An Overview of Principles and Designs of Hydraulic Fracturing Experiments and an Inquiry into the Influence of Rock Permeability and Strength on Failure Mode

Abstract

The relevance of hydraulic fracturing experiments in the analysis of subsurface flow mechanisms and interactions during fracking operations underpins past and current efforts towards designing and implementing more representative physical models. An overview has been presented that comprehensively discusses the key elements and design requirements for successful experimentations. In setting up a hydraulic fracturing experiment, it is imperative that, in line with the research objective, the physical model that includes the initial and boundary conditions, wellbore configuration, type of fracturing fluid and injection rate be a true representative of actual reservoir/underground flow environments. This investigation recognises the main elements that form the framework for effective laboratory scale experiments, which comprise the specimen, in-situ stresses, pore pressure, fluid injection, duration, and visualisation and monitoring. Furthermore, an examination of the influence of rock properties on the characteristics of fracturing and failure of rocks subjected to wellbore conditions indicates a trend highly dependent on rock strength and permeability. Soft and highly permeable rocks tend to cause an inward collapse of the wellbore cavity. Cavity size is also shown to have a considerable effect on the failure process. Wellbore stability is inversely related to cavity size; larger cavities are found to be less stable.

Use of Outcrop as Substitute for Subsurface Shale: Current Understanding of Similarities, Discrepancies, and Associated Challenges

“…42−45 fracturing process is the availability of enough specimens. 46 The resolution of wireline logs is also too low to capture many small-scale heterogeneities that affect fluid flow. Additionally, the large distance between logged wells results in uncertain long-range correlations.…”

“…Also, outcrop characterization can improve subsurface data interpretation (well logs, core, cuttings, and seismic) and identify organic-rich intervals and ductile/brittle zones. The outcrop shale cores are used as physical models to simulate hydraulic fracturing and its propagation in the wellbores since many core samples are required. − One of the crucial key elements to successful physical modeling of the hydraulic fracturing process is the availability of enough specimens . The resolution of wireline logs is also too low to capture many small-scale heterogeneities that affect fluid flow.…”

Investigating Sand Production Phenomena: An Appraisal of Past and Emerging Laboratory Experiments and Analytical Models

“…Artificial specimens can be constructed using simple materials such quartz sand, cement and water [3,45]. However, it is possible to create particles artificially, such as glass beads bonded by binders like epoxy resin mixed with acetone [46].…”

“…The cavities in both hollow cylinder and prism specimens are usually not cased or supported, and fluid flow towards the cavity is instigated by applying a pressure gradient. The pressure gradient is established by ensuring the external pore pressure is greater than the internal (cavity) pore pressure [46,47,51].…”

“…To bind natural (sand) or artificial particles (e.g., glass beads), they are mixed with binders like cement and epoxy resin [3,44,46]. Binders create the desired inter-particle bond, which contributes to the macroscopic mechanical strength properties of the specimen.…”