Porosity from Nuclear Magnetic LogS
Nuclear Magnetic Resonance Logs come in many flavours and have been around  for more than 40 years. However, they are still quite rare and only the last few years have shown themselves to be useful in identifying zones with high irreducible water that would be previously be considered too wet to be worth completing. By comparing irreducible water from NMR with total pore water from conventional logs, an estimate of water cut can be made. However, results from many NMR calculations are still ambiguous.

The chief application of the NMR tool is to determine moveable fluid volume (BVM( of a rock. This is the pore space excluding clay bound water (CBW) and irreducible water (BVI). Neither of these are moveable in the NMR sense, so these volumes are not easily observed on older logs. On modern tools, both CBW and BVI can often be seen in the signal response after transforming the relaxation curve to the porosity domain. Note that some of the moveable fluids (BVM) in the NMR sense are not actually moveable in the oilfield sense of the word. Residual oil and gas, heavy oil, and bitumen may appear moveable to the NMR precession measurement, but these will not necessarily flow into a well bore.

Micritic interparticle porosity and pin point vugs in carbonates may be included in the CBW fraction. This may cause a minor difference between effective porosity seen by conventional logs and from NMR, especially in clean rocks in which some of the porosity has very small pore size.

The Log Response Equation for modern nuclear magnetic logs is the same as for all other logs. The difference between the NMR and other porosity logs is that the Log Response Equation is solved by the service company at logging time, instead of by the analyst after the logs are delivered. This transform is illustrated below.


Transforming the precession decay time curve into the porosity domain, showing breakdown of CBW, BVI,
 and BVM. The T2 cutoff for the boundary between CBW and BVI is often chosen at 3 ms. In sandstones, the boundary between BVI and BVM is usually chosen at 32 or 33 ms, but in carbonates the cutoff could be much higher (80 to 120 ms) and varies with lithology. The cutoffs can be determined on rock samples in the lab.

T2 distribution for heavy oil (red curve) and light oil (yellow) - area under the curves are equal, so total porosity is independent of hydrocarbon density. T2 cutoff between BVI and BVM varies with lithology - total porosity does not vary with lithology, but BVI and BVM do vary with lithology.

The matrix and dry clay terms of NMR response are zero. As a result the NMR porosity is said to be independent of lithology. However, this is only true for total porosity (the total area under the shaded curve in the above illustration). The boundary between CBW and BVI, and the boundary between BVI and BVM, do depend on lithology and may vary foot by foot through the reservoir. As a result, the choice of fixed T2 cutoff times to represent these boundaries is not a good idea, and more elaborate methods are now being used.

An NMR log run today can display clay bound water (CBW), irreducible water (capillary bound water, BVI), and mobile fluids (hydrocarbon plus water, BVM), also called free fluids or free fluid index (FFI). On older logs, only free fluids (FFI) are recorded and some subtractions, based on other open hole logs, are required to obtain BVI and CBW.

A confusion is caused by the term Moveable Fluids or Free Fluids (BVM in the diagram above). The freedom to move refers to the protons in the fluid, not to the fluid itself. Some or all of the BVM may be immoveable, as residual oil, heavy oil, bitumen, or tar.

Comparison of conventional and NMR porosity shows that effective porosity
(curves to the left of the red shading) are nearly identical. Water volume from
PHIe*Sw and BVI are also very similar (right edge of red shading), indicating
that the water is irreducible and not moveable ==>

For modern NMR logs:
      1: PHIt = PHIcbw + PHIbvi + PHIbvm
      2: PHIe = PHIbvi + PHIbvm
      3: PHIuse = PHIbvm
      4: SWir = PHIbvi / PHIe
      5: IF SW >> SWir
      6: THEN zone will produce with a water cut

  PHIcbw = clay bound water (fractional
  PHIbvi = irreducible water or capillary bound water (fractional)
  PHIbvm = mobile fluids (fractional)
  PHIt = total porosity (fractional)
  PHIe = effective porosity (fractional)
  PHIuse = useful porosity (fractional)
  SWir = irreducible water saturation (fractional)

Nordegg tight oil shows large separation between the density (red curve) and neutron porosity (black short dash), even though the sand is clean according to the gamma ray log. The core porosity (blue dots) and NMR total porosity (solid grey) are about halfway between the two conventional porosity curves, which is where the Complex Lithology model would also put the porosity. The PE (black heavy dash) varies between 1.8 and 4.5 showing the variable nature of the heavy mineral content. NMR porosity is unaffected by organic carbon (TOC) so it is a good log in shale gas environments. (image courtesy Schlumberger)

Some or all of the sums defined above may be displayed on the delivered log. Log presentation is far from standard for NMR logs.

In some situations, mobile water can be separated from hydrocarbon, and sometimes gas can be distinguished from oil, by further (experimental) processing of the original signal. However, the depth of investigation and measurement volume are tiny, so the hydrocarbon indication is from the invaded zone.

For the same reason, PHIt and PHIe from NMR do not always agree with that derived from density neutron methods, which see much larger volumes of rock.

None required.


 Porosity from Older Nuclear Magnetic Logs
On older NMR logs, only free fluids (FFI) are recorded and some subtractions, based on other open hole logs, are required to obtain BVI and CBW.

For older NMR logs:
      7: PHInmr = FFI
      8: SWir = KBUCKL / PHInmr
      9: PHIe = FFI / (1 - SWir)
      10: BVWSH = (PHINSH + PHIDSH) / 2
      11: PHIt = PHIe + Vsh * BVWSH

IF  PHIe is known from some other log:
      12: PHIbvi = PHIe - PHInmr
      13: SWir = PHIbvi / PHIe

  FFI = free fluids or free fluid index (fractional)
  PHInmr = NMR porosity (fractional)
  PHIt = total porosity (fractional)
  PHIe = effective porosity (fractional)
  SWir = irreducible water saturation (fractional)
  Vsh = shale volume (fractional)
  BVWSH = bound water 100%in shale (fractional)

PHIe and PHIt should be compared to density neutron or other methods defined earlier. The significant difference in measured volume between NMR and other logs may give silly answers that need to be trimmed or smoothed.

KBUCKL is in the range 0.010 to 0.100, with a default of 0.040.

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