Formation fracturing by pore pressure drop (Laboratory study)

Abstract

Pore pressure increase in saturated porous rocks may result in its fracturing and corresponding microseismic event occurrences. Another type of the porous medium fracturing is related with rapid pore pressure drop at some boundary. If the porous saturated medium has a boundary where it directly contacted with fluid under the high pore pressure, and the pressure at that boundary is dropped, the conditions for tensile cracks can be achieved at some distance from the boundary. In the paper, the results of experimental study of fracturing of the saturated porous artificial material due to pore pressure rapid drop are presented. It was found that multiple microfracturing occurred during the pore pressure dropping, which is governed by pore pressure gradient. Repeated pressure drops result in subsequent increase of the sample permeability. The permeability was estimated on the basis of non-linear pore-elasticity equation with permeability dependent on pressure. The implementation of calculations to laboratory experiment data showed significant variation of the porous sample permeability during the initial non-stationary stage of the fluid pressure drop. The acoustic emission activity variation was found to be controlled by pore pressure gradient and changes of the number of potential fractures, which can be activated by the pore pressure gradient. It was found, that the probability distribution of these “potential fractures” could be approximated by a Weibull distribution. A way of solution of the inverse problem of local permeability defining from microseismic activity variation in a particular volume of porous medium was suggested.