An Investigation Of The Gas Producing And Storage Mechanism Of The Devonian Shale At Cottageville Field
Authors: Ancell K.L. , Price H.S. , Ford W.K. 
Published: December 2007

This paper presents the results of a simulation study of the producing and storage mechanisms of the Devonian Shales at Cottageville Field in West Virginia. The study compares the performance of a system in which the gas is stored in a macrofracture system which is also the flow channel to wells with that of a system in which the gas is stored primarily in an adsorbed state in a micropore structure and diffuses into a macrofracture system of low pore volume through which it migrates to wells. The results show that within the range of measured parameters, either system will adequately describe the twenty-six year production history at Cottageville Field. Further it production history at Cottageville Field. Further it is virtually impossible to discern between the two mechanisms even though the system postulating adsorbtion as the storage mechanism predicts a much larger resource base.

Introduction

The Devonian Shale has been producing natural gas for over a century. This resource is characterized by extremely long life with low individual well deliverability. Because of the widespread occurrence of the shale throughout the Appalachian Region, it has been identified as a potential source of increased domestic natural gas production. Because of its potential, the U. S. Department of Energy and its predecessors initiated its Eastern Gas Shales Project.

As a part of this project, a study of the producing and storage mechanisms was commissioned. producing and storage mechanisms was commissioned. Several mechanisms have been postulated. It was the objective of this study to investigate the most plausible of these postulations to see if any could plausible of these postulations to see if any could explain the long-term behavior of this type of natural gas production.

There are two widely held theories for the occurrence of the gas in the Devonian Shales. The first theory is that the shales are a conventional gas reservoir and that the porosity is created by a macrofracture system which is also the flow channel to the wells. Under this theory the gas is stored by compression only. This is known as a single porosity system.

The second theory is that the shales contain the gas as an adsorbed state in a micropore structure. The adsorbed gas diffuses into a macrofracture system of low pore volume and then migrates to the well in response to a pressure differential. The second theory is referred to as a "dual" porosity system.

A reservoir simulator which incorporates the adsorbed gas concept with conventional Darcy flow in porous medium was used to determine the properties of porous medium was used to determine the properties of the dual porosity system. An actual field example was used to test the validity of both theories.

SIMULATOR DESCRIPTION

The production of gas from a dual porosity system is believed to be dependent upon two distinctly different physical processes,diffusion from the interior of a solid particle to a crack or macropore in the rock, andDarcy flow through the fracture or macropore structure to a production well.

Diffusion of methane through solid particles of coal is thought to be a much slower process than the fracture flow. Depending on particle size, it may or may not be the controlling factor in production. Diffusivities have typically been measured by grinding rock particles to a uniformly small size and comparing particles to a uniformly small size and comparing rates of desorption to analytical solutions for diffusion in a sphere of comparable diameter.

The differential mass balance describing diffusional transport in a sphere is as follows:

............(1)

The concentration of methane, C, is expressed as moles/unit volume of rock. The boundary conditions for this equation are as follows:

......................(2)

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