Seismic Waves Do Sense Fracture Connectivity: Experimental Validation

<p>Monitoring of the subsurface with seismic methods can be improved by better understanding the attenuation of seismic waves due to fluid pressure diffusion (FPD). In porous rocks saturated with multiple fluid phases the attenuation of seismic waves by FPD is sensitive to the mesoscopic scale distribution of the respective fluids. The relationship between fluid distribution and seismic wave attenuation could be used, for example, to assess the effectiveness of residual trapping of carbon dioxide (CO2) in the subsurface. Determining such relationships requires validating models of FPD with accurate laboratory measurements of seismic wave attenuation and modulus dispersion over a broad frequency range, and, in addition, characterising the fluid distribution during experiments. To address this challenge, experiments were performed on a Berea sandstone sample in which the exsolution of CO2 from water in the pore space of the sample was induced by a reduction in pore pressure. The fluid distribution was determined with X-ray computed tomography (CT) in a first set of experiments. The CO2 exosolved predominantly near the outlet, resulting in a heterogeneous fluid distribution along the sample length. In a second set of experiments, at similar pressure and temperature conditions, the forced oscillation method was used to measure the attenuation and modulus dispersion in the partially saturated sample over a broad frequency range (0.1 - 1000 Hz). Significant P-wave attenuation and dispersion was observed, while S-wave attenuation and dispersion were negligible. These observations suggest that the dominant mechanism of attenuation and dispersion was FPD. The attenuation and dispersion by FPD was subsequently modelled by solving Biot’s quasi-static equations of poroelasticity with the finite element method. The fluid saturation distribution determined from the X-ray CT was used in combination with a Reuss average to define a single phase effective fluid bulk modulus. The numerical solutions agree well with the attenuation and modulus dispersion measured in the laboratory, supporting the interpretation that attenuation and dispersion was due to FPD occurring in the heterogenous distribution of the coexisting fluids. The numerical simulations have the advantage that the models can easily be improved by including sub-core scale porosity and permeability distributions, which can also be determined using X-ray CT. In the future this could allow for conducting experiments on heterogenous samples.</p>

The characterization of fractured rock is a critical problem in the development of nuclear waste repositories in geologic media. A good methodology for characterizing these systems should be focused on the large important features first and concentrate on building numerical models which can reproduce the observed hydrologic behavior of the fracture system. In many rocks, fracture zones dominate the behavior. These can be described using the tools of geology and geomechanics in order to understand what kind of features might be important hydrologically and to qualitatively describe the way flow might occur in the rock. Geophysics can then be employed to locate these features between boreholes. Then well testing can be used to see if the identified features are in fact important. Given this information, a conceptual model of the system can be developed which honors the geologic description, the tomographic data and the evidence of high permeability. Such a model can then be modified through an inverse process, such as simulated annealing, until it reproduces the cross-hole well test behavior which has been observed insitu. Other possible inversion techniques might take advantage of self similar structure. Once a model is constructed, we need to see how well the model makes predictions. We can use a cross-validation technique which sequentially puts aside parts of the data and uses the model to predict that part in order to calculate the prediction error. This approach combines many types of information in a methodology which can be modified to fit a particular field site.

We present a novel coupled fluid‐poroelastic model and an associated numerical upscaling procedure to calculate seismic attenuation and velocity dispersion in porous rocks induced by fluid pressure diffusion (FPD) in the presence of mesoscopic fluid‐saturated voids, such as, for example, vugs or factures. By applying appropriate interface conditions, the proposed model couples the Navier‐Stokes equations for viscous fluids with Biot's equations of poroelasticity to model the mesoscopic voids and the embedding background, respectively. A finite element method is employed to solve the coupled problem for a set of three relaxation tests, which enables us to compute the complex‐valued and frequency‐dependent equivalent stiffness matrix of the considered synthetic sample. The newly proposed fluid‐poroelastic approach is compared with a purely poroelastic one as well as a fluid‐elastic approach in a benchmark model containing interconnected mesoscopic fractures embedded in a poroelastic background. We obtain excellent agreement for the proposed approach and the purely poroelastic model by optimizing the material properties of the fractures for the latter, which demonstrates both the correctness and advantages of our method over the purely poroelastic approach for modeling fluid‐saturated mesoscopic voids. We also observe that, while the coupled fluid‐elastic approach and the proposed method provide consistent results with regard to seismic attenuation due to the fracture‐to‐fracture FPD, the latter also allows to account for the effects of fracture‐to‐background FPD. Finally, we employ the proposed methodology to explore the seismic characteristics of a synthetic “vuggy” carbonate‐type sample, for which we visualize and interpret the resulting seismic attenuation in terms of FPD between the microscopic and mesoscopic pores.

