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The Effect of Osmotic Pressure on Improve Oil Recovery from Fractured Shale Formations

Shale swelling during drilling is attributed to osmotic pressure, where low-salinity water enters the shale pores to cause swelling. Low-salinity water injected into high-salinity Bakken formation could similarly enter the matrix pores to displace oil by counter-current flow observed in core experiments. As a result, we believe, low-salinity water can potentially enhance oil recovery from oil-wet Bakken formation. In this paper, we report experimental and numerical modeling studies we conducted to evaluate the potential of lowsalinity waterflooding in Bakken. For laboratory experiments, we used horizontal core plugs drilled parallel to the bedding plane. The mathematical included osmotic pressure, gravity and capillary effects. In the mathematical model, the osmotic pressure mass transfer equations were calibrated by matching time-dependent salinities in a published laboratory osmotic pressure experiment. We also modeled oil recovery for a Bakken core using our osmotic pressure mass transport model. The results indicate that osmotic pressure promotes counter-current flow of oil from both the water-wet and oil-wet segments of the core. Introduction Osmosis is the transport of water molecules flow from low-salinity side of a semi-permeable membrane to the high-salinity side to equalize the concentration of the dissolved salts. This causes an increase of pressure on the higher-salinity side, called osmotic pressure (π), Fig. 1. In subsurface environment, high-clay shale sediments can behave as a semi-permeable membrane, thus causing osmotic water transport (Kemper; 1961, Milne et al.; 1964, Young and Low; 1965, Chenevert; 1970, Olsen; 1972, Greenberg et al.; 1973, Marine and Fritz; 1981, Fritz; 1986, Van Oort et al.; 1994, and Keijzer; 2001).