Experimental investigation of deposition of crude oil components in brine-filled pores

It is often presumed that fine-scale surface pores and roughness in reservoir rock remain water-wet. To test this assumption, deposition tendency of asphaltenes and resins during crude oil aging of brine-filled pores was investigated. Model frameworks of water-wet silica with sub-micron pores were synthesized as planar films of thickness 5 μm. These pore networks were conditioned in brine, submerged wet into crude oil for aging in the absence of applied pressure, then cleaned. Brines covering a wide range of NaCl concentrations, without or with added CaCl₂ and/or brine degassing, were analyzed. Spectroscopy and microscopy were employed to determine the amount and distribution of asphaltene/resin deposits. The results clearly demonstrate that crude oil is capable of spontaneously invading these fine pores to render them oil-wet. All samples exhibited at least some deposition, with the overall NaCl concentration having little effect, whereas the increasing presence of CaCl₂ led to a reduction in deposition. Scanning electron microscopy revealed that deposition typically took the form of uniform thin layers lining pore walls. A mechanism for spontaneous displacement of brine from tight water-wet pores, based on local rupture of convexly-curved brine thin films, is discussed.