Frequency-dependent seismic anisotropy in fractured rock

During underground coal mining, Mining-induced seismicity is a crucial factor leading to roadway deformation. Based on a case study of rib spalling caused by mining-induced seismicity in China, this paper utilizes the Universal Distinct Element Code to investigate the mechanism of rib spalling. It analyzes the stress evolution process of roadway surrounding rock and the damage process under the influence of mining-induced seismicity. Additionally, it examines the relationship between seismic wave amplitude, frequency and the damage characteristics of roadway surrounding rock. Findings show that seismic waves induce stress concentration zones in the deeper surrounding rock, causing shear failure, while stress concentration zones emerge in the top, bottom and shoulders of roadway, leading to tensile failure in the shallower surrounding rock. With increasing seismic wave amplitude, the critical crack length of surrounding rock decreases, resulting in a linear increase in crack count and distribution area. The roadway exhibits a ‘n'-shaped damage profile primarily driven by shear failure. With increasing seismic wave frequency, the crack count in the surrounding rock of the roadway exhibits a trend of initially increasing and then decreasing. This trend becomes more pronounced with larger amplitudes. Rib spalling damage is influenced by seismic wave frequency and amplitude: frequencies of 20–25 Hz and 80–100 Hz require amplitudes exceeding 3 m for damage, while 25–30 Hz and 55–80 Hz require amplitudes over 2 m, and 30–55 Hz necessitate amplitudes exceeding 1.5 m.

SUMMARYThe presence of sets of open fractures is common in most reservoirs, and they exert important controls on the reservoir permeability as fractures act as preferential pathways for fluid flow. Therefore, the correct characterization of fracture sets in fluid-saturated rocks is of great practical importance. In this context, the inversion of fracture characteristics from seismic data is promising since their signatures are sensitive to a wide range of pertinent fracture parameters, such as density, orientation and fluid infill. The most commonly used inversion schemes are based on the classical linear slip theory (LST), in which the effects of the fractures are represented by a real-valued diagonal excess compliance matrix. To account for the effects of wave-induced fluid pressure diffusion (FPD) between fractures and their embedding background, several authors have shown that this matrix should be complex-valued and frequency-dependent. However, these approaches neglect the effects of FPD on the coupling between orthogonal deformations of the rock. With this motivation, we considered a fracture model based on a sequence of alternating poroelastic layers of finite thickness representing the background and the fractures, and derived analytical expressions for the corresponding excess compliance matrix. We evaluated this matrix for a wide range of background parameters to quantify the magnitude of its coefficients not accounted for by the classical LST and to determine how they are affected by FPD. We estimated the relative errors in the computation of anisotropic seismic velocity and attenuation associated with the LST approach. Our analysis showed that, in some cases, considering the simplified excess compliance matrix may lead to an incorrect representation of the anisotropic response of the probed fractured rock.

Although earthquake lights (EQLs) are considered a scientifically proven fact (Derr 1973; Ouellet 1990), and experiments have been implemented in the laboratory to try to explain their cause (Warwick et al. 1982; Brady and Rowell 1986; St-Laurent et al. 2006), a direct relationship between seismic waves and EQLs in situ has not yet been established. EQLs are an unusual luminous aerial phenomenon, similar in appearance to the aurora borealis, that appears in the sky at or near areas of tectonic stress, seismic activity, or volcanic eruptions. Here I report a time difference correlation between seismic waves and EQLs. I have been able to determine that seismic waves start the EQL generation process only at certain points on the Earth's surface. It appears that when propagating through the Earth's crust, strong seismic waves release part of their energy, thus generating EQLs. My research, and the ensuing conclusions, was possible only because of the excellent film of EQLs made during the Peruvian earthquake …

Seismic velocity fluctuations and fracture permeability: speculation: P. Leary, Geothermal Science & Technology, 5(1–2), 1995, pp 91–109

A novel approach of two-dimensional representation of rock fracture network characterization and connectivity analysis

Taking a bedding rock slope with weak structural plane as the prototype, a shaking table test with a similarity ratio of 1:10 is designed and carried out. By analyzing the acceleration and displacement responses at different positions of the slope, the seismic response and instability mechanism of rock bedding slope under different seismic amplitudes, frequencies, and durations are studied. Before the failure of the slope, the rock bedding slope shows an obvious “elevation effect” and “surface effect” under the action of Wenchuan Wolong earthquake wave with different amplitudes. With the increase of the amplitude of the input seismic wave, the elevation effect and the surface effect gradually weaken. When the amplitude of the seismic wave reaches 0.9 g, the rock bedding slope begins to show damage, which demonstrates that the difference of PGA amplification coefficients on both sides of the weak structural plane increases significantly. Compared with the Kobe seismic wave and Wenchuan Wolong seismic wave, the excellent frequency of EL Centro seismic wave is closer to the first-order natural frequency of slope model and produces resonance phenomenon, which leads to the elevation effect of PGA amplification coefficient more significantly. Through the analysis of the instability process of rock bedding slope, it can be found that the failure mechanism of the slope can be divided into two stages: the formation of sliding shear plane and the overall instability of the slope.

