Borehole Self-Potential

Basic Concept

Self-potential (SP) borehole logging is an electrical wireline method that passively measures naturally occurring voltage anomalies along the length of an open, fluid-filled borehole. Self, or spontaneous, potential is a potential difference (i.e., voltage) that is generated by electrical current that arises in the absence of an external electric field (Beck, 1981). Though similar in theory to the surface SP method, the self potentials generated in borehole environments have different causative mechanisms and subsurface implications.

Self-potential logging detects anomalous voltages produced by electrochemical phenomena that occur within systems that have formation water and borehole fluid with contrasting ionic concentrations (i.e., salinity). The electrochemical potentials that arise in such borehole environments are influenced by the membrane- and liquid-junction potentials. Measurements of these can be used to indicate positions of permeable formations, estimate shale/clay volume in sandstones, and determine formation fluid resistivity (ρ_w ).

Theory

Self-potential producing electrical currents are generated by a combination of numerous electrochemical and electrokinetic mechanisms that occur at various scales in the subsurface. However, electrochemical mechanisms are the most relevant to self-potential logging, and electrokinetic potentials are typically assumed to be negligible. Electrochemical potential, which includes two components, is generated by the processes that occur during ion transport between the uninvaded formation water and the borehole fluid.

The first component of the electrochemical potential and the one that most influences the measured self-potential signal is the membrane potential. Membrane potentials often form across the shale beds that separate the borehole fluid/mud from permeable formations containing native groundwater. Because the clay minerals that compose shales have negatively charged surfaces, shales act as ion-selective membranes through which only cations (i.e., positively charged ions) can pass (Collier, 1993).

Groundwater ions undergo diffusion and are driven from regions of higher concentration to regions of lower concentration. Subsequently, groundwater cations travel toward the borehole fluid/mud and, after passing through the shale bed, accumulate at the shale-borehole interface. Because their movement is inhibited, anions (i.e., negatively charged ions) build up at the boundary between the shale and porous formation. Thus, a potential difference forms across the shale bed and has a polarity and magnitude related to groundwater properties (Collier, 1993).

The liquid-junction, or diffusion, potential forms across contacts (i.e. junctions) between the uninvaded-formation fluid and the mud filtrate (i.e., drilling fluid that invades the formation). Just like with the generation of membrane potentials, ions diffuse in attempt to equalize concentrations when a difference in ionic concentration/salinity exists between these two fluids. However, because cations are larger than anions and are attracted to negatively charged water molecules, cations diffuse at slower speeds than anions.

Thus, the fluid with higher ionic concentration becomes populated with the less-mobile cations, and the fluid with the lower concentration becomes occupied with the fast-moving anions. This charge polarization induces electrical current flow, the magnitude of which is proportional to the contrast in ionic concentration. However, if permeable formations contain clays, microscale membrane potentials can form and, because they have an opposite polarity, reduce the magnitude of the measurable liquid-junction potential (Mussett and Khan, 2000; Collier, 1993).

Applications

A self-potential log typically records the potential difference in millivolts (mV) between two electrodes. One electrode is placed at the surface and the second is positioned on the logging sonde and moved up the borehole. Self-potential measurements vary corresponding to the vertical changes in potential between the drilling fluid and the formation fluid within various strata. Measurements collected adjacent to shale beds remain relatively constant and form the “shale baseline” to which all other measurements are relative.

The self-potential log depends upon lithology, porewater chemistry, and drilling fluid chemistry and typically deflects left or right opposite permeable formations. Instead of an abrupt shift, the self-potential log varies on either side of a formation boundary for a distance dependent on the ratio of uninvaded-formation resistivity to drilling-fluid resistivity. Thus, contacts between formations are chosen to be the inflection point of the curve between the shale baseline and maximum deflection (U.S. Bureau of Reclamation, 2001).

Typically, the fluids contained within the uninvaded formation have higher ionic concentrations than the borehole fluids left over from drilling. In these common instances, the membrane potential creates a positive charge along the portion of the borehole that is adjacent to the shale. Additionally, the liquid-junction potential creates a negative charge along the portion of the borehole that is adjacent to the permeable formation (Collier, 1993).

The self potential changes at the boundary between the shale and the permeable formation, and this deflection is relatively negative. If the drilling fluid has an ionic concentration that is greater than the formation fluid, the polarities are reversed, and the self-potential log shows a positive deflection at the boundary. Furthermore, if an impermeable formation is adjacent to a shale bed, there is no current flow and no self-potential deflection between lithologies (Collier, 1993).

The volume of clay within in a sandstone can be determined using the static self-potential (SSP) value, which is obtained by measuring sufficiently thick beds. The SSP of a porous, permeable, and clay-free formation will be the maximum value measured within a borehole and represents “clean” sand. Because clays reduce porosity and permeability, the percentage of clay within a formation can be estimated using the SSP ratio of the formation relative to the “clean” sand.

