The equilibrium acid fracturing technique has been developed to stimulate wells in the Wasson San Andres Denver Production Unit. This new treatment technique maximizes acid contact time with the fracture faces while allowing control of the created fracture dimensions. Maximum acid contact time is essential to create highly conductive etched pathways on the fracture faces of cool dolomite formations which react slowly with acid. Control of fracture dimensions is important in the Denver Unit San Andres because fractures tend to grow uncontained in at least one vertical direction and the oil column is bounded by permeable gas bearing intervals above and permeable water bearing intervals below.
Using this technique, a fracture of desired dimensions is first created by injecting acid at fracturing rates. The volume of acid required to create the desired fracture dimensions is determined by a two dimensional fracture geometry program using design parameters determined from mini-frac testing and laboratory testing. Injection is then continued at reduced rates which maintain equilibrium with the fluid leak-off rate from the created fracture faces. By maintaining equilibrium between injection and leak-off, the created fracture can be held open without significant further fracture extension. Equilibrium is achieved in the field by maintaining the injection pressure below the fracture extension pressure, but above the fracture closure pressure determined by mini-frac testing.
The background and theory of this technique will be presented in this paper along with design procedures, field examples, results, and conclusions. A comparison of the results of the equilibrium acid fracture treatments to the other acid stimulations performed in the Denver Unit is also shown.
The Denver Unit is one·of several production units in the Wasson San Andres Field located in the western Texas counties of Gaines and Yoakum. The location of the Denver Unit within the field is shown in Figure 1. The San Andres formation is a Permian dolomite. The target interval is at a depth of about 5000'. The Wasson San Andres Field was discovered in 1936. The Denver Unit was formed in 1964 and waterflooding began at that time. CO2 flooding in the Denver Unit was started in 1984 and expansion is ongoing today.