13th International Conference on Fracture June 16–21, 2013, Beijing, China -8marked decrease in crack propagation duration is due to rapid acceleration of crack growth associated with drastic increase in stress intensity factor as crack spacing decreases. We also examine the sensitivity of crack propagation distance and excess pressure to the changes of crack spacing, as shown in Figures 6 and 7, respectively. Our numerical calculation shows that when b/a0 > 3, no crack coalescence is found for typical burial depth in the range of 3000-7000m where thermal cracking of oil to gas occurs. For b/a0 =2.0, the excess pressure decreases with time and drops sharply when the cracks are about to coalesce. Due to crack interaction, the threshold excess pressure to initiate subcritical crack growth increases with increasing crack spacing. The crack propagation reduces significantly for smaller crack spacing. For example, the crack propagation duration is about 3.39 million years for b/a0=5. It reduces to about 1.09 million years when b/a0=2. 0 1x106 2x106 3x106 4x106 1.0 1.2 1.4 1.6 1.8 2.0 C olinearcracks w ith initial depth ofburialH0 = 4000 m b/a0 = 5 b/a0 = 3 b/a 0 = 2 a/a0 Tim e (year) 0 1x106 2x106 3x106 4x106 0 1 2 3 4 5 C olinearcracks w ith initial depth ofburialH0 = 4000 m b/a0 = 5 b/a0 = 3 b/a0 = 2 Δ P (M Pa) Tim e (year) Concluding Remarks To address the coupling between gas generation and expulsion, we develop a theoretical model to study the gas migration through propagation and coalescence of collinear microfractures initially filled by oil incorporating fracture mechanics of source rocks, geochemistry of thermal cracking of oil to gas, and an equation of state for gas. Based on our numerical simulation, the following conclusions can be reached, which may provide some insight into gas migration process. (1) Gas expulsion through self-propagating microfractures is an effective mechanism for primary migration within well-sealed source rocks and excess pressure caused by thermal cracking of oil to gas serves as driving force for microfracture propagation. (2) Increasing depth of burial leads to decreasing crack propagation distance due to gas compressibility. Therefore cracks at shallower depth are more likely to form interconnected fracture network. (3) Crack spacing has a significant effect on crack propagation duration for collinear cracks with smaller crack spacing leading to shorter crack propagation duration. Figure 6. Effect of crack spacing on the crack propagation distance for collinear cracks with initial depth of burial H0 = 4000 m Figure 7. Effect of crack spacing on the crack propagation distance for collinear cracks with initial depth of burial H0 = 4000 m
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