Study On Shale Gas Reservoirs Formation Damage Mechanisms

With the rapid and persistent development of economy in china, the industrialization, urbanization and modernization are accelerating. In 2009, the oil dependence exceeded 50%, a warning limit, for the first time, which meant that our country’s petroleum risk intensified. In order to alleviate this situation, it is very necessary to take some unconventional resources as supplements or replacements. These unconventional resources include coal bed methane, shale gas and tight sand gas. In china, the shale gas resource is very abundant. For example, there is about 6.8 to 8.5 trillion stere of shale gas in Sichuan basin. Several companies did trial in Yangzi area to develop shale gas, but their work didn’t break through, for one important reason was the formation damage in the drilling and hydraulic sand fracturing. So that, to study the formation damage theory and the technology of reservoir protection for shale gas exploitation and exploration is very significant. The technology of reservoir protection can contribute to improving the single well production and the whole shale gas exploitation.This paper takes the shale in Xvjiahe formation of Eastern and Western Sichuan as an example. It analyzes the diagenesis of shale, and studies the shale gas accumulation mechanism, storage space, porosity and permeability characteristics, fluid occurrence mechanism, potential damage factors and liquid intrusion laws. According to SEM,XRD, stereo microscope and specific surface test, it analyzes pore and fracture characters and clay mineral of shale. The result indicates that, there are micro-fractures in the shale, that the main mineral is quartz, that the shale in Western Sichuan contains carbonate while the shale in Eastern Sichuan contains little carbonate, and that the clay mineral is consisted of most illite and little montmorillonite and mixed-layers. The study reveals that the types of formation damage in shale are the water lock effect after liquid phase invasion and the stress sensitivity of fractures.Moreover, this paper analyzes the mechanism of water-phase invasion under over-balanced, near-balanced and under-balanced conditions. Based on the theory, a mathematical model was built to simulate liquid-phase invasion in shale. Then, aiming at different pressure and time condition, this paper uses the model to analyze the seepage space, pressure field and seepage field of liquid phase invasion. The result shows that the liquid invasion will deepen over time and differential pressure, and the invasive liquid is able to form strong water lock area along the fracture plane. This damage by liquid invasion is irreversible.