Transport in Shale Reservoirs

Introduction

The pores in shale reservoirs are in the range of 1->100 nm so that the size of gas molecules contained in the pores (approx. 0.5 nm) is comparable with pore size. Under certain pressure and temperature conditions, the distance between hydrocarbon molecules (mean free path) exceeds the size of the pores. In such circumstances, the gas molecules might move singly through the pores, and the flow behavior in nanopores deviates from the concept of continuum and bulk flow.

Some important definitions are:

Viscous Flow (Darcy Flow): Simply stated, this is flow governed by a pressure gradient, and is typically Darcy Flow. This flow regime may also be referred as Convective flow. In other words, viscous forces dominate over inertial forces. Flow velocity at the walls of a pore is zero (no slippage).
Knudsen Diffusion: Flow is in a transition regime (partial Knudsen diffusion) or total Knudsen diffusion. In this case the mean free path of a gas molecule is large compared to pore size (I.e., the molecule can change directions [colliding with other molecules] in relatively short distances).
- A Knudsen number is often used to distinguish flow mechanisms in the non-absorbed (free gas) phase.
Klinkenberg Slippage Effect: This effect is due to slip flow of gas at pore walls which enhances gas flow when pore sizes are very small, and is related to Knudsen Diffusion. That is, in certain scenarios, gas slippage occurs at the pore wall. With respect to RTA methods, it tends to manifest itself as a reduced effective permeability.

References:

Kun Sang Lee, Tae Hong Kim, Transport in Shale Reservoirs, Gulf Publishing Company, 2019.
T. A. Blasingame, Reservoir Engineering Aspects of Unconventional Reservoirs, SPE Webinar, 27 January 2015.
Freeman, Craig Matthew (2013). Study of Multi-scale Transport Phenomena in Tight Gas and Shale Gas Reservoir Systems. Doctoral dissertation, Texas A & M University
Freeman, Craig M. (2010). Study of Flow Regimes in Multiply-Fractured Horizontal Wells in Tight Gas and Shale Gas Reservoir Systems. Master's thesis, Texas A&M University