The Effects of Non-Darcy Flow in Propped Hydraulic Fractures

Abstract

Hydraulic fracturing is being applied to progressively higher permeability formations. In many cases, productivity improvements are controlled by the achievable fracture conductivities. An analysis of the effects of non-Darcy flow in high rate oil and gas wells shows that in gas wells, fracture conductivities are dominated by non-Darcy effects and that effective conductivities are non-linearly dependent on proppant coverage. Laboratory tests to investigate non-Darcy flow effects in proppant packs have been performed at rates much closer to field conditions than hitherto reported; both single and two-phase flows are considered. Dry gas tests have demonstrated that there is a change in pressure loss behaviour and flow regime at high rates. Three types of test involving gas and water flows have been performed. First the effects water-saturated gas flowing in a proppant pack at residual water saturation were compared with results on the same pack with dry gas. Two series of experiments were then performed to examine the effects of mobile water in the gas stream. Pressure losses are shown to be very sensitive to the water rates, and the field implications are examined. Finally field data from tests performed on high rate, hydraulically fractured, gas wells are examined. Perceived fracture conductivities are related to estimated proppant coverage, and it is shown that non-uniform vertical distributions of proppant may have a large impact on the overall fracture flow capacity. INTRODUCTION With the major fields in the North Sea and on the North Slope now being on decline, the industry is being forced to consider marginally economic developments in which the performance of individual wells is of major singnificance. As a result, hydraulic fracturing is being applied to formations with far higher permeabilities than would formerly have been considered appropriate. In higher permeability formations, the productivity increases achievable by hydraulic fracturing are strongly dependent on the effective, or flowing, fracture conductivities that can be attained1. Increased fracture lengths are of little benefit in the absence of adequate fracture conductivities.

Pre-Darcy flow revisited under experimental investigation

“…This effect is generally termed as nonDarcy flow; however, in this study, we will refer to it as post-Darcy flow. Later studies (Holditch and Morse 1976;Guppy et al 1982;Martins et al 1990) have published the impact of post-Darcy flow on fractured gas wells. The literature already has effectively dealt with post-Darcy flow, and the reader is suggested to consult elsewhere for a more comprehensive treatment of the subject.…”

A Criterion for Non-Darcy Flow in Porous Media

“…It is obvious that this revised definition can be applied to all types of porous materials, as long as the permeability and non-Darcy coefficient can be determined experimentally, or empirically when no experimental data is available (Li and Engler, 2001). In fact, several researchers have expressed their preference for using this type of criterion (Geertsma, 1974;Martins et al, 1990;Gidley, 1991). For all these reasons, the Forchheimer number revised in Equation (11) is recommended as the criterion for non-Darcy flow in porous media.…”

Experiments and Modeling of High-Velocity Pressure Loss in Sandstone Fractures

“…To calculate the pinched-out skin we need the following additional data: kf =50 darcies, bf =3.7´10 5 m -1 , wf =0.03 m. The inertial resistance was obtained from the correlation 31 (34) where kf is in darcies and bf is in 1/m. Fig.…”