Experimental Study On Mechanical Properties Of Rock Under Three - Axis Confining Pressure

Our country is rich in deep oil and gas resources, and the important problems which the development of deep resources facing with are related to rock mechanics. In most oil engineering, we must know whether rocks had been damaged under a stress state, such as well wall stability, formation sand production, bits break rocks and so on. Strength criteria are fundamental theories that used to describe the function of stress state and rock strength and judge whether rocks had been broken under such stress state. In order to solve the rock mechanics problems that the development of deep resources facing with and provide sufficient scientific support of theory for engineering, it needs to find enough accurate mathematical models of rock strength criteria, it also needs enough homogeneous rock samples to establish the function relation. But the particularity of petroleum engineering is the hole size is very limited, it is hardly to get enough samples as ground engineering to satisfy the strength criteria. The lack of rock samples has become the technology bottleneck of science establishing strength criteria.The paper compares and analyzes four kinds of common strength criteria in the oil and gas well engineering through the literature data and experimental data. And the conclusion is Mohr-Coulomb criterion and Hoek-Brown criterion are not suitable for describing nonlinear strength characteristics of rock under high confining pressure, but exponential strength criterion with three parameters has obvious advantages in this respect and it can describe the strength of rock characteristics under different bottom hole pressure. The power function criterion of three parameters is also very good in accuracy, but because of its physical meanings of parameters are not clear, so the application of the power function criterion needed further study.In view of lack of sample in petroleum engineering, using the method of mathematical statistics, trying to find a method of establishing rock strength criteria accurately to reduce sample from the test means. The study found that reducing test times scientifically could meet the requirements for rule of rock strength parameters to a certain extent, the key is to selecting confining pressure level and understanding of some basic rules of rock strength. But because of many types of rock, rock heterogeneity stronger, and complex mechanical properties, there are still a lot of research work to do to find its basic rule.Single sample test is a kind of experimental methods that has got approved by the ISRM. Because of the process of experiment is difficult compared with normal experiments. So it does not have a wide range of popularization. Through the analysis of the literature data and conducting conventional triaxial compression and single sample multistage continuous load contrast experiment, we learn that single sample method loading stop point is difficult to control, and high confining pressure is difficult to load on the single sample test. The experimental results have great influence on the determination of strength criterion parameters. Single sample experiment intensity point is subdued point, it is below the breaking point, the strength criterion established by the subdued point is conservative. The key problem is the relation between subdued point and breaking point is not clear. The research of changing subdued point strength to breaking point strength to be continued.Through the comparison and analysis of rock mechanics strength criteria, and the conducting of multi-level continuous loading of the single sample, the achievements can be a very valuable reference to the improvement of strength criteria, the simplify of experimental process and the cost of the experiments can be cut down.