Target window spatial distribution prediction based on network fracability: A case study of shale gas reservoirs in the Changning Block, southern Sichuan Basin

Target window optimization has significance for marine shale gas reservoir stimulation. Taking the marine shale gas reservoir in the Longmaxi Formation of the Changning Block in the southern Sichuan Basin as the research object,argon ion polishing scanning electron microscope,Brazilian disk experiment and fracturing operation statistics were carried out and the influence of mineral size,cracking toughness and crustal stress on hydraulic fracture penetration,propagation and diversion were analyzed. In view of the inadequate fracturing-quality effect of the current methods on the prediction of target window in shale gas reservoir,a new target window prediction method based on the network fracability theory,illustrated by the morphology of logging curves,was established,using brittle-ductile index calculated by sonic,density,gamma and mineral content logging data,and stress difference index calculated by the optimal stress-stress difference coupling model derived from the mechanics theory and experiment in a fast track. The results show that the Ⅰ-type target window,showing a considerable overlap between the curves of brittle-ductile index and stress difference index with "double-trough",is the most favorable for development,followed by the successive cross-overlap as the Ⅱ-type and the discontinuous cross as the Ⅲ-type. The application in the Changning Block shows that most of target windows in each well have the "double-trough" shape,but the corresponding layer of each target window is different. The target windows from west to east have changed from the 1 layer to the 2 layer and then back to the 1 layer. The high drilling-encounter rate of various target windows in different well-controlled zone positively controlled the stimulation reservoir volume,test productivity and cumulative productivity.