Abstract

This paper describes a new laboratory technique to evaluate the properties of a naturally fractured, low permeability core sample. We specifically determine (1) the porosity of the matrix, (2) the permeability of the matrix, (3) the effective width of the fractures, and (4) the permeability of the fractures.

New laboratory equipment has been designed and constructed to conduct pressure pulse tests in either a homogeneous or a fractured core sample. Analytical solutions have been developed to model gas flow in a fractured core sample during a pressure pulse test. An automatic history matching program has been developed to analyze the laboratory measured pressure transient data using the analytical solutions. The new technique has been used to measure the matrix and fracture properties in twelve naturally fractured, Devonian Shale cores.

The technique we developed in this research is new to the petroleum industry. Our laboratory equipment is unique, and the analytical solutions have not been published in the literature. With this technique, we can measure matrix properties as low as md. This is a significant step forward in permeability measurement because the lowest permeability that most existing laboratories can measure is about millidarcies.

Introduction

Oil and gas production from naturally fractured reservoirs is an important source of energy throughout the world. Petroleum engineers need to improve their understanding of naturally fractured reservoirs to better predict oil and gas flow rates and reserves. The porosities and permeabilities of the matrix and fractures are key parameters used in reservoir simulation models to predict the performance of naturally fractured reservoirs.

The most reliable and direct way to determine the formation properties is to cut a core from the reservoir and to measure the properties in the laboratory. However, conventional laboratory methods cannot be used to measure the matrix and fracture properties in a naturally fractured core. If a core sample contains a natural fracture, existing laboratory methods can only measure the effective permeability of the core sample. The effective permeability will be the thickness weighted average permeability of the matrix and the fractures. The specific properties of the matrix and the natural fractures cannot be distinguished using existing laboratory techniques.

In 1990, Kamath et al. first showed that the pressure transient behavior of a pressure transient test in a fractured core was different from that in a homogeneous core if the equipment is properly designed. They calculated the pressure responses for homogeneous and fractured cores using a finite difference model. They conducted measurements with an artificially split sandstone sample and matched the experimental data with numerical solutions to obtain the fracture and matrix properties.

Hopkins et al. conducted an extensive numerical study on the laboratory pressure pulse testing for evaluating low permeability, naturally fractured core samples.

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