PSI - Issue 6

I. Bazyrov et al. / Procedia Structural Integrity 6 (2017) 228–235 Bazyrov ILdar et al// Structural Integrity Procedia 00 (2017) 000–000

233

6

The rest of the work is devoted to the fracture reorientation analysis. A similar approach was used in [8] to estimate the influence of various factors on stress state of rocks. The results of [8] showed that a number of factors influence on the orientation of hydraulically induced fractures, such as pressure gradient, reservoir thickness, Young's modulus, horizontal stress anisotropy, reservoir permeability and Arching effect (decrease of stresses in sand collector and increase of stresses in adjacent clays, which contributes to the strengthening of "barriers" in a roof and in a bottom of production interval). Table 1 represents a comparison between simulated parameters before and after hydrofracturing. Figure 5 illustrates stress, pore pressure and lateral anisotropy values of host medium surrounding well-candidates. Table 1 Comparison between reservoir conditions surrounding wells A and B A before A after B before B after Pore pressure, bar 270 65-110, (150 port 3) 275 97-108 (128 port 6) Shmin, bar 380-400 280-300, (325 port 3) 370-390 285-315, (325 port 6) Shmin (top), bar 421-436 433-448 400-419 408-430 Shmin (bottom), bar 414-439 425-450 385-427 390-440 Stress anisotropy, bar 10-13 0-5 10-13 4-10, (2-5 port 6) According to the complex numerical simulation, the 30-90 degree alteration of maximum horizontal stress was predicted. As a result of re-frack excursion well-candidates A and B coverage areas were increased respectively by 100-150 and 20-80 meters.

Figure 5. Comparison of the parameters surrounding well-candidates A and B (at the moment of repeated multistage refracturing): a - pore pressure map, b - minimum horizontal stress map, c - horizontal stresses anisotropy map. Faults reactivation Besides hydrofracture orientation effect, alteration of pore pressure takes a complex influence on a reservoir system and a tectonic dislocation (natural fractures and faults) in the context of faults permeability. It is known [11] that each fracture or geological dislocation can be determined by normal and tangential strength tensor components. Magnitude of these parameters in conjunction with the stress state and normal and tangential components of stress tensor affect its filtration properties. In case the point on Mohr’s diagram is under critical line – it means that the stress state of fault or fracture allows to fluid flow. Increasing of pore pressure leads to the reduction of effective stresses and the Mohr’s circle moves to the left. In addition, pore pressure reduction leads to shift circles to the right (figure 6). This fact is useful for hydro geomechanical simulations of fractured reservoirs [2].