R.P. Young

A laboratory acoustic emission experiment under in situ conditions

In this paper, we revisit acoustic emission (AE) data from an in situ rock fracture experiment conducted at the Underground Research Laboratory (URL) in Manitoba, Canada. The Mine-By experiment, a large-scale excavation response test, was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel. During the experiment a small array of 16 Panametrics V103 AE sensors enclosed a 0.7 m × 0.7 m × 1.1 m rectangular prism of Lac du Bonnet granite located in the tunnel wall. The V103 sensors were later calibrated in the laboratory, and a source parameter analysis was undertaken using a spectral fitting method. Corner frequency and moment magnitude were found to be inside the ranges 250 kHz <fc< 490 kHz and −7.5 <Mw<−6.8, respectively. Static stress drops ranged from 0.3 to 4 MPa, which is consistent with large seismicity recorded at the URL.

Hydraulic fracture energy budget: Insights from the laboratory

In this paper we present results from a series of laboratory hydraulic fracture experiments designed to investigate various components of the energy budget. The experiments involved a cylindrical sample of Westerly granite being deformed under various triaxial stress states and fractured with distilled water, which was injected at a range of constant rates. Acoustic emission sensors were absolutely calibrated, and the radiated seismic energy was estimated. The seismic energy was found to range from 7.02E−8% to 1.24E−4% of the injection energy which is consistent with a range of values for induced seismicity from field-scale hydraulic fracture operations. The deformation energy (crack opening) of the sample during hydraulic fracture propagation was measured using displacement sensors and ranged from 18% to 94% of the injection energy. Our results support the conclusion that aseismic deformation is a significant term in the hydraulic fracture energy budget.

Observation of the Kibble-Zurek Mechanism in Microscopic Acoustic Crackling Noises.

Characterizing the fast evolution of microstructural defects is key to understanding “crackling” phenomena during the deformation of solid materials. For example, it has been proposed using atomistic simulations of crack propagation in elastic materials that the formation of a nonlinear hyperelastic or plastic zone around moving crack tips controls crack velocity. To date, progress in understanding the physics of this critical zone has been limited due to the lack of data describing the complex physical processes that operate near microscopic crack tips. We show, by analyzing many acoustic emission events during rock deformation experiments, that the signature of this nonlinear zone maps directly to crackling noises. In particular, we characterize a weakening zone that forms near the moving crack tips using functional networks, and we determine the scaling law between the formation of damages (defects) and the traversal rate across the critical point of transition. Moreover, we show that the correlation length near the transition remains effectively frozen. This is the main underlying hypothesis behind the Kibble-Zurek mechanism (KZM) and the obtained power-law scaling verifies the main prediction of KZM.

Time-lapse Velocities for Locations of Microseismic Events - A Numerical Example

The accuracy in the location of microseismic (MS) events relies, among other factors, on the use of a realistic velocity model in the forward calculation of travel times. During the hydraulic stimulation of deep rock reservoirs, the physical properties of the rock are altered and therefore the velocity structure is subject to changes along the treatment. In this paper, a numerical study using the distinct element method and the cross-correlation technique is carried out to measure velocities in a naturally fractured geothermal reservoir in order to better understand the relationship between induced microseismicity and fractures, fluid pressure and seismic velocity anisotropy. The fracture damage zone and fluid permeable zone were successfully correlated with the time-lapse velocity changes extracted from the numerical model. The model offers the unique ability to examine directly the microprocesses leading to macroscopic velocity changes. Validated models could be extended to quantitatively calibrate velocities required for microseismic locations over time, and predict the fracture propagation and fluid migration within a field-scale engineered reservoir.

Automated Microseismic Event Location Using Finite Difference Traveltime Calculation and Enhanced Waveform Stacking

The Fast Sweeping Method (FSM) is a finite difference algorithm providing significant computational efficiency and modelling capability in calculating the first arrivals of seismic P- or S-waves. Based on the calculated travel-timetable, we stack waveform amplitude for every possible source location and origin time followed by semblance weighting. With three-component (3-C) data, the stacked image can be further improved by matching the waveform polarization with the modelled ray vector from FSM and thus reduce the azimuth ambiguity in the location of microseismic (MS) events. We apply the algorithm on synthetic surface and borehole data and show that the combination of FSM and enhanced waveform stacking can efficiently locate MS events with high spatial resolution even when realistic noise levels are added. Finally we test the algorithm on a set of field 3-C data from the stimulation of a sandstone reservoir monitored using two borehole arrays. The method allowed the location of twice the number of event compared to locations using each borehole individually, with both sets defining a consistent fracture structure.

A New Finite Difference Eikonal Equation Solver for Anisotropic Medium

The fast sweeping method has been proved very efficient in calculating the first arrivals in isotropic media. In this study, we extend the fast sweeping method to heterogeneous anisotropic medium by adapting the Lax-Friedrichs local scheme. The new fast sweeping method is able to solve the first arrival traveltime field for both qP and qS waves. By comparing the traveltime field to the full-waveform solution, we demonstrate that the iso-surfaces of the time field follow the constant phase of the wave and form a continuous envelope wrapping the wavefront. The iso-surfaces for the shear wave identify two continuous wavefronts one ahead of the other even in the directions where the triplication of qS-wave is developed. The rays for both qP and qS wave can be traced using the slowness vector from the traveltime field and we compared its accuracy with a two-point ray tracing method in a layered model. We show that rays from the traveltime field is nearly identical to the two-point ray tracing results. This new fast sweeping method not only avoids the multipath and shadow zone issues in complex heterogeneous media but also circumvent the multiple shear branch problems due to anisotropy.

Quantifying Reservoir Stimulation Using Passive Traveltime Tomography

Hydraulic fracturing stimulates reservoir and imposes stress changes in the surrounding rock that typically induce or trigger seismicity with a wide range of magnitudes. Seismic monitoring provides insight into the reservoir deformation and give critical feedback to the on-going stimulations. We have developed a passive seismic tomography technique adapted from earthquake seismology to jointly locate induced microseismic events and update the velocity of the reservoir illuminated by the microseismicity. We calculate travel-time based on the fast sweeping method to account for complex 3D distribution of velocity and use the adjoint method to transform the inverse problem to a forward problem which can also be solved by the fast sweeping method. In this paper, we apply our algorithm to a two-stage reservoir stimulation project and demonstrate the capability of the microseismic tomography in mapping the stimulated rock volume and in quantifying the reservoir degradation even in the absence of visible P-waves.

Numerical Modeling of Microseismicity Induced by CO2 Injection

In order to assess the feasibility and performance of geological sequestration as a long-term solution for CO2 depletion, it is crucial to evaluate and monitor the integrity and stability of the caprock. Direct comparison with results from field observation and modeled microseismicity provides a unique tool to evaluate the evolution of the treated rock reservoir. A suite of 2D DEM models with varying gas pressures, reservoir temperatures and permeability for the pre-existing fracture, were built to examine the interaction between the injected CO2 and the pre-existing fracture and caprock. Jointed models with a higher permeability for the pre-existing fracture subject to the highest injection pressure showed microseismic activity increasing with reservoir temperature, indicating higher likelihood of CO2 migration. All the induced fractures crossed the pre-existing fracture following different patterns. High-permeability jointed models with low and medium injection pressures showed no microseismicity and injected CO2 remained confined within the reservoir. Jointed models with a lower permeability for the pre-existing fracture, reproducing caprocks such as shale layers, produced low microseismicity under all modeled pressures and temperatures although CO2 saturated further through the pre-fracture. The unjointed models produced similar results to the low-permeability jointed models.