Study On The Effect Of Bottom - Down Pressure On Rock Strength

In the case of conventional, shallow, oil and gas resources which is easy to exploit are declining, deep oil and gas reservoir is an important field in exploration and development that would become an important part in energy production at present and future. But low drilling rate restrict the effective exploration and development for its poor drillability and high abrasiveness in deep reservoirs. Among all the ways to improve drilling rate, the under balanced and gas drilling technology which can reduce the bottom-hole pressure are superior to improve drilling rate. The purpose of this paper is to study the mechanism which the bottom-hole pressure influence the rock stress field in bottomhole plane, and to study the Rock strength evaluation model which can be adequate for bottomhole-rock, as the basis for analyzing the potential capacity of reducing bottom-hole pressure to improve drilling rate.The crushing of the rock is the result of the interaction by the drill bit and the bottom bottomhole-rock, Bottomhole stress field is an important factor in the decision bit broken rock effect. Investigate effects of stress on the bottomhole-rock in the specific original to stress and different bottomhole pressure conditions with finite element simulation. To consider that reduce the bottomhole pressure can reduce the stress on the bottom of the bottomhole-rock, changing the stress state of rock, which is conducive to the drill bit broken on the rocks. Conclusion:the rock of the bottomhole near the center of the most easily broken, the more close to the borehole wall the greater the stress, in the near side wall zone, about 0.8R(R is the radius of the borehole), there will be a significant stress concentration of broken rock the hardest of the region.Accurately determine the rock suffered confining pressure is an important prerequisite for laboratory testing of rock strength. There are some defects and errors with the original way to determine the pressure on rock. Finite element method as a basis for determing the pressure on bottomhole-rock, it’s calculation results closer to the real confining pressure. Also study on the relationship between the bottomhole-pressure and the density of drilling fluid, results showed that there is a good linear correlation between the two, and get a function expression.Bottom-hole rock strength assessment of deep-hole is important for optimization of drilling method, drilling parameters and design of bit-tool. The paper gives a quantitative comparison of some commonly used rock strength criterias, results showed that Mohr-Coulomb strength criterion and Hoek-Brown strength criterion are not suitable for describing the non-linear strength characteristics of rock under high confining pressure, on the other hand,the exponential type strength criterion has an obvious advantage than others in this respect. It can be used for the description of the rock strength characteristics under different bottomhole pressure. Combined with a function of confining pressure and the density of drilling fluid, the paper gets an rock strength evaluation model with drilling fluid density as the independent variable under the specific geostress conditions.At the present time, the method to obtain the rock strength criterion in laboratory is to test the strength of a number of core samples in different pressure condition, get the outsourcing network line of stress circles, then determine the expression of the strength criterion. But this method cost more cores and money and its efficiency is low. So a new test method is established in this paper, This method can use less core test for the strength criterion parameters, at same time, there are advantages of low cost and high efficiency.Multivariate drilling rate equation model is established in this paper, based on finite element analysis and evaluation modle of downhole rock strength. It can intuitively evaluate the influence of drilling fluid density on ROP, quantitative evaluation the extent or potential to improve the penetration rate by reduceing the density of drilling fluid.The results is important for understanding the mechanics of rock breakage and enriching the basic theory of improving drilling rate. The model of deep well bottom-hole rock strength evaluation can be used in drilling practices to improve the drilling efficiency. It provides theory basis for the study of the most economical, effective drilling and breakage rock technology.