The Wamsutter (Almond) formation in southwest Wyoming is a tight gas reservoir in the eastern part of the Greater Green River Basin (GGRB). The Almond formation is generally encountered between depths of 8,500 and 10,500 feet with reservoir pressures varying from initial conditions (0.54–0.58 psi/ft) in the Lower Almond to varying stages of pressure depletion in the Upper Almond. The Almond interval is generally completed via massive hydraulic fracture stimulation down casing with a series of treatments targeting the Lower, Main and Upper Almond intervals. These zones are subsequently commingled for production.
In the absence of reservoir properties such as in-situ reservoir permeability, reservoir pressure, and closure stress, it has been difficult to optimize the stimulation treatments on a zonal basis. To identify these reservoir characteristics, pre-fracture impulse injection testing has been performed in each interval. These simple, easy to perform transient tests have aided in the interpretation of the key reservoir parameters.
This work presents data from eleven wells in the Wamsutter field and describes the methodology used to optimize the stimulation treatments. Reservoir properties obtained from the impulse tests helped eliminate the addition of nitrogen to the fracturing fluid, which had been routinely included to aid fracture clean-up after fracture stimulation. This design change has led to more effective fracture stimulation treatments yielding higher gas production rates while significantly reducing completion costs.
Production from wells stimulated with the modified design is compared to production from wells that were stimulated without the benefit of reservoir properties obtained from the pre-fracture impulse tests.
Introduction
The Almond formation is the uppermost member of the Mesaverde Group in the GGRB and is one of the most prolific producers of natural gas in southwestern Wyoming. A field map showing various areas that produce from the Almond formation is shown in Figure 1. Gas wells targeting the Almond formation are hydraulically fracture stimulated in multiple perforated intervals. In this type of multilayer environment, it is very important to ascertain reservoir properties such as reservoir permeability and reservoir pressure in key intervals for optimum fracture design.
The Almond formation consists of three to four individual intervals varying in terms of depositional environment. Generally, these are named the Lower Almond, Middle Almond and the Upper Almond 1. The Lower Almond formation overlies the Ericson sandstone and is treated in the first of a three to four stage stimulation treatment. This fluvial formation is composed of interbedded siltsone, coal and sandstone. Porosity is on the order of 8–10% and the net pay thickness varies between 10.0 and 25.0 feet. A type log is shown in Figure 2.
The Middle Almond is very similar to the Lower Almond in terms of porosity and pay thickness. As coal intervals separate the Middle and Lower Almond intervals, fracture designs are developed to stimulate these separately.
The Upper Almond or the Bar sand as it is commonly known is a thick 40.0 foot marine sandstone interval that underlies the massive Lewis shale. Log porosities are higher in this pay sand and often the neutron-density log crossover effect is not noticed in this sand. The Upper Almond is laterally continuous and has a higher permeability than the Lower and Middle Almond intervals. The large number of wells completed in the Bar sand has resulted in pressure depletion in certain areas of the field. Recent wells that were completed in this interval have shown lower than original pore pressure gradients. It has been shown in various studies that the Bar sand dominates well performance and production is supplemented by the Lower and Middle Almond intervals 2–3.
