Abstract
Hydraulic fracturing technique is of vital importance to effectively develop unconventional shale resources. However, the low recovery of hydraulic fracturing fluids appears to be the main challenge from both technical and environmental perspectives in the last decade. While capillary forces account for the low recovery of hydraulic fracturing fluids, the controlling factor(s) of contact angle, thus wettability, has yet to be clearly defined. We hypothesized that the interaction of oil/brine and brine/rock interfaces governs the wettability of system, which can be interpreted using Derjaguin–Landau–Verwey–Overbeek (DLVO) and surface complexation modelling. To test our hypothesis, we measured a suit of zeta potential of oil/brines and brine/minerals, and tested the effect of ion type (NaCl, MgCl2 and CaCl2) and concentrations (0.1, 1, and 5 wt %). Moreover, we calculated the disjoining pressure of the oil/brine/mineral systems and compared with geochemical modelling predictions. Our results show that cation type and salinity governed oil/brine/minerals wettability. Divalent cations (Ca2+ and Mg2+) compressed the electrical double layer, and electrostatically linked oil and clays, thus increasing the adhesion between oil and minerals, triggering an oil-wet system. Increasing salinity also compressed the double layer, and increased the site density of oppositely charged surface species which made oil and clay link more strongly. Our results suggest that increasing salinity and divalent cations concentration likely decrease water uptake in shale oil reservoirs, thus de-risking the hydraulic fracturing induced formation damage. Combining DLVO and surface complexation modelling can delineate the interaction of oil/brine/minerals, thus wettability. Therefore, the relative contribution of capillary forces with respect to water uptake into shale reservoirs, and the possible impairment of hydrocarbon production from conventional reservoirs can be quantified.
Highlights
IntroductionDue to the decline of oil and natural gas production from conventional reservoirs, the gravitation towards unconventional resources development receives broad scientific interest [2]
Oil and natural gas remains as important primary resources in this century [1]
This is because divalent cations (e.g., Ca2+ and Mg2+) significantly compress the electrical double layer due to a higher charge density than monovalent cations (Na+) [30], lower double-layer expansion force, or even generating attractive electrical double layer force due to the opposite polarity of the zeta potential of oil/brine and brine/kaolinite [31]
Summary
Due to the decline of oil and natural gas production from conventional reservoirs, the gravitation towards unconventional resources development receives broad scientific interest [2]. Such resources are reserved in shale rocks with nano-Darcies level permeability, the presence of small natural fracture may trigger an effective permeability higher than the order of nano-Darcies [3]. Hydraulic fracture stimulation generally refers to a process of injecting high pressure fluids into the formations or rocks through wellbore to create fractures which are filled with high permeable engineered materials, termed as proppant, to keep the created fracture open. Several mechanisms have been proposed to understand why the fracturing fluids disappeared, e.g., remaining in fracture network [4,5], creating new fractures [6,7], hydration of clay minerals [8,9], capillary pressure [5,6,7,10], and osmotic flow [11,12]
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