Study of a Mechanism for Well Deviation in Air Drilling and Its Control

Air drilling applied in an oilfield of western China shows its advantages in enhancing the rate of penetration (ROP) and oil reservoir protection. However, wells deviate more severely in air drilling than in mud drilling, which constrains the its application. The main factors resulting in well deviation are bottom hole assembly (BHA) and formation. This article analyzes these two factors in air drilling. The analysis shows that there is no significant difference in the behavior of BHA during air drilling and during mud drilling. BHA is not the critical factor that results in more serious well deviation in air drilling. The fundamental reason is that the bottom hole formation character changes in air drilling. Air replacing mud in a bore hole would lead to redistribution of bottom hole stress. Greater anisotropy index of formation in air drilling leads to more serious well deviation, and the increasing invasion depth of bit teeth in air drilling aggravates the well deviation. This article will analyze the above changes quantitatively by describing rock failure of anisotropy formation in air drilling. Based on the mechanism of well deviation, two types of BHA are recommended: air hammer and pendulum assembly with air motor, which have been proven as effective measures of controlling deviation in air drilling in Tarim oilfield.