A review of the current status of induced seismicity monitoring for hydraulic fracturing in unconventional tight oil and gas reservoirs

Induced Seismicity Characterization during Hydraulic‐Fracture Monitoring with a Shallow‐Wellbore Geophone Array and Broadband Sensors

Source‐Mechanism Analysis and Stress Inversion for Hydraulic‐Fracturing‐Induced Event Sequences near Fox Creek, Alberta

Fingering, veining, channelling and focussing of porous fluids are widely observed phenomena in the Earth’s interior, driving a range of geo-processes across all scales. While observations suggest fairly localized flow patterns induced by fractures, the classical Darcian model predicts diffusive behaviour that leads to never-ending spreading and delocalization. We here investigate an alternative physical mechanism without the need to involve fractures. Decompaction weakening leads to the formation and propagation of localized flow-pathways in fluid-saturated porous media. We numerically solve the coupled equations using high-resolution 2-D and 3-D numerical modelling to predict non-linear porous flow in a non-linearly viscously deforming matrix. We show that high-porosity channels may be a dynamic and natural outcome of sufficiently resolved hydromechanical coupling and decompaction weakening. We propose an efficient solution strategy that involves an iterative pseudo-transient numerical method to solve the coupled system of equations in a matrix-free fashion. We discuss benefits and limitations of this approach that performs optimally on hardware accelerators such as graphical processing units and is well-suited for supercomputing. We benchmark the pseudo-transient routines against commonly used direct-iterative solving strategies and show convergence towards identical results. Furthermore, we use the fast solver to systematically study in 2-D the high-porosity channel propagation velocity as a function of bulk and shear viscosity ratios and report discrepancy between 2-D and 3-D configurations. We conclude that the fluid-flow rate in the channels is up to three orders of magnitude higher than expected by pure Darcian flow regimes and show that the high-porosity channels occurrence remains with strain rate dependant shear viscosity. We provide both the 2-D MATLAB-based direct-iterative and pseudo-transient routines for full reproducibility of the presented results and suggest our model configuration as a key benchmark case to validate the implementation of hydromechanical coupling in 2-D and 3-D numerical codes. The routines are available from Bitbucket and the Swiss Geocomputing Centre website, and are also supporting information to this paper

/pdf/resolving-hydromechanical-coupling-in-two-and-three-524aouuhz0.pdf

Resolving hydromechanical coupling in two and three dimensions: spontaneous channelling of porous fluids owing to decompaction weakening

S U M M A R Y A new Matched Filtering Algorithm (MFA) is proposed for detecting and analysing microseismic events recorded by downhole monitoring of hydraulic fracturing. This method requires a set of well-located template (‘parent’) events, which are obtained using conventional microseismic processing and selected on the basis of high signal-to-noise (S/N) ratio and representative spatial distribution of the recorded microseismicity. Detection and extraction of ‘child’ events are based on stacked, multichannel cross-correlation of the continuous waveform data, using the parent events as reference signals. The location of a child event relative to its parent is determined using an automated process, by rotation of the multicomponent waveforms into the ray-centred co-ordinates of the parent and maximizing the energy of the stacked amplitude envelope within a search volume around the parent’s hypocentre. After correction for geometrical spreading and attenuation, the relative magnitude of the child event is obtained automatically using the ratio of stacked envelope peak with respect to its parent. Since only a small number of parent events require interactive analysis such as picking Pand S-wave arrivals, the MFA approach offers the potential for significant reduction in effort for downhole microseismic processing. Our algorithm also facilitates the analysis of single-phase child events, that is, microseismic events for which only one of the Sor P-wave arrivals is evident due to unfavourable S/N conditions. A real-data example using microseismic monitoring data from four stages of an open-hole slickwater hydraulic fracture treatment in western Canada demonstrates that a sparse set of parents (in this case, 4.6 per cent of the originally located events) yields a significant (more than fourfold increase) in the number of located events compared with the original catalogue. Moreover, analysis of the new MFA catalogue suggests that this approach leads to more robust interpretation of the induced microseismicity and novel insights into dynamic rupture processes based on the average temporal (foreshock–aftershock) relationship of child events to parents.

Detection and analysis of microseismic events using a Matched Filtering Algorithm (MFA)

P.-F. Roux, J. Kostadinovic, T. Bardainne, E. Rebel, M. Chmiel, M. Van Parys, R. Macault and L. Pignot present an acquisition and processing technique to further decrease the noise recorded at the surface of the Earth when monitoring hydraulic stimulation.

It is well known that fluid injection into reservoirs, be it in the context of enhanced geothermal systems or for the stimulation of hydrocarbon reservoirs, generates so-called ‘induced’ seismic activity (Evans, 1966). Early on, the link between the stimulation and this activity has been established, and it has become increasingly obvious that measuring the microseismicity generated by the injection would provide a wealth of information on the mechanical processes at work during the stimulation.
Historically, downhole geophone tools have been used to monitor microseismic activity during stimulation programmes. Such tools usually offer a very high sensitivity to the microseismic sources, provided that the observation well is close enough to the treated well (Rutledge and Phillips, 2003). However, this becomes limited when more information on the source mechanism (usually termed focal mechanism and represented by the infamous moment tensor) is required. This is because of the three-dimensional nature of the focal mechanism, which means it cannot be retrieved properly using a single observation point. In addition, a poorly situated observation well may indeed lead to a reduced detection capability.

Increasing the accuracy of microseismic monitoring using surface patch arrays and a novel processing approach

To assess water-weakening effects in reservoir rocks, previous experimental studies have focused on changes in the failure envelopes derived from mechanical tests conducted on rocks fully saturated either with water or with inert fluids. So far, little attention has been paid to the mechanical behavior during fluid injection under conditions similar to enhanced oil recovery operations. We studied the effect of fluid injection on the mechanical behavior of the weakly consolidated Sherwood sandstone in laboratory experiments. Our specimens were instrumented with 16 ultrasonic P wave transducers for both passive and active acoustic monitoring during loading and fluid injection to record the acoustic signature of fluid migration in the pore space and the development of damage. Calibration triaxial tests were conducted on three samples saturated with air, water, or oil. In a second series of experiments, water and inert oil were injected into samples critically loaded up to 80% or 70% of the dry or oil-saturated compressive strength, respectively, to assess the impact of fluid migration on mechanical strength and elastic properties. The fluids were injected with a low back pressure to minimize effective stress variations during injection. Our observations show that creep takes place with a much higher strain rate for water injection compared to oil injection. The most remarkable difference is that water injection in both dry and oil-saturated samples triggers mechanical instability (macroscopic failure) within half an hour whereas oil injection does not after several hours. The analysis of X-ray computed tomography images of postmortem samples revealed that the mechanical instability was probably linked to loss of cohesion in the water-invaded region.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JB011894

Mechanical instability induced by water weakening in laboratory fluid injection tests

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Increasing the accuracy of microseismic monitoring using surface patch arrays and a novel processing approach

Mechanical instability induced by water weakening in laboratory fluid injection tests