Abstract

Generally, water saturations in the Devonian Shale are low. Production records indicate minimal, if any, water in the areas producing gas or gas and oil. This low water saturation appears to be the key to stimulating the shale, especially in the oil-producing areas.

The introduction of water in the stimulation fluid appears to reduce the permeability to oil, which is reflected in poorer production. The relative permeability reduction seems to be more of a problem than particle migration or clay swelling and could explain the good initial results from straight nitrogen treatments. However, the lack of a proppant, even with low closure stress, leads to very rapid declines.

Water-base stimulation fluids appear to increase water saturation in the Devonian Shale. Use of 90+ quality foam with sand should provide a method of minimizing saturation changes while creating a propped fracture. Initial results indicate this technique provides better sustained production increases in the Devonian Shale. production increases in the Devonian Shale. This paper defines areas of production, describes the geology and presents physical data of the Devonian Shale. It also compares results of several types of treatments that have been used in the Devonian Shale. These results indicate 90+ quality foam with sand should provide an improved stimulation technique for this formation.

Introduction

Most publications concerning the Devonian shales agree that production from the shales is largely due to storage effect of the existing natural fractures. The actual rock matrix is a secondary production source with very low permeability and porosity.

A major goal of stimulating the Devonian Shale is to propagate a long fracture which will interconnect as many of the existing natural fractures as possible. Low reservoir pressure, low water saturation and the natural fracture system are major concerns. Considerations in treatment design include low water content, good leakoff control, good recovery of fluids pumped and effective placement of proppants. The 90+ quality foam with sand appears to provide these advantages.

Discussion

The Devonian Shale has been productive primarily in Ohio, West Virginia and Kentucky (Figure 1). The shale, while being similar in formation characteristics throughout this region, can be divided into areas of gas production and oil and gas production in the Marietta area (Table 1). Both of these areas generally have low water saturations and show little or no water production.

Shooting and Waterfrac treatments with sand have been widely used as stimulation methods. Shooting seemed to work where there was a high density of natural fractures in the proximity of the wellbore. Waterfracs with sand attempted to develop more main fracture length and met with increased success. Penetration, however, was often limited by high rates of leakoff due to the natural fractures present. These early Waterfrac treatments indicated that clay swelling and plugging by particle migration should be controlled but were not the major explanation for poor production results. The low reservoir pressure also presents a problem in that a large pressure also presents a problem in that a large percentage of fracturing water is never recovered. percentage of fracturing water is never recovered. As experience grew and as fracturing techniques improved, treatments with foam and sand, a stimulation method initially developed by Blauer and Durborow (U.S. Patent No. 3,937,283), were used with improved results. A significant improvement in initial well response was seen when nitrogen alone was used in fracturing treatments. This indicated damage might be caused by treatments using even the moderate amounts of water found in conventional foam.

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