An Overview of Geomechanical Engineering Aspects of Tight Gas Sand Developments

Abstract

Tight gas sand exploration and development is a key focus as markets around the world exploit local resources for natural gas. Tight gas sands have matrix permeabilities in the micro-Darcy range and low porosities (typically ~5%). Commercial production is possible from these reservoirs when they have a conductive natural fracture system, and when they are stimulated by hydraulic fractures. However, both the formation matrix and the natural fracture systems can be stress-sensitive, i.e. they close during depletion, reducing the gross permeabilities which pose numerous challenges to establishing sustained economic flow rates. Geomechanical engineering is central to the development of these tight gas resources: Economic flow rates and commerciality of the field developments can only be achieved by stimulation, through fracturing and dilation of the reservoir formations; imposing a "man-made" permeability network on the geological system. This paper discusses geomechanical considerations of wellbore orientation, in-situ stress systems, and connectivity between hydraulic fractures with the natural fracture system, for single and multi-staged fracturing.A major consideration during field development of tight gas sands is the stress evolution accompanying drawdown and depletion. It is now well established that reservoir pressure changes have an effect on both the stress magnitudes and the stress directions in the sub-surface. Two major consequences of the stress changes are addressed: 1) Well placement: Understanding the stress evolution accompanying depletion is central to optimally plan the well locations, infill locations, and hydraulic fracture treatments. Stress changes result in fracture re-orientation following the re-completion, and an increase in the productivity and ultimate recovery from these wells. In addition, optimally placed new infill wells may become more productive than the initial wells as a result of improved connectivity with open fracture systems.2) Sweet spot enlargement: Greater production and ultimate recovery is commonly observed where the fracture system is "relaxed" or "critically stressed". As the stresses change accompanying depletion, so can the sweet spots, with potentially negative impact on development. Geomechanical engineering can be used in field development planning to define depletion strategies which produce an optimum stress evolution in the reservoir to open up these tight formations, by enhancing the conductivity of the natural fracture system. Thus, the productive sweet spots provided by nature may be enlarged to increase the available gas resources and higher ultimate recoveries, by increasing the area available for commercial development.The geomechanical engineering aspects of tight gas sands are discussed, highlighting some potential areas where tight gas sand developments can be optimised.

Hydraulic fractures productivity performance in tight gas sands—a numerical simulation approach

“…Tight gas sands are one of the most commonly produced unconventional gas resources around the world, but the low productivity and permeability provide further challenges in meeting economic production (Pankaj and Kumar, 2010). Tight gas sands are commonly classified as low permeability (less than 0.1mD) and low porosity, with high irreducible water saturation and high capillary pressures (Addis and Yassir, 2010). Naturally these sub-optimal reservoir properties have a significant impact on reservoir productivity and production performance.…”

“…Holditch (2006) defines tight gas sands to be "a reservoir that cannot be produced at economic flow rates nor recover economic volumes of natural gas unless the well is stimulated by a large hydraulic fractures". Addis and Yassir (2010) also define tight gas sands as requiring "man-made" permeability systems for economic production.…”