The quest for cleaner fracturing fluids has led to the development of sand fracturing using liquid carbon dioxide (CO2) as the proppant carrier fluid. CO2 is handled at the surface as a liquid at approximately 200 psi and -30 deg F Special blending equipment has been developed to inject proppant directly into the liquid CO at these surface conditions. The sand laden liquid CO2 is then pumped by conventional equipment. At stabilized reservoir temperature and pressure, the liquid CO2 vaporizes in a gas well or vaporizes and is partially dissolved in the reservoir oil in an oil well. The greatest potential of liquid CO2 fracturing is the potential of liquid CO2 fracturing is the elimination of most of the formation damage normally associated with fracturing fluids and the very rapid clean up and evaluation of the well following the stimulation.
CO2 is a liquified gas that is readily applicable to oil field stimulation use because of its unique phase behavior (Figure 1). CO2 can be transported as a liquid at 20 to -40 deg. As these temperatures are non-cryogenic, the CO2 can be pumped with conventional equipment and injected directly into the well as a liquid. Upon heating 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 formation to the wellbore with the lower viscosity of the gas. It 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 and slightly soluble in water. CO2 aids recovery in oil bearing formations as it is miscible with most crudes and greatly reduces oil viscosity. 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 in the 1960's in fracture stimulation of oil and gas wells. The CO2 was pumped with the oil or water based treating fluid in ratios sufficient to gas lift the liquid back to the surface after the treatment. Further development led to higher CO ratios where the liquid CO2 was effectively part of the fracture generating liquid (50%) with the proppant pumped in oil, water or methanol proppant pumped in oil, water or methanol with conventional equipment. The improvements were the reduction of the liquid volume and the ample gas energy supplied to recover the liquid from the formation.
The latest development in CO2 fracturing consists of injecting proppant directly into the liquid CO2 and using liquid CO2 as the sole carrier fluid. The obvious improvements are the elimination of residual liquid left in the formation and a fracturing fluid recovery mechanism that is not dependent upon reservoir pressure.
Fracture stimulation is based on the principal of generating a crack or fracture principal of generating a crack or fracture away from the wellbore through the producing formation, then placing proppant in the fracture to act as an efficient drainage channel back to the wellbore.