A Circulating Foam Loop for Evaluating Foam at Conditions of Use

Abstract

Abstract Foam stability is an important parameter for foamed fracturing. Bench-top testing is useful for screening but does not address the necessary conditions of temperature, pressure, pH (particularly with carbon dioxide [CO2] systems) and dynamic flow conditions which can have unexpected influence on the foam's performance. A laboratory apparatus has been constructed for measuring the rheology of circulating foam fluids to 400°F and 2000 psi. The apparatus is equipped with a circulation pump, view cells, foam generator, mass flowmeter and piping for loading a foam of the desired quality using either nitrogen or CO2. The foam rheometer is intended for evaluation of foam stability with time and comparison of various foam formulations for application in foam fracturing. The foam loop was designed to mimic shear rates found in a fracture or reservoir, which are typically 200 s–1 or less. The rheology is measured by monitoring the pressure drop across a 20-ft length of 1/4-in. tubing maintained at temperature in an oven. Flow rate is continually adjusted to ensure a constant shear rate in the tubing by the software using continuous mass flowmeter input. Results relating to CO2 and nitrogen foams are discussed with emphasis on foam persistence, bubble size and population, and the rheological behavior with time. Temperature, pressure, and additives affect both foam texture and foam stability. The adoption of a standard technique patterned after this work for evaluating foam rheology could impact the use and development of foam fluids in the future. Introduction Foamed fracturing fluids are used in approximately 40% of all fracturing stimulation treatments executed in North America. Foam fluid functional properties, such as proppant carrying capacity, resistance to leakoff, and viscosity for fracture width creation, are derived from the foam structure and the external phase properties. Moreover, the foam must have structural stability in order to maintain its performance throughout the treatment. A major objective of this work was to develop an efficient method of evaluating the time dependent properties of foam fracturing fluids under meaningful conditions. The reasons for this objective are to evaluate the effectiveness of surfactants and to determine the engineering parameters, behavior index (n') and consistency index (K'), used by fracturing simulators to estimate treatment operating parameters and fracture geometry. Foam structure is best preserved through the use of effective surfactants that enable the formation of stable interfacial surfaces, and through the use of appropriate external phase viscosifiers that reduce the rate of some foam destruction mechanisms. Foam is adequately described by three descriptors: quality, texture, and rheology. Foam quality at a given temperature and pressure is determined using the following equation.Equation 1 Gas-liquid mixtures are classified by quality: dispersions (G < 52%), wet foam (52% < G 74%), dry or polyhedral foam (74% < G < ˜96%) and mist (G > ˜96%) as depicted in Fig. 1. Foam density is related to the foam quality using the appropriate equation of state. Foam texture refers to the bubble size distribution of the dispersed gas phase. Qualitatively, foam texture can be described as fine texture (small bubbles) or coarse texture (large bubbles) and as homogeneous or heterogeneous, i.e., comprised of similar- or dissimilar-sized bubbles. Quantitatively, one can measure and tabulate the bubble size and frequency as illustrated in Fig. 2. Common statistical parameters such as mean and standard deviation can then be used to compare different foams.