Fluid pressure heterogeneity during fluid flow in rocks: New laboratory measurement device and method

TL;DR

This paper introduces a novel, miniaturized transducer for measuring local pore fluid pressure in rocks under high confining pressures, enabling detailed analysis of pressure heterogeneity and flow dynamics.

Contribution

A new passive transducer design capable of high-pressure, localized pore pressure measurements in rocks, adaptable and tested on sandstone and granite samples.

Findings

Accurate measurements up to 80 MPa with minimal deviation.

Quantification of permeability decrease during faulting.

Tracking pore pressure fronts during transient flow.

Abstract

We present a new type of transducer capable of measuring local pore fluid pressure in jacketed rock samples under elevated confining pressure conditions. The transducers are passive (strain-gauge based), of small size (7 mm in diameter at the contact with the rock and around 10 mm in length), and have minimal dead volume (a few mm$^3$). The transducers measure the differential pressure between the confining fluid and the internal pore pressure. The design is easily adaptable to tune the sensitivity and working pressure range up to several hundred megapascals. An array of four such transducers was tested during hydrostatic pressurisation cycles on Darley Dale sandstone and Westerly granite. The prototypes show very good linearity up to 80 MPa with maximum deviations of the order of 0.25 MPa, regardless of the combination of pore and confining pressure. Multiple internal pore pressure measurements allow us to quantify the local decrease in permeability associated with faulting in Darley Dale sandstone, and also prove useful in tracking the development of pore pressure fronts during transient flow in low permeability Westerly granite.