Determination and prediction of thermodynamically-controlled micro-fracture zones in organic-matter rich rocks

Abstract

Abstract Temperature, organic material properties, and in situ stresses are first order factors in micro-fracture zone determination and migration prediction in organic-rich strata. High temperatures acting on organic material lead to gas generation, and this can lead to high pore pressures. This high pore pressure generates a condition where the least effective stress become zero; then, pressures exceed the critical stress intensity of the rocks and lead to the propagation of various flaws and defects which develop into micro-fractures or a micro-fracture network. This thermodynamically-induced fracturing process takes place over a long period of geological time; therefore, to study thermal generation and fluid migration history, knowledge of pressure and in situ stress evolution over time is necessary. Thermal history in this study was obtained by using low-temperature geo-thermometers such as vitrinite reflectance and fluid inclusion response. Current formation temperatures were collected from downhole temperature data. FEM simulation was used to generate the thermal stratification sections based on the above data. Acoustic emission (AE) during loading was used to estimate peak stresses that the strata had experienced. Identification of paleo-stresses and present in situ stress required knowledge of geological loading history, aided by features of drilling performance. Fluid inclusion homogeneous pressure was used to provide paleo-pore-pressure determination. This method was successfully used to determine the fracture zones for Xingouzhui strata, an Eocene Age formation in the Tuoshi Oil Field of central China.