Historically, carbon dioxide (CO2)-foamed fracturing fluids were used to stimulate wells in the Waltman field in Wyoming—due to the low formation permeability and rock properties—and have been proven effective, but still not perfect. Limitations on the amount of proppant placed near water zones and formation damage from polymer residuals were the main drawbacks. A never ending quest for efficiency and higher production rates called for different options. One of those options was the recently developed CO2 viscoelastic surfactant (VES) fluid system. It has recently been employed to eliminate the disadvantages of the traditional polymer-based fluid. This VES-CO2 fluid system combines the benefits of viscoelastic surfactant-based fluid—such as low formation damage, superior proppant transport, and low friction pressures—with carbon dioxide advantages of enhanced cleanup and better hydrostatic pressure. This fluid was recently selected for the fracturing treatments on three wells. Initial production from these wells was observed in the range of 5 to 7 MMcf/D, significantly greater than neighboring wells' gas rates of 2 MMcf/D stimulated with polymer-based fluid.
The Waltman-Cave Gulch field complex is located on the northeast flank of the Wind River Basin, 50 miles west-northeast of Casper, Wyoming (Kuuskraa et al. 1996), Figure 1. Production from this complex was established in 1959 from Lance formation. The full extent of the Lance sands in the Cave Gulch (shallow) structural accumulation was rediscovered in 1994. The production boundaries of Waltman-Cave Gulch are not yet established. Current development priorities are defining the deep Frontier, Muddy, and Cloverly reservoirs and extending the Cave Gulch (shallow) accumulation to the southeast and northwest. The Waltman-Cave Gulch complex is located on the Waltman Anticline, a north-south trending asymmetric fold of Laramide age. The fold is bound on the west by a high angle reverse fault. The complex produces from three geologically distinct accumulations, the Waltman (stratigraphic), Cave Gulch (shallow), and Cave Gulch (deep), Figure 2. All the wells discussed in this paper are located in Waltman (stratigraphic) accumulation, and completed in Lance formation.