Multiphase Hydrodynamic Model Predicts Important Phenomena in Air-Drilling Hydraulics

Abstract

Multiphase Hydrodynamic Model Predicts Important Phenomena Predicts Important Phenomena in Air-Drilling Hydraulics Summary. Wellbore hydraulics poses difficult problems for the drilling engineer designing a drilling program. Intricate interactions between the drill cuttings, the transport fluid (mud or air), the wellbore, and the drillstring cause the difficulties. Lack of understanding of the physics involved and the lack of a fundamental descriptive capability physics involved and the lack of a fundamental descriptive capability inhibit the development of an appropriate predictive model. This problem is more apparent in air drilling because only limited data are available on which empirical correlations can be based. This paper addresses wellbore hydraulics with a fundamental hydrodynamic multiphase-flow model. The model incorporates the fundamental physics involved in the pneumatic transportation of solids cuttings in the drillstring/wellbore annulus. This model forms the basis for a predictive tool for the optimal lifting velocity, an essential ingredient in the optimal design of an air-drilling program. Available correlations are, at best, gross approximations; more program. Available correlations are, at best, gross approximations; more important, they fail to account for the physical phenomena observed in the pneumatic transport involved in air drilling (e.g., choking and clumping). pneumatic transport involved in air drilling (e.g., choking and clumping). The model accommodates nonuniformity in particle sizes. Extensive parametric analysis of the system explores the predictability of some of parametric analysis of the system explores the predictability of some of the phenomena associated with air drilling. The model is capable of predicting the pressure-drop profile in the annulus under various simulated predicting the pressure-drop profile in the annulus under various simulated drilling conditions. Also, results demonstrate that the model can predict many phenomena associated with lifting cuttings out of the hole during air drilling. Model prediction shows very good agreement with experimental data. Finally, the model possesses good scale-up capability. Introduction Drilling with air instead of mud has several advantages, including significantly higher penetration rates, substantial savings in cost and rig-up time, and elimination of problems common in mud drilling (e.g., lost circulation). These advantages have made significant differences in several field cases. The benefits of air drilling are well documented; its potential is even more far reaching if the technology is used effectively. Even though air drilling is not new, its hydraulics is radically different from that of conventional mud drilling, thus preventing direct application of the experience accumulated for mud drilling to air drilling. Fundamental technical information on air-drilling hydraulics is scarce, even though the technology is widely used in the eastern U.S. to drill oil and gas wells and in the western U.S. for geothermal wells. In fact, some claim that air drilling could be used effectively for more than 30% of the wells drilled in the U.S., resulting in tremendous cost and time savings. The major block to its acceptance and widespread use is the absence of systematic predictive and design procedures. Rules of thumb and empiricism are still the dominant methods used by engineers to design air drilling programs. The only systematic approach used to design air drilling programs is Angel's procedure. Unfortunately, this procedure, because of several simplifying assumptions made in the theory development, cannot predict the phenomena inherent in air drilling and the optimal design parameters. We think this lack of a fundamental systematic approach is the result of a lack of understanding of the wellbore hydraulics of air drilling. Several investigators have examined this problem. More recently, researchers began new studies to answer many unanswered questions, to understand the physics involved, and to create a rational basis for the development physics involved, and to create a rational basis for the development of a fundamental predictive strategy. Air drilling is used to drill wells in hard, relatively dry, formations. As the circulating fluid, air performs all the traditional functions of drilling mud, particularly hole cleaning. The importance of hole cleaning cannot be overstated in determining drilling efficiency. For air, or any drilling fluid, to be effective in hole cleaning, it must be circulated in adequate quantities. On the other hand, circulating more fluid than required causes unnecessary additional compression or pump load. The key to realizing a cost reduction is to determine optimal fluid volumetric flow rate. One main drawback of Angel's method (and its offspring) is that it assumes a constant air velocity of 3,000 ft/min (915 m/min] for all operating conditions. Experimental measurements and field observations have demonstrated that the optimal cuttings lifting velocity is not constant but dictated by operating and lithological parameters. Note that Machado and Ikoku's and Supon and Adewumi's models are empirical and thus may not be generalized. Also, Supon observed several phenomena that cannot be explained or predicted by existing models. This problem and the need for a predicted by existing models. This problem and the need for a fundamental physics-based predictive/design model for wellbore hydraulics in air drilling were the impetus for this work. A multiphase hydrodynamic approach is used to describe air-drilling wellbore hydraulics. Basically, wellbore hydraulics is dictated by the intricacies of particulate transport in the drillstring/wellbore annulus. Hydrodynamic Approach Adewumi and Tian applied multiphase-hydrodynamics theory to simulate air-drilling wellbore hydraulics. The encouraging results led to a thorough system analysis. The analyses reveal some interesting phenomena that should be important in the design of an optimal air-drilling program. A brief review of the salient elements of the basic framework for the hydrodynamic description of air-drilling wellbore hydraulics provides a focal point for our discussion. Hydrodynamic Variables. The main difference between air and conventional drilling is that in air drilling air is used as the circulating fluid instead of mud, a mixture of liquids and several solid additives. One major purpose of this circulating fluid is the transportation of solids cuttings from the bottom of the hole to the surface (i.e., the hole cleaning). The basic practical design variables for an air-drilling program include minimum air volumetric flow rate and power requirements. Understanding air-drilling wellbore hydraulics depends on an understanding of pneumatic transport of solid particles in the wellbore. Several independent variables that characterize the solids transport in the wellbore must be considered:the physical properties of the drill cuttings, including size and size distribution, properties of the drill cuttings, including size and size distribution, shape, and density;thermophysical properties of the transporting fluid, such as density and viscosity;penetration rate, because it determines the feed rate of solids cuttings or solids loading;the geometrical configuration of the wellbore/drillstring annulus and sizes of the various components; andfluid transport velocity. The basic design parameters dictated by these independent variables include optimal transport velocity and associated pressure loss. Also, the ability to predict choking phenomenon is needed because of its importance in this system. Adewumi and Tian developed a hydrodynamic approach that considered all these variables. SPEDE P. 145

Enhancing cuttings removal with gas blasts while drilling on Mars

“…Flushing of cuttings with a continuous flow of gas has been modeled quite successful before by Tian and Adewumi [1991, 1992], Temple et al [1996], and Puon and Ameri [1984]. These models were based on the empirical equations developed by studying the actual flow of gas in the controlled environment.…”