| The unconventional oil and gas resources in China has huge potential.Accelerating the exploitation of unconventional oil and gas is of great significance to alleviating the contradiction between supply and demand of oil and gas in China.Tight oil and gas are the important unconventional oil and gas resources currently and it is difficult to obtain economic benefits by conventional hydraulic fracturing.With reference to the development model of shale gas,simulation reservoir volume(SRV)fracturing technology is currently the primary technology for commercial development of tight reservoirs.The SRV fracturing technology is used to communicate natural fractures to form complex fracture networks,increase the sweep area further.The development of natural fractures can further increase the effect of hydraulic fracture.But if the natural fractures are not well connected with the main hydraulic fractures,they will contribute little to the improvement of production.Therefore,increasing the connectivity of fractures is the key to improve the oil and gas recovery of tight sandstone reservoirs effectively.The nature fractures in the reservoir are difficult to observe,the fracture connectivity and the simulation reservoir volume are difficult to calculate.Hence,the influence of fracture connectivity on the effect of fracturing reform needs study further.In order to solve the difficulty of describing the connectivity of natural fractures in Songbei tight sandstone reservoirs,and the unclear understanding of the propagation mechanism of hydraulic fractures in the reservoirs with complex natural fractures,in this paper,the fracture connectivity and fracturing propagation mechanism of Songbei tight sandstone reservoir were studied by combining the methods of literature research,laboratory experimental theoretical model establishment and numerical simulation.The main research work is as follows:(1)The geomechanical characteristics of the Songbei tight sandstone reservoir were tested,including rock mechanics parameters,in-situ stress characteristics,brittleness characteristics,and natural fracture development.Through investigation and comparison of geological characteristic data of the SRV fracturing technology blocks successfully implemented,the Block A in Songbei tight sandstone reservoir is medium-elastic and medium-hard rock.Its elastic modulus,Poisson's ratio,and compressive strength are similar to those in Daniudi gas field.The results of evaluation show that the in-situ stress difference coefficient is low,the brittleness index is moderately high,natural fractures are developed.Block A has the geological characteristics of forming complex fractures.(2)On the basis of elasticity theory and fracture mechanics,the fracture propagation mechanism considering the effect of wellbore and the stress distribution near the wellbore are analyzed.Besides,a semi-analytical mechanical model of the fracture tip stress intensity factor and fracture width is proposed.The calculation method of stress intensity factor and fracture width of hydraulic fracture tip considering the influence of collinear natural fractures was established.Also,the effects of rock mechanical parameters,stress anisotropy,and injection pressure on fracture propagation morphology are discussed.The researches show that the existence of natural fractures can promote the expansion of hydraulic fractures.(3)Combined with fractal theory and topology method,the natural fracture network model and node model are established by applying Monte Carlo simulation method and probability distribution function.Based on this model,the types of natural fracture nodes,the connectivity of natural fractures and their influencing factors are analyzed.Our investigations confirm that the number and development of natural fractures are influenced by the fractal dimension,group number and initial number.On the one hand,the connectivity of natural fractures increases as the fractal dimension increases.On the other hand,the number of fracture groups and the included angle have a significant effect on fracture connectivity.Also,the threshold of fracture connectivity is controlled by the fractal dimension,the initial number of fractures and the initial length.(4)Considering maximum principal stress fracture criterion,a dynamic hydraulic fracture propagation model is established with a cohesive element to simulate the nature fracture,which simulates the hydraulic fractures propagation process with non-connected natural fractures and connected natural fractures of X-type nodes.The effects of natural fracture angle,in-situ stress,elastic modulus,and Poisson's ratio on hydraulic fracture propagation morphology were analyzed.The natural fracture angle and stress difference are important factors affecting the fracture morphology when a hydraulic fracture intersects a natural fracture.(5)Based on the natural fracture network model,the numerical simulation method of embedded cohesive units globally is applied to simulate the dynamic fracturing process of hydraulic fracturing in natural fractured reservoirs.The research on hydraulic fracture propagation law of multi-fractured reservoirs and the influencing factors of hydraulic fracture reforming effect were carried out.Then,the effects of natural fractures with different fracture groups and different fractal dimensions on hydraulic fractures propagation are analyzed.The results show that when the fracture group and the fractal dimension increases,the fracture connectivity increases,and the fracture propagation morphology(6)The true triaxial hydraulic fracturing experiments of cement-wrapped full-diameter rock cores of Block A in Songbei tight sandstone reservoir were carried out.Besides,the effects of natural fractures and stress differences on fracture morphology and connectivity were analyzed.Finally,the hydraulic fractures morphology in the test well was analyzed.The experimental results prove that,when natural fractures develop and the stress difference is low,hydraulic fracturing is easy to form complex fractures,the hydraulic fracture communication area is large,the fracture connectivity is better.The above research results revealed the rock mechanical properties,natural fracture distribution and hydraulic fracture propagation law of Songbei tight sandstone reservoir deeply.It provides a theoretical basis and technical support for the effective development and improving oil and gas recovery of Songbei tight sandstone reservoir. |
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