Seismicity pore pressure

Shapiro, S., Patzig, R., Rothert, E. Rindschwentner, J. 2003. Triggering of seismicity by pore pressure perturbations permeability-related signatures of phenomenon. Pure and Applied Geophysics, 160, 1051-1066. 

The necessary sequence of operations to predict effective pressure and overburden and hence, pore pressure from seismic data starts with detailed velocity analysis (Dix, 1955). The usual steps for velocity analysis (VA) are listed below ... 

The predicted pore pressures from seismic are compared with those predicted from sonic log in Fig. 14. The curve marked lithostatic is the overburden pressure obtained from integrating the seismically... 

These case studies revealed that (i) active migration pathway of fluids can be imaged by 2-D/3-D effective stress maps using seismic velocity data, and (ii) the predicted pore pressures at the well using both seismic and sonic data are in agreement with each other and with an independent set of data the RFT measurements. 

The BP technology for prediction of pore pressure using seismic velocities has a number of merits ... 

Drilling experience has shown that this technology can predict pressures to within 0.75 ppg at target depths, provided the low-frequency trends of seismic interval velocities are of good quality and are within 5-10% of well velocities. This has been observed by numerous case studies and applications within BP s exploration and exploitation community. The current technique predicts effective stress quantitatively and directly, unlike any other method. The method is completely pre-drill in nature it does not use trend data and it is not tied to block-to-block well calibration. However, it does require an understanding of the local geology and in particular, of rock properties. In addition, the reliability of the predicted effective stress and pore pressure is limited by the resolution of the seismic velocity. 

This research uses observations of reservoir induced seismicity (RIS) at A u reservoir. NE Brazil, to investigate the spatial and temporal evolution of effective stress in the region and its relationship to fault permeability. A u reservoir was constructed in 1983 and has a capacity of 2.4 x lO m maintained by a 34 m high earth-filled dam constructed on Precambrian shield. Annual reservoir variation is 3-6 m which results in annual seismic activity due to a proposed mechanism of pore pressure diffusion (Ferreira et al. (1995), do Nascimento et al. (2003a)). Digital data at A u... 

It is considered that some fractures opened with increasing pore-pressure maybe after shear slip because the maximum wellhead pressure is about lOMPa. However, Mode I fractures can not radiate seismic events having enough energy, and we can not detect these seismic events. Therefore, we consider only shearing fractures for estimation of critical pore-pressure using induced microseismic events. 

Finn, W.D.L., and Bhatia, S.K. 1981. Prediction of seismic pore-water pressures. In Proceedings, of the 10th International Conference on Soil Mechanics and Foundation Engineering 3. Rotterdam, the Netherlands 201-206. 

When the downstream dam site is considered, the retained reservoir level will be 2475 a.s.l. The specified phreatic surface in this model was selected to be the same level of water damming. The colluvium and heavily weathered bedrock were considered to be permeable while the bedrock was assumed to be impermeable. The shear strength parameters of the saturated deposit are selected as 0.7 times of the dry condition referring to the research of Liu (2009). The hydrostatic pressure on the slope was also considered. Figure 9 shows the pore pressure of the colluvium and the phreatic line can be obtained from the result. Figure 10 illustrates the displacement contour and vectors after 1000 iteration steps. Figure 11 shows the cross sectional map of the displacement in Y direction after partial saturation. It can be concluded that, unlike the seismic condition, the... 

On the other hand, most fine-grained soils experience initial excess pore on the same order of magnitude as the applied stress even when static permanent loads are applied at the rates normally expected of. Whereas, when the loads are applied at a faster rate such as those expected of for the day-to-day hve loads and for the seismic loads, little or no excess pore pressure generates in most low-permeable finegrained soils. 

M3 in Fig. 11a). Flow here represents quasistationary creep including episodic high activity, depending on infiltration and the pore pressure (Fig. 11b). The distribution of velocities in space and time within the landslide mass justifies its macroscopic description as a ductile medium. However, as the following two examples will prove, brittle deformations or interactions between pore fluid and the rock/soil mass also exist and resemble seismic sources. 

The equation demonstrates that a high pore pressure plays the same role as a low external stress, causing the compressional wave velocity to be reduced. This is used in the estimation of abnormal pore pressures from logs or seismic velocities (e.g. Japsen et al., 2006). Since the expected trend in a homogeneous formation would be a monotonous increase of velocity with depth (because the effective stress increases with depth), an overpressure zmie shows up as a low-velocity zone breaking the expected trend. 

Within the interconnected pores, there is fluid pressure equilibrium and no pore pressure gradient as a result of passing waves. Thus, the low frequency allows an equilibration of the pore pressure within the pore space. Therefore, Gassmann s equation works best for seismic frequencies (<100 Hz) and high permeability (Mavko et ah, 1998). 

Note BSR, Bottom-Simulating Reflection Chloride Content of Pore Water CH4, High Methane Content Logs, Well-Log Response PCS, Pressure Core Sample Geophysical, Seismic Evidence of Past Occurrence of Gas Hydrate. 

Fig. 14.16 Comparison of AT anomalies (blue lines) to gas hydrate content estimated dissolved chloride distribution (red lines), and given as percent occupancy of the pore space. Leg 204. Green lines denote estimates based on data from pressure core barrel deployments, the depth of seismic reflectors corresponding to the bottom of the GHSZ (BSR). Location of Insert B shows the temperature profile derived from an infrared image in the vicinity of from Site 1245, and the corresponding chloride concentration in closely-spaced pore water depth between the two graphs is due to the removal of core as gas expansion voids between collected and the pore water samples were taken (modified from Trehu et al. 2004a).

Fig. 14.17 A. Diagram illustrating the double BSR observed in seismic data in the vicinity of Site 1247. B. Chloride concentration in pore waters from site 1247, compared with expected values derived from a diffusive attenuation model following gas hydrate dissociation. The assumed hydrate content at time zero has a width of 10 m and a magnitude comparable to the anomaly observed just above the present BSR (BSRp). The data suggest that the hydrate dissociation occurred 5000 yrs ago. The authors postulate that pressure and temperature changes in the period of 8000 to 4000 years ago, led to a shift in the depth of the hydrate stability zone, creating the double BSR (modified from Bangs et al. 2005).

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