Low Fluid Residual Fracturing

Abstract

ABSTRACT A new process of fracturing virtually eliminates the requirement for loadfluid recovery. Liquid carbon dioxide makes up 80 per cent of the total fluidvolume injected, and this fluid gasifies after a short, period of time in thereservoir. Propping agents are carried at a high concentration in a smallside-stream of gelled alcohol, and then blended with the liquid CO2 pumped into the well. After the C02 gasifies in the formation, onlythe alcohol remains for recovery as a fluid or vapor. The process is primarilydesigned for fracture stimulation of low-permeability gas wells. INTRODUCTION FRACTURING OF LOW-PERMEABILITY Reservoirs has always presented the problemof fluid compatibility with the formation core and formation fluids, particularly in gas wells. For example, many formations contain clays whichswell when contacted by aqueous fluids causing restricted permeability, and itis not uncommon to see reduced flow through gas-well cores tested with variousoils. In the past, the approach to the swelling problem has been the use ofchemical additives, such as salts or pH control additives, to water forfracturing water-sensitive formations. Where hydrocarbons are used, lightproducts such as gelled condensate have seen a wide degree of success, but arerestricted in use due to the inherent hazards of pumping volatile fluids. Amore recent technological improvement to the fracturing ofpermeability-sensitive reservoirs essentially eliminates contact of theformation with any fracturing fluids at all. Stimulation is effected byfracturing the well with liquid carbon dioxide as the basic fracturing fluid.Propping agents are blended in a separate stream of gelled alcohol and blendedwith the C02 in the well. The net effect is that the well isfractured with a base fluid with liquid properties. After a short period oftime in the formation, however, the CO2 gasifies, leaving only a lowfluid residual (LFR) of alcohol to recover. This alcohol, in turn, is solublein reservoir gas (methane) and is essentially returned as a vapor. TREATMENT MECHANICS Earlier experimentation with LFR fracturing indicated that the alcohol hadto be gelled to a viscosity of 20 centipoise or higher in order to besufficient to carry the high concentration of proppant agent through theblender and pumping equipment. Alcohol was chosen as the proppant-carryingagent in view of its compatibility with most gas reservoirs, its low freezingpoint and its potential chemical benefit to the stimulation. Alcohol is alsocompletely compatible and miscible with CO2 for immediate blendingof the two fluids.