Predictive modelling of NAPL injection tests in variable aperture spatially correlated fractures

In preparation for a field experiment where a NAPL will be injected into a fractured sandstone aquifer, a 2D invasion percolation model of DNAPL migra tion in a single horizontal fracture with varying aperture has been develop ed. This simulation investigated the effect of spatially correlated fractur e aperture fields on pressure-saturation relation!;hips as a function of va riable aperture mean, standard deviation, and spatial correlation statistic s under hydrostatic conditions. Results from spatially correlated variable aperture fields can be significantly different from random fields. Longer r anges decreased entry pressures and higher standard deviations decreased no nwetting phase saturations. Mean aperture is the major control on displacem ent pressure, entry pressure and the form of the: pressure-saturation curve . Simulation results using statistical parameters for a variable aperture n atural sandstone fracture us described by Yeo et al. [International Journal of Rock Mechanics: and Mining Sciences 35 (1998) 1051] closely resemble a Brooks-Corey pressure-saturation function: and exhibit a distinct entry pre ssure followed by a rapid increase in nonwetting phase saturation. Graphica l estimates of entry pressure provide a good approximation of the critical aperture controlling the formation of a continuous nonwetting phase pathway in a variable aperture fracture. Consequently, we show that multiphase now concepts developed for porous media can successfully he applied to variabl e aperture fractures. Entry pressure correlates well to the mean aperture i n these simulations when using a Gaussian distribution of fracture aperture . Interpreted aperture distributions from NAPL injection experiments do not fit the true distribution well at low nonwetting phase saturations, but do at higher saturations above the entry pressure. Consequently, true, mechan ical aperture variation within a fracture plane cannot be determined from N APL injection experiments either in the field or laboratory. (C) 2001 Elsev ier Science B.V. All rights reserved.