Pore-pressure prediction

It Is now well established experimentally that the solvation force, fg, of confined fiuld Is an oscillating function of pore wall separation. In Figure 4 we compare the theoretical and MD results for fg as a function of h. Given that pressure predictions are very demanding of a molecular theory, the observed agreement between our simple theory and the MD simulations must be viewed as quite good. The local maxima and minima In fg coincide with those In n y and therefore also refiect porewldths favorable and unfavorable to an Integral number of fiuld layers. 

Figure 2. Capillary hysteresis of nitrogen in cylindrical pores at 77 K. Equilibrium desorption (black squares) and spinodal condensation (open squares) pressures predicted by the NLDFT in comparison with the results of Cohan s equation (the BJH method) for spherical (crosses and line) and cylindrical (line) meniscus.

Figure I Relation between filling pressure and pore width predicted by the modified Kelvin equation (MK), the Horvath-Kawazoe method (HK), density Junctional theory (DFT), and molecidar simulation (points) for nitrogen adsorption in carbon slits at 77 K [8].

Prediction of pore pressure prior to drilling can be critical at several stages in the exploration, and development process. It can be used during exploration ... 

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. 

By associating aquifer gradients (Fig. 12) with first-order spatial pressure domains and depth of burial, aquifer pressures for individual prospects can be predicted. Retention capacity as dictated by the pressure difference between the reservoir aquifer pressure and seal pore-pressure or fracture envelope can then estimated. The critical stage in this method is the selection of the correct aquifer pressure. Of the other variables required, the crestal elevation of the prospect is usually known with a reasonable degree of confidence, and seal pore-pressures are coincident with the fracture gradient which in turn is confirmed by measured (LOT/FIT) data. Application of this method within the GEA suggests that pre-Cretaceous seals retain hydrocarbon columns within the range from 200 to over 750 m. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

For the low rock permeability case, a correct prediction of the pore pressure in the rock mass requires a full THM analysis, as shown on Figure 9 for point B6 of the rock adjacent to the buffer. At early times, due to its low permeability, the rock could not supply water to the buffer at a sufficient rate, leading to a drop in pore pressure. Rebuilding of the hydrostatic pressure starts at about 30 years for the THM case, and about 60 years for the HM case, while it is still unsaturated after 100 years in the TH case. 

Effective stresses are affected in two different ways, an increase in total stress due to the mechanical loading of the ice sheet and a decrease in effective stress due to the increase in pore pressure. Consequently, changes in effective stresses are much less than expected from the mechanical stresses due to the weight of the ice sheet. Even in the dead-ended horizontal fracture zone in Section 2 Configuration 6 there is no hydraulic Jacking, i.e., no effective tensile stress. No shear failure has been predicted. There is practically no rotation of principal effective stresses. 

I (Takahashi, 2000). The peak shear stresses obtained from the triaxial compression tests are also plotted in Fig. A2. In the plot of Fig. 5, there is general agreement between the two types of the experimental results. Thus, it is thought that the occurrence of the shear fracture in the simulated hydraulic stimulation tests can be approximately predicted by the Coulomb criterion. Based on the comparison, the critical condition for the shear fracture due to hydraulic stimulation was estimated using the experimental results of the triaxial compression tests, as given in Fig. A2, and the averaged value of pore pressure. The detailed discussion of the triaxial compression tests can be found elsewhere (Takahashi, 2000.). 

A realistic prediction of the permeability distribution in three dimensions in sedimentary basins seems impossible given the wide ranges of values for different types of sediments and the heterogeneities of the basins. Pore pressures and fluid fluxes in three dimensions can not be modelled reliably. When the fracture pressure is reached at high overpressure, the fluid flow becomes decoupled from the permeability of the rock matrix and is mostly a function of the permeability of very thin hydrofractures. The permeability is then coupled to the fracture spacing and width which again is a function of the fluid flux and the rate of compaction. 

Figure 9.4 Comparison of pore diameters obtained from capillary hysteresis of nitrogen in cylindrical pores at 77.4 K. Equilibrium desorption and spinodal condensation pressures predicted by the NLDFT method in comparison with the resuits of the BJH method. (Reprinted with permission from A. V. Neimark and P. 1. Ravikovitch., Microporous Mesoporous Mater. 2001, 44-45, 697. Copyright 2(X)1 Elsevier.)...

Ratio of measured residual pore pressure to predicted reference residual pore pressure UJU ... 

Pore pressure as a function of cyclic shear strain illustrating a threshold strain of about 0.01%, below which no excess pore pressure are developed. (After Dobry, R. et al.. Prediction ofPorewater Pressure Buildup and Liquefaction of Sands during Earthquakes by the Cyclic Strain Method, NBS Building Science Series 138, U.S. Department of Commerce, National Bureau of Standards, 1982.)... 

The pressures calculated using the above approaches generally use the linear (Airy) theory and expect the pore pressure response to be in phase with the surface waves. This is in agreement with what has been measured for sandy bottoms. In contrast, measurements made by Hirst and Richards (1978) showed that bottom pressures can be much larger than predicted by linear theory for soft clayey bottoms. In Sections 9.6.3.1 and 9.6.S.2 the case of a rigid seabed (i.e., sandy bottom) will be considered first followed by the case of a deformable seabed (i.e., soft clayey bottom). 

A typical plot of pure water flux vs. pressure difference for hydrophilic SPG membranes is shown in Figure 16.6 (solid lines). Under these conditions, Unearly increases with increasing the transmembrane pressure, which is in correspondence with Equation (16.10) for laminar flow regime in the pores. As predicted from Equation (16.10), the water flux was higher for the larger mean pore size. If the SPG membrane was hydrophobic, the water flux was not observed below a critical pressure (dotted lines in Figure 16.6). After reaching the critical pressure, the water flux sharply increases and approaches a value for the same hydrophilic membrane. 

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