The Stability Prediction Of Boreholhe After Laser Breaked Rocks

Laser rock-breaking technology is expected to be applied to petroleum exploration and production operation in the vertical drilling, horizontal drilling, casing window, well completion and other advanced engineering. The world’s industrialized countries are currently engaged in the race to research the laser theory of breaking rock and related art.The basic principle of laser rock breaking is the use of high-engergy beam rapid heating the localized rock, so the rock-solid pahse instantly transition to the state of melt and vaporize and form the heterogeneous mixture of gas, liquid as well as solid,which would be carried and exclulded by the high-speed assisted air flow or othewise.After laser drilling, borehole temperature due to the formation of the role of heat generated by the laser, whihc resulting in the changes of rock mechanical properties and borehole stress,thus affecting the borehole stability when laser drilling.In this paper, the thermal-mechanical coupled equations of laser rock breakking was establised action based on the basis of many research literatures and the thermal stress in elasticity theory. Besides, depended on the deep rock experimental data of Tahe Oilfied, as well as the Coulomb-Mohr strength theory as a criterion,using ANSYS software to predict the wellbore stability.The main results and knowledge obtained as follows:(1) Generally speaking, the specific energy was used to measure different rock-breaking efficiency during laser rock breaking.From the perspective of oil industry,the main practical value of lasers are eight species.However, the fiber lasers become the first choice during the downhole rock breaking because it has small size, does not need maintenance, easy to spread by underground cable and the advantages of transmission efficiency. Through the exprimental data analysis about the impact of laser beam density, laser frequency, laser irradiation time and laser power to laser rock-breaking efficiency which indicated that the key factors of affecting the success of laser rock breaking is laser irradiation time and laser power.(2) After laser drilling, the laser light energy covert into heat energy.The physical and mechanical properties of rocks have changed under the effect of temperature.In the analysis of the Elastic modulus, Poisson’s ratio, thermal conductivity, thermal expansion coefficient of sandstone at different temperatures,which obtained that the rock mechanical parameters are very different under various temperature.So,the temperature on the rocks physical and mechanical properties can’t be ignored in the actual engineering calculations.(3) Compared with conventional drilling,due to the heat and the heat exchange (4) between the surrounding wall of the well section of laser drilling,thereby causing the changed temperature of surrounding rock,which will result in additional thermal stress, and thus changes in stress distribution around the borehole.In this paper, the laser rock breaking temkperature field model was built based on the thermal stress elasticity theory and by using the finite element method, and the wellbore mechanics equations was established through a comprehensive consideration of suti stress-drilling circulating medium-temperature of this three combined effect on the wall.(5) The deep rocks of formations are in the three-dimensional stress state, the destruction of rock often associated with the value and ratio of three principal stresses.Therefore, choosing a suitable faillure criteria for various stress states and to meet the actual needs of the project is essential. When the drilling medium is a gas in the laser drilling, so the laser drilling column pressure is approximately zero, and there is only borehole collapse phenomenon in the process and without wellbore rupture. Currently, there are many wellbore strength criterias, because that the Mohr-Coulomb strength theory not only can reflect the brittle fracture of rock, but also reflect the characteristics of plastic failure, and the predicted results are also considered to be the most realistic, so it should be selected to judge whether the wellbore is instability.(6) The rock suffered situ stress which could be expressed with thress main ground stress, they are a vertical main stress, and the two mutually perpendicular horizontal stress which are called the maximum horizontal stress and minimum horizontal stress.The establish of situ stress model is the key to accurately predict the wellbore stability, based on the15wells of yield fracturing data of Tahe Oilfield and the deep stress model was established in this paper.(7) To evaluate the wellbore stability by using the ANSYS software to establish the temperature-stress finite element model in the laser drilling, analysis and consideration of the effects of different temperatures on the wellbore of rock breaking,and arrive at the distribution of main stress and temprature under different temperatures.Meanwhile according to Mohr-Coulomb criterion to predicte the stability at a depth H=5148m.