Multiphase Hydrodynamic Model Predicts Important Phenomena In Air Drilling Hydraulics

Abstract The problem of wellbore hydraulics in drilling operations poses an intractable problem to the drilling engineer attempting to design a drilling program. Intricate interactions between the drill cuttings, the transport fluid (be it drilling mud or air), the wellbore and the drill string constitute the source of the difficulties. Lack of understanding of the physics involved coupled with the lack of fundamental descriptive capability, inhibits the development of appropriate predictive capability. This problem is more apparent in the case of air drilling since only a limited amount of data is available on which empirical correlations can be based. A systematic study of this problem especially utilizing a fundamental approach is lacking. This study addresses this important problem using a fundamental hydrodynamic multiphase flow model. The model incorporates the fundamental physics involved in the pneumatic transportation of solid cuttings in the drill string-wellbore annulus. This model forms the basis for a predictive tool for the optimal lifting velocity, an essential ingredient in the optimal design of the air drilling program. Drilling engineers often experience frustration due to lack of models with adequate predictive capability to help generate this basic design parameter. Available correlations are at best gross approximation and more importantly, they woefully fail to account for the physical phenomena that are observed in pneumatic conveying involved in air drilling such as choking, clumping, etc. We present a fundamental wellbore hydraulics model based on the understanding of the physics involved in the pneumatic transport of solid cuttings in the drill-string/wellbore annulus. The model accommodates non-uniformity in particle sizes. Extensive parametric analysis of the system is performed to explore the predictability of some of the phenomena associated with air drilling. The model is demonstrably capable of predicting the pressure drop profile in the annulus under various simulated drilling conditions. In addition, results demonstrate the capability of the model in being able to predict a number of phenomena that are associated with lifting cuttings out of the role during air drilling. Model prediction shows very good agreement with experimental data. Finally, the model possesses good scale up capability. Introduction Drilling with air in place of mud has several advantages including significantly higher penetration rate, substantial savings in cost and rig-up time, and elimination of some of the problems commonly associated with mud drilling such as lost circulation, etc. These advantages have made very a significant difference in several field cases which are well documented in the literature [ e.g. Bowen and Parkhouse (1978), D'Agostino (1974), Hook et. a1. (l977 a, b, c), Parkhouse and Teesdale (1984) ]. The benefits associated with air drilling in comparison with conventional mud drilling are well documented and its potential is even more far reaching if this technology can be effectively utilized. Even though the technology of air drilling is not new, its hydraulics are radically different from those of conventional mud drilling. While extensive technical information is available for mud wellbore hydraulics and significant advances have been made in its synthesis, the same cannot be said of air drilling hydraulics.