Method For Fracability Evaluation Of Tight Reservoirs And Verification Of Its Validation Through Experiments

Hydraulic fracturing has been widely used in the development of conventional and unconventional oil and gas resources in tight reservoirs.Among them,the fracability of tight reservoirs and the evaluation of fracturing effect are particularly important to the cost and productivity efficiency of oil and gas development.Therefore,this paper takes the reservoir fracability evaluation index and its quantitative method as the foundation firstly.And then,set up the fuzzy comprehensive evaluation model of the fracability and put forward the evaluation method basing on the AHP-entropy method.Finally,the reliability of compressibility evaluation results is verified by hydraulic fracturing test of sandstone and shale reservoirs,which provides theoretical support for fracturing and high efficiency production of tight oil and gas reservoirs.The results of the study are as follows:(1)The differences of physical and mechanical parameters of different lithology are compared and analyzed.According to the scientific,systematic,maneuverability and relative independence,the parameters of the compressibility model are uniaxial compressive strength?uniaxial tensile strength?Young's modulus?Poisson's ratio? cohesion coefficient and horizontal stress difference coefficient.In order to evaluate the fracability,an improved hyperbolic tangent function is proposed to normalize each index,so that each index can fully reflect the contribution of the size of each index to reservoir compressibility fracturing.(2)The basic theory of tight reservoir compressibility evaluation method is systematically expounded,and the fuzzy comprehensive evaluation method of compact reservoir compressibility based on AHP-entropy method is established.The weight of each index can be determined by AHP-entropy method,which can not only avoid the subjectivity of AHP,but also overcome the abnormal sensitivity of entropy method,and make the evaluation result close to the reality to a great extent.The compressibility of tight sandstone reservoir and tight shale reservoir are evaluated by fuzzy comprehensive evaluation method.(3)The differences and similarities of hydraulic fracturing test between sandstone and shale under the same conditions are compared and analyzed in combination with the model set up in this paper.The results show that the number of acoustic emission events of shale is much higher than that of sandstone,which indicates that shale is damaged more and crack propagation is more in unit time.After fracturing,the number of shale acoustic emission events has only a few large fluctuations,the reason is that hydraulic fractures penetrate the natural bedding,so there are new events.And hydraulic fractures are interlaced to form a fracture network with moderate compressibility.After fracturing,the number of acoustic emission events of sandstone fluctuates in a low range,indicating that no fractures occur,and that hydraulic fractures have a single shape and extend perpendicular to the direction of minimum principal stress,and the degree of compressibility is poor.The reliability of the model is verified by agreement with the results of the compressibility evaluation model established in this paper.