Effects of Texture on Rheology of Foam Fracturing Fluids

Abstract

Summary A new laboratory technique was developed to measure changes in foam texture (bubble-size distribution) vs. foam viscosity in a recirculating pipeline viscometer. Fluid variables studied included foam quality, surfactant type and concentration, gelling-agent concentration, shear history, system pressure, and liquid-phase chemical type. The viscous properties of foam fluids are determined primarily by quality (internal-phase volume) and liquid-phase properties. The viscosity of an equilibrated foam is influenced by texture to a lesser extent. Foams are shear-history-dependent fluids. The bubble size and degree of dispersion will adjust to an equilibrium state that depends on time at a given shear rate. Finer-texture foams are produced by higher shear rates, higher surfactant concentrations, and higher pressures. Viscosity measurements at low pressure may not adequately simulate field use at high pressure. The liquid-phase chemical type influences texture. The structure of methanol and hydrocarbon foams may result in sensitivity to degradation at high shear rates. Introduction in a number of theoretical and experimental studies of foam behavior, the influence of bubble-size distribution or texture has been recognized. Although polydisperse distributions have been recognized as a more general case than monodisperse distributions, there has been no agreement on how various degrees of dispersion relate to dynamic properties. For this reason, an empirical approach was undertaken to determine the effect of real bubble-size distributions on the rheological properties of foam. Results are limited only by the ability to generate various types of distributions that retain their identities long enough to be measured. To assess the influence of bubble size on the behavior of foam fluids, one must gain an appreciation of the bubble sizes that actually exist in real foams. It is common to speak of either coarse or fine-texture foams, referring to relatively large or small bubble sizes, respectively. Different samples are readily compared by reference to the average diameter, or mean, of each sample. But the real samples considered here have a range of sizes in each sample. How broad the range of sizes is in each bubble-size distribution may be equally as important as the mean size of the distribution. The particular type of mean (volume, surface volume, surface area, or number) used for comparison may also be important. Several opposing factors influence the eventual bubble-size distribution at equilibrium. Shear forces reduce large bubbles to smalr sizes. Sufficient surfactant must be present to stabilize the newly created surfaces. The smallest mean bubble-size distributions are created by high shear rates. There is probably a limiting upper shear rate for a given quality and liquid-phase chemical system beyond which the foam structure will collapse. At low shear rates, the bubbles tend to grow larger and coalesce as a result of gas diffusion between bubbles. At very low shear rates, liquid drainage also becomes significant. A proper balance of all factors must be considered in the determination of the physical properties of such a dynamic fluid as foam. Experimental Foams were prepared and circulated through a foam pipeline loop viscometer as described previously. Foaming surfactant and any viscositying agent [hydroxypropyl guar (HPG)] were added to the liquid phase of the fluid before introduction to the loop. N, gas was added through a small orifice to liquid circulating in the loop. When the specific gravity for the desired foam quality was attained, N2 addition was stopped. Foam fluids were recirculated by a Zenith QM BLB pump, having 22 teeth per gear. Spacing between teeth was much larger than the bubble diameters. Experiments with different sizes of tubing in the flow loop indicated that shear effects were attributable to shear rates in the pipe and not to the pump. Wail slippage effects were considered negligible. A new instrument developed for measuring foam texture sampled the entire foam contents within a shown length of tubing, rather than looking at a possibly skewed sample near the surface of a glass Bubble-size-distribution measurements were made by taking foam samples from the recirculating loop and then conditioning and counting the separated bubbles. The apparatus is shown in Fig. 1. The valving arrangement (Fig. la) allowed a fresh foam sample to be taken from a flowing stream in the loop. Foam flowed through two plug valves and a specially built three-way valve to fill the sample plug valves and a specially built three-way valve to fill the sample tube. Once the sample valves were fined with fresh fluid, valves were switched, as shown in Fig. lb, to isolate approximately 2.8 mL of foam. A bubble-separator chamber, an HIAC (Ref. 17) E-2500 particle-detector cell, and a liquid-phase reservoir were placed particle-detector cell, and a liquid-phase reservoir were placed vertically in line above the three-way sampling valve. A continuous column of the liquid phase was necessary in the HIAC detector cell at all times for proper counting. A liquid-equalizing tube around the detector cell helped maintain this liquid column. As soon as the three-way valve was positioned as shown on the right side of Fig. 1, liquid began to displace foam from between the two valves. Foam separated into individual bubbles, with the largest bubbles rising fastest through the bubble-separator chamber. It is assumed that diluting a foam with a fluid identical to its liquid phase does not alter the volume or size distribution of the discrete, internal-phase bubbles. This assumption requires that the liquid phase in the counter be saturated with N2 before introduction of the foam sample. Separated bubbles were columnated, rose into the detector cell, and were counted photoelectrically. Electrical impulses were separated into six size ranges by a multichannel analyzer. These partitioned sizes were converted mathematically to volumes, and geometric standard deviations and volume means were calculated. Immediately after the sample was taken for texture determination, rheograin measurements were made on foam circulating in the loop. Next, static drain time was measured by closing valves immediately before and after a 10-in. [25.4-cm] -high visual cell and observing the liquid/foam interface rise from the bottom. The time required for one-half of the liquid in the sample to drain to the bottom was recorded.

Rheology of Aqueous Foams: a Literature Review of Some Experimental Works

“…By varying gas and fluid flow rate ratio, it is possible to change foam quality. This porous medium is often constituted of sand (Burley and Shakarin, 1992;Enzendorfer et al, 1995;Raza and Marsden, 1967) or of stacked glass beads (Patton et al, 1983;David and Marsden, 1969); Harris (1983Harris ( , 1985Harris ( , 1987Harris ( , 1990Harris ( , 1994Harris ( , 1996 produces foam by circulating first the fluid in the recirculating loop, and then injecting gas into fluid through a small opening (the fluid in excess is allowed to escape through a backpressure regulator). When using a mixer (Hanselmann and Windhab, 1996), foam quality can't be controlled.…”

“…Measurements made at different shear rates (i.e., made at different flow rates) are carried out on foams having different textures. Patton et al solved this problem by imposing a constant flow rate at the porous medium entrance: the foam texture is then stable; a bypass pipe at the capillary entrance (i.e., at the porous Experimental results of Harris (1985): texture and foam rheology. medium exit) allows the flow rate to vary (and therefore the shear rate) in the capillary pipe without changing foam texture (Fig.…”

“…From Enzendorfer et al (1995). Harris (1983Harris ( , 1985Harris ( , 1987Harris ( , 1990Harris ( , 1994Harris ( , 1996 also studied foam rheology under pressure (69 bar) and high temperature (149°C) in a recirculating flow loop. However, there are very few results on foam behavior under high static pressure.…”

“…Foam is described as a non-newtonian fluid: -pseudoplastic power-law (Thondavadi and Lemlich, 1988;Raza and Marsden, 1967;David and Marsden, 1969;Patton et al, 1983;Sanghani and Ikoku, 1983;Enzendorfer et al, 1995); -Bingham plastic (Khan et al, 1988); -Herschel-Bulkley (Calvert and Nezhati, 1986;Burley and Shakarin, 1992;Harris, 1985). A power law seems therefore appropriate to describe foam behavior.…”

“…Viscosity variations with pressure have been studied by Harris (1985) and by Hanselmann and Windhab (1996). The apparent viscosity of foam decreases with increasing static pressure, especially at low shear rates.…”