Logging interpretation of brittle-plasticity parameters based on stress-strain curves for continental shale reservoirs in Xinchang field, Western Sichuan Basin, China

Abstract

The brittleness index is widely used to evaluate the hydro-fracturing potential of shale gas reservoirs, and often interpreted based on the elastic parameters or mineral composition in well logs. To determine an effective brittleness method for the target area, 12 samples were collected for brittleness analysis, and a fracturing well was selected to assess the application effects in logging. First, thematerial compositions were obtained fromtotal organic content (TOC) testing and X-ray diffraction analysis. Then, brittle-plasticity characteristics were obtained from axial compression tests under simulated reservoir conditions combined with acoustic velocity tests. Results show that: (i) all samples are divided into sandstone, siltstone and shale. Sandstone is distinguished from siltstone by mineral distribution and TOC, while shale has great differences from sandstone and siltstone in the content and distribution of minerals and organic matter. (ii) All samples can be divided into brittle, semi-brittle and plastic according to stress-strain behavior. As the lithology changes from sandstone to shale, mechanical properties change from brittle to plastic. However, there is little correlation between elastic parameters and brittle-plasticity. (iii) Indoor tests showed a strong exponential correlation between Peakmodulus and P-wave velocity. A new method for calculating plastic strain and the strain ratio in logging was proposed, in which brittle-plasticity parameters were derived from Young's modulus, peak modulus and compressive strength. (iv) The new logging interpretation method for brittle-plasticity was assessed in a hydro-fracturing well. It showed better estimations for hydraulic fracture propagation detected by acoustic emission monitoring than the traditional methods.