Abstract

A stable mixture of 98% liquid CO2 and 2% of an organic fluid has been developed. The fluid has a viscosity of up to 26 cp at -26 °C for generation of fracture width, control of leak-off and sand carrying capability. The non-damaging aspects of fracturing with liquid CO2 are maintained.

Viscosity and fluid behavior have been examined in the laboratory from -26 °C to the critical temperature. These results are used to calculate sand-fall times and to generate simulated fracture profiles.

Several field treatments are reviewed with respect to sand con cent rat i on achieved, sand placed and post-fracture production.

Introduction

A fracturing process which used 100% liquid CO2 as the sole carrying fluid was first introduced by Canadian Fracmaster Ltd. in 1981.

The principal advantages of this fracture stimulation process are: compatibility of liquid CO2 with reservoir fluids, elimination of formation damage and residues associated with conventional fracturing fluids, and easy and rapid clean-up.

The major disadvantage of liquid CO2 compared to other fracturing fluids is its low viscosity. Because of this, major differences exist between conventional fluids and liquid CO2: lower sand concentration, smaller sand sizes, higher leak-off and higher tubular friction. 1.2

A research program directed towards improvement of the rheological properties of liquid CO2 has been carried out in our laboratories for several years. This included, polymer addition, in-situ polymerization, and more recently, emulsification of liquid CO23

This report describes further attempts made on direct viscosification and emulsification of liquid CO2.

DIRECT VISCDSIFICATIDN OF LIQUID CO2
POLYMER ADDITION

Historically, viscosification of liquids has been achieved by means of polymer addition, either directly or through in-situ polymerization. Both approaches have been tried by Canadian Fracmaster as well as by the New Mexico Petroleum Recovery Research Center4. Only limited increases in viscosity have been noted (an increase from 0.08 cp to 0.8 cp).

Six new polymers from ICI, Organic Division, UK were tested as possible viscosifiers of liquid CO2. 5 of these were caprolactone polymers including either aliphatic alcohol or methoxy acetic acid terminations and the other was a copolymer condensate of caprolactone and ethyleneimine.

All samples were insoluble in liquid CO2.The use of a co-solvent: dichloromethane (CH2CI2), up to a maximum concentration of 15% v/v, did not give better results. Upon addition of liquid CO 2, the polymers precipitated out of solution as solids or oils.

These results suggested that a different approach was required. Some of the new ideas are described below.

USE OF FUMED SILLICAS

The use of fumed si1icas hydrophobic) as thickening agent is well documented in the literature. These materials are widely used in industry as thickeners of water and hydrocarbon based mixtures, for example, adhesives, coatings, cosmetics, lubricants, rubbers, pharmaceuticals, silicone oils, etc. Two fumed silicas were tested as possible viscosifiers for liquid CO2: Cabosil PTG (hydrophilic) and Cabosil N70 - TS (hydrophobic), both from Cabot Corporation.

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