Analysis of wellbore hydraulics is motivated by the need for improved understanding of this system so as to enhance air drilling operation. A viable wellbore hydraulics model is utilized as the basis for this study. Several important parameters that influence air drilling operations are analyzed, such as drill cutting size and size distribution, hole size changes, attrition, and particle shape. This work defines several areas that need special investigation and sheds light on several previously unanswered questions. The most salient findings are that hole size changes, cuttings size and size distribution and particle shape affect the wellbore hydraulics very significantly. The analysis led to the evolution of a clear explanation for choking in air drilling.
The benefits associated with air, as a drilling fluid compared with conventional drilling mud, is well-documented in the literature [e.g., Wilson (1951), Jackson (1961), D'Agostino (1974), Hook et al. (1977), Bowen and Parkhouse (1978), Parkhouse and Teesdale (1982, 1984), Supon and Adewumi (1989)]. Several field cases have been reported where air drilling has been successfully used where either conventional drilling had failed or where drilling costs would have been otherwise prohibitive. Of course, there are a number of limitations to the use of this technique, but wherever suitable, air drilling is very economical and effective. In fact, reliable estimates claim that air drilling could be effectively used for more than 30% of the wells drilled in the United States. Equally true, however, is the fact that where air drilling is currently used, achievement of an optimal drilling program is hampered by a lack of understanding of the wellbore hydraulics involved. This fact was recognized by some of the earliest investigators of this problem [e.g., Angel (1957)]. Various researchers have focused on the single design parameter of the air requirement; nevertheless, the recurring theme is that wellbore hydraulics is a prerequisite for an optimal drilling program.
It is important to note that there are two fundamental differences between wellbore hydraulics associated with air drilling and conventional drilling. The first is the high compressibility of air compared with drilling mud, and the second is the large density difference between air and drill cuttings. These differences preclude the wide experience already acquired for conventional drilling from being directly applied to air drilling.
The major function of the circulating fluid in drilling is to clean the hole and thus effect a good penetration rate. In order to effectively perform this task, air, which is the circulating fluid in air drilling, must be circulated in adequate quantity. On the other hand, circulating excessive air will result in unnecessary additional pressur loss in the wellbore and hence compression power will be wasted. In addition. maximum penetration rate is achieved at minimum bottomhole pressure, which will correspond to the optimum air volumetric flow rate. Furthermore, higher air velocity will impart an unnecessarily high velocity on the particles which could cause faster equipment erosion and hence greater maintenance costs. It is indisputable that the key to achieving the optimal drilling rate is to use an optimal air volumetric flow rate. It has been demonstrated through experiments and field observations that optimal air velocity is dictated by operating and lithological parameters.
Several attempts have been reported in the literature to develop a systematic design procedure for predicting wellbore hydraulics in air drilling, particularly the minimum air flow rate required [Angel (1982), Machado and Ikoku (1982), Gray (1958), Supon and Adewumi(1989)]. The common denominator of these attempts is that they all use empirical or semi-empirical approach. The models presented by these workers lack unifying theory, even though they provide some concrete engineering methods. However, the several limiting assumptions usually made preclude the consideration of important parameters. The first setback in using any of these approaches is that they deal with the air volumetric requirements without due cognizance of the variables involved. Secondly, they all have the lack of generality since they are not physics-based. For instance, Angel's method, which is one of the most commonly used techniques for designing air drilling programs, assumes a constant air velocity of 3000 ft/min for all operating conditions. Several subsequent investigations have found this assumption not to be ideal.