Air drilling has many advantages over mud drilling in increasing penetration rate, decreasing formation damage, eliminating lost circulation, reducing pressure differential sticking and so on. Hole cleaning and pipe erosion failure are the two key problems for air drilling. Hence proper injecting parameters should not only meet the requirement of carrying cuttings, but also avoid the erosion of drillpipe. For example, if the standpipe pressure was much larger, high-speed air flow with cuttings would cause the erosion of drillpipe and destroy its internal coating, which is the main reason of shorting service time of drillpipe. Other parameters such as drillpipe rotational speed, borehole deviation, eccentricity, cutting size and cutting density also affect the cuttings transportation and pipe erosion. Previous published papers are limited to physical model dimensions and operating parameters, or no exact calculation results due to too many assumptions.
In this paper, a model of gas-solid two-phase flow was built and a new approach to determine the injecting parameters was introduced. Based on the computational fluid dynamics and aerodynamics theory, the flow properties in 311.5mm wellbore, 1500m long, with 127mm drillpipe had been simulated with variable parameters. Then the distributions of pressure, velocity and cuttings volumetric concentration were obtained in drillstring and annulus. By analyzing multi-group calculation results, injecting parameters were optimized to meet the needs of the drilling job. Calculation results coincided well with the actual field data, and had been used to guide air drilling in Da-Wan 3 well, Lao-Jun 1 well, and Qing-Xi 1 well for Chuanqing Drilling and Exploration Corp.