Abstract

This paper presents a new test procedure designed to measure individual layer reservoir properties (permeability, skin factor, and reservoir pressure) in a single wellbore and in a reasonable amount of time for low permeability gas wells. The method involves (1) isolating a small interval with packers, either in an open- or cased-hole, (2) injecting nitrogen into the interval at a rate such that the bottomhole pressure builds up to a predetermined value (below fracturing pressure), and (3) shutting in the well, preferably downhole, and monitoring the pressure falloff. The pressure falloff data is then analyzed to determine reservoir properties. Essentially, this method can be described as a nitrogen slug test and can be applied to any formation where single-phase gas is produced.

We have found that average reservoir properties determined with conventional analyses of a single pressure buildup test conducted over a large interval can lead to optimistic predictions of the future performance of a well producing from a layered reservoir system. Therefore, being able to quantify individual layer properties can lead to better reservoir characterization, modified completions, and improved performance projections. Existing techniques, such as multiple pressure buildup tests, are inadequate for measuring reservoir permeability in several intervals in a single wellbore because, often in low productivity gas wells, either a pre-stimulation rate is not attained or the time required to reach the correct semilog straight line for a conventional well test analysis is too long. Knowledge of layered reservoir properties impacts completion decisions (where to perforate), fracture design considerations, and post- fracture analyses.

Introduction

In 1991, the Gas Research Institute (GRI) began a three-well research program in the Devonian Shales and Berea Sand in the Appalachian Basin of Eastern KY. From a reservoir engineering perspective, the primary objective of this research was to obtain a better understanding of the physical properties controlling well performance. Based on prior research, we learned that knowledge of the permeability-thickness (kh) distribution within the completion interval is important to understanding and predicting the pre- and post-fracture productivity in multilayered reservoirs. Typically, pressure buildup tests in the Shales conducted over large perforated intervals, on the order of 200 to 300 ft and sometimes even up to 700 ft, result in an average permeability over the entire interval. Although this value is useful, it normally leads to optimistic performance projections. It has been shown that an equivalent single-layer reservoir model is not as accurate when describing a well completed in a multilayer reservoir. Usually, the single-layer model will be optimistic. So, in order to improve the reservoir characterization of the Devonian Shales, one of our main goals was to measure or determine permeability in smaller intervals. To accomplish this goal, we set out to design a test procedure that met two fundamental requirements:

  1. The test had to be relatively short, on the order of hours as opposed to weeks, since we wanted to test many intervals in a single wellbore, and

  2. The test interval had to be small, since one of our goals was to describe the reservoir in as much detail as possible.

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