Study On Influencing Factors And Evaluation Methods Of Rock Brittleness In Shale Gas

As a new type of clean energy, the development value of shale gas attracts increasing attention. The main properties of shale gas layer-tight, low porosity and ultra-low permeability, makes it hard to produce gas flow huge enough for industrial production. So the long horizontal well technique and effective fracturing stimulation have to be applied to improve the permeable condition for commercial development. Researches show that the brittleness is the key standard of rock compressibility and reservoir sweet spot assessment. Brittleness also affects rock drilling ability and borehole stability. Besides it has influence on device availability and the selection of proper mud density. Hence, increasing attention has been paid to the study of shale gas layer brittleness domestic and overseas and the research is still active. Most of the nowadays brittleness assessment methodologies are proposed for scholars’own study aim. So far, there’s no uniformed methodology.This paper is based on the research of existing assessment methodologies of brittleness, taking the shale gas layer of Longmaxi Formation of Sichuan Basin as object of the study. After analyzing different methodologies with studies of related experiments, a new assessment methodology, based on compressive test, has been explored. With the combination of numerical simulation and physical experiments, the effective factors of brittleness have been studied. Besides, two methodologies for field prediction have been proposed. One is based on rock mineral constituent. The other one is based on petrophysical parameters, porosity, density and elasticity modulus. Several understandings have been reached through the study:(1) The paper suggests a new brittle index to characterize brittleness, the secant modulus of peak point of stress-strain curves. The tests show that low strain with high strength at peak point means the rock has high brittleness. According to the method, the paper also proposed the rank scheme of brittleness.(2) The paper analyzes the effects of rock beddings, strength parameters, confiding pressure and work liquid on rock brittleness. With the bedding angle increasing, the brittleness first decreases then increases, and the brittleness has the lowest value at60°; the brittleness increases with the interval between bedding adding; the brittleness increases with the increase of strength reduction factor. The brittle index has a linear positive correlation with elasticity modulus. While the relation between the index and Poison ratio is not clear, so is the relation between the index and the peak strength. The index decreases with the increase of confining pressure. And the index decreases after soaking in water and working solution.(3) Several components’contents of Longmaxi shale-clay, quartz and pyrite, have good relation with the brittle index, while feldspar and carbonate are not. Based on above, using multiple regression, the paper establishes a brittleness prediction method with rock mineral components. Besides, as the density, porosity and elasticity modulus have good relation with brittleness index, the paper also established a prediction method using the three parameters. Based on the log data of Longmaxi Formation, the paper also verified the two prediction models. Results show that the prediction models are practical.The paper is supported by the project of National Natural Science Foundation of China Petrochemical joint fund key projects Basic research of low-cost and efficient drilling and completion techniques of shale gas (U1262209) and the project of National Natural Science Foundation of China Prime Rock Mechanics Study of Well Fracture Form Regulation and Control of Hard Brittle Shale Formation (51274172).