Spontaneous potential
Encyclopedia
Spontaneous potential, also called self potential, is a naturally occurring electric potential difference
in the Earth, measured by an electrode relative to a fixed reference electrode. Although SP differences have been measured along the Earth's surface in mineral exploration, today it is used almost exclusively by the oil and gas industry for formation evaluation
in boreholes. SP was first recognized by Conrad Schlumberger, Marcel Schlumberger, and E.G. Leonardon in 1931, and the first published examples were from Russian oil fields.
The origin of SP across formation can be attributed to two processes involving the movement of ions:
Streaming potential originates from the flow of an electrolyte (water) over naturally charged solids (i.e., surfaces that acquired electrokinetic or zeta potential). The streaming potential appears when mud filtrate is forced into the formation under the differential pressure between mud column and formation. The streaming potential is produced when the flow takes place across mud-cake in front of permeable formations, across permeable formations being invaded, and across shale beds. It is generally accepted that the streaming potential across the mud-cake is compensated by that across the shale. As such, in most cases, the spontaneous potential measured is only related to the electrochemical potential.
Electrochemical potential (EC) is the sum of liquid junction or diffusion potential (EJ), and membrane potential (EM)
Liquid junction potential is established at the direct contact of the mud filtrate and formation water at the edge of the invaded formation. Ions Na+ and Cl- diffuse from either solution to the other, but at different rate due to different mobilities. Na+ tends to be less mobile due to its affinity for water molecules.
where:
Membrane Potential develops when two electrolytes of different ionic concentrations, such as mud and formation water, are separated by shale. The clay minerals in shale are usually made up of atom Al, Si, and O. O2- ions occupy the outer layer and cause a net negative charge. Na+ ions from solution are attracted and allowed to pass through the shale, while Cl- ions are repelled. Na+ ions will migrate between the two solutions, with a net influx from the more saline to the less.
where:
The total electrochemical potential is thus summarized as EC = EM + EJ = K log10(aw/amf)
Since spontaneous potential is a measure of electrochemical potential and the ionic activity of a solution is inversely proportional to its resistivity, the above equation can be simplified as SP = EC = K log10 (Rmfe/Rwe), where Rmfe and Rwe are equivalent mud filtrate resistivity and equivalent formation water resistivity respectively.
The ideal spontaneous potential across clean bed is known as Static SP (SSP), and defined as follow:
, SP can only be recorded in conductive mud.
Static SP (SSP) can be obtained directly from the SP curve if the bed is clean, thick, porous, permeable, and only moderately invaded. When these conditions are not met, the recorded SP will need to be corrected. Various correction charts are available for this purpose.
To convert the measured mud filtrate resistivity Rmf into an equivalent mud filtrate resistivity Rmfe, the following rules are employed:
Schlumberger Chart SP-2 can then be used to convert Rwe to obtain Rw.
, or mV), typically with the goal of identifying the path of groundwater
flow in the subsurface, or seepage from an earthen dam
. A voltmeter
measures the voltage between a fixed liquid-junction electrode and a mobile one (rover), which is moved along a dam face or over an area of investigation to collect multiple readings. Anomalies observed may indicate groundwater movement or seepage.
at the Earth's surface (called surface electrode) and the other probe in the borehole (called downhole electrode), where the SP is to be measured. In fact, logging tools employ exactly this method. Since this measurement is relatively simple, usually SP downhole electrode is built into other logging tools.
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
in the Earth, measured by an electrode relative to a fixed reference electrode. Although SP differences have been measured along the Earth's surface in mineral exploration, today it is used almost exclusively by the oil and gas industry for formation evaluation
Formation evaluation
In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one"....
in boreholes. SP was first recognized by Conrad Schlumberger, Marcel Schlumberger, and E.G. Leonardon in 1931, and the first published examples were from Russian oil fields.
Physics
Spontaneous potentials (SP) are usually caused by charge separation in clay or other minerals, due to presence of semi-permeable interface impeding the diffusion of ions through the pore space of rocks, or by natural flow of a conducting fluid through the rocks.The origin of SP across formation can be attributed to two processes involving the movement of ions:
- Streaming potential (Ek)
- Electrochemical potential (Ec)
Streaming potential originates from the flow of an electrolyte (water) over naturally charged solids (i.e., surfaces that acquired electrokinetic or zeta potential). The streaming potential appears when mud filtrate is forced into the formation under the differential pressure between mud column and formation. The streaming potential is produced when the flow takes place across mud-cake in front of permeable formations, across permeable formations being invaded, and across shale beds. It is generally accepted that the streaming potential across the mud-cake is compensated by that across the shale. As such, in most cases, the spontaneous potential measured is only related to the electrochemical potential.
