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Abstract

In order to evaluate the effectiveness of a hydraulic fracture treatment, one must be able to correctly analyze post-fracture production and pressure buildup data. In many cases, conventional analysis techniques or simple single-phase, two-dimensional models are all that is required to obtain satisfactory results. However, in complex reservoir systems, such simplified techniques may provide incorrect solutions. As part of a research program sponsored by the Gas Research Institute (GRI) a substantial engineering effort is being exerted towards better understanding of complex reservoir systems. The thrust of this work has been in the Travis Peak formation of east Texas which is a multi-layered reservoir that normally produces both gas and water. To properly analyze this formation, a three-dimensional and/or two-phase reservoir simulator is needed.

This paper presents a thorough discussion of the problems associated with the analysis of complex reservoirs and illustrates these problems with numerous field examples. Examples are included in which short, high conductivity fractures are calculated when conventional analysis techniques are used. However, a longer, more realistic estimate of fracture length is computed when one uses a multi-phase, multi-dimensional reservoir model to accurately history match the gas and water flow rates as well as the wellbore pressures.

Introduction

For several years, the Gas Research Institute (GRI) has been conducting research directed at increasing the recovery efficiency and reducing the production costs of gas from low permeability reservoirs. One of the primary goals of the research is to improve the technology surrounding hydraulic fracturing. However, in order to better understand the hydraulic fracturing process, it is first necessary to improve our understanding of the reservoirs that are being fracture treated and the rock layers surrounding the main productive interval. To reach this goal comprehensive geological, coring, logging, well testing, fracture treatment monitoring and fracture diagnostic studies are being performed on selected cooperative research wells.

The Travis Peak formation in east Texas and north Louisiana has been the primary focus of this research. The Travis Peak is a complex sand dispersal system that is typically 1500-2000 ft. thick and contains both lenticular and blanket sandstones. Some of the sandstones are high permeability intervals, but a vast majority of the sands are actually low permeability. On a regional basis, most of the Travis Peek is gas bearing; however, a few sandstone intervals in almost every well will produce water. It is very little wonder that almost every engineer and geologist who has dealt with the Travis Peak has quickly concluded that it is a very complex system of reservoirs.

In order to effectively analyze a multilayered reservoir system, it is absolutely essential that the formation be clearly described in three dimensions. Three-dimensions reservoir models are thus sometimes needed to analyze the effect of hydraulic fractures in these layered reservoirs. In formations where substantial volumes of water or oil are being produced along with the natural gas, multi-phase reservoir models may be needed to correctly analyze the production and pressure buildup data.

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