Abstract:
Measuring permeability through tight fractured rock specimens in the laboratory using fluid injection is known to be challenging. We have collectively performed an extensive series of laboratory fracture permeability experiments using unconfined, triaxially confined, and true-triaxially confined specimens and various flow control methods. Tested materials have included shale, granite, cement, and acrylic with fluid flow occurring through both the rock matrix and stimulated fractures. Darcy and cubic law approximations anticipate unique steady-state flow and pressure correlations while our experiments observed method-dependent permeability measurements. Generally, constant flow rate control was found to produce greater variation and increased stabilization times relative to constant pressure control. Constant pressure control typically produced a lower bound permeability measurement. Repeatability of measurements through a given specimen was found to improve when using constant pressure control. Stepped constant pressure and stepped constant flow rate schedules were used to evaluate time-dependence of flow and hysteresis. Most recently, a sinusoidal flow experiment was performed to investigate dynamic flow behavior. Results from these experiments suggest that constant pressure control should be used, when possible, for evaluation of low permeability specimens. The results also indicate that flow history can significantly affect permeability measurements but this effect is not necessarily permanent.
Introduction
Accurately measuring the permeability of fractured rock is challenging, yet also crucial for predictively modeling fluid flow through the subsurface. Such predictions are used to optimize hydraulic fracturing jobs for oil, gas, and geothermal well-production, design dewatering systems for tunnels and excavation pits, estimate leakage potential from underground fluid storage sites, and to explore the suitability subsurface waste storage facilities.
It can be hypothesized that fractures and porous media (i.e. rocks) possess many complex traits that can affect permeability at any given time. Perhaps most importantly, fracture geometry can greatly affect hydraulic connectivity through and between other fractures and pores, therefore exhibiting a likely strong effect on permeability. However, even highly localized blockages in these fractures (e.g. grout, cement, fines, or contact-seals) can result in a significant permeability reduction in some cases (e.g. filter clogging and shear fracture compaction) or significant permeability gains in others (e.g. hydraulic fracture proppant and shear fracture dilation). In addition, other potentially relevant issues include chemical dissolution/ precipitation, mechanical creep, mineral swelling/compaction, thermomechanical alteration, and solid-fluid compressibility (i.e. Biot's constant). To make matters ever more complex, scale effects and discontinuity are not simple to account for when comparing measurements. While the specific mechanisms influencing permeability are not to be the focus of this study, it is important to acknowledge them as they offer some better understanding in regard to the importance of fluid flow and boundary condition control methods, for which many options are available.
