Summary The use of drilling foams is increasing because foams exhibit properties that are desirable in many drilling operations. A good knowledge of cuttings transport efficiency under downhole conditions is essential for safe and economical foam drilling. Previous cuttings transport studies with foam are limited to low pressure and ambient temperature conditions. No experimental study has been conducted under downhole (i.e., elevated pressure and temperature) conditions. This paper presents an experimental study of cuttings transport with foam in a horizontal annulus under simulated downhole conditions. Experiments were conducted to determine the effects of polymer additives, foam quality, flow velocity, temperature, and pressure on foam cuttings transport. Experiments were carried out at elevated pressures (100 to 400 psi) and temperature (80 to 170°F) conditions in a unique full-scale flow loop with a 73-ft long test section (5.76×3.5 in. concentric annulus). A field-tested commercial foam system consisting of surfactant (1% v/v) and hydroxylethylcellulose polymer (HEC) was used in the experiments. Three different polymer concentrations (0.0, 0.25, and 0.5% v/v) were tested. Foam quality was varied from 70 to 90%. During a test, cuttings were injected continuously to the flow loop until a steady state condition was established in the test section. In-situ cuttings volumetric concentration (i.e., the ratio of the volume of cuttings in the suspension and the cuttings bed to the volume of the annulus) in the test section was determined using nuclear densitometers, load cell measurements, and by weighing cuttings flushed out of the flow loop. Test parameters recorded during the experiments were: liquid and gas injection rates, cuttings weight in injection and removal towers, mixture density, friction pressure loss, and pressure and temperature in the annulus. Two flow patterns, stationary cuttings bed, and fully suspended flow, were observed during the cuttings transport tests. The flow pattern depends on polymer concentration, foam quality, and annular velocity. Annular flow velocity, foam quality, and polymer concentration all affect cuttings transport efficiency and frictional pressure loss. This paper will help to better design foam drilling and cleanup operations.

Summary An experimental investigation on polymer-based drilling foams was carried out. Rheology tests were performed with foams that have different concentrations of hydroxylethylcellulose (HEC) and 1% commercial surfactant. Experiments were conducted in a large-scale flow loop that permits foam flow through 2-, 3-, and 4-in. pipe sections, and a 6×3.5-in. annular section. During the experiments, frictional pressure losses across the pipe and annular sections were measured for different gas/liquid flow rates, polymer concentrations (0, 0.25, and 0.5%), and foam qualities (70, 80, and 90%). Significant rheological variations were observed between aqueous foams containing no polymers and polymer-thickened foams. Experimental data show three distinct flow curves for the 2-, 3-, and 4-in. pipe sections, which indicates the presence of wall slip. The Oldroyd-Jastrzebski approach was used to calculate the wall slip velocity and determine the true shear rate. It has been found that wall slip decreases as the foam quality or polymer concentration increases. Two foam hydraulic models, which use slip-corrected and slip-uncorrected rheological parameters, have been proposed. These models are applicable for predicting pressure loss in pipes and annuli. Model predictions for the annular test section are compared with the measured data. A satisfactory agreement between the model predictions and measured data is obtained. This paper will help to better design foam drilling and cleanup operations. Introduction The use of drilling foams is increasing because foams exhibit properties that are desirable in many drilling operations. In practice, aqueous and polymer-based foams have been used with commercial success. However, drilling-foam rheology and hydraulics are still not sufficiently understood to minimize the risk and costs associated with foam drilling. It is generally accepted that the addition of polymers to the liquid phase affects the viscosity and stability of foams. However, the degree to which the bulk properties of drilling foams are enhanced by polymers has not been well understood and is difficult to predict. For safe and economical foam drilling, accurate knowledge of bottomhole pressure is essential. However, foam rheology and pressure drop predictions are not accurate enough to provide adequate hydraulic design information such as equivalent circulation density. This problem is more pronounced when polymers are added, because the apparent foam viscosity of polymer-thickened foams can be significantly higher than aqueous foams. It becomes apparent that there is a need for polymer foam rheological characterization in order to improve the knowledge of foam rheology and hydraulics. Foam rheological characterization was carried out using large-scale, single-pass pipe viscometers (composed of 2-, 3-, and 4-in. pipe sections). Foam qualities were varied from 70 to 90%. Test pressure and temperature were 100 psig and 80°F. Two foam hydraulic models were considered, assuming both no-slip condition at the wall and slip condition at the wall. The first model assumes no-slip boundary conditions in both pipes and annulus. By assuming no slip condition at the wall, slip-uncorrected foam rheological parameters were obtained from the pipe viscometer measurements. It has been found that if we plot friction factors vs. Reynolds numbers for all test data, regardless of pipe diameters, foam qualities, and flow rates, a single curve is obtained. This curve is similar to that obtained for incompressible fluid flow. Pressure drop in the annulus is calculated with the proposed model, and satisfactory predictions are obtained. The second model is based on the assumption that there is wall slip in both pipes and annulus. Rheological parameters and wall-slip coefficient corrections were first obtained using Oldroyd-Jastrzebski approach. The annular pressure losses are predicted based on slip-corrected rheological parameters and wall-slip coefficient correlations.

