Nanoscale grain boundary channels in fracture cement enhance flow in mudrocks

Abstract

Hydrocarbon production from mudrock or shale reservoirs typically exceeds estimates based on mudrock laboratory permeability measurements, with the difference attributed to natural fractures. However, natural fractures in these reservoirs are frequently completely cemented and thus assumed not to contribute to flow. We quantify the permeability of nanoscale grain boundary channels with mean apertures of 50–130 nm in otherwise completely cemented natural fractures of the Eagle Ford Formation and estimate their contribution to production. Using scanning electron imaging of grain boundary channel network geometry and a digital rock physics workflow of image reconstruction and direct flow modeling, we estimate cement permeability to be 38–750 nd, higher than reported permeability of Eagle Ford host rock (~2 nd) based on laboratory measurements. Our results suggest that effective fracture‐parallel mudrock permeability can exceed laboratory values by upward of 1 order of magnitude in shale reservoirs of high macroscopic cemented fracture volume fraction.

Fracture capture of organic pores in shales

“…In any laboratory study of fracture generation, there is always the question of which fractures were generated experimentally, which were generated during core retrieval and handling, and which were generated in situ in the subsurface. Distinguishing among these different types of fractures, even with high‐resolution images, is extremely difficult [e.g., Landry et al ., ]. In our adsorption experiments, we see evidence of deformation in pores smaller than 100 nm, and the SEM images do indicate fractures intersecting those pores.…”

Fractures in mudrocks: Advances in constraining timing and understanding mechanisms

“…It is generally accepted that the sealing of fibrous veins keeps up with fracture opening, such that little or no fracture porosity is present during the growth process (Bons et al, 2012). Landry et al (2016) noted that despite the low porosity of mineral fills, they could nonetheless represent sites of enhanced permeability, relative to the host mudrocks. In any case, the nature of the permeability-transient and long-lasting-of mineralized fractures in mudrocks is still largely unresolved.…”

Occurrence and Origin of Nanoscale Grain Boundary Channels Under Diagenetic Conditions

“…A sample mean was calculated based on 1–38 single grain boundary mean apertures (Figure 3c). Because core retrieval and handling can cause microfracturing and dilation of preexisting microfractures and grain boundaries (Landry et al., 2016), we only measured apertures of partially open grain boundaries containing intact cement bridges that completely traverse the grain boundary, thus ascertaining that the apertures are representative of in situ subsurface conditions. The exception are grain boundaries in barite that are generally unbridged.…”

“…This does not rule out that stress release during exhumation and possibly during core retrieval affects and possibly widens grain boundaries. Landry et al (2016) defined textural criteria to distinguish grain boundaries widened or formed by stress release from grain boundaries largely unaffected by stress release. They found that mating unbridged grain boundaries may have formed by stress release.…”

“…Building on the work by Landry et al. (2016), this study sets out to investigate whether NGBCs are inherent and common features of fracture‐filling mineral cement in diagenetic sedimentary systems and to describe and quantify the morphology and aperture of NGBCs. In addition of creating an inventory of grain boundary textures in these systems, we also attempt to shed light on the processes that may control their aperture and morphology, thus potentially preserving connected flow pathways.…”