Probabilistic Evaluation of Global Technically Recoverable Tight Gas Resources
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 165704, ’Probabilistic Evaluation of Global Technically Recoverable Tight Gas Resources,’ by Zhenzhen Dong, SPE, Schlumberger, and Stephen A. Holditch, SPE, and Walter B. Ayers, SPE, Texas A&M University, prepared for the 2013 SPE Eastern Regional Meeting, Pittsburgh, Pennsylvania, USA, 20-22 August. The paper has not been peer reviewed.
There are no reliable estimates of technically recoverable resources (TRRs) for unconventional reservoirs outside North America, and many countries lack the advanced technology such as horizontal drilling and hydraulic fracturing necessary to develop unconventional resources. To encourage investment in these technologies, new resources estimates are needed. The objective of this study was to develop a methodology and software for probabilistic resource estimation and to use this new technology to estimate global technically recoverable gas resources in tight sands.
Introduction
In this paper, tight gas is gas that is present in low-permeability sandstone or limestone formations. In the US, tight gas reservoirs are generally defined as having less than 0.1-md permeability. To produce gas at economic rates from reservoir rocks with such low permeability, massive hydraulic fracturing is necessary.
Petroleum Resources Management System (PRMS). The terms “resources” and “reserves” have been and continue to be used to represent various categories of mineral or hydrocarbon deposits. In March 2007, SPE, the American Association of Petroleum Geologists, the World Petroleum Council, and the Society of Petroleum Evaluation Engineers jointly published the PRMS to provide an international standard for classification of oil and gas reserves and resources. It is important to remember that the broadest categories are also the least precise. As one moves toward the categories at the bottom of the PRMS, the associated estimates of the amount of natural gas in those categories become more and more uncertain. However, technically and economically recoverable resources are not formally classified in the system.
The Energy Information Administration’s (EIA) Classification System. According to the EIA, TRRs are the subset of the total resource base that is recoverable with existing technology. The term “resources” represents the total quantity of hydrocarbons that is estimated, at a particular time, to be contained in (1) known accumulations and (2) accumulations that have yet to be discovered (prospective resources). Economically recoverable resources are those resources for which there are economic incentives for production. It is important to note that economically unrecoverable resources may, at some time in the future, become recoverable as soon as the technology to produce them becomes less expensive or when the characteristics of the market are such that companies can ensure a fair return on their investment by extracting the resources. The authors of the original paper considered TRRs to be the resources that can be produced within a 25-year time period.
The authors rearranged categories of the PRMS, and presented an overview of how the estimates of technically and economically recoverable resources are broken down. Those commercial resources, including cumulative production and reserves, are economically recoverable resources. TRRs are the subset of the total resource base that includes commercial resources, contingent resources, and prospective resources. Estimated ultimate recovery is not a resources category but a term that refers to the quantities of petroleum that are estimated to be potentially recoverable from an accumulation, including those quantities that have already been produced.