Measurements of Streaming Potential for Downhole Monitoring in Intelligent Wells
a report by
MZ Jaafar,1 MD Jackson,2 CC Pain,2 JH Saunders2 and J Vinogradov2
1. Technological University of Malaysia; 2. Imperial College London
Permanently installed downhole sensors are increasingly being deployed to provide ‘realtime’ reservoir data during production. These data help to reduce uncertainty in the reservoir description and contribute to reservoir management decisions.1,2
Where wells are
equipped with inflow control valves, it is possible to develop a feedback loop between measurement and control to optimise production.3
Wells equipped with downhole sensors and control valves are often described as ‘intelligent’ or ‘smart’ and it is widely recognised that they have the potential to significantly enhance production.4
the boundary layer are transported with the flow, giving rise to an electrical current termed the streaming current. At steady state, the streaming current is countered by a conduction current through the fluid (and rock, if it is conductive) to maintain overall electrical neutrality. An electrical potential termed the streaming potential is associated with this conduction current. Downhole measurements of the streaming potential, which is generated when water flows in the reservoir during production, provide information on water saturation at some distance from the well.
Inflow control to a well can be ‘reactive’ or ‘proactive’. Reactive strategies control inflow in response to changes measured within the well or the adjacent reservoir, while proactive strategies control inflow in response to changes measured or predicted in the reservoir at some distance away from the well.5
The advantage of proactive control is that problems, such as the approach of unwanted fluids, can be mitigated before they affect production from the well. The management of water or gas displacement fronts to prevent early breakthrough, by balancing inflow along the length of a well, is a typical example of proactive control.6 However, proactive control requires knowledge of fluid saturation changes at some distance from the well. Most downhole sensors only sample the formation immediately adjacent to the wellbore. The search is therefore on to identify monitoring technologies that can detect saturation changes in the reservoir and facilitate proactive control. To date this search has largely focused on methods based on the interpretation of seismic data acquired using either surface or downhole geophones,7
installed downhole electrodes.8
or resistivity data acquired using permanently Another approach that shows promise
is based on measurements of streaming potential, which can also be acquired using permanently installed downhole electrodes.9–11
Streaming Potentials in Porous Media
Streaming potentials in fluid-saturated porous media are one of the phenomena included under the term ‘spontaneous potential’. They result from the presence of an electrical double layer at the solid–fluid interface.12
Streaming Potential During Oil Production
Streaming potentials generated during oil production can be illustrated using a numerical model of a simple production scenario in which water encroaches on a vertical production well in a homogenous sandstone oil reservoir.9,10,13
The selected rock and fluid
properties yield a shock-front-dominated displacement. Ahead of the shock front, oil flows in the presence of connate water; behind the shock front, water flows in the presence of residual oil (see Figure 1). At the front, water saturation varies rapidly over a limited spatial interval. This type of displacement of relatively low-viscosity oil by water is observed in many reservoirs. As oil is extracted from the production well, the encroaching waterfront moves towards it.
The streaming potential is associated with the flowing oil and water. It has a peak at the location of the waterfront and decays towards zero (measured with respect to a distant reference electrode) ahead of and behind the front. The streaming potential falls to zero ahead of the front because there is no streaming current associated with the flow of oil, which in this model is non-polar and so contains no electrical charge; it falls to zero behind the front because the streaming current is constant where the water saturation is constant. The peak electrical potential follows the waterfront as it moves through the reservoir. It is associated with the front because the divergence of the streaming current becomes non-zero where the saturation changes, so the waterfront acts as a current source.
The solid surfaces become electrically charged when they react with the adjacent fluid; in typical reservoir conditions, the mineral surfaces of hydrocarbon reservoir rocks are negatively charged.13 Electrostatic forces attract counter-ions in the fluid that have the opposite charge to the surface and repel co-ions that have the same charge as the surface. This gives rise to a boundary layer adjacent to the mineral surface that contains an excess of counter-ions. Within this boundary layer, the concentration of excess counter-ions decreases away from the solid surface until the fluid becomes electrically neutral. The thickness of the boundary layer is typically tens of nanometers.12
If the fluid is induced to flow relative to the solid surfaces by an external potential (pressure) gradient, some of the excess counter-ions within
44
Measurements of streaming potential are therefore an ideal method for monitoring fluid fronts when there is a significant change in the streaming current between the displacing and displaced fluids because the peak potential is associated with the front. A waterfront approaching a production well can be detected and monitored before it arrives because the streaming potential decays slowly with distance away from the location of the front. As soon as the leading edge of the streaming potential signal caused by the moving front arrives at the well, a change in potential is recorded that can be interpreted (see Figure 2). The potential measured at the well increases as the distance to the waterfront decreases because the peak of the electrical signal associated with the front moves closer. At water breakthrough, the peak signal arrives at the well.
© TOUCH BRIEFINGS 2010
Drilling & Well Technology
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