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

摘要

High performance foamed fracturing fluids are required toenhance cleanup during applications in low permeability, lowpressure gas reservoirs and mature, depleted hydrocarbonreservoirs. Moreover, accurate foamed fluid rheologicalproperties, obtained under field conditions, are necessary toenable complex fracturing treatments to be executedsuccessfully. Standard laboratory tests used to evaluateproperties of foamed fluids include the foam half-lifemeasurement and the characterization of the water-basedpolymer fluid rheology that serves as the external aqueousphase of the foam. These tests are generally limited by theabsence of CO2. In addition, the rheology tests are commonlyperformed with the pH of the water-based fluid adjusted withacid to a value consistent with that expected for CO2 foamunder reservoir conditions.This study evaluates the relevance of conventionallaboratory tests in indicating the performance of CO2 foamfracturing fluids formulated with linear and zirconatecrosslinked carboxymethylhydroxypropyl guar (CMHPG)polymer. Specifically, the effects of the water-based polymerfluid pH, crosslink delay and corresponding shear sensitivity,temperature, and foam quality are evaluated under typicalconditions encountered in a fracture. Relatively newcapabilities to measure pH in the presence on CO2 under highpressure(up to 1,500 psig) and high-temperature (up to 280oF)indicate pH values in the range of 3.5 to 4.1 for CO2 foamunder down hole conditions. Results demonstrate that thecommon practice of performing Model 50 rheology tests withwater-based polymer fluids adjusted to low pH to simulate theeffects of CO2 are not indicative of CO2 foam fluidperformance, as the foam viscosity was not dependent on thecrosslink delay or crosslink pH. Furthermore, standard shearhistory tests with zirconate-crosslinked CMHPG fluids did notcorrelate well with foam performance at qualities greater than52% CO2. This work demonstrates the conditions under which CO2 foam rheology is dominated by foam propertiesversus water-based polymer fluid properties.