Imbibition: The Mechanism by which Gas Shale Sequesters Residual Treatment Water
Production of gas from shale by horizontal drilling and massive slickwater hydraulic fracturing (i.e., fracking) offers a suite of benefits including environmental while prompting other environmental concerns. Concerns involving water quality arise because as much as 2 x 104 m3 of water-based fluid with additives (i.e., treatment water) is injected into gas shale to open fractures in the vicinity of each horizontal well. Less than half of the treatment water with its natural components is recovered as flowback or later production brine. While recovered treatment water is safely managed at the surface, the water left in place, called residual treatment water (RTW), slips beyond the control of engineers. Some recent papers imply that gas shale like the Marcellus is leaking now, naturally without any human assistance, and that if water-based fluid is injected into these cross-formational pathways, that leakage, which is already ‘contaminating’ the aquifers with salt, could be made much worse. Gas shale in general imbibes RTW at such a rate that it is unlikely to migrate out of the formation. While 104 m3 of RTW may seem like a large volume, the reservoir volume that is stimulated by hydraulic fracturing is much larger. If imbibed pervasively in the stimulated reservoir volume, the RTW could be absorbed by a porosity of roughly 0.06%. Imbibition into cutting samples of as-received Marcellus (contact angle = 72°), in this approximately 2 grams from the Union Springs Member of the Marcellus, show a characteristic square-root time curve that assures sequestration of RTW. Imbibition of 100 microliter beads of tap water into 2-gram chips of the Union Springs Member takes less than 2 hours. Imbibition experiments under standard conditions must account for evaporation which accounts for about 30% of the volume loss of the tap-water bead in two hours. During the 2-hour experiments counter-current imbibition is indicated by the growth of methane bubbles within the tap-water beads.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013