Page 227 - Enhanced Oil Recovery in Shale and Tight Reservoirs

210 Enhanced Oil Recovery in Shale and Tight Reservoirs

dolomite (CaMg(CO 3 ) 2 ) and pyrite (FeS 2 ). Pyrite in shale reacts with per-
sulfate to generate sulfuric acid by the following oxidation reaction:
2FeS 2 þ 15S 2 O 2 þ 16H 2 O ¼ 2Fe 3þ þ 34SO 2 þ 32H þ (8.5)
8 4
Ammonium persulfate is a very strong oxidizer that is commonly used to
decompose the gelling agent in hydraulic fracturing. The resultant sulfuric
acid is a strong acid whose solution pH can be less than 2. Then this acid
reacts with calcite and dolomite to precipitate gypsum crystal:
þ
CaCO 3 þ 2H þ SO 2 þ H 2 O ¼ CaSO 4 $2H 2 O þ CO 2 [ (8.6)
4
þ
CaMgðCO 3 Þ þ 2H þ SO 2 þ H 2 O
4
2
¼ CaSO 4 $2H 2 O þ MgSO þ 2CO 2 [ (8.7)
4
The above reactions (processes) are called replacement reactions (pro-
cesses), as the calcite and dolomite are dissolved by the acid and a new
replacing mineral, gypsum, is precipitated in the immediate vicinity of the
dissolved carbonate mineral surfaces. As gypsum has a higher molar volume
(74.4 mL/mol) than calcite (36.9 mL/mol) and dolomite (64.3 mL/mol),
the local replacement reactions can generate internal swelling stress that
may fracture the surrounding shale matrix. The reaction-induced stress
from crystallization pressure is on the grain scale. The crystallization pressure
can easily exceed 30 MPa, which sufficiently causes intensive shale
microfracturing.
Fig. 8.32 shows the induced fractures when unconfined cylindrical shale
samples were exposed to deionized water (a), H 2 SO 4 solution (b), and
(NH 4 ) 2 S 2 O 8 solution (c). Few fractures were seen on shale samples exposed
to deionized water. In contrast, many fractures could be seen when the
samples were exposed to the chemical solutions. These induced fractures
propagated parallel to the lamination. It took 3 and 5 days for those fractures
to form for H 2 SO 4 solution (b) and (NH 4 ) 2 S 2 O 8 solution (c), respectively,
indicating the H 2 SO 4 solution reaction was faster than the (NH 4 ) 2 S 2 O 8 solu-
tion. Interestingly, the gypsum deposited in the induced fractures, and not on
sample surfaces, indicating that those fractures were caused by the increased
gypsum volume.
(NH 4 ) 2 S 2 O 8 is a very strong oxidizer, whereas the dilute sulfuric acid
solution is only a strong dibasic acid not an oxidizer. Oxidation reaction
may significantly change the organic fraction and the color of black shale.
Their experiments did not show obvious color change, and the
fracture-generation behavior is similar in both the H 2 SO 4 solution and