Page 215 - Enhanced Oil Recovery in Shale and Tight Reservoirs

198 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 8.22 Tensile fractures in organic-rich shale with different boundary conditions:
(A) no confinement, (B) displacement confinement, and (C) stress confinement. The red
lines (black lines in print version) represent tensile failures, whereas the blue lines (dark
gray lines in print version) show shear failures (Yang et al., 2015).

shale specimen. However, lab data shows that most shales are very heteroge-
neous at the microscopic scale; they have some acid-soluble minerals. Some
shales like Caney have high content of calcite (Grieser et al., 2007).
In a hydraulic fracturing operation, an acid (HCl) preflush is commonly
practiced to low the rock compressive strength near the wellbore, to remove
drilling and completion damage, and to enhance microfracture connectivity
by removing calcite (Fontaine et al., 2008). Typical acid concentrations are
0.08%e2.1% of the total fluid pumped in shale formations (McCurdy,
2011). Morsy et al. (2015) observed that the shale rock mechanical strength
(Young’s modulus and breakdown pressure under uniaxial compressive
tests) was decreased, and the porosity became higher, when shale rock sam-
ples were immersed in low-concentration (up to 2%) HCl solutions. The oil
recovery from spontaneous imbibition increased with HCl concentration.
Grieser et al. (2007) reported that when acid pads and sand slugs were
alternatively injected in an S.E. Oklahoma Woodford frac job, unexpected
pressure drop was observed when the acid pads hit the perforations, and gas
rate was increased. Table 8.2 shows the pump schedule in the Woodford