Page 159 - Fundamentals of Gas Shale Reservoirs

7

PORE PRESSURE PREDICTION FOR SHALE FORMATIONS
USING WELL LOG DATA

Abualksim Ahmad and Reza Rezaee
Department of Petroleum Engineering, Curtin University, Perth, WA, Australia

7.1 INTRODUCTION with the surface during burial. Therefore, the pore fluids
are squeezed out by the normal compaction, and as a
Pore pressure in any sedimentary formation is defined as result, a normal hydrostatic pressure regime is established.
the pressure of the fluid contained in the pore space of the In normally pressured sediments, the vertical effective stress
rocks, and can be either normal or abnormal pressure. continues to increase as the depth increases. The normal
Abnormal pressure is subclassified into abnormal high hydrostatic pressure and gradient can be calculated by using
pressure (overpressure) and subnormal pressure. Knowledge Equations 7.1 and 7.2, respectively, and graphic illustration
of pore pressure regimes in any sedimentary basins is an of the normal pore pressure regime is presented in Figure 7.1:
integral part of the formation evaluation process in gas shale
formations (Gretener, 1979). Appropriate evaluation of pore P w g z (7.1)
pressure is crucial for drilling and completion planning
(Tingay et al., 2003). In terms of pressure gradient,
In this chapter, definitions of important pore pressure‐
related terms are presented first, and then overpressure‐
generating mechanisms are explained in detail followed by dP w g (7.2)
overpressure estimation methods. In addition, the relation- dz
ships between pore pressure distribution and tectonic
elements in sedimentary basins are presented herein. These where P is the pore fluid pressure, ρ is the pore water
w
relationships were observed in a recent study that was density, g is the gravitational acceleration, and z is the
conducted on the potential gas shale formations in the Perth vertical depth of the formation. For freshwater with a density
3
Basin, Western Australia. Finally, the origins of overpressure of 1 g/cm , the hydrostatic pressure gradient is 0.433 psi/ft.
in these shale intervals are explained and some examples
are presented.
7.1.2 Overpressure
Overpressure is defined as a formation pressure that is greater
7.1.1 Normal Pressure
than the normal hydrostatic pressure of a column of pore
The normal hydrostatic pressure at any depth is defined as fluids that reaches from the surface to the vertical depth of the
being the pore pressure equivalent to the hydrostatic pressure formation. Sediment compaction is mainly caused by an
due to an open column of pore fluids that reaches from increase in overburden stress, and the theory of compaction
the surface to the vertical depth of the formation. In normally was well described by Terzaghi et al. (1996). The authors
pressured formations, pore fluids communicate efficiently established an equation of equilibrium (Eq. 7.3). It was

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.