Effect of the presence of organic matter on bubble points of oils in shales

Abstract

The relative amounts of oil and gas produced in prolific plays like the Eagle Ford are affected by the oil bubble point. The oil and kerogen (organic matter) are found in the same rock and the oil may remain in contact with the kerogen. Bulk experiments and molecular simulations clearly show that kerogen preferentially absorbs hydrocarbons. The absorbed oil phase remains in multi‐component equilibrium with the expelled oil produced at the surface. Results from a model proposed to calculate the bubble points (at 400 K) of in situ oils (absorbed + free) in the presence of kerogen indicate suppression of about 4150–16,350 kPa from the original value of 28,025 kPa of produced Eagle Ford oil. These calculations depend on the type and level of maturity of kerogen. The prediction of accurate saturation pressures has key implications on volumes of recovery and rates of production from liquid rich shales. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3083–3095, 2017

Kerogen Swelling and Confinement: Its implication on Fluid Thermodynamic Properties in Shales

“…The sorbed oil has different PVT properties compared to the free oil. The effect of swelling of kerogen is modeled for Eagle Ford shale and oil in a recently published article 12 by the authors, however, the current work goes a step beyond to experimentally prove this effect. This effect is different from the previously well-studied 13 – 22 effect of confinement of fluids residing in the nano pores of the kerogen, on their PVT properties.…”

“…Such fluids inside the kerogen matrix (partly made up of indistinguishable sorbed fluids and retained fluids) would be called sorbed fluids from here on, for the purpose of this manuscript. The sorbed fluids stay in multi-component equilibrium with free but confined fluids in the nano pores 5 , 12 . The sorbed fluids cause a net expansion in the volume of kerogen.…”

“…In that paper, authors have found that the critical properties of fluids are suppressed in the confined spaces due to dominant interactions with the pore walls. The sorbed hydrocarbons that swell kerogen have different thermodynamic properties than the free but confined hydrocarbons because kerogen preferentially sorbs polars and aromatics more than saturates leading to the change in composition of sorbed oil 5 , 12 . Thus, the fluid thermodynamic properties of both, confined free oil in nano pore and sorbed oil in shales are different than the bulk oil due to the effect of confinement and swelling of kerogen, respectively.…”

“…TGA and DSC experiments were performed on kerogen and single component liquid (pure decane) to assess the effect of confinement only, on the phase change of decane as the effect of swelling of kerogen can only be assessed in a multi-component hydrocarbon mixture like oil. The multicomponent mixture allows the kerogen to uptake components to different extents based on preference (polars and aromatics are preferred more than saturates) so that the multicomponent mixture is split into a sorbed and a free fraction 5 , 12 , 43 – 45 . The sorbed fraction has a different composition than the free fraction and is richer in heavier, polars and aromatic compounds.…”

“…The sorbed fraction has a different composition than the free fraction and is richer in heavier, polars and aromatic compounds. Such fractionation of oil by the kerogen is discussed in details in the recent article 12 by the authors. This compositional split does not occur in a single component and thus, the TGA and DSC results would only show the effect of confinement in the kerogen-decane sample.…”