Abstract

This paper critically assesses the importance of various inputs that are used for a common methodology to develop a simulator model of hydraulic fractures in geologically complex, fluvial, tight gas reservoirs. A planar 3-D fracture simulator is used with a fully coupled fluid/solid transport simulator. The geomechanical rock properties from logs (Young's modulus, Poisson's ratio and Biot's constant) and diagnostic mini-frac injection tests of individual sandstone reservoirs were investigated to assess their importance in developing a valid stress model.

The work describes the investigations using a model previously matched using both net surface pressure and microseismic/tiltmeter data. From these results it is possible to get a better understanding of how fracs grow and interact with complex fluvial reservoirs, allowing operators to better optimize field well performance and completion methods in these geologic settings. Additionally, the minimum critical data recommendations necessary to develop such a model have been identified and will aid operators in developing their data acquisition programs. Although developed in the Rocky Mountain region, the presented technique can be extrapolated to other similar geologically complex reservoirs world-wide.

Introduction

Currently, engineers have to undertake ever more complete hydraulic fracture (HF) evaluation in order to fully evaluate the well potential, as well as the effectiveness of the treatment design for creating the desired fracture. Geologically complex reservoirs require both an in-depth knowledge of fluid mechanics and the reservoir rock mechanics. It is only recently that the hydraulic fracture engineer has had the tools available to effectively model reservoirs using fully three-dimensional models, together with improvements in reservoir analytical techniques.

The overall objective of this study is to undertake sensitivity analyses of a model previously matched using a methodology practitioners consider to be accurate for developing a threedimensional (3D) hydraulic fracture (HF) simulation of a geologically complex reservoir. Previous work has been carried out in the Piceance basin1 and other geologically similar tight gas reservoirs2 but the techniques used in this study are those presently applied to generate input data for an accurate model of hydraulic fractures in complex, fluvial, tight gas reservoirs. The simulator outputs were initially matched with direct diagnostic results from fracture mapping, to help constrain the model and aid in the simulator output matching process.3 In that model, six different intervals containing several production zones each were hydraulically fractured and analyzed. This work is part of ongoing research to help operators and researchers identify theminimum data necessary to effectively model and optimize hydraulic fracture treatments in geologically complex reservoirs.

The Department of Energy (DOE)/National Energy Technology Laboratory (NETL) sponsored a number of projects during the 1980's and a significant amount of the research work was undertaken at the Multi-Well Experiment (MWX) site near Rifle, Colorado4. This multi-disciplinary research carried out in the Piceance basin has gone a long way in helping stimulation technology development in tight gas reservoirs. However, further development of stimulation technology requires models that can be used to analyze, target and optimize hydraulic fracture treatments as well as predict well production. This study assesses the inputs required to model a well in these complex geological systems and make a recommendation for the minimum data necessary to develop a fully 3D model of a hydraulically fractured well.

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