Rheology of Aqueous Drilling Foam Using a Flow-through Rotational Viscometer

Abstract

Abstract Safe and economical foam drilling requires a good knowledge of foam rheology and hydraulics. Compared with traditional incompressible fluids, foam is a thermodynamically unstable fluid and its rheology is rather complex. Compressibility, quality, liquid and gas phase properties, slippage at the wall, etc., can all affect foam rheology. The true rheology of foam has been hidden in many past foam rheology experiments due to the confounding effects of wall slip, mode of foam generation, foam bubble size and size distribution, etc. This study describes foam rheology experiments conducted using a Foam Generator and Viscometer Apparatus and Process designed and developed at the University of Tulsa (US Patent number US 6,807,849), This process utilizes a flow-through Couette viscometer with roughened cups and rotors at gauge pressure 1.72×105 Pa (25 psig) and temperature 25°C (77°F). This apparatus generates foam with controllable properties and allows the foam to flow through a modified Couette-type rotational viscometer. The flow rate is regulated so that rheology of the foam is determined under constant foam quality (in-situ gas flow volume fraction), pressure and temperature. A visualization cell coupled to an image acquisition device permits structure characterization of the foam in parallel with the rheological measurements. Wall slip and rheological properties of different foams were studied for different rotor and cup surface roughnesses. Results relating to foam rheology are discussed with emphasis on the effects of foam bubble size, quality and wall roughness. Significant rheology differences are observed for different surface roughnesses, that was explained by wall slip phenomena. Introduction Drilling foam is a low density, high viscosity fluid, which provides good cuttings carrying capacity and ECD management capability. The use of foams as drilling fluids has experienced a large growth in underbalanced drilling operations. During underbalanced drilling operations, foam properties, such as density and rheology, need to be properly controlled to obtain the desired equivalent circulation density (ECD). Foam is also used to avoid severe problems of lost circulation in naturally fractured formations or low-pressure zones, which are a major concern with respect to conventional drilling fluids. Foam can also be a good candidate for deep water drilling applications, where the margin between fracture gradient and pore pressure gradient is very small. The major advantage of foam fluid compared with other drilling fluids is that it offers a better control over ECD by varying the liquid injection rate, air injection rate, liquid phase composition, and backpressure at the top of wellbore. In addition, foam can be advantageous in remote areas and areas where logistics are difficult because compared with traditional drilling fluids, foam contains only a small fraction of liquid phase to be treated. Nevertheless, foam is a compressible non-Newtonian fluid with complex structure. The analysis of foam flow behavior is difficult, because many variables such as quality, liquid phase viscosity and wall slip affect its flow behavior. Moreover, foam generation method, shear history, foam texture, and type of surfactant play significant roles in determining the flow behavior of foam. Many efforts have been directed toward the fundamental understanding of the rheology of foam since the early studies on froth and firefighting foams. A variety of standard viscometric instruments and specially fabricated equipment has been developed and used to study foam rheology1,2. A careful examination of these instruments reveals that "foam viscometers" can be classified into two main categories: pressure driven pipe viscometers and rotational viscometers. Rotational viscometers used in foam rheology study include Couette-type viscometers, parallel disk and cone and plate viscometers. One commonly used method to measure foam rheology is to use a pipe viscometer. Designs of pipe viscometers can be quite different, but the basic idea is to pass foam through pipes, and measure frictional pressure loss and volumetric flow rate. The flow in the pipe viscometers should be laminar and under constant temperature and foam quality conditions. By applying conventional viscometric analysis, that is correlating the wall shear stress with appropriate nominal Newtonian wall shear rate, the foam rheology can be determined.