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Neutron & Density corrections
Neutron porosity corrections:-
Some of the most important and most complex environmental corrections are required for neutron porosity measurements. Neutron porosity is affected by a number of factors, most importantly temperature. If these effects are left uncorrected, then neutron porosity will be in error and as a result porosity will be wrongly calculated. Also, with neutron corrections, it is vital that the appropriate chart be used for the particular type of tool run.
Depth of investigation of Neutron tools is difficult to establish, since it varies with lithology, porosity, formation temperature, formation salinity and formation capture cross-section.
There are lots of correction that needs to be applied on neutron porosity as given in next paragraph,most of them are small.The largest correction on neutron porosity are for borehole temperature and bore hole diameter.
Neutron correction is applied for hole diameter, mud cake thickness, borehole mud salinity, mud weight, borehole temperature, borehole pressure, tool standoff and formation water salinity.
Parameters for neutron corrections are calculated as follow:
Thickness of mud cake at various depths is calculated using the following relationship:
hmc=bitsize –caliper/2
where : hmc is thickness of mud cake.
Borehole mud salinity is calculated using mud resistivity provided by field personnel
Mud weight is provided by field personnel .
Bore hole temperature at all depths can be calculated using Arps equation as given below:-
where:
R2 = resistivity value corrected for temperature
R1 = resistivity value at known reference temperature (T1)
T1 = known reference temperature
T2 = temperature to which resistivity is to be corrected
k = temperature constant
k = 6.77 when temperature expressed in 0F
Bore hole pressure can be calculated using the relation ship given below as :
Pressure (psi)=mud weight (lb/gal) X Depth (feets) /19.27
Each of the individual influences calculated for, the resulting correction factors must be summed in order to obtain the total effect . This is accomplished by adding the sum of the individual correction factors to the uncorrected value of neutron limestone porosity.
Total CF =Borehole effect CF+ Mudcake CF + Salinity CF + Mudweight CF + Temperature CF+Pressure CF
Where CF is Correction factor
Bulk density corrections:-
Mud cake effect:-The depth of investigation of density tool is very small (1.5in) so it is always effected by the mud cake .This effect of mud cake is eliminated by using spine and rib plots ,which is a plot of log short spacing detector counts in the x-axis and log of long spacing detector counts in the y-axis. This plot should be a straight line ,any deflection from straight line is attributed to the effect of mud cake density.
Rugosity effect:-As The depth of investigation of density tool is very small ,the count rate will always be effected if borehole wall is not smooth .The result is that Rugosity can be seen on the log as an increase in DRHO .As the value of DRHO increases ,the density at that depth becomes less credible .This is because the density correction algorithm is designed to compensate for mud cake ,not washouts.
Density correction need not be applied in SDL responses by analyst in office as this correction is generally applied by engineer in the real time.