The
neutron log is widely used to estimate porosity. The method
works well when shale volume and matrix rock neutron
response are accurately known. Errors as large as 8%
porosity can occur, and 2 to 4% are common if incorrect
assumptions are made.If both density and neutron logs are available, a superior model that does not require matrix rock properties is the Shale Corrected Density Neutron Complex Lithology Crossplot Method. The Meta/Kwik spreadsheet for this model is available at Downloads and Spreadsheets. Neutron Log RESPONSE EQUATION The response equation for the neutron porosity log also follows the classical form: 1: PHIN = PHIe * Sxo * PHINw (water term) + PHIe * (1 - Sxo) * PHINh (hydrocarbon term) + Vsh * PHINsh (shale term) + (1 - Vsh - PHIe) * Sum (Vi * PHINi) (matrix term) Where: PHINh = log reading in 100% hydrocarbon PHINi = log reading in 100% of the ith component of matrix rock PHIN = log reading PHINsh = log reading in 100% shale PHINw = log reading in 100% water PHIe = effective porosity (fractional) Sxo = water saturation in invaded zone (fractional) Vi = volume of ith component of matrix rock Vsh = volume of shale (fractional)
Porosity
From the Neutron Log 1. Radioactive Well Log Interpretation J.P. Campbell, A.B. Winter, Lane Wells, 1946 2. Experimental Basis for Neutron Log Interpretation J.T. Dewan, L.A. Allard, Petroleum Engineer, 1953 3. Sidewall Epithermal Neutron Porosity Log J. Tittman, H. Sherman, W.A. Nagel, R.P. Alger, JPT, 1966 4. Dual Spacing Neutron Log - CNLi R.P. Alger, S. Locke, W.A. Nagel, H. Sherman, AIME, 1971 A chart to solve this equation, along with the lithology shifts can be found below.
KN
is in the range of 1.0 to 3.0 depending on depth of invasion,
gas density and logging tool type. Use local experience. Apply
this correction only if gas is known to be present and log reading
is still too low after lithology corrections.
NOTE: If neutron
log is on a scale (sandstone, limestone) that does not match the
rock mineral, lithology corrections are required. PHIN = 0.28 PHINSH = 0.30 Vsh = 0.33 Neutron log type = CNL CNL / FDC units = Sandstone Rescaling is not required, as log is in correct units. No gas correction is required. PHInc = 0.28 - 0.33 * 0.30 = 0.18
A large number of charts for specific tools, spacings, borehole conditions and rock types were available from service companies, such as the one shown below. These may no longer be easily found today, and the semi-logarithmic approach described below works well except in very low porosity .
There were three source types used (RaBe, PuBe, and AmBe) and several source - detector spacings (15.5 and 18.5 inches were common), combined with hole size, mud weight, and casing variations, leading to a plethora of transforms. Some service companies didn't have a lot of faith in their charts - one used the term "Strata Index" instead of "Porosity" on the Y-axis. If no appropriate chart exists, or if you don't believe in them, it is expedient to use the "High porosity- Low porosity" method. A logarithmic scale can be applied algebraically with the following formulae using the high porosity/low porosity method. Calculate slope and intercept of semi-logarithmic relationship
between neutron count rate and porosity:
Calculate total porosity:
A
large number of charts for specific tools, spacings, borehole
conditions and rock types are available from service companies. PHILO should be in the range 0.01 to 0.05, and cannot be zero.
PHIHI = 0.30 PHILO = 0.01 NCPS = 2500 cps CPSHI = 1500 CPSLO = 4500 SLOPE = (log (0.30 / 0.01)) / (1500 - 4500) = - 0.000492 (rounded to - 0.0005) INTCPT = 0.30 / 10 ^ (1500 * (-0.0005)) = 1.6432 PHIn = 1.6432 * 10 ^ (-.0005 * 2500) = 0.096
NEUTRON
ENVIRONMENTAL CORRECTIONSEnvironmental corrections are based on the following equations,
taken from “Experimental Determination Of Environmental
Corrections For A Dual-Spaced Neutron Porosity Log”, by Dan M.
Arnold and Harry D. Smith, Jr., SPWLA June 1981, updated
January, 1982. These are for the Welex-Halliburton tool.
Equations for other tools will vary somewhat due to differences
in tool design and experimental error.In open hole, borehole size corrections are normally made automatically during logging based on the caliper log, For cased hole, borehole size, casing thickness, and cement thickness corrections are made automatically, based on input parameters. The equations for the remaining corrections are often used in computer post-processing of the log. All porosity values and corrections are in fractional porosity units.
