Shale content

Select a tungsten carbide bit with high offset and chisel inserts if the shale content of the formation increases and/or the mud density is high. Use bit type 5-2 or 5-3. 

The salinity of formation waters is often calculated using electrical-resistivity and spontaneous-potential (SP) logs, and the values obtained are reasonable, except in geopressured zones with high shale content (Hearst and Nelson, 1985 Rider, 1996). It is often possible to determine vertical variations in salinity over a distance of several kilometers from a single log. 

Direct observations of sub-surface pressure allow a calibration to be made between the SGR and seal capacity. Ideally, an in situ measurement of the pore-pressure in the reservoir and that inside the fault zone would allow the capillary entry pressure of the fault to be calculated. However, fault-zone pressures are rarely available. Instead, the pressure difference between the two walls of the fault is a more general parameter that can be derived from pressure measurements in pairs of wells across the fault. Fig. 7a shows one such calibration, based on the Nun River dataset of Bouvier et al. (1989). From their strike projections of Fault K , values of SGR have been calculated on a dense grid across the fault surface. On the same grid, minimum across-fault pressure differences have also been derived, using the proven distribution of hydrocarbons in the footwall sands to calculate buoyancy pressures. Fig. 7a shows a cross-plot of these two parameters for the areas of sand-sand contact at the fault surface. The dashed line indicates the inferred relationship between SGR and seal capacity. At SGR < 20%, no fault-sealed hydrocarbons are observed the shale content of the slipped interval... 

Both anisotropy and heterogeneity are terms connected with the scale of consideration and definition of a property. A small sample size (core, plug) from a sandstone layer may be homogeneous, whereas a sandstone bed with variation of grain size, shale content, and porosity is heterogeneous. The scale is also fundamental for anisotropy. A typical example is a laminated sand it may be that the individual small layers are isotropic, but a section as resolved by an electrical logging tool sees an anisotropy (macroscopic anisotropy, see Section 8.6). 

Rowan et al. (2003) derived porosity-depth profiles from log data of 19 offshore wells. For the three main sediment groups—sand, silt, and shale—they used the shale content Vsh from a Gamma log as a parameter for classification. The following mean equations result ... 

Carbonate rocks also show a decrease of porosity under the influence of depth or overburden pressure, respectively. Brown (1997) analysed the influence of carbonate mineralogy, shale content, and fabric on the porosity versus depth correlation. For the study, argillaceous limestone, limestone, dolomitic limestone, and dolomite of the Mississippian Madison Group in the Williston... 

FIGURE 2.18 Permeability as a function of shale content (West Sak Reservoir). Data from Vernik (2000). 

For laminated shaly sand, a dramatic decrease of permeability in the vertical direction can be expected because the flow is COTitroUed by the low shale permeability, whereas in the horizontal direction, the magnitude of permeability is still controlled by the sand fraction for moderate shale content also. This creates permeability anisotropy. 

For dispersed shaly sand, there are various relationships and models that describe the dependence of permeability on porosity, shale content, and shale properties. Examples are ... 

Schon and Georgi (2003) developed a capillary-based model for dispersed shaly sand that shows an analogy to the Waxman-Smits equation (Section 8.5.3) for electrical properties. This model (Fig. 2.32) accounts for the reduction in porosity and decrease in the pore cross-sectional area with the content of clay and the associated immobile water. Starting with Hagen-Poiseuille s law, the flow rate for a cross-section is reduced by a film of clay particles. Permeability for dispersed shaly sand can be written as a functimi of the clean sand permeability sd and the dispersed shale content Fsh... 

If a spectral gamma measurement is available, the use of thorium and potassium for shale content derivation is recommended. Fertl (1983) noted that the thorium curve of a spectral gammalog allows a quantitative clay volume estimate despite the presence of varying amounts of uranium and potassium... 

In sedimentary rocks, clean carbonates and sandstones normally show the lowest values. Radioactivity increases with the shale content. The highest values are from black marine shales. But there are some important specific cases of high radiation ... 

A shale content estimate can be derived from a gamma measurement based on the correlation between shale cmitent and radioactive isotope content, which originates natural gamma activity. It is assumed that only shale or clay is responsible for the radiation no other radioactive minerals are present. 

Transformation of the gamma-ray index into shale content For the transformation, empirical equations are recommended for various formations. The following list and Fig. 5.4 give a selection. Linear correlation in all cases delivers the highest shale content (Fig. 5.7). 

FIGURE 5.7 Relationships between gamma-ray index /qr and shale content Vsh- As example gr=0.35 (dotted Une) results in Ksh=0.35 for the linear equation, 7sh=0.21 for the Larionov equation/Tertiaiy elastics, Vsh=0.l2 for the Larionov equation/Mesozoic and older elastics. 

Thus, for an exact porosity calculation from a neutron measurement, the knowledge of the neutron response of the fluid, the neutron response of the matrix, the neutron response of the shale and the shale content is necessary. 

Shale content shifts the points from the sand line to the right (neutron porosity increases) it shifts apparently towards the dolomite. 

Kirchberger (2001) analysed logging data from the Vienna Basin and used the gammalog for characterizing the shale content F haie the density log... 

Figure 8.23 shows the resistivity / , as a function of the water saturation 5w at different shale contents Vsh calculated. 

Figure 8.25 shows a comparison of the shale effect on the result of water saturation calculation. Input are the measured formation resistivity / t, the shale content Psh, the shale resistivity Rsh, the water resistivity the porosity (j), and the Archie parameters m,n. 

Shale content Preferred are shale content calculations based on gamma-log and/or neutron-density combination. 

Input data from a resistivity measurement are the two macroscopic resistivities The unknown properties are the laminated shale content Vsh.iam... 

Solution A Additional input is the laminated shale content Fsh (determined from an independent source, for example, nuclear measurements, Thomas-Stieber technique). For the two unknown (microscopic) resistivities, the results are ... 

Solution B Additional input is the shale resistivity / sh (derived from an adjacent thick shale layer). Then the sand resistivity and laminated shale content results ... 

Investigations on sandstone with no or low shale content show also a kind of interface conductivity that is controlled by the specific internal surface. 

Fuchs and Forster (2013) analysed log and sample measurements from the North German Basin and implemented the shale content into the regression equations ... 

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