| Comparing to conventional fluid drilling, Gas drilling as a rapid developed under-balanced pressure drilling technology, has great advantages with improving the penetration rate, preventing leakage, protecting reservoirs from pollution and reducing the total drilling cost.At the same time, auxiliary equipment and techniques are constantly improving. So gas drilling technology has been applied more and more widely at home and aboard, and is playing a more and more important role in oil exploration and development. But, on the one hand, research on gas drilling in the domestic start lately, research on characteristics of the circulation system pressure distribution, fluid flow characteristics and optimization design of drilling parameters is not proficient. On the other hand, most of our equipment import from abroad and the localization rate is low, dependence on foreign technology is strong. This not only increases the cost of drilling, but also put us in a position of the enslaved. Therefore, strengthening the basic theory of gas drilling research and related equipment localization is the top priority of the drilling industry. The purpose of this paper is to provide validation in theory of the main parameters in gas drilling through the further study of gas drilling theories.The key technology of gas drilling is the proper design of the gas injection volume, to clean bottom hole and carry cuttings with least gas, and ensure the carry efficiency and drilling safety. So, research on wellbore pressure system and the injection volume becomes the key object in the process of gas drilling.This paper, based on making a lot of investigation and theory analysis, finally defines Angel’s mixed fluid theory as a foundation, divides the hole orbit into vertical section, bending section and stable inclination section, establishes the model respectively and analyzes the distribution of wellbore stress, gas velocity and density. Through the force analysis of cuttings in annual, the migration rule of cuttings is deeply understood. The minimum gas volume and injection pressure is determined based on the requirements of carrying cuttings at the key point. In the paper, the Angel’s model is optimized, gas compression characteristics and slippage between debris and gas is considered, and the optimized model is more in line with the actual theoretically. Finally, a software which is compiled by Visual Basic programming language is used to make an example analysis. Following issues are discussed:the circulation system pressure in the wellbore, the theory of minimum volume, the origin and the characteristics, the principle of selecting cuttings size, advantages and disadvantages of gas drilling and application range, the layout of the well site, the influential factors of safe drilling, the disposal of abnormal conditions during drilling, etc. |
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