New evaluation methods using conventional logging data for gas shale in southern China

Abstract

Because of characteristics of compactness, rich in organic matter and gas adsorption, the parameters that decide shale quality are not only porosity and permeability but also the abundance of organic matter, mineral composition, crack, and mechanical properties. Logging-evaluation methods of conventional reservoirs are not valid in gas shale. Advanced technology of core measurements can obtain more accurate parameters of gas shale. However, because it is restricted by heterogeneity and the high price of core, this method cannot be used widely to develop a continuous section. Based on conventional wireline logging data, the natural properties of rocks underground are calculated using the improved evaluation model, including mineral composition, clay constituents, total-organic-carbon (TOC) content, porosity, permeability, crack porosity, and mineral brittleness index. Moreover, based on the rock-physics model with rich organic matter, elastic properties are achieved to calculate mechanical properties, which include maximum and minimum horizontal principal stress, pore pressure, fracture pressure, differential horizontal-stress ratio (DHSR), and elastic brittleness index. By combining those calculated parameters with characteristics of high-quality gas shale, the target formation which is suitable for hydraulic-fracturing stimulation can be determined. The predicted results in a well in the Jiannan area of southern China prove the feasibility of the methods.