Abstract

As a part of our research program aimed at further improving the rheological and thermodynamic properties of liquid CO2, for use as a hydraulic fracturing fluid, the behaviour of several systems of liquid CO2. a second, usually immiscible hydrocarbon oil and various surfactants/emulsifiers was investigated.

A mixing cell was constructed in which the emulsion created could be observed far quality and stability. From this cell the emulsion could be transferred directly to the couette of a Rheometrics Pressure Rheometer for determination of its viscosity versus shear rate/shear stress behaviour.

Such measurements were determined at temperatures of 20 °-24 °C and pressures from 4 mPa to 7 mPa (750–1000 psi) at which CO2 exists primarily as a liquid. No values were determined for CO2 foams.

It was found that very few systems are stable at a high phase volume (>80%) of liquid CO2. Sam. systems were found at a phase ratio of 85/15 liquid CO2/oil mixture that have viscosities greater than 2 cp and stabilities of at least a few minutes at rest. No system was observed wherein liquid CO2 became the internal phase of an emulsion.

Introduction

CO2 has been used for over twenty years as a component of fluids used for hydraulic fracturing (1). Initially, it was, and indeed still is, used to provide drive and lift for cleanup of fracture fluids from low pressure formations (2). Improvements led to the use of higher CO2 ratios so that the liquid CO2 was a part of the fracturing fluid (30% - 60%) but the proppant still had to be added via the second phase (gelled water, oil or methanol). A further refinement (3) has provided the ability to fracture hydrocarbon (mainly gas) - bearing formations and placing of proppant by means of virtually 100% liquid CO2.

A method of increasing the viscosity of liquid CO2 has been developed (4). but the viscosity obtainable is still not sufficient to enable fracturing of deep oil wells and placing of large mesh size proppant.

The present work was undertaken as part of a continuing commitment to improve the rheology of liquid CO2, by emulsifying it in another compatible but immiscible oil. A second benefit Which may be derived is the alteration of some of the thermodynamic properties of liquid CO2 (e.g. critical temperature, equilibrium vapor pressure, etc). Fig 1 shows the phase behaviour of liquid CO2 as a function of temperature and pressure. The formation of an emulsion and consequent stablization of the liquid/liquid interface by surfactant interaction should raise the critical temperature and lower the vapor pressure. This will effectively increase the liquid portion of the phase diagram so that the CO2 will be present longer as a liquid during the fracturing process but will still vaporize for effective cleanup.

A further benefit of a high phase volume liquid CO2 emulsion system would be that conventional liquid CO2/sand blending equipment, already in the field, could be used without major modification.

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