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.

Abstract Experiments were carried out in a unique full-scale flow loop which includes a 73-ft (22.25 m) long annular section of 6-inch (152 mm) casing and 3.5-inch (89 mm) concentric drillpipe at elevated pressures and elevated temperatures (EPET) ranging from 185 to 500 psi (1.28 to 3.45 MPa), and 80 to 175°F (26.8 to 79.44°C) respectively. The gas-liquid ratio of the aerated fluids varied from 0.0 to 0.38. The in-situ cuttings concentration (i.e. volumetric concentration) was determined by using a special designed multiphase measurement system consisting of an air expansion tank, quick-closing valves, cuttings weighing system and two nuclear densitometers. The following test parameters were recorded during the experiments: liquid and gas flow rates, cuttings weight in the annulus, liquid holdup, mixture density and pressure losses. The results clearly show that in addition to liquid flow rate and gas-liquid ratio (i.e. injection gas volume fraction calculated at test temperature and pressure), temperature essentially affects the cuttings transport efficiency and the associated frictional pressure drop. The volume of cuttings which accumulated in the annulus was very sensitive to the liquid flow rate. Elevated temperature was found to cause a significant increase in the cuttings concentration at given flow conditions. The injection of gas has a positive effect on the cuttings transport at high liquid flow rates (greater than 150 gal/min). However, it was found that at lower liquid flow rates (less than 150 gal/min), increasing gas-liquid ratio (GLR) results in a decrease in cuttings transport efficiency. A mechanistic model for cuttings transport with aerated fluids under EPET conditions has been developed to predict frictional pressure loss and cuttings concentration in the annulus. The model is based on mass and momentum conservation equations and wall equations. Comparisons between the predictions of the model and experimental results show satisfactory agreement. In summary, this paper presents several important new aspects of cuttings transport that will be very useful for practical underbalanced drilling (UBD) design. Introduction The need for technologies to reduce cost and improve recovery from existing hydrocarbon reserves is well known. One of the most effective methods of cost reduction relies on improvements in drilling technologies. Particularly, development of UBD technology is beneficial for drilling partially depleted reservoirs and as well as and re-entry wells. During conventional (overbalanced) drilling, mud filtrate penetrates the near-wellbore formation because of high equivalent circulation density (ECD). This alters near wellbore pore-flow properties. As a result, well productivity decreases significantly. As a result, UBD is often used to minimize problems associated with formation damage, lost circulation and differential sticking. It has great potential to reduce drilling time and cost. High rates of penetration and longer bit life can be obtained using UBD. This technology is also important in offshore, deep water drilling to avoid fracturing of unconsolidated formations. In field applications, many different techniques are available for achieving underbalanced conditions. These mostly involve circulating low density fluids such as aerated mud. Nonetheless, multiphase flow behavior of aerated muds is complex and it is difficult to predict cuttings transport efficiency of aerated muds. As a result, cuttings accumulate in the borehole when this technique is applied. In-situ concentration of cuttings in the wellbore is not equal to the concentration near the drill bit. Similarly, in-situ gas mass fraction is not the same as the injection gas mass fraction. This will significantly affect flow behavior of the fluid. Using low density fluids alone does not always guarantee underbalanced conditions. Excessive frictional pressure loss due to poor hole cleaning may result in overbalanced conditions even with low density fluids. Therefore, transport mechanisms of cuttings with aerated fluids should be well understood to control ECD and optimize hole cleaning and hydraulics. Several solids transport models have been proposed in the literature to predict solids transport in pipe flow. However, very few of them are related to aerated muds. Compared to the pipe flow, little work has been done for flow through annuli. To our knowledge, no studies are in the literature concerning cuttings transport using aerated fluids at elevated pressures and elevated temperatures.

A discussion about horizontal foam-flow behavior in pipes and annular geometry under elevated pressures and temperatures is presented. The study is empirically based and covers the effects of foam quality, foam texture, pressure, temperature, and geometry of the conduit on the rheological response of foams.

Abstract Extensive aerated mud experiments were performed in a unique field-scale elevated pressure and elevated temperature flow loop (6″ × 3.5″ annular test section, 73 ft length, horizontal configuration without drillpipe rotation). A view port was installed to observe flow patterns in the test section. Two nuclear densitometers were used to measure steady state mean void fraction. During test runs, the liquid and gas phase flow rates were in the range of 50-250 gal/min and 50-150 scf/min, respectively. For all the test runs, measurements of pressure drop and average liquid holdup over the entire annular section were carried out. The two-phase flow patterns were identified by visual observations through the view port. Stratified and slug flow were the two flow patterns observed over the range of the chosen test matrix. The presence of slug flow does not justify many existing simulation practices, which assume a homogeneous gas-liquid flow. A mechanistic model has been developed for aerated mud hydraulics based on conservation equations and existing two-phase pipe flow correlations. An extensive sensitivity analysis is presented to quantify the influence of mud properties and flow parameters on the bottom-hole pressure. Comparisons between the predictions of the model and experimental measurements show a satisfactory agreement. The present model is particularly suitable for the design of underbalanced coiled tubing applications.