Page 222 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 205

change with temperature-induced stress change at 200 ft from the injector.
The pore pressure increases decreases the effective stress and shifts the
Mohr circle to the left. The temperature decrease causes negative strain or
tension in the system, and it creates significant shear stress due to the mechan-
ical strain contrast in the vertical and horizontal directions. As a result, the
Mohr circle is shifted to the left. Thus, both the increase in pore pressure
and the decrease in temperature shift the Mohr circle toward the failure en-
velope, synergistically leading to rock failure. However, the effect of temper-
ature decrease is more important than that of pore pressure increase in a typical
water injection case, because the pressure is generally maintained as injection
and production continues, but the temperature keeps declining as more cold
water is injected and hot reservoir fluids are produced.
The above paragraph discusses the concept of water-cooling effect. More
meaningfully, we want to know whether such effect is significant in a typical
shale and tight reservoir, and how much more oil can be produced from
such effect. Fakcharoenphol et al. (2013) used a sector model of Bakken
shale to study the effect. Fig. 8.28 shows the rock failure indicator. The pos-
itive value indicates the rock failure potential. From this figure, there does
not appear to be extensive rock failure sites. Probably cooling the reservoir
in a high temperature drop requires a huge amount of water.
Taghani et al. (2014) studied reactivation of existing microfractures by the
difference between the fracturing fluid and the reservoir fluid. Owing to the
fracture fluid leak-off, the increased pore pressure and decreased fracture pres-
sure reduce the formation of effective stresses; and the cooling effect induced
the rock tensile stresses. Collectively, these phenomena may open existing
natural microfractures and increase the fracture complexity. The effect of
such reactivated fractures depends on the number of existing fractures. If
the number is large, the increased area open to flow is large and the produc-
tivity or injectivity could be significantly increased. However, the changes in
reservoir pressure may close or reopen microcracks; hence the effectiveness of
these microcracks could be dynamic.

8.12 Reaction-induced fractures

Kelemen et al. (2017) filed a US patent that fluids are chemically
reacted within pores. Crystallization of solid minerals in pore space will
lead to compressive stresses and fracturing of rocks with crack spacing close
to the pore scale. Examples of fluids may be: (1) dissolved lime (CaO, as
Ca(OH) 2 , in solution) and carbon dioxide (CO 2 , as HCO 3 in solution) to