Study On The Method For Fracability Evaluation Of Shale-gas Reservoirs

Hydraulic fracturing is the core technology of the development of shale gas,through the massive volume fracturing of shale reservoir,the shale reservoir form complex fractures,thus enabling the industrial development of shale gas stored in ultra low permeability shale reservoir.Compressibility is an important content of shale gas reservoir evaluation,we can know the difficulty degree of effective transformation and effective evaluation of the reservoir compressibility through the formation of fracture network of shale reservoir under hydraulic action.So far,the evaluation method of compressibility at home and abroad is not perfect,especially the fracture development is not introduced into the compressibility evaluation,which is of great importance to the formation of fracture network.At present,we characterize the compressibility of shale reservoir with the brittleness index.Although there has been up to 30 kinds of rock brittleness evaluation method,there are mainly two evaluation methods commonly used according to the ease of the actual data acquisition,one is the modulus method,using elastic modulus of the reservoir rock and Poisson ratio to calculate shale brittleness,the rock with high elastic modulus and low Poisson ratio has high brittleness and compressibility;the other is the mineral composition method,which uses the content of brittle mineral components to calculate the brittleness of rock,the higher the content of quartz and other brittle minerals is,the stronger the brittleness and compressibility of the rock is.However,most of the time the results of the two methods is not consistent,and no answers are given to the difference,while the natural fracture has an important influence on the formation of hydraulic fracture network,and it has a decisive influence on some cases.So far,the effect of the fracture on the compressibility has not been introduced into the model of compressibility evaluation.On the basis of analyzing the influence of fracture on the formation of fracture network and elastic modulus,the study suggests that the modulus and brittleness index calculated from mineral composition reflect different properties of reservoir rock,mineral composition represents the brittleness of matrix material,and elastic modulus is more influenced by the fracture,which represents the ability of resisting deformation.Currently,the two methods of compressibility evaluation essentially consider the reservoir as homogeneous and fracture-free medium,the compressibility coefficients calculated from the two methods are approximately the same for the reservoir of undeveloped fracture,while for the reservoir of developed fracture,calculation of compressibility coefficient of modulus method is less than that of the mineral method due to the impact of fracture.Brittleness of rock matrix and fracture development both are of great importance to the compressibility,however,there is no distinction between the influence of matrix and fracture in current model of compressibility evaluation,and the impact of fracture is inadequately considered.We propose a new model of compressibility evaluation by considering fracture as an independent influencing factor,which combines the influence of matrix brittleness and fracture development.K=?1K1+?2K2,where K1 is matrix brittleness coefficient,K2 is influence coefficient of fracture,?1 and ?2 are weight factors of the influence of matrix and fracture,indicating the relative size of the two influencing factors.According to the mineral composition,matrix brittleness coefficient K 1 can be calculated from the relative proportion of brittle minerals,fracture development coefficient K2 can be characterized according to the difference degree of the rock modulus and fracture-free matrix material modulus and obtained by logging data or laboratory test.For fracture development influencing factor K2,we can estimate matrix material modulus by the Voit-Reuss-Hill model in rock physics according to the mineral composition firstly,and correct the effects of porosity and TOC,and draw a theoretical modulus of fracture-free shale matrix.Then we can use wave velocity to obtain the measured dynamic modulus of shale,which takes the impact of fracture into consideration.The theoretical modulus of fracture-free matrix is used to reduce the measured dynamic modulus,and we can know fracture development influencing factor K2 by the ratio of the difference and the theoretical modulus of matrix.In this paper,the compressibility evaluation method and model are applied and verified by the core of shale gas reservoir and lo gging data in C hongqing,and the compressibility evaluation results and the relative size of Longmaxi,Lujiaping,Niutitang and etc.reservoirs are obtained as well.The comparison between the calculated results and imaging logging results shows that the new model is more reasonable compared with the existing models.