Abstract
Shale fracture conductivity can be reduced significantly due to shale-water interactions. Factors that may influence shale fracture conductivity include shale mineralogy, proppant embedment, shale fines migration, proppant fines migration, brine concentration, longer term rock creep, and residual water in the fracture. The study of excessive proppant embedment has been reported in our previous work (Zhang et al. 2014a). This paper presents the studies of the rest of these factors.
Laboratory experiments were run to understand each of these factors. To study the effect of rock mineralogy, recovered fracture conductivities after water damage for the Barnett Shale, the Eagle Ford Shale, and Berea Sandstone were measured. During conductivity measurements, water flow directions were switched to study the effect of shale fines migration. The size of shale fines was measured by microscopic imaging techniques, and scanning electron microscopic observations are also presented. Proppant fines migration was examined by placing two colors of sand on each half of the fracture surface and a microscope was used to identify the migrated crushed sands of one color mixed in the other color sand. Fresh water and 2% KCl were injected to study the effect of brine concentration. After water injection, the proppant pack was either fully dried or kept wet to investigate the damage by residual water.
Results showed that clay content determines the fracture conductivity damage by water. Fines generated from the shale fracture due to fracture face spalling, slope instability, and clay dispersion can migrate inside the fracture and are responsible for approximately 20% of the conductivity reduction. There is no evidence of crushed proppant particle migration in this study. Longer term rock creep accounts for a 20% reduction of the fracture conductivity. Fresh water does not further damage the fracture conductivity when initial conductivities are above 65 md-ft. Removal of the residual water from the fracture by evaporation helps recover the fracture conductivity to a small extent.
A theoretical model of propped fracture conductivity was extended to include the effects of water damage on fracture conductivity. An empirical correlation for the damage effects in the Barnett shale was implemented in this model.
A new theoretical method to calculate shale fracture conductivity based on the population balance equation
