Modeling Production and Evaluating Fracture Performance in Unconventional Gas Reservoirs

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract This article examines issues with forecasting and evaluating production from unconventional gas reservoirs such as those in the Barnett shale. How can reservoirs be commercial with matrix permeability measured not in millidarcies or even microdarcies (10-3md), but nanodarcies (10-6md)—typically, 10 to 100 nanodarcies? The key is maximizing the reservoir area that is connected to the wellbore by creating a very large manmade- fracture network. But how do we create a fracture network? The answer is large-volume high-rate hydraulicfracture treatments that use water and small-mesh proppant to "activate" or stimulate the existing natural fractures or rock fabric. The created fractures are far from the classic "planar" model; they are large, complex flow networks that typically encompass 50 acres or more where the rock has been "broken." Gas production is a function of the number and complexity of fractures created, the effective fracture conductivity, and the matrix permeability. Understanding the relationship between gas recovery and fracture complexity, fracture conductivity, and matrix permeability is a key component of economic shale-gas development. This article will highlight the application of reservoir simulation to model production in shale-gas reservoirs, providing significant insights into these relationships that can improve stimulation designs, completion practices, and field-development strategies. Currently, most shale-gas resources are being developed with horizontal wells, and the reservoir simulations in this article focus on horizontal completions. Introduction The exploitation of unconventional gas reservoirs has become an ever increasing component of the North American gas supply. Gas shales are organic-rich shale formations and are apparently the source rock as well as the reservoir. The gas is stored in the limited pore space of these rocks, and a sizable fraction of the gas in place may be adsorbed on the organic material. The success of the Barnett shale has led to the development of other shale plays in North America, including the Woodford, Haynesville, Fayetteville, and Marcellus. The natural-gas resource potential of gas shales is estimated to be between 500 and 1,000 Tcf (Arthur et al. 2008).