The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic-Pacific moisture flux (DF a ) and atmospheric moisture diffusivity (K q ). In a large ensemble of spinup experiments, the values of DF a and K q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor ''On'' and ''Off'' states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of DF a and K q , reveal the coexistence of two stable THC states (Conveyor ''On'' and ''Off'')-i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a ''Conveyor On'' state, irreversibly collapses to a ''Conveyor Off'' state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.

Summary Spontaneous potential (SP) is routinely measured using wireline tools during reservoir characterization. However, SP signals are also generated during hydrocarbon production, in response to gradients in the water-phase pressure (relative to hydrostatic), chemical composition, and temperature. We use numerical modeling to investigate the likely magnitude of the SP in an oil reservoir during production, and suggest that measurements of SP, using electrodes permanently installed downhole, could be used to detect and monitor water encroaching on a well while it is several tens to hundreds of meters away. We simulate the SP generated during production from a single vertical well, with pressure support provided by water injection. We vary the production rate, and the temperature and salinity of the injected water, to vary the contribution of the different components of the SP signal. We also vary the values of the so-called "coupling coefficients," which relate gradients in fluid potential, salinity, and temperature to gradients in electrical potential. The values of these coupling coefficients at reservoir conditions are poorly constrained. We find that the magnitude of the SP can be large (up to hundreds of mV) and peaks at the location of the moving water front, where there are steep gradients in water saturation and salinity. The signal decays with distance from the front, typically over several tens to hundreds of meters; consequently, the encroaching water can be detected and monitored before it arrives at the production well. Before water breakthrough, the SP at the well is dominated by the electrokinetic and electrochemical components arising from gradients in fluid potential and salinity; thermoelectric potentials only become significant after water breakthrough, because the temperature change associated with the injected water lags behind the water front. The shape of the SP signal measured along the well reflects the geometry of the encroaching waterfront. Our results suggest that SP monitoring during production, using permanently installed downhole electrodes, is a promising method to image moving water fronts. Larger signals will be obtained in low-permeability reservoirs produced at high rate, saturated with formation brine of low salinity, or with brine of a very different salinity from that injected.

The injection of cold water into a hydrocarbon reservoir containing relatively warmer, more saline formation brine may generate self-potential anomalies as a result of electrokinetic, thermoelectric, and=or electrochemical effects. We have numerically assessed the relative contributions of these effects to the overall self-potential signal generated during oil production in a simple hydrocarbon reservoir model. Our aim was to determine if measurements of self-potential at a production well can be used to detect the movement of water toward the well. The coupling coefficients for the electrochemical and thermoelectric potentials are uncertain, so we considered four different models for them. We also investigated the effect of altering the salinities of the formation and injected brines. We found that the electrokinetic potential peaked at the location of the saturation front (reaching values of 0.2 mV even for the most saline brine considered). Moreover, the value at the production well increased as the front approached the well, exceeding the noise level ($ 0.1 mV). Thermoelectric effects gave rise to larger potentials in the reservoir ($10 mV), but values at the well were negligible .0:1 mV ð Þ until after water breakthrough because of the lag in the temperature front relative to the saturation front. Electrochemical potentials were smaller in magnitude than thermoelectric potentials in the reservoir but were measurable > 0:1 mV ð Þat the well because the salinity front was closely associated with the saturation front. When the formation brine was less saline ($1 mol=liter), electrokinetic effects dominated; at higher salinities ($5 mol=liter), electrochemical effects were significant. We concluded that the measurement of self-potential signals in a production well may be used to monitor the movement of water in hydrocarbon reservoirs during production, but further research is required to understand the thermoelectric and electrochemical coupling coefficients in partially saturated porous media.

We have examined the behavior of the streaming potential under multiphase conditions, and under conditions of varying temperature and salinity, to evaluate the feasibility of using downhole streaming-potential measurements to determine fluid distributions in a reservoir. Using new insights into the pore-scale distribution of fluids and of electric charge, we found that the saturation dependence of the streaming potential coupling coefficient is important in determining the resulting streaming potential. Through examination of the four independent physical parameters which comprise the coupling coefficient, we developed an understanding of the behavior of the coupling coefficient under conditions of elevated temperature and brine salinity. We found that although increasing salinity substantially reduces the magnitude of the coupling coefficient, and therefore also the magnitude of the predicted streaming potential, increasing temperature has only a small effect, showing about a 10% change between 25°C and 75°C, depending on salinity.

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