Liquid CO2 for the Stimulation of Low-Permeability Reservoirs

Abstract

Abstract The effects of residual fluid in the stimulation of low permeability reservoirs has long been a problem. The use of conventional fracturing fluids (water or oil) in tight reservoirs, especially those considered to be dry gas sands, generally results in the loss of 50-75 % of the fracturing fluid. Total recovery of the conventional fracturing fluid is usually never accomplished as the fluid becomes immobile at irreducible saturation. The use of liquid carbon dioxide (CO2) as a fracturing fluid was developed to combat the problems of fluid imbibition that are problems of fluid imbibition that are encountered with conventional fluids. The liquid CO2 fracturing system is essentially a non-damaging system that reverts to a gaseous state at static formation conditions. The CO2 is then returned to the surface under controlled rates as a gas. The result being a more rapid cleanup and evaluation of the well following the treatment. Introduction The use of hydraulic fracturing has been available to the oil and gas industry since 1947. Since the initial use of this method to increase production rates, the industry has seen a continuing advancement in fracturing theory, fluids and techniques. These advancements have principally been towards answering or understanding two basic problems. These are reduction of formation problems. These are reduction of formation damage as well as efficient and predictable proppant placement. The successful proppant placement. The successful development and use of these advances has proven to be a major challenge to the industry. The use of carbon dioxide since early in the 1960's has continued to be a significant part of these advances 1. CO2 was first part of these advances 1. CO2 was first used in the stimulation of oil and gas wells as an aid in fluid recovery. It was pumped with oil or water based treating fluids in ratios sufficient to gas lift the liquid back to the surface after the treatment. Further development led to higher CO2 ratios where the CO2 was effectively part of the fluid system (50-75%) with the proppant pumped in oil, water or methanol. The improvement was the reduction of the total liquid volume and sufficient energy for fluid recovery. At stabilized reservoir pressure and temperature, CO2 will vaporize to the gaseous state. The CO2 is slightly soluble in water and very soluble in oil CO2 will aid the recovery of oil as it is miscible with post crudes and greatly reduces oil viscosity 4. The dissolving of CO2 in water will form weak carbonic acid which has a pH of 3. There has been no indication that CO2 or the resultant carbonic acid does any damage in sandstone reservoirs. The latest development for the use of CO2 consists of using only liquid CO2 s a fracturing fluid for proppant transport?. The CO2 eliminates most of the damage associated with fracturing fluids. The greatest potential of this fracturing system is the creation of a propped fracture without the related permeability damage from residual fluid retention. The use of CO2 also offers a fluid recovery mechanism that is not dependent on reservoir pressure. THEORY The process of hydraulic fracturing is based on the principle of creating a fracture away from the wellbore through the formation. Once the fracture is created, a proppant is then placed to act as an efficient drainage channel back to the wellbore. P. 145