Method for brittleness index prediction based on grey correlation and analytic hierarchy process:A case study of the tight reservoirs in the Lucaogou Formation of the Jimusaer Sag,Junggar Basin

Owing to the poor physical properties,the tight reservoirs in the Jimusaer Sag can yield industrial oil flow only through hydraulic fracturing.The research on mechanical properties and the brittleness assessment of rocks can provide a certain reference for hydraulic fracturing.This study obtained the mechanical properties of the tight strata in the Jimusaer sag using triaxial mechanical tests and determined the log parameters potentially sensitive to rock brittleness by analyzing the correlation between the sensitivity of the brittleness index and logs.Then,based on the grey correlation theory,this study determined the initial sequence of sensitivity parameters and normalized the parameters selected.Then,it quantitatively correlated the selected parameters with the potential sensitivity to the brittleness index and determined the degrees of correlation and their order.On this basis,this study established a matrix for the pair-wise comparison of the sensitivity parameters using the analytic hierarchy process (AHP) and determined the weight vector.Then,it established the functional relationship model between the brittleness index and the sensitivity parameters,thus developing a new prediction model for the brittleness index.Finally,this study compared the log models with the brittleness model established based on mechanical properties and the brittleness index determined through laboratory tests.The study results are as follows.The tight strata in the Jimusaer Sag have high brittleness,and the comprehensive brittleness index characterized using the whole-process stress-strain curve agreed with the actual brittleness characteristics of rocks.The degree of correlation of the sensitivity parameters determined using the grey correlation method was in the order of natural gamma-ray(GR)>resistivity(R_t)>density(ρ)>neutron(CNL)>sonic interval transit time(ΔT),which had a weight coefficient of 0.33,0.22,0.18,0.16, and 0.11,respectively in the new prediction model.The prediction method proposed in this study was applied to the Lucaogou Formation in the Jimusaer Sag.Compared with that determined through laboratory tests,the brittleness index predicted can reflect the actual brittleness of the formation,exhibiting a high consistency.As shown by the results from well tests,the productivity index of oil was proportional to the brittleness index,and a higher brittleness index was associated a high production capacity after fracturing.Therefore,the new method provides a new approach to brittleness index prediction and guides the parameter selection for the fracturing of reservoirs.