ON THE DEFORMATION OF FRACTURES AND MICROFRACTURES IN POROELASTIC ROCKS

摘要

Some volcanic hydrothermal systems contain portions of rockwhere fractures and microfractures appear closed. Thisphenomenon could have two origins: the first one is selfsealingby mineral deposition during water -rock interactions.The second one results from the combined action of both fluidrockcompressibility and rock geomechanical deformation.Under geothermal conditions the rock can be dry, wet orcompletely saturated. From experimental results and directobservations in cores it is well known that the cohesivestructure of rocks is weakened by the presence of liquid in thepores. The mechanical parameters of fractured volcanic rockare also influenced by its cohesion and directly affected byboth pressure and composition of the fluid located in pores,fractures and microfractures. In some type of volcanic systemsthe fluid could be scarce since the beginning of its formation,producing a natural closure of fissures. In this paper wedevelop a practical explanation and a theoretical description ofsuch geomechanical phenomenon of closure of fissures on thebasis of some fundamental poroelastic equations. Recentstudies carried out on a prototype of this type of systems (LosHumeros, México) allow to conclude that at the moment ofbeing formed, out of unknown reasons, this reservoir wasunable to store abundant fluid as in other similar reservoirs.Our central hypothesis is that such lack of fluid caused thecollapse and closing of fractures and faults in some portions ofthe reservoir, originating a global permeability drop andpermitting, at the same time, the coexistence of strong pressuregradients between the matrix blocks and the few open fissures.The lack of liquid and the poor global permeability are alsosupported by geochemical evidence, because in most drilledzones in this reservoir chemical equilibrium between waterand rock has not been attained.