Chapter%206 - Petrophysics MSc Course Notes The Borehole...

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Petrophysics MSc Course Notes The Borehole Environment Dr. Paul Glover Page 65 6. THE BOREHOLE ENVIRONMENT 6.1 Introduction Wireline logging has a single clearly defined purpose: to give accurate and representative data on the physical properties of the rock formations and fluids encountered in a borehole. The tools used to take these readings have to cope with extremely tough conditions downhole, particularly, high temperatures and pressures, inhospitable chemical conditions and the physical constraints imposed by the physics of the measurements and the borehole geometry. It should also be remembered that we are interested in the properties of the rocks in undisturbed conditions, and the act of drilling the borehole is the single most disturbing thing that we can do to a formation. 6.2 Overburden Pressures The formations in the sub-surface are at raised pressure, and are occupied by fluids which are also at high pressure. The pressure that a rock is subjected to at a given depth is determined by the weight of the rock above it, and hence the density of that rock. This is called the overburden pressure or sometimes the lithostatic pressure (note that, to a first approximation, the overburden pressure is the same in all directions ( isotropic )). We can write an equation to describe the overburden pressure (6.1) where, P over = the overburden pressure at depth h r rock = the mean rock density above the depth in question g = the acceleration due to gravity h = the depth to the measurement point. Clearly the rock above a given depth will have a varied lithology and porosity and hence a varying density. A more accurate determination of the overburden pressure can be obtained by summing the pressure contributions for each density by writing Eq. (6.1) for i different rock densities, each with thickness h i . (6.2) The overburden pressure/depth curve is called the geobar or lithostat . An example is given in Fig. 6.1. It should be noted that in actuality this pressure is not isotropic but operates vertically. The pressures horizontally depend upon the overburden pressure, but are modified by additional large scale sub- horizontal tectonic forces (in tension and compression), and are affected by local inhomogeneities in the crust, such as fractures. However, to a first approximation the pressure at depth can often be considered to be isotropic (hydrostatic). h g P rock over r = ) ( = i i i over h g P r
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Petrophysics MSc Course Notes The Borehole Environment Dr. Paul Glover Page 66 Figure 6.1 Lithostatic, hydrostatic and overpressures. 6.3 Fluid Pressures The pressure in a fluid occupying a formation depends upon many forces. If there is a continuously connected pathway of fluid from the surface to the depth in question, the fluid pressure depends primarily upon the weight of the fluid above it, in a direct analogy to the situation for rocks. As the density of fluids is approximately one third of that for rocks, the fluid pressure will be approximately
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Chapter%206 - Petrophysics MSc Course Notes The Borehole...

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