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

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 nun 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 pressurization 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.

Automated summary

The new type of transducer is capable of measuring local pore fluid pressure in rock samples under high confining pressure conditions, with high accuracy and flexibility in design. The prototypes show good linearity up to 80 MPa with minimal deviations, and are useful for tracking and quantifying changes in permeability associated with faulting and transient flow in different types of rock.