Loading...
Seismic investigation of laboratory hydraulic fracture growth in rocks
Butt, Awais
Butt, Awais
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2024
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2025-11-26
Abstract
Hydraulic fracturing (HF) technique is commonly employed to enhance the permeability of reservoir rock for cost-effective resource extraction (oil, gas, heat, etc.). While this technique has been frequently employed for the past decades, the influence of various controlling factors such as rock properties, operational parameters, and in-situ stress, and their relationship to the fracturing processes is not clearly understood. It is important to develop techniques that can accurately monitor and interpret complex HF behavior under various conditions and, therefore, assist in better control and optimization of the stimulation operation.
This work aims to enhance the comprehension of HF physics and to advance the use of acoustic monitoring techniques for characterizing fluid-induced fracturing in brittle rocks. This involved exploring the impact of various factors (fracture propagation regimes, injection parameters such as viscosity and rate, rock types, pre-existing fracturing, and injection protocols) on HF behavior through detailed seismic analyses. This goal was achieved through the following objectives: (1) Identify and analyze the fracturing patterns and their evolution, through various classification criteria, during laboratory HF of granitic rock following different fracture propagation regimes; (2) estimate the energy budget including seismic source parameters and seismic efficiency during laboratory HF of granitic rocks following different dominating fracture propagation regimes; (3) characterize the aseismic and seismic HF processes in granitic rocks through simultaneous active and passive seismic monitoring; (4) investigate the influence of rock-type (granite and sandstone) on the HF processes and the associated seismic response; (5) explore the effect of pre-existing fractures on the generated HF and the associated microseismicity; and (6) investigate the influence of fluid injection protocol on the HF behavior through seismic monitoring techniques.
To achieve the research objectives, we conducted laboratory HF experiments in true-triaxially loaded cubic specimens of Barre granite and Lyons sandstone using high (oil) and low (water) viscosity injection fluids. Custom loading/sensor housing plates were fabricated to facilitate real-time combined active (ultrasonic) and passive (acoustic emission, AE) monitoring of the HF experiments. Firstly, scaled HF experiments were conducted on Barre granite to bridge the gap between laboratory and field-scale HF operations. These experiments were designed to simulate specific conditions corresponding to different propagation regimes encountered in the field. The hydro-mechanical response, fracture profile, and the associated microseismicity or AE were investigated for two varying HF propagation regimes, viscosity and toughness dominated. Detailed AE analysis included spatio-temporal localization of AE events, frequency-magnitude distribution (b-value) for different fracturing phases, and source mechanisms from different classification criteria. Besides distinguishing the variance in fracture processes and the associated source mechanisms for different dominating propagation regimes, this study emphasized the dissimilarity in the proportions of fracture mechanisms based on the classification criteria employed. Seismic source characterization involved determining spectral and seismic parameters for each AE event, which required the calibration of individual AE sensors through the ball drop test. By determining the actual displacement spectra, various seismic parameters (magnitude, source dimension, stress drop, and radiated seismic energy) were estimated for viscosity and toughness-dominated HF experiments. The seismic source parameters were further compared with large-scale natural and induced earthquakes to explore potential scale invariability.
To deepen our understanding of the complete HF phenomenon, concurrent investigations of passive (AE) and active (ultrasonic) techniques were conducted to identify and differentiate the aseismic and seismic HF processes. HF experiments were performed in both Barre granite and Lyons sandstone to identify the variations in fracturing processes based on the rock-type. A methodology was proposed to eliminate the contamination of ultrasonic scans from the AE monitoring results. The differences in AE monitoring detected inelastic fracturing including AE locations, source mechanisms, and b-values, were contrasted between granite and sandstone specimens. The sensitivity of active signal attributes (velocity, amplitude, energy, and frequency) was explored in conjunction with their employment direction for characterizing various HF processes in granite and sandstone. A joint study of passive microseismicity and active signals attributes permitted a detailed comprehension of the various HF processes (aseismic deformation, fracture initiation and propagation, fluid permeation, and leak-off) and their dependence on the specific rock type.
The study also investigated how two factors, thermally induced micro-fractures and injection schemes, influenced the generated HF and the associated seismic response. Different levels of thermal micro-fractures were induced before HF using high-temperature treatments (200-800ºC) and quantified using non-destructive (ultrasonic) and destructive (microscopic thin section analysis) techniques. The fracturing behavior was evaluated by measuring various HF parameters, both hydromechanical (borehole pressure evolution, injected fluid volume, etc.) and microseismic using 16 calibrated AE sensors. The effect of thermally induced micro-fractures on the spatio-temporal evolution of the AE events, along with b-value, source mechanisms, and seismic source parameters, was examined in temperature-treated granite specimens. Lastly, the HF behavior under monotonic (continuous fluid injection) and creep (constant pressure below failure) injection schemes was assessed using simultaneous active and passive seismic monitoring. The evolution of HF initiation and propagation during stable and unstable HF phases were explored for both injection schemes using coupled active and passive monitoring. The increase in time to breakdown, fracture morphology, AE (spatial distribution, source mechanisms), and ultrasonic (velocity, amplitude, energy) characteristics was evaluated jointly to better assess the HF phenomena involved with monotonic and creep injection protocols.
The findings of this thesis demonstrated the effectiveness of acoustic monitoring techniques in providing valuable insights into the impact of various factors on the behavior and dynamics of HF processes. The advancements in the monitoring techniques, offering a more thorough and precise approach, represent a significant step towards optimizing HF practices and ensuring sustainable resource extraction.
Associated Publications
Rights
Copyright of the original work is retained by the author.