Page 163 - Fundamentals of Gas Shale Reservoirs

OVERPRESSuRE-GEnERATInG mEchAnISmS 143
diagenetic processes, smectite is stable and at least two water Pressure/stress Transit time Density
layers are preserved. There would be no loss of the interlayer
water (dehydration) as the temperature of the interlayer
water is below the threshold temperature of 71°c (colten‐
Bradley, 1987). Within a temperature range of 71–81°c, clay Depth Overburden stress A A

v
A
becomes unstable and one of the interlayers water is released. Pore pressure B B
For the other interlayer to be released, it requires a tempera- B C C C
ture range of 172–191°c (Boles and Franks, 1979; hower
et al., 1976). In other words, the conversion of smectite to
illite eliminates a considerable amount of the smectite FIGURE 7.6 Schematic diagram of the responses of wireline
interlayer surface, which was hydrated when the clay was in logs to overpressure generated by unloading mechanisms.
the smectite phase. As a result, the volume of the shales’
intergranular water increases and this increases the pore would be no reversal in density log, and it often continues
pressure thereafter. however, the increase in water volume to increase but may reverse slightly at the bottom of the
resulting from clay transformation processes cannot generate overpressured section. The responses of wireline logs to
a high magnitude of overpressure unless a perfect sealing overpressure caused by unloading are presented in Figure 7.6.
exists (Osborne and Swarbrick, 1997). The related chemical The pore structure is classified into storage pores (pore
reaction of this transformation produce major changes in spaces) and connecting pores (pore throats) (Bowers and
the behaviors of subsurface rocks due to the release of a Katsube, 2002). The effective porosity is the sum of all
significant amount of water into the pore system (draou and interconnected pores, whereas the total porosity is the sum
Osisanya, 2000). The chemical reaction of the transforma- of interconnected pores and the isolated pores. The storage
tion of smectite to illite is presented by Boles and Franks pores affect the total porosity and the bulk density of a
(1979) and stated in Equation 7.4. certain formation. These two petrophysical properties are
attributed to the total volume of the net pore; thus, the storage
Smectite K Illite SilicaH O (7.4) pores are the major porosity contributor of shale. On the
2
other hand, the connecting pores that control the flow within
All the processes of clay diagenesis are subject to temperature the pore system make very minor contributions to porosity.
and create overpressure through the transfer of load‐bearing When overpressure in shale is generated by fluid expansion
into pore fluids and through the fluid expansion process, for mechanisms, the response of the fluid expansion is basically
example, release of water process. an elastic opening (widening) of the connecting pores as a
result of effective stress reduction (Bowers and Katsube,
7.2.2.3 Heating As depth increases, temperature increases 2002; cheng and Toksöz, 1979). This response is due to the
and causes expansion of both the rock matrix and the fact that connecting pores have a low aspect ratio, and they
pore fluids. According to miller (1995), the increase in are mechanically flexible and more harmonious than the
volume resulting from the rock expansion is one order less storage pores. As a result, the porosity increases only by a
in magnitude than the increase in volume resulting from the very small amount (hermanrud et al., 1998). In contrast, the
expansion of pore fluids. hence, the increase in volume aspect ratio of the storage pores is high and they are
resulting from rock expansion can be ignored. If pore fluids mechanically inflexible and scarcely affected by fluid expan-
are heated while they are efficiently sealed, pore pressure sion. moreover, the bulk density is hardly influenced by
could increase significantly. however, luo and Vasseur fluid expansion responding to the low magnitude of porosity
(1992) concluded in their study that the expansion of pore increase. Bowers and Katsube (2002) and hermanrud et al.
fluids due to heating is not a significant contributor for (1998) stated that the connecting pores have significant
generating a high magnitude of overpressure. The authors impacts on transport properties such as sonic velocity and
stated that in order to maintain overpressure generated by electrical resistivity and thus affect sonic transit time and
heating, the pore fluids must be sealed effectively. however, electrical resistivity logs. On the other hand, they have insig-
this condition cannot be met in real situations as there is no nificant effects on density and neutron porosity logs.
formation with zero permeability and when there is a leaking
of the fluids in the system, this mechanism is neglected.
7.2.3 World Examples of Overpressures
7.2.2.4 Wireline Logs’ Response to Unloading Mechanisms Overpressure exists in almost every geological environment of
The response of wireline logs to overpressure generated by all ages. The event appears in all parts of the world. It is believed
unloading mechanisms is a decrease in effective stress, that the mechanism of under‐compaction is the main cause of
which produces a reversal in sonic transit time moving to a overpressure in young geological environment that experiences
higher sonic transit times as depth increases. however, there rapid sedimentation rates, for example, uS Gulf coast region