Volumetric Requirements for Foam and Mist Drilling Operations

Abstract

Summary State-of-the-art foam and mist drilling suggests a need for predictive models for volumetric requirements that properly account for predictive models for volumetric requirements that properly account for frictional losses caused by the solid phase in solids/foam slurry flow, settling velocities of such solids, and pressure drop across bit nozzles during foam flow. The objective of this paper is to fulfill this need. A model that predicts pressure drop across bit nozzles for foam and mist is presented. It accounts for the compressibility of foam but assumes negligible pressure losses resulting from friction and change in elevation. A model has been developed for predicting minimum volumetric requirements for foam and mist drilling operations. It accounts for the frictional losses caused by the solid phase, pressure drop across bit nozzles, and particle-settling velocity. The technique offers a high degree of flexibility in the selection of wellhead injection pressures and volumetric injection rates. Field application of this work can be accomplished by two primarily graphical methods that depend on compressor specifications: variable-backpressure and constant-backpressure schedules. Charts are presented for 7.875- and 9.00-in. [20.0- and 22.9-cm] hole sizes, and for presented for 7.875- and 9.00-in. [20.0- and 22.9-cm] hole sizes, and for 0.500-, 0.75-, and 1.00-in. [1.27-, 1.91-, and 2.54-cm] cutting sizes. Penetration rates range from 30 to 90 ft/hr [9 to 27 m/h]. Penetration rates range from 30 to 90 ft/hr [9 to 27 m/h]. Results indicate that volumetric requirements increase with increasing hole size, depth, and particle size. Increases in penetration rate cause only minor increases in volumetric requirements. All foam-drilling and well-cleanout operations can be accomplished within the laminar flow region with adherence to 0.55 minimum bottomhole and 0.96 maximum annular foam quality. Annular backpressures greater than atmospheric pressure are needed to maintain a bottomhole foam quality of 0.55 or more while reaching reasonable depths. To maintain constant depth as backpressure increases, however, both wellhead injection pressure and gas injection rate must be increased, and liquid flow rate decreased. Introduction Foam is defined here as a fluid that consists of tap water, a surface-active agent, and air. The aqueous solution of water and the surface-active agent constitutes the continuous phase, with air appearing as discontinuous bubbles. On the other hand, mist is defined as a fluid that comprises identical components; air is the continuous phase, and aqueous solution appears as discontinuous droplets. In rotary drilling operations, other gases-such as nitrogen, natural gas, CO2, and inert gases derived from engine exhausts-are used sometimes in lieu of air. CO2, however, produces a foam of poor stability because of its high solubility and very reactive nature. 1 Additives such as polymers, graphite, and asphalts can be added to the foam solution as required, as stabilizers, corrosion inhibitors, shale inhibitors, and lubricants. However, most preformed (i.e., generated in the absence of solid and preformed (i.e., generated in the absence of solid and liquid contaminants naturally found in wells) stable-foam drilling and cleanout operations are done with simple foamer and water. Foam may be generated either at the injection point, which is known as in-situ generation, or by passing the various fluid components through a porous medium or coiled tubing generator. Foam must be preformed to be used effectively as a circulation medium. A preformed stable foam is able to withstand much more contamination than one that is generated in situ. Terms that generally apply to foam are foam quality, which is defined as the ratio of gas volume to the total volume, and foam texture, which is related to the size and distribution of the gas bubbles. Fine foams have small bubbles, while coarse foams consist of large bubbles. Low-quality foams are referred to as wet foams, while high-quality foams are called dry foams. Gas and liquid volumes, injection pressure, and annulus backpressure must be controlled in practical foam application in rotary drilling. Foam must lift all solids and liquid from the borehole without breaking down because of a simple water or slug flow or without eroding the borehole because of excessive flow velocity. The carrying capacity of any fluid, however, is a function of its flow velocity, density, and rheological attributes. The volume and injection pressure requirements to drill economically and safety to a certain depth at some penetration rates are functions of the fluid's carrying capacity. A clear understanding of the rheology of foams and lift mechanisms of solid/liquid systems is important to predict various operating parameters for foam drilling. Frictional losses caused by the solid phase and the settling velocities of such solids must be taken into account to predict minimum air and liquid volumetric requirements accurately for foam drilling operations. SPEDE P. 71