Abstract
We describe a new (or under-reported) type of deformation feature that has some of the textural characteristics of both a fracture and a shear band. The examples described occur in experimentally-deformed source-rock materials, and in tight limestones, both of which are constituents of many shale reservoirs. The deformation features, which emerge at very low magnitudes of bulk strain, create new dilative zones within the rock, and thus enhance the flow characteristics. Direct observation of fluid flow, involving neutron-tomography experiments of these experimental samples, reveals flow behaviours that lead to the inference that the features have an unusual set of properties: both high capillary pressure and high permeability. Detailed textural observations generate insights that lead to hypothesized physical explanations for the surprising flow characteristics. Our present understanding is that these features can form in the low-strain (and low energy-cost) conditions that can be achieved in hydraulic stimulation operations. If such deformations do occur in the suitable rock types within shale sequences, their role in fluid flow may be significant but heretofore unrecognized.
Introduction
Hydrocarbons in ‘shale’ reservoirs require stimulation to be effectively accessed, nominally by means of hydraulic fracturing (HF). There is growing appreciation that HF provokes a distributed response through the rock mass, involving rock breakage and movement over a large volume of the potential reservoir, including both displacements along natural fractures and new deformations. Thus, HF in unconventionals leads to the need to understand if deformation features, such as those studied herein, may be located within suitable rock types, and how the resulting textures and patterns of the features may impact fluid flow. Here, we describe the textural and property characteristics of experimentally-created ‘shear fractures’ in mudrocks and fine-grained carbonates, which are commonly components of the inter-layered sequences of some current shale plays.
The lab-induced deformations exhibit local dilational volume changes, and on that basis the local deformations would be expected to serve as flow conduits. In micro- and nano-scale investigations, however, the features are not seen as clean openings, as expected of ‘fractures’. Instead, they are filled with a newly-created ‘fault-rock’ equivalent material that has textures reminiscent of the gouge that occurs in shear-bands affecting siliciclastic rocks. Such shear bands in sandstones have historically been assumed to serve as flow barriers. However, in some of the examples here, the bands do operate as flow conduits – as revealed by direct flow observations using time-lapse neutron tomography experiments. The bands in the lab samples are inferred to be a result of local shear strain, plus or minus volumetric dilations or compactions. Numerical simulations of the experiments, which involve an enforced shear motion across an initially-intact layer, produce the same patterns of volumetric and shear strains inferred from the post-experiment textural examinations, and thus the simulations are judged as capturing the same operative phenomena, and the physical understanding that is derived from the simulations may be applied to the experimental outcomes. The emerging concept model is one in which localized shear features may develop in poor-quality rocks subjected to low values of bulk strain, creating previously-unanticipated flow pathways.
