Fluid flow and coupled poroelastic response in low-permeability rocks

Abstract:

Hydraulic transport properties of reservoir rocks are traditionally defined as rock properties, responsible
for the passage of fluids through the porous rock sample, as well as their storage. These properties are
also called permeability and storage capacity. The evaluation of both is an important part of any reservoir
characterization workflow. A vivid example of the importance of the transport properties is the blooming
business of unconventional oil and gas production. Tight formations with ultra-low permeabilities and storage
capacities, which have never been perceived as reservoir rocks, today are actively exploited for oil and gas.
This tremendous achievement in petroleum science and technology was only possible due to hydraulic frac-
turing, which is essentially a process of enhancing permeability and storage capacity by creating a swarm
of microcracks in the rock. The knowledge of hydraulic and poroelastic properties is also essential for proper simulations of diffusive pore fluid
flow in petroleum reservoirs, as well as aquifers.

This work is devoted to an integrated study of low-permeability rocks’ hydraulic and poroe-
lastic properties as measured with the oscillating pore pressure experiment. The oscillating pore pressure
method is traditionally used to measure hydraulic transport properties. We modified the method and built
an experimental setup, capable of measuring all aforementioned rock properties simultaneously. The mea-
surements were carried out for four sub-millidarcy rock samples at a range of oscillation
frequencies and effective stresses. An apparent frequency dependence of permeability was observed. Measured frequency dispersion of drained poroelastic properties
indicates an intrinsically inelastic nature of the porous mineral rock frame. Standard Linear Model demon-
strated the best fit to the experimental dispersion data. We established that hydraulically-measured storage capacities
are in good agreement with elastically-derived ones. We also introduce a novel method, which allowed
us to estimate the permeability from the full range of acquired frequency data by utilizing a nonlinear least-
squares regression. The results of numerical simulation of oscillatory fluid flow confirm both the analytical
solution and the experimental data.