Research On Brittleness Evaluation Of Rock Based On Damage Theory

With proven reserves of oil and gas resources to maintain the time becomes shorter, to ensure sustained and stable production of oil and gas, unconventional oil and gas exploration is the development trend in the future. As a high quality of unconventional oil and gas resources, coal-bed methane, China has very rich reserves, accounting for nearly 13% of the world, however, capacity of coaled methane in China and the United States is almost 20 times. Hydraulic fracturing is the main measure of CBM development at present, can form a complex network of internal cracks in coal seam by fracturing, is the key to improve the yield of coaled gas.Previous studies show that the brittleness of rock on rock drill ability, wellbore stability, rock burst characteristics and reservoir reconstruction is closely related, effective evaluation of brittle rock to guide oil and gas exploration and reservoir reconstruction has important significance. However, its definition is not clear, the evaluation method is different, the lack of unified standards and understanding, there are some theory support. In this paper, by investigating the existing brittleness evaluation methods, analysis of the most recent and commonly used evaluation indicators, combined with damage mechanics theory to establish the evaluation method of rock brittleness. Based on the rock brittleness evaluation method, the structural characteristics and mechanical characteristics of coal are analyzed, and the influence of coal rock structure and surrounding rock on the brittleness of coal is analyzed. The research results are as follows:(1) According to the constitutive characteristics of rocks and the damage characteristics of rock under loading, the re definition of brittleness is the damage characteristics of rock under the combined action of internal and external factors. The damage degree of rock is hard to be damaged, that is, the rock damage rate is fast and the degree is high, and the rock is brittle.(2) To establish a new evaluation method of brittle rock, the rock brittleness evaluation index to characterize the brittle, brittle rock is weak, brittle index tends to zero, more brittle, brittle index is bigger.(3) Does not consider the stress of coal and rock brittle effect under the premise of coal rock in perpendicular to the bedding, parallel to face cleat, showed the highest brittle; parallel to bedding, vertical to face cleat, showed the lowest brittleness. With a cleat system of coal and rock specimens of brittle random distribution, showed no obvious regularity.(4)In the overall distribution of coal and rock brittleness in 0.4~0.8 between coal and rock with confining pressure increases. Among them, perpendicular to bedding, parallel to the surface of the cut of coal and rock structure of brittle rock is greatly reduced, decreased linearly; parallel bedding, vertical in the face cut of coal and rock structure of brittle rock in low stress drop low faster and in stress increasing decreased slowly, exponentially; parallel bedding, vertical to the face cutting of coal and rock structure of brittle rock in low stress decreased slowly, under high stress decreased rapidly and exponentially.