Some volcanic hydrothermal systems contain portions of rock where fractures and microfractures appear closed. This phenomenon could have two origins: the first one is self-sealing by mineral deposition during water rock interactions. The second one results from the combined action of both fluid-rock compressibility and rock geomechanical deformation. Under geothermal conditions the rock can be dry, wet or completely saturated. From experimental results and direct observations in cores it is well known that the cohesive structure of rocks is weakened by the presence of liquid in the pores. The mechanical parameters of fractured volcanic rock are also influenced by its cohesion and directly affected by both pressure and composition of the fluid located in pores, fractures and microfractures. In some type of volcanic systems the fluid could be scarce since the beginning of its formation, producing a natural closure of fissures. In this paper we develop a practical explanation and a theoretical description of such geomechanical phenomenon of closure of fissures on the basis of some fundamental poroelastic equations. Recent studies carried out on a prototype of this type of systems (Los Humeros, MÈxico) allow to conclude that at the moment of being formed, out of unknown reasons, this reservoir was unable to store abundant fluid as in other similar reservoirs. Our central hypothesis is that such lack of fluid caused the collapse and closing of fractures and faults in some portions of the reservoir, originating a global permeability drop and permitting, at the same time, the coexistence of strong pressure gradients between the matrix blocks and the few open fissures. The lack of liquid and the poor global permeability are also supported by geochemical evidence, because in most drilled zones in this reservoir chemical equilibrium between water and rock has not been attained.

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