Characterization And Mechanical Behavior Of Natural Fractures In Continental Shale Reservoirs

Continental shale reservoirs have various types of natural fracture systems with a large number and random distribution,which affects the uncertainty of 3D geological modeling,the wellbore stability and the complexity of hydraulic fracture network formation in the process of exploration,drilling and production.These restrict the safe and efficient development of continental shale oil and gas.However,what type of natural fractures and how it affects wellbore stability and fracture network formation are the difficult problems to be solved urgently.The study of natural fracture characterization and mechanical behavior in continental shale reservoirs is the key to solve this problem.Therefore,natural fracture characterization and mechanical behavior of continental shale reservoir are studied in this paper,and the conclusions and understandings are as follows.(1)Quantitative characterization of natural fractures in continental shale reservoirsA new method for quantitative characterization of natural fractures in continental shale reservoirs based on fractal theory,topology theory and statistical method was proposed.The development degree,type,connectivity and distribution of natural fractures were characterized by fractal dimension,topological node type,topological structure parameters and fracture occurrence quantitatively.Firstly,the overall development degree of natural fractures is low and the fractal characteristics are poor.Secondly,the natural fractures can be divided into three types:natural fractures with type Ⅰ nodes,natural fractures with type Ⅱ nodes and natural fractures with type Ⅲ nodes.Thirdly,the natural fracture network is mainly dominated by typeⅠ nodes,the connectivity between natural fractures is poor.Fourthly,natural fractures with typeⅠ,Ⅱ and Ⅲ nodes are distributed at angles of 10°,5° and 5°with the direction of maximum horizontal principal stress,respectively.(2)Stress distribution around wellbore in continental shale reservoirs with typical natural fracturesNew three-dimensional stress distribution models around vertical and horizontal wellbores were established in shale reservoirs with natural fractures,the finite element method was used to analyze stress distribution around wellbore in continental shale reservoir with typical natural fractures and the effect of typical natural fractures on the stress around the wellbore under boundary in-situ stress.And wellbore stability and the prediction of whether hydraulic fractures can be initiated on wellbore wall was carried out.Firstly,circumferential stress and radial stress are distributed orthogonally and symmetrically around vertical wellbore in isotropic reservoirs,but they are distributed non-orthogonally and asymmetrically around vertical wellbore in laterally isotropic reservoirs,circumferential stress and radial stress are distributed non-orthogonally and asymmetrically around horizontal wellbore in isotropic and laterally isotropic reservoirs,and the tangential stress distribution is disordered.Secondly,stress value and its stress concentration degree near horizontal wellbore are larger compared with vertical well,and borehole wall of horizontal wellbore is the most unstable.Thirdly,natural fractures have a greater effect on the near-wellbore stress,but less on the far-field stress.Natural fractures with type Ⅱ and type Ⅲ nodes have the greatest effect on circumferential stress,radial stress and tangential stress around wellbore,borehole wall is the most unstable.(3)Fractures mechanical behavior in continental shale reservoirs with typical natural fracturesThrough physical simulation,combined with numerical simulation,the law of hydraulic fracture initiation and propagation under the influence of typical natural fracture groups,and also the interaction mechanism between hydraulic fracture and typical natural fractures were clarified for the first time in horizontal and vertical direction of horizontal wells of continental shale reservoirs.Firstly,the initiation position of the hydraulic fracture is symmetrical in naturally fractured reservoir with type Ⅰ nodes,while the initiation position is asymmetrical in naturally fractured