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Abstract

In this paper, we present a case history of integrating numerical and analytical well test analysis methods to interpret the pressure and production history of a complex, multilayer, multiphase reservoir. Some engineers believe that recent improvements in analytical models have made numerical modeling for well test interpretation all but obsolete. Others believe just as strongly that numerical models should be used for routine well test interpretation. Still others believe that both tools are useful, and that a combination of the two approaches provides the best general strategy for reservoir characterization.

This field case focuses on the Gas Research Institute (GRI) Staged Field Experiment (SFE) No. 2. This well was hydraulically fractured. A variety of different analysis techniques were applied in order to evaluate the consistency between reservoir and fracture properties estimated by the different methods of analysis.

Analysis methods applied to the flow and buildup tests performed on this well include type curve matching using analytical solutions, numerical modeling with single-phase, multi-layer radial and Cartesian grids, and numerical modeling with multi-phase, multi-layer radial and Cartesian grids. The simpler models were able to match segments of the production and pressure history; only the full multiphase, multilayer model was able to satisfactorily match the complete production and pressure history.

We have found that an approach combining analytical and numerical models has been the most successful strategy in our work. As in this field case, analysis begins with the simple models and proceeds through more complex models as necessary to match the reservoir behavior in the desired detail.

Introduction

At the May, 1998, SPE Pacific-Asia Forum on “integrating Well Testing and Reservoir Characterization,” there was an entire session devoted to use of numerical models in well test analysis. The reason for the session was that some engineers feel that, given the increasing sophistication of analytical models available in commercial software, we can obtain a reservoir description consistent with available data in most if not all caaes. That is not the same as saying it is the “correct” reservoir description; that, we will never know because we will never have sufficient data. However, some feel that a sufficient analytical model can almost always be found.

Other engineers believe that numerical models are all we need today, and that we can use them to history match test data to obtain the reservoir description we desire. Ths camp believes implicitly that real reservoirs have complexities than cannot be captured with the most comprehensive analytical models. Therefore, we should include in our model all the complexities of which we are aware, and proceed to adjust reservoir parameters until we match the data, completely bypassing the step of test interpretation with analytical models.

To still other engineers, a combination of the two approaches is of considerable value in certain situations with complex reservoir geometry or multiphase fluid flow. This paper discusses a case history in which the combination approach proved valuable. The case history summarized in this paper is an analysis of a hydraulically fractured, low-permeability gas reservoir. The analysis was done by Peter Bastian and Jan Sherman and is summarized in reference 1 and discussed in more detail in reference 2. I have selected this particular case history to illustrate how analytical and numerical methods can be combined because the case is typical of reservoir characterization needs in tight case formations; i.e., the formation characteristics in this example arise repeatedly in practice.