The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16–18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206.
The development of stabilized foam has created the potential for a widely suitable fluid in water-sensitive and/or under-pressured reservoirs. Foams, to date, have proved to be effective fracturing fluids in shale, coal and tight sands, but proppant placement has been a problem. With the use of a stable foam pad, proppant can be transported with a crosslinked fluid at optimum concentrations for maximum fracture conductivity. The stable foam pad has excellent fluid-loss characteristics and will retain energy to flow back the gelled water stage. With the use of a stable foam rod and crosslinked proppant-carrying fluid, the advantages of both fluids can be utilized in creating a highly conductive fracture with minimal formation damage.
In coal seams where proppant may not be needed due to low closure stresses, stable foam alone can be an efficient fracturing fluid. With higher viscosities and better fluid-loss control properties than previous foams, a wider and longer fracture can be generated with similar volumes of fluid.
Foams have already proved to be cost-effective fluids for stimulation in the northeastern United States. Their use in shales and low-pressure sand formations has been extensively tested. The ability of foams to clean up rapidly with minimal formation damage is one of its most beneficial attributes.
The small percentage of liquid phase present in foams, usually 15 to 45%, contributes to its non-damaging status. This, combined with the high energy potential imparted to the system due to the compressed gas, creates a highly mobile fluid capable of rapid cleanup.
Formation fluid incompatibilities are minimized and changes in oil-water saturations are also diminished by the fact that less liquid is available to react. Since the pad fluid is the portion most likely to penetrate the deepest into the matrix, this is a significant benefit.
All of the physical properties that are attributed to foam have made it an excellent stimulation fluid. The only serious drawback has been the inability to inject high concentrations of sand into the system due to the small volumes of liquid available with which to work. In a 75-quality foam, a proppant surface concentration of 20 lb/gal in the liquid phase is required to achieve five pounds per gallon after foaming in the formation due to the dilution of the slurry by the gas.
This may or may not be a serious limitation, depending on the fracture conductivity requirements needed for the desired flow. With low-permeability pay zones, one would not anticipate the need for packed fracture widths of 0.1 to 0.2 in. But due to increased effective permeabilities created by natural fractures and possible sand pack damage, permeabilities created by natural fractures and possible sand pack damage, higher fracture conductivities may indeed be required. Some of the potential prop-pack problems are formation softening, clay migration, potential prop-pack problems are formation softening, clay migration, sludges, paraffin plugging and precipitation of salts from formation water. All of these should be considered when designing fracture conductivity requirements.
In order to compensate for these damaging problems, a wide sand pack can be created which will withstand much of the possible damage and still be a suitably conductive channel.
One way to take advantage of foam properties and place a high-pound-per-gallon slurry in the formation is to use a stable foam as the pad fluid and use a highly viscous prop-carrying fluid to place the proppant. With a low amount prop-carrying fluid to place the proppant. With a low amount of fluid-loss in the pad fluid and an efficient prop transporting fluid, nondamaged, highly conductive fractures can be created.