Research

Hydraulic Fracturing

Hydraulic fracturing is a method routinely used to exploit oil and gas, in pilot Enhanced Geothermal Systems and in Artificial Groundwater Recharge. Despite its widespread use, there are environmental impacts, such as induced seismicity and contamination of rock formations and subterranean aquifers that may hinder the production of important subsurface energy and water resources.

 

Broader impact

Our group aims to understand, at a fundamental level, the physical processes involved in hydraulic fracturing, with the ultimate goal of minimizing the environmental impacts of this controversial technology.

What we study

In our lab, we study the mechanisms involved in the initiation and propagation of hydraulic fractures, with emphasis in their connectivity and in the interaction between existing and newly-created fractures, as well as in the impact of field variables in the propagation and/or arrest of hydraulic fractures. These field variables include the in-situ state of stress, injection rate and viscosity of the fracturing fluid, among others.

How we study

We study hydraulic fracturing experimentally by designing and building customized devices which allow one to simultaneous monitor the fracturing processes visually and through acoustic emissions, while applying specific loadings to rock specimens.

Device designed and built at NJIT to apply a 3D state of stress and pressurize pre-fabricated flaws in prismatic granite specimens with simultaneous image and acoustic emission monitoring

Video of a hydraulic fracture propagating in one of the tests conducted in our lab. (Real duration of video is 2 ms)

Induced Seismicity

Induced seismicity is a co-product of hydraulic fracturing. Enhanced Geothermal Systems projects, such as Basel, in Switzerland, have been shut down due to the micro-earthquakes created during the stimulation (hydraulic fracturing) of the target rock.

Broader impact

Our goal is to evaluate how induced seismicity is affected by variables that can be controlled in the field, such as the injection rate and fluid viscosity, in order to elucidate which parameters should be primarily tuned to minimize induced seismicity.

What we study

We study the micro-seismicity caused by the damage processes that occur during hydraulic fracturing tests. These processes can occur before, during and after the initiation and propagation of macro (e.g. visible) fractures

How we study

We use acoustic emission sensors to capture and record the waveforms produced during the hydraulic fracturing experiments. These waveforms carry information that allows us to locate micro-seismic events, estimate their magnitudes and source mechanisms (tension, shear, combination tension/shear). This information is also integrated with the visual data (macro- and microscopic images), in order to more accurately interpret the micro-seismic activity.

Location and amplitude of first P-wave arrivals of acoustic emission events during hydraulic fracturing tests with three distinct injection rates: 0.3ml/min, 3ml/min and 30ml/min. 

Micro-scale Rock Fracturing

Rock fracturing is a multi-scale process. While macro, or visible, fractures are of interest to most subsurface applications (e.g. oil and gas exploitation, enhanced geothermal systems, artificial groundwater recharge) and in civil engineering (e.g. rock slope stability, stability of underground openings in rock), their propagation occurs through the coalescence of micro-cracks that develop at and near the tip of the macro-fractures, in the so-called Fracture Process Zone.

 

Broad impact

Identifying and interpreting the microscopic fracturing mechanisms that occur prior to the development of macroscopic fractures will provide us with important information to predict when failure is on the verge of occurring (for civil engineering) and how natural and man-made fractures may have developed or will develop over time.

 

What we study

We investigate how, when and where micro-cracks initiate and develop and how they evolve into macro-cracks. We also identify the mode of propagation of the micro-cracks (i.e. opening, shearing, combination) as well as their morphology (e.g. aperture, relative surface displacement, roughness).

 

How we study

We subject rock specimens to different loading conditions and observe the damage zones that develop prior to macro-crack development. We use Scanning Electron Microscopy, micro-CT scanning and Atomic Force Microscopy to observe these damage zones, with particular focus on micro-cracks. In order to better determine the mechanisms involved in micro-crack development, we also relate the imaging data to the source mechanisms of recorded acoustic emission events.

Scanning Electron Microscope image of damage region in granite specimen showing zigzagging micro-cracks

Micro-CT scan image of hydraulic fractures in a granite specimen (scan performed at NJIT’s Otto H. York Center). Images are being used to determine the effect of stress states in the fracture roughness of hydraulically-created fractures in granite.

Modeling of Fracturing Processes

Fracturing of rock and rock-like materials is complex to predict. Continuous advancement of theoretical and numerical models is necessary to more accurately simulate fracturing.

Broad impact

Having models to predict the initiation, path of propagation and type of coalescence of fractures, as well as the stresses at which these phenomena occur is crucial to design hydraulic fracturing operations for oil and gas and EGS operations, as well as to analyze the stability of rock slopes and underground openings in rock.

What we study

We numerically predict the location of initiation, path of propagation and type of coalescence of fractures subjected to various loading conditions. We are also interested in the mode of propagation of the fractures (opening vs sliding) and in the stresses at which fracture initiation, propagation and coalescence occur.

How we study

Our research group develops and implements fracture initiation and propagation criteria in numerical models to simulate fracture development in rock and rock-like materials subjected to various loading conditions, such as vertical compressive loads and hydraulic pressures inside existing flaws. The experimental work is used as benchmark to validate the numerical models and fracturing criteria proposed. Recent numerical work has been conducted using a Displacement Discontinuity Method (FROCK) initially developed by the Rock Mechanics group where Dr. Gonçalves da Silva worked at MIT.

Videos showing comparison between numerical prediction using a stress-based fracturing criterion in FROCK and hydraulic fracturing test in granite

Thermal-induced Damage of Rocks and Rock-like materials

Thermal action can result in the alteration of the mechanical properties of rocks and rock-like materials. This is particularly relevant in subsurface applications, such as the storage of nuclear waste, and in civil engineering tunnels.

Broad impact

Nuclear waste, which can be stored in deep underground caverns and openings, generates heat that may damage the surrounding rock and compromise the seal originally provided by it. Assessing this thermal-induced damage would provide scientific support to the selection of underground sites to store nuclear waste, as well to determine the safe amount of waste that can be stored and the best disposal procedures to implement in these sites. Civil engineering tunnels, on the other hand, may also be subjected to thermal damage induced by fires, which may result in significant financial losses, time-consuming repairs and even loss of lives

What we study

Our research group is quantifying the fire-induced spalling in tunnel liners using different materials and fire scenarios. We are also evaluating cost-effective and user-friendly techniques to assess the fire-induced damage of rocks and tunnel liners, using the color changes and crack density in rocks and concrete.

How we study

Using a furnace, we apply thermal loads to concrete sections subjected to compressive loads. We observe and physically interpret changes in color, cracking and spalling during this process, relating these visual and morphological changes to the degradation in strength and elastic properties of the material. While this project is primarily civil engineering-oriented, the methodologies being developed could also be applied to deep openings in rock without liners, which would be relevant to the storage of nuclear waste.

 
 
 
 
 

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