Combined Effect of Non-Darcy Flow and Formation Damage on Gas Well Performance of Dual-Porosity and Dual-Permeability Reservoirs
Authors: Pereira Carlos A. , Kazemi Hossein , Ozkan Erdal 
Published: October 2006

Summary
This paper addresses the combined effect of formation damage and non-Darcy flow in naturally fractured reservoirs using simplified analytical solutions and a 2D numerical simulator. Pressure drawdown, buildup, and isochronal tests simulated in this work indicate that, despite high fracture permeability, skin damage may accentuate the non-Darcy flow effect and drastically influence pressure-transient characteristics of low-pressure, naturally fractured reservoirs. In high-pressure reservoirs, this effect is significant only at high rates. Non-Darcy flow does not usually mask the typical pressure-transient characteristics of dual-porosity and dual-permeability reservoirs, but the conventional interpretation of the early-time data may lead to erroneous results. If the exponent, n, of the isochronal tests approaches 0.5 while the matrix permeability is low and flow rate is rather high, this would indicate the predominance of fracture flow. Under these conditions, small contributions from skin damage may greatly reduce gas-well performance in naturally fractured reservoirs.

Introduction
High velocity flow through porous media and fractures causes a higher pressure drop than predicted by the Darcy equation. This phenomenon, generally referred to as non-Darcy flow, was first described by Forchheimer (1901). Since then, it has been well established that the main variables that affect non-Darcy flow are the velocity, density, and saturation of the fluid and the permeability and porosity of the reservoir. Reservoir properties may be correlated to a single parameter, known as the non-Darcy flow coefficient, beta. Very little is known about the effect of other parameters, such as physical skin damage, on non-Darcy flow and their consequences in well performance. In fact, a recent literature review on non-Darcy flow by Li and Engler (2001a) indicates that most of the work has been focused on finding an accurate correlation for the non-Darcy flow coefficient, beta. There is also the issue of non-Darcy flow in dual-porosity and dual-permeability reservoirs, where high local velocities are prominent in the fractures. This paper pertains specifically to this issue.
In general, the lower the formation permeability, the greater the non-Darcy pressure gradient. Formation damage in the near-wellbore region causes a drastic reduction in formation permeability, which potentially could be even more prominent in naturally fractured reservoirs. Thus, a greater non-Darcy flow effect could result in the wellbore region of a dual-porosity reservoir. The literature explaining the combined effect of physical damage and non-Darcy flow in single-porosity reservoirs is abundant (Berumen-C. et al. 1989; Camacho-V. et al. 1993; Fligelman et al. 1981); however, there is little information about such effects in dual-porosity and dual-permeability reservoirs.
A finite-difference, 2D simulator in cylindrical coordinates was constructed to simulate pressure-drawdown and -buildup tests. By analyzing the simulated pressure drawdown and buildup tests, it was possible to decipher the combined effect of the skin damage and non-Darcy flow in fractured reservoirs. Both dual-porosity and dual-permeability idealizations of fractured reservoirs were considered.