The quest for cleaner frac fluids has led to the development of sand fracturing using liquid carbon dioxide (CO2) as thesand carrying fluid- CO2 is handled at surface as a liquid at approximately 1 MPapressure and −35 °C. Special blending equipment has been developed to inject proppant directly into the liquid CO2 at these surface conditions. The sand ladenliquid CO2 is then pumped by conventional frac equipment. At stabilized reservoir temperature and pressure the liquid CO2 vaporizes in a gas well, or vaporizes and is partially dissolved in the reservoir oilin an oil well. The greatest potential of liquid CO2 fracturing is the elimination ofmost of the formation damage normally associated with fracturing fluids and the very rapid clean up and evaluation of thewell following the stimulation.


CO2 is a liquid gas that is readily applicable to oil field stimulation use because of its unique phase behaviour(Figure 1). CO2 can be transported as a liquid at −20 ° to −40 ° C. As these temperatures are non cryogenic, the CO2 can be pumped with conventional frac equipment and injected directly into the well as aliquid. Upon heating up to reservoir equilibrium temperature and expanding to reservoir equilibrium pressure, CO2 will vaporize to the gaseous state. The gaseous CO2 will then, flow back from the formationto the well bore with the lower viscosity of gas, will not exhibit any surface tension as liquids do and will aid formation liquid production from the well by reducing the pressure gradient up the tubing.

CO2 is very soluble in crude oil 11 and slightly soluble in water 12 CO2 aidsrecovery in oil bearing formations as it is misible with most crudes and greatly reduces oil viscosity 6 CO2 dissolved in water will form weak carbonic acid which has a pH of 3. There is no indication that CO2 or the resultant carbonic acid does any formation damage in sandstone reservoirs.

CO2 has been used since early the 1960's in fracture stimulation of oil and gas wells 7. The CO2 was pumped with theoil or water base frac fluid in ratio's sufficient to gas lift the liquid back to surface after the frac. Further development led to higher CO2 ratios where the liquid CO2 was effectively part of the frac generating 1iquid (50%), with the proppant pumped in oil, water or methanol through conventional frac equipment. The improvements were the reduction of the liquid fracfluid volume and the ample gas energy supplied to recover the liquid from formation. The latest development in CO2 fracturing is injecting proppant directly into the liquid CO2 and using liquid CO2 as the sole frac fluid. The obvious improvements are the elimination of any residual liquid left in the formation from the frac fluid, and a frac fluid recovery mechanism that is not depel1dent on reservoir result.

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