Study On The Main Controlling Factors Of Brittleness And Compressibility Classification Evaluation For Tight Reservoirs

In this paper,by studying the coupling law of fracture formation and energy release,a general method for quantitatively characterizing the brittleness and brittle-ductile transition characteristics of rock is established.The brittleness characteristics and main controlling factors of tight reservoirs were evaluated through the proposed brittle-ductile index(BDI);Based on the brittle-ductile transition characteristics of the rock,the tight reservoir rock mechanics unit layer is divided;By studying the BDI range,the reservoir characteristics,mineral components content,diagenesis types,surrounding rocks stress conditions and rock mechanical characteristics in the different mechanical units layer,the tight reservoir compressibility classification evaluation system are constructed.The specific research work and achievements of the project include the following aspects:First,a general method for evaluating rock brittleness and brittle-ductile transition characteristics is established by studying the coupling law of fracture formation and energy release.This method can continuously describe the evolution process of rock from brittle to ductile deformation.By analyzing the macro-fracture behavior of various rock samples and the development types of micro-fractures after fracturing,it is confirmed that the BDI index can more accurately evaluate the brittle fracture characteristics of the rock,and it can quantitatively characterize the brittle-ductile transition behavior.The rock fracture behavior is divided into 3 stages: brittle deformation stage(BDI> 0.5): the rock failure modes are mainly split cracks or low-angle shear fractures,brittle ductile transition stage(0 <BDI<0.5): conjugate fractures or local fragmentation appear in the rock failure mode,and ductile deformation stage(BDI<0): the rock deformation has no macroscopic failure characteristics,and develops scattered fine cracks or fragmentation.Secondly,For tight reservoir shale and sandstone,under the experimental conditions of loading surrounding rock pressure,the rock mechanical parameters of the samples are tested;The influence of reservoir physical properties,mineral composition,diagenesis,organic matter content and surrounding rock stress conditions on rock mechanical characteristics is analyzed.The BDI index was used to evaluate the rock brittleness,and the main controlling factors affecting brittleness were analyzed based on the Partial Least Squares Regression(PLSR)method.Thirdly,Based on the brittle-ductile transition characteristics of rock,the rock mechanics unit layer is divided.The evolution mechanism of rock failure mode and the distribution characteristics of mechanical properties under different surrounding rock stress conditions within the the mechanical unit layer are studied.Considering BDI,rock mechanics parameters and surrounding rock stress conditions,an evaluation model for dividing the tight reservoir rock mechanics unit layer is established.Based on the analysis of the main controlling factors affecting brittleness,study the BDI range,reservoir physical characteristics,mineral composition content,diagenesis type,surrounding rock stress conditions and rock mechanics characteristics in different mechanical unit layers,and construct a classification evaluation system of tight reservoir compressibilityFinally,taking the Chang 6 tight reservoir in the Ordos Basin as an example,based on the logging interpretation of BDI,surrounding rock stress and mechanical parameters,the mechanical unit layer is divided in the buried depth area.Based on the tight reservoir compressibility grading evaluation system,combined with the observation and description of down-hole core fractures,the Chang 6 tight reservoirs are evaluated for the compressibility grading.On this basis,based on the log interpretation data of 81 wells,the3 D visual evaluation model of compressibility in the study area is constructed.In a word,starting from the proposed brittle-toughness index BDI,evaluate the brittle characteristics and main controlling factors for tight reservoirs.Based on the rock brittle-ductile transition characteristics,the tight reservoir mechanical unit layers are divided.Through analyzing the distribution characteristics of the main controlling factors on brittleness in different mechanical unit layers,the tight reservoir compressibility grading system is constructed.And evaluate the compressibility characteristics of the Chang 6 tight reservoir in the Ordos Basin.The research content of this project open up a new idea and research method for the evaluation of compressibility of fractured rock mass and the evolution mechanism of fracture propagation system of tight reservoir.It is of great significance and academic value to clearly understand the fracture extension and network system connectivity law of tight reservoir fracturing,and provides a reference for the subsequent research of tight reservoirs based on brittleness characteristics.It lays a scientific foundation for the selection of fracturing section and evaluation of reservoir reconstruction effect.