Unconventional gas reservoirs including tight gas, shale gas, and coalbed methane are becoming critically important components of the current and future gas supply. However, these reservoirs often present unique stimulation challenges. The use of water-based fracturing fluids in low-permeability reservoirs may result in loss of effective fracture half-length caused by phase trapping associated with the retention of the introduced water into the formation. This problem is increased by the water-wet nature of most tight-gas reservoirs (where no initial liquid hydrocarbon saturation is or ever has been present) because of the strong spreading coefficient of water in such a situation.
The retention of increased water saturation in the pore system can restrict the flow of produced gaseous hydrocarbons such as methane. Capillary pressures of 10 to 20 MPa or higher can be present in low-permeability formations at low watersaturation levels. Inability to generate sufficient capillary drawdown force using the natural reservoir-drawdown pressure can result in extended fluid recovery times, or permanent loss of effective fracture half-length. Furthermore, use of water in subnormally saturated reservoirs may also reduce permeability and associated gas flow through a permanent increase in water saturation of the reservoir. Secondary costs such as rig time for swabbing can add to the negative economic impact. Significant additional costs for N2 and coiled tubing may also be incurred.
Gelled liquid petroleum gas (LPG) based fracturing fluids are designed to address phase trapping concerns by replacement of water with a mixture of LPG and a volatile hydrocarbon fluid. Once the well is drawn down for flowback, some of the LPG portion of the fluid may be produced back as a gas, dependent upon temperature and pressure. The remaining LPG remains dissolved in the hydrocarbon fluid and is produced back as a miscible mixture using a methane drive mechanism. By eliminating water and having LPG as up to 80-90% of the total fluid system, cleanup is greatly facilitated, even in wells having very low permeability and reservoir pressure.
The effects of fracturing-fluid retention on gas flow in the fracture face can be as important as fracture conductivity when designing a treatment. It is possible to have a conductive fracture with good half-length in the desired productive zone and still not realize economic or optimum gas production if phase trapping and/or relative permeability effects are restricting gas flow. 1
Gelled LPG-based fracturing fluids make up a unique hydrocarbon-based fracturing fluid system designed for gaswell stimulation. They use up to 100% gelled LPG for the pad and flush. The sand slurry stages are a mixture of up to 90% LPG with the balance of the volume being a volatile hydrocarbon base fluid. All chemicals and proppant are added to the base fluid at the blender. LPG, which composes the balance of the downhole fluid volume, is injected at the wellhead where it forms a miscible mixture with the base oil. It is important to note that under pumping pressures, this is a single-phase gelled fluid system, similar to gelled oil in rheology, friction pressures, proppant transport, and leakoff control.
