Role of Natural Fractures and Slot Porosity on Tight Gas Sands

Abstract

Abstract Naturally fractured tight gas sands have come of age in the United States. They will also come of age in Canada within the next few years and in the rest of the world within the next few decades. This work discusses the role of natural fractures and slot porosity on tight gas sands as they relate to geoscience, drilling, completion, stimulations, petrophysics, well testing, and reservoir engineering. It is concluded that natural fractures and/or slot porosity play, and will continue to play, an important role in the successful and economic production of gas from tight sands. Research leading to innovating technologies that presumably will reduce capital and operating costs will play a key role in the development of this resource. Understanding the rocks and the regional variability of fracture distribution and diagenesis is the cornerstone of this research. Introduction Tight gas sands are part of what is usually known as unconventional gas which also includes coal bed methane, shale gas and natural gas hydrates. Tight gas sands have been defined in different ways by different organizations but a unique definition has proven elusive.1 The original definition dates back to the U.S. Gas Policy Act of 1978 that required in-situ gas permeability to be equal to or less than 0.1 md for the reservoir to qualify as a tight gas formation.2 At present this is probably the most commonly accepted definition. A second U.S. legal definition indicates that in a tight reservoir an average sustained un-stimulated initial gas rate is less than the maximum specified for a given depth class. However, it is important to understand that, although convenient, not only permeability and/or depth play a role in gas production from tight gas reservoirs. A cursory examination of the pseudo steady state, radial flow equation illustrates that gas rate is a function of many physical factors including pressure, fluid properties, reservoir and surface temperatures, permeability, net pay, drainage and wellbore radius, skin and non-Darcy constant.3 And this is without adding the effect of natural fractures in the equation. As a result an alternative preliminary definition 1 indicates that tight gas is "contained in lowpermeability sandstone and carbonate reservoirs where reservoir stimulation or specialized drilling technology is required to establish economic flow rates and recovery." Another view 3 indicates that "a tight gas reservoir can be deep or shallow, high pressure or low pressure, high temperature or low temperature, blanket or lenticular, homogeneous or naturally fractured, and can contain a single layer or multiple layers. Independent of the definition,4 "natural fractures are extremely important to unconventional gas reservoirs, and the assessment and characterization of these fractures (and other determinants of permeability) in unconventional plays is a high priority R&D need."