Real-Time Measurements of Spontaneous Potential for Inflow Monitoring in Intelligent Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 135146, ’Real-Time Measurements of Spontaneous Potential for Inflow Monitoring in Intelligent Wells,’ by M.D. Jackson, SPE, M.Y. Gulamali, E. Leinov, J.H. Saunders, and J. Vinogradov, Imperial College London, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.
Spontaneous-potential (SP) signals are generated during hydrocarbon production because of gradients in the water-phase pressure (relative to hydrostatic), chemical composition, and temperature. Measurements of SP during production, taken with permanently installed downhole electrodes, could be used to detect water that is encroaching on a well while the water is several tens to hundreds of meters away.
Introduction
SP measurements, logged before production or injection using wireline tools, are primarily electrochemical (EC) in origin. Contrasts in chemical composition between formation and drilling fluids give rise to “junction” or “diffusion” potentials in permeable beds, while the exclusion of (typically) negative ions from the pore space of fine-grained rocks (e.g., mudstones and shales) results in “membrane” potentials. Together, these EC potentials typically dominate the SP log, although in some cases, electrokinetic (EK), or streaming, potentials, which arise from gradients in fluid pressure (relative to hydrostatic), also may contribute. However, gradients in fluid pressure, chemical composition, and temperature also will be present during hydrocarbon production, particularly during waterflooding or steamflooding where colder or hotter water of a chemical composition different from that of the formation brine is injected into the reservoir. Consequently, if the casing is either nonmetallic or is insulated electrically from the formation, SP signals will be observed during production. This study characterized the magnitude of these SP signals to determine if their measurement, using permanently installed downhole electrodes, might be useful in monitoring fluid flow.
SP Components
The SP acts to maintain overall electro-neutrality when a separation of electrical charge occurs in response to gradients in pressure, chemical composition, or temperature. In a water-wet reservoir rock, charge separation occurs at the mineral/water interface because the water reacts with the mineral surfaces to leave an excess of (typically) negative charge on the mineral surface and an excess of positive charge in the water adjacent to the mineral surface. This arrangement of charges at the mineral/water interface is known as the electrical double layer. The negative charge on the mineral surface is immobile, but some of the excess positive charge in the adjacent water is mobile and will move with the fluid. If the water is subjected to a pressure gradient, which causes it to flow relative to the mineral surfaces, then some of this positive charge is transported with the flow. The net excess of positive charge moving with the flow gives rise to a so-called streaming current. To balance this streaming current, a conduction current is established and the electrical potential required to maintain this conduction current is the EK, or (more specifically) streaming, potential. The term EK is used here for consistency with the terminology used to describe the EC and thermoelectric (TE) potentials.