Combining Continuous Improvements in Acid Fracturing, Propellant Stimulations, and Polymer Technologies to Increase Production and Develop Additional Reserves in a Mature Oil Field
Authors: Whisonant R.J. , Hall F.R. 
Published: December 2007

The use of water shut-off and acid fracture stimulation techniques in combination with propellant technology has resulted in increased production and oil recovery. These three technologies have been combined and continually improved during workover operations of over 30 wells in the Oregon Basin Field, located in Park County, Wyoming, with plans for a number of additional well treatments.

Acrylamide-polymer/CrIII-carboxylate (Gel) technology has been used successfully in Oregon Basin since the mid-1980s to improve conformance in injection wells and to reduce water production from producing wells. Acid fracture stimulations have historically played a significant role in maintaining production from Oregon Basin. In 1996, acid fracturing technology was combined with proven Gel technology and relatively new propellant stimulation tools for treatment of wells in Oregon Basin. The combination of the three different technologies allowed treatment of a number of wells that had not previously been considered stimulation candidates due to their high total fluid production rates. This paper will briefly discuss the evolution of the Gel and acid fracture treatments, and the reasons for combining the two treatments with new propellant technology. The results of the 1996 program will be discussed as well as ongoing changes in treatment design. During the course of the project, the propellant stimulation techniques were significantly modified to reduce job problems and lower cost by development of a combination perforating/propellant system. Data is presented detailing success in allowing stimulations to remain in zone, and subsequent production results clearly show improved water oil ratios (WORs) and higher total oil production, while minimizing incremental water production.

Introduction

It has been understood for some time that the upper Embar formation in Oregon Basin has been poorly depleted in comparison to adjacent zones of the same formation. The most obvious measure of this characteristic has been processed openhole logs from recent infill wells (Fig. 1). This lack of depletion has been attributed in large part to significant vertical permeability variations and subsequent inefficiencies in waterflooding operations (conducted in Oregon Basin since 1961). Additionally, stimulation problems posed by natural fracturing and mechanical rock property variations are believed to have contributed significantly to the lack of depletion of the upper Embar.

Gel treatments have been widely used to reduce excessive water production and improve areal water injection conformance in the Embar carbonate. These treatments have resulted in significant incremental reserve recovery from the Embar reservoir at Oregon Basin.

Separately, acidizing the Embar formation has been performed since the early 1940s. These treatments have evolved from early small volume 'matrix' type treatments to larger volume acid fracturing treatments starting in the late 1980s. Recently, foamed acid fracturing has been successfully applied using gravity segregation within the fracture to effectively stimulate the upper, highly oil saturated interval. This paper describes a process whereby the two different proven technologies, Gel treatments and foamed acid fracturing, have been used in combination with propellant technology in the same wellbore to further develop the remaining upper Embar oil target. The process has been applied to both production and injection wells within the Oregon Basin Field.

Field and Reservoir Description

The Oregon Basin Field is located in northwestern Wyoming in the Big Horn Basin. The field consists of two anticlines, called the North and South Domes. The first oil producing zone, the Embar, was discovered in 1927. P. 393^