Rheological Evaluations of CO2 Miscible Hydrocarbon Fracturing Fluids

Abstract

Previous publications(1–4) described the theory and application of CO2 miscible hydrocarbon fracturing fluids to gas well stimulation. The fluids are ideally suited to gas reservoirs susceptible to phase trapping due to high capillary pressures when waterbased fluids are used. Reservoirs particularly prone to phase trapping are those with low permeability (less than 0.1 md), those that are subnormally water-saturated, and those that are under pressured. In order to conduct meaningful rheological evaluations for etermination of fluid properties and required chemical concentrations, it is essential that downhole conditions be accurately duplicated. The most fundamental requirement is that the gelled hydrocarbon and liquid CO2 be combined and homogenized below the critical temperature of CO2 (31 °C) and at a pressure above the bubble point of the resulting fluid mixture to ensure one miscible phase. This normally requires a minimum initial pressure f at least 20 MPa. Temperature is increased to the bottomhole static temperature of the well under consideration as the test progresses. This requires that the rheometer have a pressure rating high enough to withstand the increased pressure caused by expansion. This paper presents several different testing methodologies designed to provide representative rheology vs. time with all chemicals present, and to provide varying insights into fluid behaviour. The first utilizes a conventional bob and sleeve configuration in a heated pressure chamber rated to 102 MPa and 204 °C(5). This allows one to gather shear stress vs. shear rate data as a function of time. Normal Power Law n' and k' parameters are calculated and are in turn used to calculate apparent viscosity vs. time. Secondary mixing is provided by a helical fin attached to the outside of the sleeve, which creates a rotational flow pattern in the rheometer. The second methodology utilizes a capillary tube viscometer allowing for precise and accurate control of the shear rate. The capillary tubes used are capable of 68 MPa at 204 °C and are sized according to the expected viscosity range and range of desired shear rates. The fluid of interest is displaced through the tube using a push-pull system of two positive displacement syringe (Ruska) pumps?one in injection and one in extraction mode to keep the system pressure at the desired level. Varying the injection rate allows one to vary the shear rate as desired. Shear stress vs. shear rate data are collected as a function of time, again allowing normal power law parameters as well as apparent viscosity to be calculated. The third methodology uses an oscillating (sinusoidal) strain. The resulting stress has an elastic component and a viscous component. G' is the elastic component and can be thought of like a spring constant. A Newtonian liquid has no storage modulus (G'). Elastic behaviour is important for suspending proppant at low shear rates. This methodology therefore provides insights into fluid behaviour that complement those obtained using shear stress vs. shear rate measurements.

Thickening CO2 with Direct Thickeners, CO2-in-Oil Emulsions, or Nanoparticle Dispersions: Literature Review and Experimental Validation

“…Unfortunately, when the cross-linking solutions designed for propane or hexane/heptane are blended with either ethane–phosphate ester or CO 2 –phosphate ester solutions, the resultant cross-linked network immediately precipitates. In order to address this limitation for CO 2 , Taylor and colleagues − proposed using a single-phase mixture of 30–80% CO 2 and 70–20% blended hydrocarbons (mostly C6–C11) thickened with a gelling agent at a concentration of 0.1–2.5 wt % (based on the hydrocarbons) for hydraulic fracturing. The gelling agent consists of a “ferric iron or aluminum polyvalent metal complex of an unsymmetrical dialkylphosphinic acid or an orthophosphoric acid ester” .…”

Direct Gas Thickener

“…diesel and kerosene) by 2-100-fold [93,94,97]. Furthermore, phosphate-based esters could be used as gel agents for CO 2 and hydrocarbon liquid mixtures [98,99]. Lee and Dhuwe et al [82,88,89] studied blends of cross-linked phosphate esters (CPE) (phosphate ester (HGA 70-C6) and cross-linker (HGA 65)) with NGL components (ethane, propane, and butane) at temperatures ranging from 298 to 373 K and pressures ranging from 13.8 to 62 MPa.…”

Small associative molecule thickeners for ethane, propane and butane

“…. However, due to an apparent desire to protect proprietary compositions and techniques, solubility and viscosity data for gelled LPG is relatively scarce[32,34].Although TBTF, HAD2EH and CPE have been reported to be potential thickeners for one or more components of NGL, a thorough comparison of the performance of these components in ethane, propane and butane has not been presented. For example, TBTF is known to thicken propane and butane while exhibiting minimal solubility in ethane, but the solubility of TBTF in ethane at pressures above 20.68 MPa has not been evaluated.…”