Page 166 - Fundamentals of Gas Shale Reservoirs

146 PORE PRESSuRE PREdIcTIOn FOR ShAlE FORmATIOnS uSInG WEll lOG dATA

7.3 OVERPRESSURE ESTIMATION METHODS Transit time

Techniques used to detect abnormal pore pressure can be
classified as (i) predictive and before drilling methods from
offset wells and seismic data; (ii) during drilling from mud
log data such as kicks, drilling ROP, and flow line tempera-
ture; and (iii) postdrilling methods from well logging data B
(draou and Osisanya, 2000; Pikington, 1988).
In other words, formation pressure can be determined NCT
either by direct or indirect methods (lesso and Burgess,
1986). direct pressure measurements include the repeated
formation tests (RFTs), drill stem tests (dSTs), and the Depth
modular formation dynamic tester (mdT). The direct Effective stress method
measurements provide promising results in permeable
formations where the measurement tool is placed along the
formation and sufficient time is allowed for reaching
pressure equilibrium. however, pore pressure in imperme-
able formations such as shale cannot be measured by direct A
measurement due to their associated operational difficulties Eaton’s method
such as high cost of rig time because low‐permeability
formations need very long time to reach pressure
equilibrium. An additional problem associated with the use FIGURE 7.10 diagram of Eaton’s method and the effective
of direct pressure measurement methods in shales is the stress method.
risk of differential pipe or tool sticking. Therefore, indirect
methods that are based on compaction and porosity‐ stress, and the knowledge of porosity‐dependent parameters
dependent parameter concepts such as the applications of (Terzaghi et al., 1996).
well logging and drilling data, where pressure‐dependent Prior to explaining the overpressure estimation methods,
parameters can be used to infer pore pressure (Alixant and an overview of the compaction theory is presented.
desbrandes, 1991; lesage et al., 1991). According to
Tanguy and Zoeller (1981), well log data provide the 7.3.1 Overview of the Compaction Theory
lithological information and appropriate petrophysical
properties needed to estimate pore pressure in shale The basic concept of compaction theory is illustrated in
formations. Figure 7.11. The difference between the pressure exerted by
As described in Sections 7.2.1.3 and 7.2.2.4, wireline the overburden stress (σ ) and the vertical effective stress
ob
logs respond to normal pressure trends and overpressure (σ ) is the pore pressure (P ) (Terzaghi et al., 1996).
v
p
phenomenon in different ways. In normally pressured inter- According to Alixant and desbrandes (1991), there are two
vals, wireline log parameters follow the ncTs as a result of limitations associated with the application of compaction
normal sedimentary environments and normal compac- theory in determining pore pressure: “(1) the determination of
tion of sediments. On the other hand, in overpressured the normal trend is a subjective task that may be troublesome
formations, the responses of wireline logs depart from the without a regional experience. (2) An empirical correlation
ncTs whether the overpressure‐generating mechanisms are between petrophysical measurements and fluid‐pressure gra-
loading or unloading. The departure of wireline logs from dients must be established on the basis of a regional data set.”
the ncT is used as a key parameter for predicting overpres- despite these limitations, the use of compaction theory is
sure in sedimentary rocks. hence, both an appropriate a general practice evaluation method in the industry to eval-
formation evaluation process and a proper drilling and well‐ uate pore pressure. An overview of the compaction theory is
completion design are achieved (Tingay et al., 2003). described before explaining the overpressure estimating
In fact, shale is quite sensitive to the compaction process methods.
and therefore, it has been used as a key parameter for the As depth increases, sediments compact, resulting in a
determination of pressure profile in sedimentary rocks reduction of porosity. many researchers studied the porosity–
(muir, 2013). The most popular prediction methods for pore depth relationship and developed many correlations. The
pressure are (i) the effective stress, also called the equivalent most cited experimental relationship between porosity and
depth method and (ii) Eaton’s method (Fig. 7.10). The depth was presented by Athy (1930) (Eq. 7.5):
fundamental concepts for estimating pore pressure in shale
formations are the knowledge of overburden stress, effective 0 e bz (7.5)