Page 200 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 183

Figure 8.9 Swelling strain of Mancos shale cores during water imbibition under
different isotropic confining pressures.
strain that was considered the total core swelling increased significantly first
and then gradually stabilized. Swelling strain reduced due to compressive
stress as the confining pressure was increased. It can be understood that a shale
core can expand with water imbibition without confining pressure, probably
resulting in larger pores or higher permeability. However, with a high
confining pressure but the rock expanding, pores must shrink. The confining
pressure has a significant effect on the water-rock interaction. These results are
consistent with the observation from Ewy and Stankovic (2010) that
confining pressure can significantly prevent induced swelling from occurring;
there exists a threshold confining pressure below which swelling decreases
with confining pressure.
Roshan et al. (2015) studied fracturing under isotropically confined condi-
tions (by hydraulic pressure). A fracture was observed on a bedding plane when
a shale core was immersed in deionized water for 40 h under 1000 psi, but the
fracture formation took a longer time than the case without confinement.
Note that the core could still expand under such hydraulic confinement.
To study the effect of anisotropic stresses, an anisotropic core holder was
needed by Liu and Sheng (2019). Fig. 8.10 is the schematic of an anisotropic
core holder which can be used under CT scan. The part 7 controls the
confining pressure in the radial direction of the core, while the part 9 con-
trols the axial pressure.
For water to be able to contact a core quickly, a hole of 13 mm diameter
and 15 mm in length was drilled in a core of w38 mm in diameter and
w51 mm in length, as shown in Fig. 8.11 (Liu and Sheng, 2019). The sam-
ple was put in an FCH aluminum wrapped triaxial core holder. A fluid could
flow into the core sample through the hole and the core end face. An axial