Study On Damage Of Drilling And Unloading Of Shale Gas Reservoir And Its Influence On Wellbore Stability

With the rapid development of the national economy and growing requirements of environmental protection.our demand of natural gas is growing.and the foreign dependency is rising gradually. Exploration and development of shale gas is still at the starting stage, there are many problems to face up with.One of those engineering technical problems which needs to be solved first is how to safely drill long lateral-section horizontal wells.There are many restriction factors of wellbore stability of shale gas reservoir rocks.for example.rock mechanical properties.situ stress environment.construction technology and so on.Studies have shown that shale gas rock has strong brittle characteristic. Brittle rock induced stress and unload seam is vulnerable to drill stress perturbations.caused wellbore structural damage of rock formations. While current domestic and international study of this also grossly inadequate. Thus, solve the problem of shale formations wellbore stability, system-depth study of failure mechanism of brittle shale under the unloading conditions, it has important practical significance to solve wellbore instability problems and achieve safe and efficient development of shale gas reservoirs.it has important practical significance to solve wellbore instability problems and achieve safe and efficient development of shale gas reservoirs.This paper relies on the National Natural Science Foundation of China(U1262209), treat the Lonamaxi Shale rock as the research object, based on loading and unloading indoor rock mechanics experiments, combine calculation of formation collapse pressure, carry out research of the influence of wellbore formation rock mechanical behavior from shale gas formation drill unloading and unload damage effects of wellbore stability. Some knowledge and conclusions are acquired as follows:(1) Tests of uniaxial compression and triaxial compression shows that the shale chosen for this research with high strength and obviously brittle characteristics. This shale has low clay mineral content, and high brittle quartz mineral content.Uniaxial compressive failure showed typical characteristics of brittle splitting failure. Under the loading conditions, the compressive strength of rock samples increased with increasing confining pressure, the rock sample shows plasticity. Failure mode of rock sample by splitting failure gradually become simple shear damage.(2) Based on a large number of loading and unloading mechanical experiments. shale rock exhibit different deformation and failure characteristics during loading and unloading process. It occurs expansion more intense than the loading process of the when shale sample is damaged, showing a stronger brittleness. Low confining pressure on the rock sample unload more sensitive and easier to produce sheets of damage. Radial strain level can reflect the extent of damage in the rock sample unloading process. (3) Unloading rate, unloading margin for dense brittle rock failure characteristics and intensity has a significant effect. It is found in tests of different unloading margin, with unload amplitude increases, the degree of destruction of rock samples showed an increasing trend after the first decrease.the greater the unload degree confining pressure is,the greater the damage to the rock samples, and the sample damage severely. Unloading will significantly exacerbate the damage evolution dense brittle rock and damage. For safety drilling in this formation, it should be fully aware of the other geomechanical environment, infrastructure characteristics, the optimization of drilling speed and fluid density, minimizing drilling rock week caused destruction and wellbore instability due to unloading.(4) Based on geomechanical parameters on the actual work area, the study of the unloading influence on borehole collapse pressure. The result shows that the formation borehole collapse pressure after considering the unloading impact was significantly increased. The use of the mechanical properties of the rock load stability analysis of rock wells week was larger than the actual, especially under the high vitu stress, high formation pressure and high-voltage values of the drilling situation.And based on unload mechanical parameters impact analysis and comparison of Mohr-Coulomb strength criterion and Hoek-Brown strength criterion for formation collapse pressure. Combined with the current actual performance of shale gas drilling projects, it is considered that the calculated collapse pressure based on Hoek-Brown strength criterion more accurately reflect the real situation formations.