TABLE OF CONTENTS Production from tight gas reservoirs is possible in many cases thanks to the presence of natural fractures. Many of these reservoirs are also multi-layered adding a great deal of complexity to the evaluation. The problem we face from a practical point of view is that productivity is usually low and as a consequence it is non-economic to spend significant amounts of money in all wells to rigorously characterize the reservoir.
Fortunately funding provided by the U.S. Department of Energy (DOE) and the Gas Research Institute (GRI) has led to research and improved techniques to evaluate and forecast performance of tight gas reservoirs in the United States. These techniques should also prove valuable in other countries.
Tight reservoirs can be grouped into (1) Devonian shales and (2) tight gas sandstones. Because of the low productivity of tight reservoirs I place emphasis on techniques that in my experience have proven cost-efficient although they are not necessarily highly rigorous.
Fractured shales have produced gas since the early 1900s along the western margin of the Appalachian Basin. The amount of gas-in-place in this basin was originally estimated at 460 quadrillion scf (Schumaker, 1976). Subsequent estimates indicates values of gas-in-situ ranging between 585 and 2500 trillion scf (USGS report 82 474, 1982). More recent estimates range between 200 and 1860 trillion scf (de Witt, 1986). The point is that there is a large uncertainty with respect to the original volumes of gas-in-place and even more uncertainty with respect to the recoverable volumes. An extensive...
