Experimental Research On Shale Gas Reservior Fracability Evaluation

Shale-gas reservoir is poor in porosity and permeability,and generally need to be hydraulic fractured to get commercial production.So fracability evaluation of this type of reservoir is particularly important,it's the basis of favorable zone selection for shale gas prospecting,and hydraulic fracturing design.At present the methods of fracability evaluation are almost based on brittleness.We summarize the concepts and evaluation methods of brittleness,point out that current brittleness index evaluation methods: laboratory analyses methods,mineral methods,and mechanical methods are all have obvious defects.A completely new set of experiment and analysis technique to evaluate fracability of rock sample was proposed,which based on the concept of brittleness in shale-gas development—complexity of post-frac rock.First,we take modified triaxial compressive strength experiment on rock sample,which can control the degree of fracturing,the samples after experiment were fractured but not broken to pieces-they have comparable fracturing degree.Mechanical properties of sample were also obtained.Then collect external and internal image of post-frac rock sample with optical scanning and X-CT scanning method.To 2D external fracture images,develop a method to redraw fractures artificially to make the information of fractures can be obtained easily.To 3D X-CT images,first we rebuild 3D digital rock sample,then 3 steps was take to screen fractures.Parameters of every fracture were extracted,to calculate surface fracture porosity(or fracture porosity)and dispersion of fracture dip angle,normalize the two parameters and calculate the mean value-fracture complexity of post-frac rock sample.For the fractures of the post-frac rock sample are more complex,and the rock strength is lower,the core will be fractured easier.Fracturing index(FI)is proposed: the ratio of fracture complexity of post-frac rock sample and its strength.The strength used here can be calculated by eliminate size effect with triaxial compressive strength.We demonstrated with experiment that the fracability of the rock sample can be reflected by FI more accurately than by mineralogical and mechanical BI.And compared with micro-seismic monitoring during hydraulic fraturing,FI of rock sample can indicate fracability of reservoir well.The relationship of FI and minerals in shale is accord with experience summarized in shale-gas development all over the word.This technique has a very wide application foreground in shale-gas reservoir fracability evaluation.