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Abstract
Most of the major unconventional gas plays in the U.S. have multiple productive intervals. Wells are completed with multiple fracture treatments, and are produced commingled.

An earlier paper1 described a single-well, multilayer, analytical reservoir simulator* for estimating individual layer properties in multilayer unconventional gas reservoirs by automatic history matching production and spinner survey data simultaneously. The present paper describes several interactive graphical aides that greatly facilitate the history matching process when used in conjunction with the analytical, multilayer reservoir simulator. These graphical tools include an allocation graph and layer and cumulative production log graphs. These graphs allow the engineer to quickly review the quality of the match at any point during the history matching process.

Use of these graphical tools has improved the history matching workflow to the point that a complete history match of a well with 20 frac stages, two years of daily production, and three spinner surveys can be performed in as little as four to six hours, from construction of the initial data set through completion of the final match.

Introduction
Wells in low permeability and other unconventional gas reservoirs are often completed by commingling production from multiple intervals, as shown in Fig. 1. In many cases, intervals are too far apart vertically to be produced by a single hydraulic fracture, so multiple fracture treatments must be used. Estimating reservoir and fracture properties for individual layers in such reservoirs is crucial to evaluating fracture treatment effectiveness, optimizing future stimulation treatments, and forecasting future performance.

Fig. 1-Gas well in a multilayer unconventional gas reservoir with commingled production.

In single-layer unconventional gas reservoirs, production data analysis has been one of the most useful tools for estimation of reservoir and fracture properties. Single-layer production data analysis methods include type curve matching using constant pressure type curves2-5, type curve matching using constant rate type curves6-9, and automatic history matching using an single-layer analytical reservoir simulator10-13.

To estimate individual layer properties for wells with commingled completions, production log (spinner survey) data must be incorporated into the production data analysis workflow to separate the contributions of the individual layers. One method that has been proposed is to allocate production to individual layers by interpolation using the spinner survey data, then analyze the resulting reconstructed individual layer rate histories using single-layer methods14-16. Unfortunately, if there is a strong contrast in layer properties, the allocation method will not accurately reproduce the individual layer flow rates, thus leading to erroneous estimates of layer properties1.