Successful Stimulation of a Thick, Low-Pressure, Water-Sensitive Gas Reservoir By Pseudolimited Entry
Authors: Mathias Paul J. 
Published: December 2007

Thick, water-sensitive gas reservoirs with very low pressures and permeabilities are especially difficult to exploit. Nonetheless, a permeabilities are especially difficult to exploit. Nonetheless, a pseudolimited-entry technique been successful in stimulating such a pseudolimited-entry technique been successful in stimulating such a reservoir in northwestern Colorado. The method incorporates liquid carbon dioxide in a fracture fluid consisting of two percent potassium chloride in fresh water.

Introduction
With an increased demand for natural gas, operators in the Rocky Mountain region are devoting more attention to certain gas-bearing Tertiary and Cretaceous formations in the area - formations that, until the past 15 years, had been largely considered non-commercial. Typical of these are reservoirs occurring in the Green River, Wasatch, Mesa Verde, Mancos and Dakota sequences and their geologic equivalents throughout the region. Portions of these formations are often thick and porous enough to contain substantial gas reserves, but are also so impermeable that payout of well expenses is long or impossible without effective stimulation. To make matters worse, formation pressures are frequently less than 1,000 psi and the reservoir rock may be susceptible to damage from extraneous water. Fortunately, the advent of air drilling and improved hydraulic fracturing techniques has corresponded to the expanding gas markets, and these innovations are largely responsible for the success to date in providing commercial gas completions in these formations. providing commercial gas completions in these formations. They are not cure-alls, however, and billions of cubic feet of potential reserves will be untapped until new or improved stimulation techniques are developed. To this end, stimulation by chemical and nuclear explosives has gained attention. Continental Oil Co. owns or operates leases on considerable acreage located on the Douglas Creek Archin Rio Blanco County, northwestern Colorado (Fig. 1). in the early 1960's, the company began working to perfect a hydraulic fracturing technique applicable to Mancos "B" formation gas wells in the area. The Mancos "B" reservoir rock not only has low permeability but also is thick, water-sensitive, and of permeability but also is thick, water-sensitive, and of subnormal pressure. By applying principles only recently developed, these problems were overcome with a pseudolimited-entry fracturing technique, which is pseudolimited-entry fracturing technique, which is discussed here.

The Mancos "B" Reservoir
For the most part, the Cretaceous Mancos formation consists of a gray to black bentonitic marine shale, and on the Douglas Creek Arch it is about 4,000 ft thick. Somewhat below midway through the formation, the shale becomes thinly laminated with a very fine-grained, argillaceous, quartz sandstone containing 10 to 20 percent carbonates. This sand-shale sequence, which is about 400 ft thick, has been termed the Mancos "B" zone at Douglas Creek and is gas-productive on the Central Arch. Fig. 2 shows a section of typical Mancos "B" core. The top of the Mancos "B" is encountered 2,000 to 3,000 ft below the surface, depending on structural position and surface elevation. The average effective permeability over the 400-ft gross interval is about 0.7 md, with porosities of 10 to 11 percent and 50 percent water-saturation as determined from core data. Significantly, the reservoir pressure is an abnormally low 437 psia (0.17 psi/ft). In addition, the formation rock is extremely susceptible to damage from extraneous fluids.
JPT
P. 185