reservoir with type Ⅱ and type Ⅲ nodes.Secondly,natural fractures can induce hydraulic fracture propagation,among which natural fractures with type Ⅱ nodes have the strongest inducing ability to hydraulic fracture propagation,followed by natural fractures with type Ⅲ nodes,and natural fractures with type Ⅰ nodes have the weakest inducing ability under the influence of typical natural fracture groups.Thirdly,hydraulic fracture fails to pass through the natural fractures,but opens the natural fracture at the intersection of the fractures.Among them,natural fractures with type Ⅱ and type Ⅲ nodes open more fracture branches than natural fractures with type Ⅰ nodes,and then turns and extends along the opened natural fracture when hydraulic fractures interact with natural fractures.(4)Fracturing effect optimization in continental shale reservoirs with typical natural fracturesFinite element analysis method was used to reveal the effect of fracturing parameters on mechanical behavior of fractures under typical natural fracture groups in horizontal and vertical direction of horizontal well of continental shale reservoirs based on single factor analysis and orthogonal experimental design,then the main and secondary factors affecting the mechanical behavior of fractures and the optimal fracturing scheme were determined by the complexity of fracture network and hydraulic fracture length.Firstly,too high or too low fracturing fluid displacement,fracturing fluid viscosity and ground stress difference will inhibit hydraulic fracture propagation,hydraulic fractures cannot effectively open natural fractures or beddings,which reduce the fracturing effect.Secondly,the optimal fracturing fluid displacements respectively are 0.015m3/s,0.015m3/s,and 0.015m3/s,the optimal fracturing fluid viscosities respectively are 0.009Pa·s,0.001Pa·s,and 0.005Pa·s,and the optimal in-situ stress differences respectively are 6×106Pa,6×106Pa and 9×106Pa for hydraulic fracturing in horizontal direction of horizontal well in naturally fractured reservoirs with type Ⅰ nodes,typeⅡ nodes and type Ⅲ nodes.For hydraulic fracturing in vertical direction of horizontal well in naturally fractured reservoirs with type Ⅰ nodes,type Ⅱ nodes and type Ⅲ nodes,the optimal fracturing fluid displacements respectively are 0.015m3/s,0.015m3/s,and 0.015m3/s,the optimal fracturing fluid viscosities respectively are 0.001Pa·s,0.009Pa·s,and 0.009Pa·s,and the optimal in-situ stress differences respectively are 6×106Pa,3×106Pa and 6×106Pa.Thirdly,for hydraulic fracturing in horizontal direction of horizontal well in naturally fractured reservoirs with type Ⅰ nodes,type Ⅱ nodes and type Ⅲ nodes,fracturing fluid displacement is the main factor affecting fracture mechanical behavior,followed by fracturing fluid viscosity,and the in-situ stress difference is the least.For hydraulic fracturing in vertical direction of horizontal well in naturally fractured reservoirs with type Ⅰ nodes,fracturing fluid displacement is the main factor affecting fracture mechanical behavior,followed by fracturing fluid viscosity,and the in-situ stress difference is the least.However,in naturally fractured reservoirs with type Ⅱ nodes and type Ⅲ nodes,fracturing fluid displacement is the main factor affecting fracture mechanical behavior,followed by in-situ stress difference,and the fracturing fluid viscosity is the least.Fourthly,the optimal combinations of fracturing operation parameters in horizontal direction of horizontal well are 0.015m3/s,0.001Pa·s and 6×106Pa in naturally fractured reservoir with type Ⅰ nodes,0.015m3/s,0.001Pa·s and 9×106Pa in naturally fractured reservoir with type Ⅱ nodes,0.015m3/s,0.005Pa·s and 9×106Pa in naturally fractured reservoir with type Ⅲ nodes.Meanwhile,the optimal combinations of fracturing operation parameters in horizontal direction of horizontal well are 0.015m3/s,0.005Pa·s and 6×106Pa in naturally fractured reservoir with type Ⅰ nodes,0.015m3/s,0.009Pa·s and 3×106Pa in naturally fractured reservoir with type Ⅱ nodes,0.015m3/s,0.001Pa·s and 9×106Pa in naturally fractured reservoir with type Ⅲ nodes.The research in this paper can provide theoretical and technical guidance for the safe and efficient development scheme of continental shale oil and gas.