Advances in the study of naturally fractured hydrocarbon reservoirs: a broad integrated interdisciplinary applied topic

Abstract

Naturally fractured reservoirs, within which porosity, permeability pathways and/or impermeable barriers formed by the fracture network interact with those of the host rock matrix to influence fluid flow and storage, can occur in sedimentary, igneous and metamorphic rocks. These reservoirs constitute a substantial percentage of remaining hydrocarbon resources; they create exploration targets in otherwise impermeable rocks, including under-explored crystalline basement, and they can be used as geological stores for anthropogenic carbon dioxide. Their complex fluid flow behaviour during production has traditionally proved difficult to predict, causing a large degree of uncertainty in reservoir development. The applied study of naturally fractured reservoirs seeks to constrain this uncertainty and maximize production by developing new understanding, and is necessarily a broad, integrated, interdisciplinary topic. Some of the methods, challenges and advances in characterizing the interplay of rock matrix and fracture networks relevant to fluid flow and hydrocarbon recovery are reviewed and discussed via the contributions in this volume.Global estimates of conventional hydrocarbon resources are typically subdivided based on lithological reservoir types for example, carbonate or siliciclastic (e.g. Roehl & Choquette 1985). However, many of these sedimentary rock reservoirs may contain fractures to a greater or lesser degree. The recent boom of unconventional reservoirs highlights once again the key role that natural fractures can play in helping production of fluids. It also requires an improved understanding of the geology and physics of natural fracture networks to meet public expectations regarding safety issues. Moreover, fracture networks can be present in otherwise impermeable crystalline basement rocks (e.g. Sanders et al. 2003;Murray & Montgomery 2012;Slightam 2012) and igneous intrusions (e.g. Gudmundsson & Løtveit 2012), also allowing these rocks to form potential fractured reservoirs. Historically, fractured crystalline basement rocks have been under-explored as potential hydrocarbon reservoirs. Naturally fractured reservoirs constitute a substantial percentage of remaining hydrocarbon resources. Naturally fractured reservoirsA reservoir fracture is a general term used to describe a 'naturally occurring macroscopic planar discontinuity in rock due to deformation, or physical diagenesis' (Nelson 2001). Reservoir fractures encompass both extensional ( joints) and shear (faults) structures. Fractures formed by brittle tectonic deformation are the most common focus for studies of naturally fractured reservoirs. However, reservoir fractures may also include structures that formed by desiccation (e.g. shrinkage cracks) and syneresis (e.g. chickenwire texture) in sediments, and structures that formed by thermal , as used here. Naturally fractured reservoirs are generally defined as such when the fracture network has a significant influence on fluid flow in the reservoir such that: (1) the fracture networ...

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…They are also the most difficult to detect or measure. Moreover, the impact of chemical processes is less widely appreciated in the development of these fractures compared to faults, where a range of chemical effects have been described (Davies & Cartwright, 2007;Di Toro et al, 2012;Eichhubl et al, 2009;Fisher et al, 2003;Fossen et al, 2018;Noiriel et al, 2010;Spence et al, 2014).…”

Mechanical stratigraphy and structural control of oil accumulations in fractured carbonates of the Irati Formation, Paraná Basin, Brazil

“…Structural discontinuities, such as joints and faults, are the product of tectonic or non-tectonic processes and represent deformational features usually present in rock massifs. In the last two decades, these structures have been studied as paths to the fluid circulation in aquifers (Berkowitz 2002, Cooke et al 2006, Morin et al 2007 and in conventional or non-conventional petroleum systems (Di Naccio et al 2005, Spence et al 2014, Tavener et al 2017. Thus, discontinuities are natural conduits for fluid circulation and may be associated with permeability barriers (e.g., cementation) and routes (e.g., dissolution), which affect fluid migration between the source rock and the reservoir.…”

“…Consequently, coarse-grained rocks tend to be brittle and have lower tensile strength than fine-grained ones with ductile characteristics (Renshaw et al 2003, Cooke et al 2006. Tsang (1984) demonstrates that less persistent fractures are not capable of generating active migratory paths; however, highly persistent fractures produce effective conduits for the storage and migration of large volumes of water or different types of hydrocarbons (Cooke et al 2006, Questiaux et al 2010, Spence et al 2014.…”

“…These aspects allow evaluating and predicting fluid behavior through open fractures. The main controls include porosity, permeability, viscosity, wettability, and capillary pressure as parameters affected during fracture generation and propagation (Morin et al 2007, Blunt et al 2013, Spence et al 2014.…”

Causes of bias and uncertainty in fracture network analysis

“…Structural geologists commonly supply data about fracture networks to modellers to develop models for fluid flow within fractured reservoirs (e.g., Spence et al, 2014). It is incumbent on geologists to inform the modellers about the biases and uncertainties in the data they have collected, and it is incumbent on the modellers to challenge the geologists about the veracity of the datasets they have supplied.…”