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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. This paper describes a new fully-coupled reservoir/wellbore single-well analytical simulator for multilayer unconventional gas reservoirs that automatically history matches production and production log data simultaneously.

The layers in the simulator are independent, with different temperatures, pressures, completion types, and reservoir geometries. The simulator rigorously accounts for transient and boundary-dominated flow in each layer and pressure drop in the wellbore, including pressure drop between adjacent layers and pressure drop from the reservoir to the surface.

By history matching data from multiple production logs as well as surface production data, the simulator provides accurate estimates of individual layer properties such as permeability, fracture length, and drainage area. The simulator may be used for reservoir and fracture treatment evaluation, restimulation candidate selection, fracture design optimization, and performance forecasting in multilayer unconventional gas plays.

Introduction

Most gas wells in North America produce from low-permeability or other unconventional gas reservoirs. These reservoirs present many challenges in drilling, completions, and reservoir evaluation. To produce at economic rates, gas wells in unconventional reservoirs must be stimulated, typically by hydraulic fracturing. Stimulation treatments represent a significant fraction of the total cost of drilling and completing the well. Thus, whether or not a frac job was successful is a question of great interest to the operator.

For single-layer tight gas reservoirs, fracture and reservoir properties are usually estimated by analyzing production data. Methods for such single-layer analysis include advanced decline curve analysis using constant terminal pressure type curves,[1,2,3] type-curve matching using constant terminal rate type curves[4,5,6,7] and automatic history matching using a single-layer analytical reservoir simulator.[8,9,10] Single-layer analysis methods may provide estimates of in-situ permeability to gas, fracture half-length, fracture conductivity, and drainage area. These properties are used in evaluating the success of a fracture treatment, in selecting restimulation candidates, in optimizing future fracture treatments, in forecasting future performance, and in estimating reserves.