontal principal stress of shale reservoir.At last,we considered the influence of formation parameters on the fracturing process in shale gas reservoir.The research work and progress of this paper mainly include the following contents:(1)Based on the YPD equation and the Gardner density and velocity relationship,a new equation(YP approximate equation)for calculating the P-wave reflection coefficient approximate equation is derived with the assumption of plane wave incidence,which establishes the linear quantitative relationship between Young’s modulus reflectivity,Poisson ratio reflectivity and the P-wave reflection coefficient.We analyzed the accuracy and applicability of the new equation.In addition,we tested the reliability of the new inversion formula by the reasonable inversion method,which provides a reliable method for the identification of the “sweet spot” of shale gas.(2)This paper introduces the horizontal bedding-fracture Hudson model and the horizontal bedding-fracture Schoenberg model theory,then establishes the prediction model for in-situ formation stress in shale reservoirs based on the structural characteristics and petrophysical properties,and on the basis of previous studies,the theoretical expression of the maximum and minimum horizontal principal stress of shale gas reservoir is obtained by using the “linear slip theory”.The theoretical expression is composed of vertical principal stress,Young’s modulus and Poisson’s ratio.The differential horizontal stress ratio(DHSR)is a parameter to evaluate the horizontal stress state without knowing the reservoir stress,which provides the basis for shale reservoir fracturing and horizontal well drilling.(3)In addition to the advantages and disadvantages of several common fracture network models,this paper analyzed the stress conditions of the formation of complex network fractures and the change of stress in the hydraulic fracturing of shale reservoirs,and it is found that the direction of horizontal principal stress will be reversed in the process of hydraulic fracturing under certain conditions.In the actual fracturing production,we can use this phenomenon to optimize the existing fracturing method.(4)By using the theory of fluid mechanics,percolation mechanics and elastic mechanics,we analyzed several geological factors(horizontal stress difference,Young’s modulus,Poisson’s ratio and distribution of natural fractures)that affect the stimulated reservoir volume(SRV),and the results show that these four parameters have a great influence on the final shale fracturing reservoir reconstruction volume.