Fractured Gas-Reservoir Performance
Authors: Thomas F.B. , Bennion D.B. 
Published: March 2010

Gas production from fractured reservoirs can be very prolific despite the extremely low matrix permeability that may be encountered. In order to quantify production, the interaction between matrix and fracture must be well understood. Knowing the limitations to production is important for operators who drill into very tight matrix reservoirs with varying degrees of natural fractures.

This paper describes a laboratory protocol for assessing matrix-fracture interaction and discusses field implications. The well-known pulse decay technique was used for determining matrix-fracture interaction by fitting parameters from a simple numerical model to experimental data. On this basis, the values of four parameters were regressed: fracture permeability, matrix permeability, mass transfer from matrix to fracture and the fraction of the porous media that was fracture. A full discussion of the mathematical approach, optimization technique and experimental protocol is included.

It was concluded that the technique has significant non-unique convergence characteristics which make the interpretation of core parameters dependent on the shape of the experimental production history and, therefore, introduces some subjectivity. Results indicate that fracture permeability is relatively insensitive to water saturation whereas matrix permeability and the matrix-fracture transfer function are extremely sensitive to increasing water saturation; the latter two parameters become rate-limiting once early-time fracture production is complete.

Background

The impetus for this work is experimental. The authors ’ objective was to investigate an experimental approach, known as pulse decay, in order to try and deduce parameters associated with gas flow from fractured porous media and to do so quickly and accurately. This would involve gas flow in the absence of any liquid phase, as well as gas flow in the presence of immobile water saturation. These measurements are important to operators of dry gas fractured reservoirs, especially in the presence of mobile water either naturally occurring or induced during drilling or hydraulic fracturing. Quantifying phase interference effects is routinely done empirically, but the state-of-the-art leak-off tests are time consuming and can be expensive. The authors wanted to investigate the merit of being able to obtain a series of relevant parameters through a simple, inexpensive approach such as pulse decay.

Single-phase flow in porous media is well known. For gas flow in porous media, potential areas for obfuscation include gas-solid slip at the wall and turbulence effects. The Klinkenberg protocol is a very effective means of removing the effects of gas-slippage which causes deviation from the assumptions inherent in using the Darcy equation (laminar flow with no flow at the wall). Figure 1 shows a standard Klinkenberg plot done for a sample analyzed in this work. The significance of this correction is that if the pore pressure is high enough, the gas will act more like a liquid and, at low rates, will behave in a laminar manner which requires the development of a quadratic velocity profile in the flow conduits and a no-slip condition at the solid surfaces. Figure 1, with its associated trend-line, shows that by extrapolating to infinite pore pressure where any gas would condense, the effective permeability for this sample is 41.2 mD.