The Evolution of Acid Fracturing Treatment Design in the Embar Carbonate, Oregon Basin Field, Wyoming
Authors: Hall F.R. , Whisonant R.J. , Dickerman R.P. 
Published: December 2007

Abstract
This paper describes stimulation treatment design changes and improvements over the past few years in the Embar carbonate formation in the Oregon Basin Field of Park County, Wyoming. These design changes have increased productivity and reserves, reduced producing water/oil ratios (WORs), and decreased workover costs. The Embar is characterized by a low-permeability, low-porosity interval immediately above an interval with higher permeability and porosity. The lower Embar has largely been swept during waterflooding operations, with the upper Embar very near original oil saturation. Acid fracture treatments have been modified over time to allow for more effective stimulation of the upper Embar interval, resulting in the recovery of reserves that were not previously being depleted. This paper discusses the evolution of treatment design, the methodology behind the design changes, and the continued improvement in results.
Treatment design has evolved from gelled acid to nitrogen-foamed acid at relatively high pump rates (12 to 15 bbl/min) to foamed acid at low pump rates (4 to 10 bbl/min). The acid used in most of the treatments has been 28% HCl. The intent of the current foamed acid treatments is to maximize stimulation of the highly oil-saturated upper Embar while minimizing stimulation of the more highly water-saturated lower Embar.

Introduction
Processed openhole logs from infill wells in the Oregon Basin Field indicated that the uppermost portion of one of the primary producing intervals was not being depleted by waterflooding operations, and the lower portions of the interval were exhibiting higher water saturations (Fig. 1). Acid fracturing techniques have been modified and optimized over the past few years to target the higher oil saturation interval through the use of nitrogen-foamed acid. The resulting treatments generate improved oil recoveries at reduced costs and have allowed a number of wells to be stimulated that would not have been if previous stimulation techniques had been used.
This paper discusses the characteristics of the reservoir interval being stimulated, its fluid properties, and the evolution of treatment design over the last few years. It also discusses the reasoning behind the changes that were made and the result of the changes compared to previous gelled and ungelled acid treatments. The basic premise behind the nitrogen-foamed treatments is the use of gravity segregation within the created (or existing) fracture to more effectively stimulate the upper portion of the fracture. Additionally, treatment design has been modified to achieve a constant-to-slightly-increasing bottomhole pressure throughout the pumping of the treatment.

Field and Reservoir Description
The Oregon Basin Field is located in the Big Horn Basin of northwestern Wyoming. The field consists of two doubly plunging asymmetrical anticlines, called the North and South domes. The formation being treated with the acid fracture stimulations discussed in this paper is the Embar (or Phosphoria as it is called in other parts of the Big Horn Basin). It is productive in both domes and throughout the saddle area between the North and South domes. The Embar is unitized with and produced commingled with the underlying Tensleep sandstone throughout the field. The Embar interval in Oregon Basin has been waterflooded since the early 1960s, originally with peripheral injection and later being expanded to five-spot with some line drive. Portions of the field were polymer flooded in the 1980s. Although the field-wide waterflood has been highly successful, the natural fracturing in the field makes it less efficient, with very rapid breakthrough occurring in some directions. Cycling of water through high-porosity/high-permeability intervals, and induced or natural fractures has resulted in those intervals being very well swept, while the lower-porosity intervals are relatively undepleted.
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