Spontaneous Potential - Physics

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:

  1. Streaming potential (Ek)
  2. 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)

Read more about this topic:  Spontaneous Potential

Other articles related to "physics":

Whistled Language - Physics
... In a non-tonal language, segments may be differentiated as follows Vowels are replaced by a set of relative pitch ranges generally tracking the f2 formant of spoken language ... Stress is expressed by higher pitch or increased length Consonants are produced by pitch transitions of different lengths and height, plus the presence or absence of occlusion ...
Physics - Current Research
... Further information List of unsolved problems in physics Research in physics is continually progressing on a large number of fronts ... In condensed matter physics, an important unsolved theoretical problem is that of high-temperature superconductivity ... In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear ...
Information - As A Property in Physics
... Information has a well-defined meaning in physics ... Bekenstein claimed that a growing trend in physics was to define the physical world as being made of information itself (and thus information is defined in this way) (see Digital physics) ...
Indeterminism - Science - Classical and Relativistic Physics
... The idea that Newtonian physics proved causal determinism was highly influential in the early modern period ... For instance, Newtonian physics admits solutions where particles accelerate continuously, heading out towards infinity ...
Institute For Theoretical And Experimental Physics
... The Institute for Theoretical and Experimental Physics (ITEP Russian Институт теоретической и экспериментальной физики) is located in Moscow, Russia as a MinAtom ... as a community of people studying topics ranging from theoretical physics and mathematics to biology and chemistry ... Research areas in theoretical physics concentrate around quantum field theory including the string theory ...

Famous quotes containing the word physics:

    But this invites the occult mind,
    Cancels our physics with a sneer,
    And spatters all we knew of denouement
    Across the expedient and wicked stones.
    Karl Shapiro (b. 1913)

    The labor we delight in physics pain.
    William Shakespeare (1564–1616)

    The fundamental laws of physics do not describe true facts about reality. Rendered as descriptions of facts, they are false; amended to be true, they lose their explanatory force.
    Nancy Cartwright (b. 1945)