It has been recognized that stress perturbations in sediments induced by salt bodies can cause elastic-wave velocity (seismic velocity) changes and seismic anisotropy through changing their elastic parameters, thus leading to difficulties in salt imaging. To investigate seismic velocity changes and seismic anisotropy by near-salt stress perturbations and their impacts on salt imaging, taking the Kuqa depression as an example, we have applied a 2D plane-strain static geomechanical finite-element model to simulate stress perturbations and calculate the associated seismic velocity changes and seismic anisotropy; then we used the reverse time migration and imaging method to image the salt structure by excluding and including the stress-induced seismic velocity changes. Our model results indicate that near-salt stresses are largely perturbed due to salt stress relaxation, and the stress perturbations lead to significant changes of the seismic velocities and seismic anisotropy near the salt structure: The maximum seismic velocity changes can reach approximately 20% and the maximum seismic anisotropy can reach approximately 10%. The significant changes of seismic velocities due to stress perturbations largely impact salt imaging: The salt imaging is unclear, distorted, or even failed if we exclude near-salt seismic velocity changes from the preliminary velocity structure, but the salt can be better imaged if the preliminary velocity structure is modified by near-salt seismic velocity changes. We find that the locations where salt imaging tends to fail usually occur where large seismic velocity changes happen, and these locations are clearly related to the geometric characteristics of salt bodies. To accurately image the salt, people need to integrate results of geomechanical models and stress-induced seismic velocity changes into the imaging approach. The results provide petroleum geologists with scientific insights into the link between near-salt stress perturbations and their induced seismic velocity changes and help exploration geophysicists build better seismic velocity models in salt basins and image salt accurately.

TEMPORARY REMOVAL: Pre-drill pore pressure modelling and post-well analysis using seismic interval velocity and seismic frequency-based methodologies: A deepwater well case study from Mississippi Canyon, Gulf of Mexico

We present the spatio-temporal evolution of seismicity recorded by eight three-component digital seismographs in operation continuously during a 3 yr period (1994 August to 1997 May) at Açu reservoir, NE Brazil. The Açu dam is a 34 m high earth-filled dam constructed in 1983 May on an area of Precambrian shield. Based on seismic monitoring between 1987 and 1989 using single-component analogue seismographs, previous workers concluded that the seismic activity was a case of reservoir-induced seismicity (RIS) associated with diffusion of pore fluid pressure beneath the reservoir. The digital data presented here reveal the seismic activity in remarkable detail with vertical and horizontal location errors ≈0.1 km. A total of 286 events were recorded by three or more stations and all occurred at a depth of <5 km. Using these data we demonstrate that the majority of the earthquake activity is clustered within several well-defined zones and that individual zones are active over discrete periods of time. Over the entire period of seismic monitoring between 1987 and 1997 there is no simple correlation between reservoir level and number of seismic events. Lateral migration of the locus of seismic activity in an unpredictable fashion is shown to be partly responsible for the poor correlation, as event detection is not uniform through time. We also show that the time delay between maximum water level and a subsequent increase in seismic activity varies systematically; longer time delays correspond to activation of an earthquake cluster with a greater average hypocentral depth. However, within any one cluster there is no correlation between time delay and depth. The 3-D distribution of seismic activity through time may only be explained in terms of triggering by the diffusion of pore fluid pressure if the rock properties (e.g. permeability, strength) are heterogeneous.

The plane frame structure of three-span, six-story reinforced concrete structure, is adopted to study whether wave passage effect should be considered in the seismic responses of frame structures or not. The influence factors of seismic wave's frequency and wave passage effect on the structure are studied. The earthquake responses of the frame structure to wave passage excitations and uniform excitations are computed respectively for the class III site seismic wave by using Midas Gen software. The results of column shear forces obtained from the wave passage excitations are compared with those of the uniform excitations. And the frequencies of seismic waves are analyzed. Numerical results showed that wave passage effect caused the shear force of the first-story column facing the seismic wave to increase remarkably for the plane frame structures. For the seismic waves with wave passage effect, the main peaks of frequency amplitude spectrum of seismic wave arise in low frequency domain.

A coupled displacement-pressure model for elastic waves induce fluid flow in mature sandstone reservoirs