High performance foamed fracturing fluids are required to enhance cleanup during applications in low permeability, low pressure gas reservoirs and mature, depleted hydrocarbon reservoirs. Moreover, accurate foamed fluid rheological properties, obtained under field conditions, are necessary to enable complex fracturing treatments to be executed successfully. Standard laboratory tests used to evaluate properties of foamed fluids include the foam half-life measurement and the characterization of the water-based polymer fluid rheology that serves as the external aqueous phase of the foam. These tests are generally limited by the absence of CO2. In addition, the rheology tests are commonly performed with the pH of the water-based fluid adjusted with acid to a value consistent with that expected for CO2 foam under reservoir conditions.
This study evaluates the relevance of conventional laboratory tests in indicating the performance of CO2 foam fracturing fluids formulated with linear and zirconate crosslinked carboxymethylhydroxypropyl guar (CMHPG) polymer. Specifically, the effects of the water-based polymer fluid pH, crosslink delay and corresponding shear sensitivity, temperature, and foam quality are evaluated under typical conditions encountered in a fracture. Relatively new capabilities to measure pH in the presence on CO2 under high-pressure (up to 1,500 psig) and high-temperature (up to 280°F) indicate pH values in the range of 3.5 to 4.1 for CO2 foam under down hole conditions. Results demonstrate that the common practice of performing Model 50 rheology tests with water-based polymer fluids adjusted to low pH to simulate the effects of CO2 are not indicative of CO2 foam fluid performance, as the foam viscosity was not dependent on the crosslink delay or crosslink pH. Furthermore, standard shear history tests with zirconate-crosslinked CMHPG fluids did not correlate well with foam performance at qualities greater than 52% CO2. This work demonstrates the conditions under which CO2 foam rheology is dominated by foam properties versus water-based polymer fluid properties.
Foamed fracturing fluids are used in approximately 40% of all fracturing stimulation treatments performed 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.
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.
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. Common statistical parameters such as mean and standard deviation can then be used to compare different foams.