Transparency 1 (tittle)

Transparency 2 (semi-empirical method)

Semi-empirical method of the neutron porosity tool calibration has been developed by professor Jan Andrzej Czubek since the late eighties and has been used in Geofizyka Kraków since 1995. The method is based on experimental measurements completed with mathematical calculations. As the result the semi-empirical method gives the full set of correction charts.

The first step of the calibration procedure is a determination of the general calibration curve, the function which corelates tool readings with a General Neutron Parameter according to the equation.

The GNP is a function of apparent neutron parameters: apparent migration length, apparent absorption cross-section and apparent probability of absorption escape during the slowing down process.

All unknown constants are determined during a fitting procedure employed to the first equation in a logarithmic form.

Four computer programs created by professor Czubek are necessary to use the semi-empirical method. The programs are as follows:

NEWCROSS, SLOWN22A - which allow to find true parameters for the slowing down process

for a given medium (rock, borehole)

Ls - the slowing down length

Ds- the diffusion coefficient during the slowing down process

Pr - the probability of absorption escape during the slowing down process

NEWCROSS, NEROTH2 - calculate true parameters for thermal neutrons

Ld- the diffusion length

D- the diffusion coefficient

S a- the absorption cross section

LMBRINN22 - calculates apparent values of neutron parameters in two-region (rock-borehole) geometry

Transparency 3 (general view)

The calibration of neutron well logging tools is performed in Zielona Góra Calibration Station. General view of the Station is presented in the picture. 21 calibration standards with different borehole sizes are available there: 8 limestones, 2 dolomites, 8 sandstones and 3 ceramic standards. An example of the rock standard is presented in the picture.

Transparency 4 (standard)

Here an example of measurement results is presented. Count rates have been measured in standard blocks of different lithologies and different porosities. The measurement has been taken every 5 cm in two orientations of the tool starting from the bottom of the standard.

Transparency 5 (measurements)

Transparency 6 (general calibration curve)

A full sequence of calibration measurements and calculation was performed for the two-detector thermal neutron tool PKNN3 in 1996. Fig. presents the general calibration curve. All points are closely distributed around the best fit curve and the correlation coefficient is satysfying (0.994). Please, notice the very good correlation, which is reached. The experimental points were obtained for different lithologies, different borehole sizes and different borehole fillings. The high correlation confirm correctness of the semi-empirical calibration method.

Transparency 7 (borehole corrections)

The LMBRIN22 program was used to create some correction charts. The figure presents an example of two sets of the borehole diameter and salinity correction charts for the PKNN3 tool.

The influence of the litology and S a is presented in the second figure.

Transparency 8 (lithology corrections)

The next step is to find correction charts for the influence of the presence of the mudcake on the borehole walls or casing.

Transparency 9 (three-region geometry)

The figure presents a scheme of the three-region geometry together with the contours of constant values of the neutron flux. Here the borehole is filled with water. It is surrounded by the intermediate region (e.g. mudcake) and finally surrounded by the geological formation.

The task is to find formulae for the apparent slowing down and migration lengths in such complicated geometry. Professor Czubek gave a fundamental solution for this problem.

The computer program MOM3 written on the base of this solution allows to calculate apparent slowing down and migration lengths in the three-region geometry.

Transparency 10 (tests and conclusions)

A comparison of MOM3 amd LMBRIN22 which calculates the apparent neutron parameters in two -region geometry gives satisfying results.

There were made some calculations of the influence of the intermediate layer on the neutron parameters. The figure presents results of this calculations.The apparent slowing down length is calculated for the three-region borehole geometry when the iron pipe of different thickness is put as the intermediate zone. The plots are presented as functions of the rock porosity.

There were made some measurements at the Calibration Station in Zielona Góra with an iron pipe in the borehole, but as till now there is insufficient amount of the measurements to discuss them.

The apparent migration length and the apparent slowing down length can be calculated using MOM3 program, but to find the GNP the apparent probability of absorption escape during the slowing down process and apparent absorption cross section should also be known. The research on this problem is going on. We are working on a proper theoretical description of the apparent absorption cross section in three-region borehole geometry. Correctness of the theoretical solution will be checked by measurements at the calibration station in Zielona Góra.

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