Impact of Water-Based Polymer Fluid Characteristics on CO2 Foam Rheology

摘要

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 280deg 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.