Some Thoughts on Existence and Importance of Natural Microfractures in Unconventional Shale-Oil and Gas Systems

Abstract

There has been much debate on the existence of natural microfractures in shale reservoirs. Many studies have addressed them, but few have documented them with reliable criteria or have documented their abundance. Microfractures have previously been defined as fractures up to a few millimeters long with an aperture up to a few tens of micrometers. Examples of proposed microfractures have been shown in the literature, but in nearly all of these examples the microfractures do not show cement, therefore, they cannot be conclusively shown to be natural fractures. Note that gypsum cement is not a reliable indicator of a natural fracture as it commonly forms post-coring. The few studies that show examples of natural fractures with cement, also indicate that these microfractures are very rare. In order for microfractures to be a permeability pathway, they must be abundant enough to be connected. Based on their size, if the microfractures were at the abundance level needed to form a permeability pathway, they would readily show up in a blue-fluorescent-dye impregnated thin section that generally covers an area of approximately 800 square millimeters. It is very easy to epoxy impregnate a sample with submicrometer-sized openings. This density of microfractures in thin sections have not been documented or has the density been documented with SEM-based samples. Beside direct evidence, other researchers have based the existence of microfractures on production performance. This is not reliable evidence as at this time we are still debating the type of flow within nanodarcy reservoirs. With the evidence we presently have, we do not have a well-documented case history of natural microfractures composing a shale reservoir. What we do not know is whether hydraulic fracture stimulation can create a shattered zone of induced microfractures. A post-stimulation core would be necessary to address this possibility. Therefore, at present, there is no conclusive evidence that natural microfractures contribute significantly to flow in shale reservoirs or whether they even exist in the necessary abundance to contribute to flow. We have documented that shale reservoirs have matrix nanopores (interparticle, intraparticle, and organic-matter) and we need to better understand how flow occurs in this type of pore network.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016