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Fluid-rock interactions                                      211


              (A)  Sample A, before and after 10 day exposure to deionized water
               0 d              10 d              10 d



                  Lamination      Hydration-induced
                   direction         fracture

              10 mm

              (B)  Sample B, before and after 10 day exposure to 10 wt.% H SO solution
                                                                   4
                                                               2
               0 d              10 d              10 d


                 Lamination       Reaction-induced
                  direction          fracture

              10 mm

              (C)  Sample C, before and after 10 day exposure to 10 wt.% (NH ) S O solution
                                                                 4 2 2
                                                                       8
               0 d              10 d              10 d


                 Lamination
                  direction
                                   Reaction-induced
                                      fracture
              10 mm

              Figure 8.32 Macroscopic observations of fractures induced by (A) clay hydration and
              (B) H 2 SO 4 solution and (C) by (NH 4 ) 2 S 2 O 8 solution in the unconfined shale samples
              (Chen et al., 2018).


              the (NH 4 ) 2 S 2 O 8 solution, suggesting that the fractures were induced by the
              above reactions, and they were not affected by the organic fraction at the
              experimental time scale.
                 Note that from the first of the above three reaction equations, pyrite reacts
              with ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) to produce the acid solution of
               þ
                      2
              H þ SO : In an anoxic depositional environment, pyrite commonly exists
                      4
              in a shale formation. Even if there is no pyrite, a sulfate-bearing acid solution
              like H 2 SO 4 can react with carbonate minerals to precipitate gypsum, and the
              above volume increase-induced fractures should be created. However,
              the above experiments were conducted without confinement.
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