Abstract
Gas-producing mudrock systems are playing an important role in the volatile
energy industry in North America and will soon play an equally important role
in Europe. Mudrocks are composed of very fine grained particles, and their
pores are very small, at the scale of nanometers. Gas production from these
strata is much greater than what is anticipated given their very low Darcy
permeability. In this paper, images of nanopores obtained by Atomic Force
Microscopy (AFM) are presented for the first time. Gas flow in nanopores cannot
be described simply by the Darcy equation. Processes such as Knudsen diffusion
and slip flow at the solid matrix separate gas flow behaviour from Darcy-type
flow. We present a formulation for gas flow in the nanopores of mudrocks based
on Knudsen diffusion and slip flow. By comparing this new gas flow formulation
and Darcy flow for compressible gas, we introduce an apparent permeability term
that includes the complexity of flow in nanopores, and it takes the form of the
Darcy equation so that it can easily be implemented in reservoir simulators.
Results show that the ratio of apparent permeability to Darcy permeability
increases sharply as pore sizes reduce to smaller than 100 nm. Also, Knudsen
diffusion's contributions to flow increase as pores become smaller. Unlike
Darcy permeability, which is a characteristic of the rock only, permeation of
gas in nanopores of mudrocks depends on rock, gas type and operating
conditions.
Introduction
In general, very fine grained sediments (<62.5 µm) are collectively
referred to as mudrocks, which show no fissility (paperlike parting) and are
commonly classified as mudstones; those that show fissility are commonly
classified as shales. The reader is referred to Folk(1), who
developed a simple classification of mudrocks. The term mudrocks, rather than
shales, for unconventional gas-producing strata is used in this paper to be in
line with the scientific classification acceptable in the geosciences.
The existence of nanopores in mudrocks has been revealed recently by ultra-high
pressure mercury injection(2, 3) and back-scattered scanning
electron microscopy(4). In this paper, for the first time we show
nanopores and nanogrooves detected in mudrocks using atomic force microscopy
(AFM)(5). Now that we are confident that such small pores exist in
mudrocks, the challenge is to understand and develop governing equations to
describe gas flow in these small pores. We present new formulations for gas
flow that include some complexities that were ignored in developing the Darcy
equation.
At equilibrium, gas molecules are distributed throughout strata, as illustrated
in Figure 1. Gas molecules occupy pores as compressed gas, cover the surface of
the kerogen materials as adsorbed gas and disperse in the kerogen materials as
dissolved gas. Drilling a well or inducing a fracture disturbs the equilibrium,
and gas molecules start flowing toward the low pressure zone. First, the freely
compressed gas in the pores is produced. Then, the gas molecules on the surface
of the kerogen walls desorb and increase pore pressure(2). Gas
desorption changes the concentration equilibrium between the bulk of the
kerogen and its surface, as illustrated in Figure 1.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
15 June 2009
- Manuscript approved:
23 June 2009
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