Fluid-rock geomechanical interactions: Micro-mechanics and fractured reservoirs

Micro-mechanics models of the process interactions, occurring between a rock framework and its contained pore fluids, provide phenomenological understanding of bulk behaviour. When fluid pressure is increased, the rock+fluid system is induced to expand, but real-world constraints normally inhibit this. As a consequence, stress components increase, and the rock experiences an increase intensive energy in both the fluid and solid components. Applying this understanding to fractured rock, the increased pressure tends to cause fractures to close, an opposite behaviour to the rule usually adopted. Conversely, when pressures are lowered, the rock loses energy, and stresses diminish, with opportunities for fractures to open, or blocks to move. The observed flow response of a fractured reservoir is explained as follows. During production of a well, the nearby rock mass is unloaded, allowing shifts between fracture-bounded blocks, enhancing bulk fluid flow. During near-well rock mass relaxation, the distant response is similar to what happens during drilling, where rocks move (slightly) towards the reduced-energy region. A 'ring' of high-strain-energy rock forms, like a horizontal, circular(ish) arch, reducing the potential flow paths from the far-field to the well area. Thus, a well initially produces at a high rate, and then a sharp decline is observed as the pore fluids in the protected region are depleted. The process described cannot be correctly simulated at present, because existing simulators are based on continuum laws (and extensions of them) that are not valid. The 'law of effective stress' used in geomechanics is physically wrong, further degrading the value of present simulations. Using approximate models of fractured-rock behaviour, recognising that those models lack correct fluid effects, some strategies emerge for consideration. By judicious use of depletion locations and timings, the responses noted here could be deliberately provoked to cause a new arrangement of flow paths and isolations to be induced.