Furthermore, static self-potential data can help estimate values of formation fluid resistivity (ρw ) (Mussett and Khan, 2000). Along with fluid resistivity, saturation (Sw ) can be calculated using the resistivity of the formation when saturation is 100% (ρo ) and the true formation resistivity (ρ_t ). Thus, the self-potential method, especially when used in conjunction with complimentary tools (i.e., borehole resistivity, borehole electromagnetic induction), can help estimate vital hydrogeologic parameters. Additionally, the self-potential method has been used to:

Distinguish shale layers
Correlate stratigraphy
Calculate formation water resistivity in ideal conditions
Identify permeable zones
Calculate clay volume in sandstones
Pick bed boundaries
Estimate variations in borehole diameter

Examples/Case studies

Jackson, M.D., Butler, A.P., and Vinogradov, 2012, Measurements of spontaneous potential in chalk with application to aquifer characterization in the southern UK: Quarterly Journal of Engineering Geology and Hydrogeology, v. 45, p. 457-471, doi:10.1144/qjegh2011-021.

Abstract: We report the first measured values of the streaming potential coupling coefficient in chalk samples saturated with natural groundwater, and preliminary field measurements of the spontaneous potential (SP), at both ambient and pumped conditions, at a test site in the Berkshire Chalk aquifer in the southern UK. The ultimate aim of the work is to use measurements of SP, in conjunction with borehole data, to characterize groundwater flow and aquifer properties. Laboratory measurements yield a value of the streaming potential coupling coefficient of −60 ± 4 mV MPa−1 and a corresponding zeta potential of −13 ± 1 mV. A negative zeta potential contrasts with previous published open-system measurements on artificial calcite, and may reflect the presence of organic material in the natural chalk samples or HCO3 and SO4 ions in the groundwater. Field measurements at ambient conditions show temporal variations in SP consistent with flow processes within the aquifer, but no coherent spatial variations. Measurements during water abstraction demonstrate that voltages at the ground surface and in monitoring boreholes become more positive during pressure drawdown and more negative during pressure build-up, consistent with the negative values of streaming potential coupling coefficient and zeta potential observed in the laboratory. Moreover, the magnitude of the change in voltage is similar to that estimated using the laboratory value of the coupling coefficient. Our results suggest that measurements of SP may make a valuable contribution to characterizing groundwater flow in the UK Chalk aquifer.

Linder-Lunsford, J.B. and Bruce, B.W., 1995, Use of Electric Logs to Estimate Water Quality of Pre‐Tertiary Aquifers: Groundwater, v. 33, no. 4, p. 547-555, doi:10.1111/j.1745-6584.1995.tb00309.x.

Abstract: Electric logs provide a means of estimating ground‐water quality in areas where water analyses are not available. Most of the methods for interpreting these logs have been developed for the petroleum industry and are most reliable in saline aquifers (concentration of dissolved solids as sodium chloride greater than about 50,000 mg/l). The resistivity‐porosity and spontaneous‐potential methods were evaluated to determine if they could be applied to identify zones of fresh water (concentration of dissolved solids as sodium chloride less than 1,000 mg/l) in three potential aquifers in central Wyoming. The potential aquifers have different lithologies–sandstone, clayey sandstone, and carbonate. The two methods generally were reliable predictors of water quality in the sandstone and carbonate potential aquifers. In the clayey sandstone potential aquifer, predictions of the dissolved‐solids concentration using the two methods differed by more than an order of magnitude in several cases. When the resistivity values are corrected for the presence of clay and shale as identified on a natural gamma log, the agreement between the results of the two methods improved by an average of 58 percent.

Salazar, J.M., Wang, G.L., Torres-Verdín, C., and Lee, H.J., 2008, Combined simulation and inversion of SP and resistivity logs for the estimation of connate-water resistivity and Archie’s cementation exponent: Geophysics, v. 73, no. 3, p. 1MJ-Z46, doi:10.1190/1.2890408.

Abstract: Knowledge of initial water saturation is necessary to estimate original hydrocarbon in place. A reliable assessment of this petrophysical property is possible when rock-core measurements of Archie’s parameters, such as saturation exponent n and cementation exponent m, are available. In addition, chemical analysis of formation water is necessary to measure connate-water resistivity Rw. Such measurements are seldom available in most applications; if they are available, their reliability may be questionable. We describe a new inversion method to estimate Rw and Archie’s cementation exponent from the combined use of borehole spontaneous-potential (SP) and raw array-induction resistivity measurements acquired in water-bearing depth intervals. Combined inversion of resistivity and SP measurements is performed assuming a piston-like invasion profile. In so doing, the reservoiris divided into petrophysical layers to account for vertical heterogeneities. Inversion products are values of invaded and virgin formation resistivity, radius of invasion, and static spontaneous potential (SSP). Connate-water resistivity is calculated by assuming membrane and diffusion potentials as the main contributors to the SSP. Archie’s or dual-water equations enable the estimation of m. The new combined estimation method has been successfully applied to a data set acquired in a clastic formation. Data were acquired in a high permeability and moderately high-salt-concentration reservoir. Values of Rw and m yielded by the inversion are consistent with those obtained with a traditional interpretation method, thereby confirming the reliability of the estimation. The method is an efficient, rigorous alternative to conventional interpretation techniques for performing petrophysical analysis of exploratory and appraisal wells wherein rock-core measurements may not be available.

References

Beck, A.E., 1981, Self-potential or spontaneous polarization, in Physical Principles of Exploration Methods: London, Macmillan Publishers Limited, p. 31-41.

Collier, H.A., 1993, The SP Log, in Borehole Geophysical Techniques for Determining Water Quality and Reservoir Parameters of Fresh and Saline Water Aquifers in Texas: Austin, Texas, Texas Water Development Board, v. 1, p. 266-288.