Electrochemical potential (EC) is the sum of liquid junction or diffusion potential (EJ), and membrane potential (EM)
Liquid junction potential is established at the direct contact of the mud filtrate and formation water at the edge of the invaded formation. Ions Na+ and Cl- diffuse from either solution to the other, but at different rate due to different mobilities. Na+ tends to be less mobile due to its affinity for water molecules.
- EJ = K1 log10(aw/amf)
where:
- K1 = 11.6 mV at 25 °C
- aw = formation water ionic activity
- amf = mud filtrate ionic activity
Membrane Potential develops when two electrolytes of different ionic concentrations, such as mud and formation water, are separated by shale. The clay minerals in shale are usually made up of atom Al, Si, and O. O2- ions occupy the outer layer and cause a net negative charge. Na+ ions from solution are attracted and allowed to pass through the shale, while Cl- ions are repelled. Na+ ions will migrate between the two solutions, with a net influx from the more saline to the less.
- EM = K2 log10(aw/amf)
where:
- K2 = 2.3 RT/F, where:
- R = ideal gas constant
- T = absolute temperature in kelvins
- F = Faraday constant
- aw = formation water ionic activity
- amf = mud filtrate ionic activity
The total electrochemical potential is thus summarized as EC = EM + EJ = K log10(aw/amf)
Since spontaneous potential is a measure of electrochemical potential and the ionic activity of a solution is inversely proportional to its resistivity, the above equation can be simplified as SP = EC = K log10 (Rmfe/Rwe), where Rmfe and Rwe are equivalent mud filtrate resistivity and equivalent formation water resistivity respectively.
The ideal spontaneous potential across clean bed is known as Static SP (SSP), and defined as follow:
- SSP = −K log10 (Rmfe/Rwe)
Applications in Boreholes
The most useful SP component is the electrochemical potential, since it can cause a significant deflection opposite permeable beds. The magnitude of the deflection depends mainly on the salinity contrast between borehole and formation fluid, and the clay content of the permeable bed. The SP log is therefore useful in detecting permeable beds and to estimate formation water salinity and formation clay content. Due to the nature of the electric currentElectric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
, SP can only be recorded in conductive mud.
Determination of Rw
As established earlier, static SP is defined as follow:- SSP = −K log (Rmfe/Rwe)
Static SP (SSP) can be obtained directly from the SP curve if the bed is clean, thick, porous, permeable, and only moderately invaded. When these conditions are not met, the recorded SP will need to be corrected. Various correction charts are available for this purpose.
To convert the measured mud filtrate resistivity Rmf into an equivalent mud filtrate resistivity Rmfe, the following rules are employed:
- If Rmf at 75 °F is greater than 0.1 ΩOhmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
·m, use Rmfe = 0.85 Rmf at formation temperature. - If Rmf at 75 °F is less than 0.1 Ω·m, derive Rmfe from Rmf using Schlumberger Chart SP-2 or equivalent.
Schlumberger Chart SP-2 can then be used to convert Rwe to obtain Rw.
Applications on the surface
Electrodes can be placed on the ground surface to map relative changes in the SP value (in millivoltsVolt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, or mV), typically with the goal of identifying the path of groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
flow in the subsurface, or seepage from an earthen dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
. A voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
measures the voltage between a fixed liquid-junction electrode and a mobile one (rover), which is moved along a dam face or over an area of investigation to collect multiple readings. Anomalies observed may indicate groundwater movement or seepage.
Interpretation
SP can be affected by several factors that complicates the interpretation. Beside petrochemical component, SP is also affected by electrokinetic potential and bimetallism. Besides, SP is also affected by the following factors:- Bed thickness (h); Since SP is a measurement of electrical potential produced by current in the mud, its amplitude approaches the SSP value only when the resistance to current due to formation and adjacent beds is negligible compared with that of the mud. This condition is met only in thick bed. In thin beds, the SP is proportionally reduced.
- True resistivity (Rt) of permeable bed; As Rt/Rm increases, the SP deflection decreases, and the bed boundaries are less sharply defined. Presence of hydrocarbons also attenuates SP.
- Resistivity of invaded zone (Rxo) and mud resistivity (Rm); SP increases with increase of Rxo/Rm
- Diameter of invasion (di); SP decreases as invasion deepens
- Ratio of mud filtrate to formation water salinities: Rmf/Rw
- Neighbouring shale resistivity (Rs); SP increases with increase of Rs/Rm
- Hole diameter (dh); With increasing hole size, the value of SP is reduced
Measurement technique
Spontaneous potential can be measure by placing one probe of a voltmeterVoltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
at the Earth's surface (called surface electrode) and the other probe in the borehole (called downhole electrode), where the SP is to be measured. In fact, logging tools employ exactly this method. Since this measurement is relatively simple, usually SP downhole electrode is built into other logging tools.