Development Of A Rationale For Stimulation Design In The Devonian Shale
Authors: Komar C.A. 
Published: December 2007

Natural gas productivity of eastern Devonian shale wells depends on the density and extent of natural fractures within the formation, the organic content of the shale matrix and the effective communication of the wellbore with the natural fracture system. Stimulation techniques are normally required to develop this communication. A strategy for testing stimulation techniques is currently being developed. The strategy for a geologic region involves the selection of the best stimulation technique for specific geologic conditions. This selection will be based on the thickness of the organic rich shale units within the formation (recognized as the source of gas) and the degree of fracture density (recognized as the reservoir) as controlled by various geologic elements.

Limited production history exists to support a strategy with certainty and confidence in the Devonian shale. However, data on initial openflow rates are available to compare explosive and hydraulic fracturing in both low and high frac density regions in the shale. These data have been accumulated on field tests conducted at random in DOE/industry projects. Tests included fracturing with displaced liquid explosives and also tests wherein cryogenic, water, and foam fracturing techniques have been used in both conventional and massive quantities. Analysis of this data shows that for regions of low fracture density, hydraulic fracturing is a slight improvement over borehole explosive stimulation treatments. Results from hydraulic fracturing may be further enhanced whenever retention time of the fracturing fluid in the formation is reduced. This suggests that low residual fracturing fluids such as foam or cryogenic treatments may be preferred for the low pressure shale reservoir. In regions of higher organic content, chemical explosives and hydraulic fracturing with cryogenic fluids are improvements over borehole explosive stimulation treatments, but the degree of improvement may not be justified by the required added costs. These observations constitute the early trend. Additional data and more detailed studies will be required in regions of different organic thickness and fracture density to reduce the uncertainty associated with this stimulation rationale.

Introduction

The objective of stimulation in the Devonian shale formation is to initiate and propagate fractures which will intersect the natural fractures that are thought to be the primary source of gas containment. The prime goal is to intersect as many natural fractures as possible to provide a network by which natural gas can flow to the borehole.

Factors governing natural gas accumulations in the Devonian shale are thought to be dependent on earth stresses affecting primary and secondary gas migration and on the types of gas traps in the area. Primary gas migration consists of the transfer of the gas from the source rocks whereas secondary migration consists of rearrangement of the gas within the reservoir and accumulation into the structural traps to form producing fields. Knowledge of the mechanisms of primary and secondary gas migration is important in the exploration process. process. Much of the shale formation is naturally micro-fractured. It has a high secondary, or fracture porosity. Formation discontinuities consist of porosity. Formation discontinuities consist of microfractures, joints, bedding plans, faults and slickensides. These voids offer an excellent system of circulation conduits for gas movement. Thus, production of gas from fractured shales warrants high priority. Production experience from naturally fractured regions has shown that formation stimulation by wellbore enhances drastically the gas flow rates. This is evident from historical production in counties located within eastern Kentucky and western West Virginia (Table 1).