Experimental Research On Rock Breaking By Impacting-cutting Drilling Under High-temp And High-press Condition

Geothermal development has been paid attention to by more and more countries, and has been put into practice. China is rich in geothermal resources of hi-temp rock body. The resources available in Chunteng of Yunnan area and Yangbajin of Tibet area are more than 1.56×1011Kw.a, whose development is of great strategic significance to improving the energy structure of our country and ensuring the energy safety of our country. Since geothermal resources of hi-temp rock body are mainly deposited in the granite, drilling hole in it is necessary to construct the artificial deposit space under hydraulic cracking technique in order to get thermal resources from it. Mechanical manner for drilling hole includes mainly three as follows: impacting, cutting, and impacting plus cutting. Under normal temperature, impacting is generally used to drill the hard rock, and cutting is used to drill the soft one.As far as the drilling engineering is concerned, rock breaking efficiency means the higher drilling speed. However, it is affected most by the property of the rock. As the drilling depth increases during the geothermal drilling process, temperature of rock increases gradually and thermal cracking arises inside the rock, leading to the variation of the rock property. That is, its intensity decreases and its plasticity increases. Study on variation of rock breaking efficiency caused by using different manners with the increase in temperature, and study on which manner can lead to the best cracking effect under different temperatures are of significant engineering meaning.In the drilling process of geothermal development to hi-temp rock body, stability of wall rock is influenced by temperature stress, property of drilling fluid, and virgin rock stress, etc. The wall rock may become neck-closed, distorted and instable, and even wall breaking down under the coupled influence from temperature field, seepage field, and stress field. This shall result in the enormous increase in the cost of drilling and wall maintenance, making the engineering program impossible to carry out.In this paper, crashing rule of granite (φ200mm×400mm) under hi-temp and hi-press (4000 meter depth and 500℃limited) is tested and studied. Through theoretical analysis, variation trend of crashing efficiency under different crashing manners is concluded; relationship between drilling speed, energy cost, and drilling parameter under high temperature is concluded; the best crashing manner under different temperature is advised. Moreover, wall stabilizing principle in hi-temp rock drilling process is discussed; and its wall instability condition is studied. Density of drilling fluid and its fluid loss capacity can greatly influence the stability of the wall, so, this article also discusses the property and the manufacturing manner of the hi-temp drilling fluid.Research and conclusion of this study mainly includes:a. This paper studies the variation of rock-breaking progress against temperature rising at higher surrounding pressure. It also studies the relationship between drilling progress and temperature variation in those three rock-breaking manners at static hydraulic pressure in 4000 meter depth. Through orthogonal experiment on those three rock-breaking manners at different parameter group in a particular temperature condition, the following conclusion can be reached: with the temperature rising, granitite becomes thermal cracking, and drilling progress of those three manners is improved greatly. Meanwhile, impact drilling progress is seldom improved at 150℃above.b. This paper studies unit energy consumption against temperature rising at higher surrounding temperature. It also studies the relationship between unit energy consumption and temperature variation in those three rock-breaking manners at static hydraulic pressure at 4000 meter depth. Through the calculation of power consumption to unit cubic rock, the following conclusion can be reached: with the temperature rising, the intensity of granitite becomes weak, and power consumption of those three manners decreases.c. This paper studies the relationship between drilling progress and drilling parameters under hi-temp and hi-press condition. Through the analysis of experiment, the following conclusion can be reached: under the condition of static hydraulic pressure at 4000 meter depth, and temperature of 300℃, but a limited drilling weight range, drilling progress of those three rock-breaking manners is improved with the increase in drilling weight, impact power, and drilling revolution; simply increasing drilling weight or impact power can't improve the drilling progress effectively.d. This paper studies the relationship between drilling parameter and unit power consumption at hi-temp and hi-press condition. Through the analysis of experiment data, the following conclusion can be reached: under the condition of static hydraulic pressure at 4000 meter depth, and temperature of 300℃, power consumption of those three rock-breaking manners decreases with the increase in drilling weight; power consumption in cutting manner increases with the increase in revolution; power consumption in cutting plus impacting manner decreases with the increase in impact power.e. Impacting drilling technique is available in lower temperature (within 150℃) for hard rock, and is not available in higher temperature condition. Cutting drilling technique is available in higher temperature (300℃above) condition, whereas the hot-endurable pit and drilling fluid discharging is required. Cutting plus impacting technique can achieve a better drilling progress under temperature range of 150℃to 300℃, and the harder rock condition.f. This paper describes wall satiability principle in drilling operation. Through the experiment under condition of static hydraulic pressure at 4000 meter depth, and temperature of 300℃, the following conclusion can be reached: the granite creeps at a faster rate, and is obviously damaged under 500℃condition. g. The property and the manufacturing manner of the hi-temp drilling fluid is discussed in this article. Through the test developed high-density non-polar polysulfide drilling fluid saturation owe salt in 220 degrees Celsius can achieve good under the liquidity and smaller filtration.