Field tests have demonstrated that oil production from sandstone reservoirs increases when injected water salinity is low, i.e. ~1500 ppm total dissolved solids (TDS). In core plug tests performed at reservoir conditions, low salinity flooding has been responsible for incremental recoveries ranging from about 5 to 38%. Previous work has suggested that for the low salinity effect to manifest itself, the oil must contain polar components, the formation water must contain divalent cations and clay must be present in the reservoir, but a clear understanding of the mechanism, from fundamental chemical and physical principals, is still subject to debate.In the work reported here, an atomic force microscope (AFM) has been used in force spectroscopy mode to investigate the nature and magnitude of the interaction between hydrocarbon molecules with carboxylic acid end groups and the pore surfaces of oil reservoir sandstones. By functionalizing the AFM tip with polar molecules we have been able to measure, quantitatively, the adhesion forces between these molecules and the mineral surfaces under 36,500 and 1500 ppm TDS artificial seawater (ASW) solutions.Collecting these measurements in two-dimensional arrays, known as force maps, revealed that adhesion was highest on the quartz grain surfaces during exposure to the high salinity solutions and it decreased when salinity decreased in nearly all cases. The drop in adhesion was observed through several high to low salinity cycles. We interpreted certain small features that were visible on the quartz surfaces to be clay that had grown directly on the sand grains from solution during diagenesis. Adhesion on these clay surfaces also changed with modifications in salinity. We observed no difference in behaviour whether the sandstone was preserved or cleaned; both types of core demonstrated a clear low salinity response.
G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by promoting guanine nucleotide exchange. Here, we investigate the process of functional association between G proteins and GPCRs and describe the events that ultimately lead to the ejection of GDP from its binding pocket in the Gα subunit. In atomic detail, we reveal the temporal progression of structural rearrangements of GDP-bound heterotrimeric Gs protein (GsGDP) upon coupling to the β2-adrenergic receptor (β2AR) using molecular dynamics simulations. The binding of GsGDP to the β2AR is followed by long-range allosteric effects that significantly reduce the energy needed for GDP release, the rate-limiting step during G-protein activation. In particular, the opening of α1-αF helices, displacement of the αG helix, and the opening of the α-helical domain weaken the interaction between GDP and the G protein. Signal transduction to the G protein occurs via a novel receptor interface, confirmed by site-directed mutagenesis and functional assays. From this β2AR-GsGDP intermediate, the G protein must undergo an in-plane rotation along the receptor axis to reach the β2AR-Gsempty state. The simulations shed light on how the structural elements at the receptor-G-protein interface interact to transmit the signal over 30Å to the nucleotide-binding site. Our analysis extends the current limited view of nucleotide-free snapshots to include additional states and structural features responsible for signaling and G protein coupling specificity.
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