Basic Porosity
Equation
Where:
Borehole Size
Correction This is the equivalent of limestone porosity (PHIN_LS) before any other corrections are made and is what we read from a compensated neutron log. Further corrections and lithology conversion are shown below..
Mud Cake
Correction
Where:
Formation
Salinity Correction
Where:
Borehole
Salinity Correction The reference paper uses the same salinity for both formation and borehole (MS = WS) – this only true if invasion is fairly deep.
Mud Weight
Correction
Where:
Standoff
Correction
Temperature
Corrections
Where:
CASED HOLE
CORRECTIONS
Cement
Thickness Correction
Where:
Limestone
Porosity Borehole size, casing, and cement corrections are normally made automatically during logging, DO NOT APPLY THEM AGAIN. Salinity and temperature corrections may or may NOT have been applied while logging. Some commercial software packages apply ALL corrections with no options to apply only those that are needed. When in doubt, apply NO corrections. See next section to apply Lithology corrections.
For SNP tools only, mud cake thickness correction is needed first: 1: PHINmc = PHIN - 0.01 * (max (0, CAL - BITZ) / KX1) Where: BITZ = bit size (inches or mm) CAL = caliper (inches or mm) KX1 = 1.0 for English Units (inches) KX1 = 25.4 for Metric Units (millimeters)
QUICKLOOK METHODS FOR
PHINmSandstone
porosity units to limestone units.CASE 1a: PHIN_LS = PHIN_SS - 0.03 SNP Log CASE 1b: PHIN_LS = PHIN_SS - 0.04 CNL Log Limestone porosity units to sandstone units. CASE 2a: PHIN_SS = PHIN_LS + 0.03 SNP Log CASE 2b: PHIN_SS = PHIN_LS + 0.04 CNL Log Limestone porosity units to dolomite units. CASE 3a: PHIN_DL = PHIN_LS - 0.03 SNP Log CASE 3b: PHIN_DL = PHIN_LS - 0.03 CNL Log CAUTION: IF final porosity < 0.05, ose more rigorous model shown below.
MORE RIGOROUS METHODS FOR
PHINmIf the log is recorded in limestone units and a more accurate
lithology correction is desired, use the following formulae.
CAUTION: There are ,many tool variations with specific charts
for each. The equations below are two published versions and are
not guaranteed to be correct or appropriate for your case -
check the specific chart for your tool to be sure.If lithology is quartz and tool type is SNP: CASE 4a: PHIN_SS = 0.222 * PHIN_LS^2 + 1.021 * PHIN_LS + 0.024 Schlumberger SNP CASE 4b: PHIN_SS = - 0.14 * PHIN_LS^2 + 1.047 * PHIN_LS + 0.0482 Dresser Atlas SNP If lithology is dolomite and tool type is SNP: CASE 5a: PHIN_DL = 0.60 * PHIN_LS^2 + 0.7490 * PHIN_LS - 0.00434 Schlumberger SNP CASE 5b: PHIN_DL = 0.34 * PHIN_LS^2 + 0.8278 * PHIN_LS - 0.01249 Dresser Atlas SNP If lithology is quartz and tool type is CNL: CASE 6a: PHIN_SS = 0.222 * PHIN_LS^2 + 1.021 * PHIN_LS + 0.039 Schlumberger CNL CASE 6b: PHIN_SS = PHIN_LS + 0.04 Dresser Atlas CNL CASE 6c: PHIN_SS = - 0.4778 * PHIN_LS^2 + 1.220 * PHIN_LS + 0.0311 Welex/Halliburton CNL If lithology is dolomite and tool type is CNL: CASE 7a: PHIN_DL = 1.40 * PHIN_LS^2 + 0.389 * PHIN_LS - 0.01259 Schlumberger CNL CASE 7b: IF PHIN_DL < 0.10 THEN PHIN_DL = 3.11 * PHIN_LS^2 + 0.102 * PHIN_LS - 0.00133 Dresser Atlas CNL ELSE PHIN_DL = - 0.06 CASE 7c: PHIN_DL = 1.397 * PHIN_LS^2 + 0.345 * PHIN_LS - 0.0152 Welex/Halliburton CNL If no lithology correction is needed. CASE 8: PHINm = PHIN_LS Finally: PHINm = SELECT CASE Where: PHIN_LS = original neutron log reading in Limestone units PHINm = apparent neutron log porosity corrected for lithology (fractional)
The charts and math shown above are for older CNL and SNP type neutron logs (NPHI curve). More modern CNL tools have additional porosity transforms (eg TNPH). Refer to the appropriate service company correction chart for newer tools. In particular, epithermal CNL logs have considerably less lithology correction. |
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