Henderson  Petrophysics

Resistivity of Solutions From Water Analysis Data
Using Microsoft EXCEL with Visual Basic Macros

Introduction

Formation water resistivity at reservoir temperature is a parameter required for calculating water saturation. Schlumberger chart Gen-9 and algorithms derived from this and similar charts is used to convert known salinity (ppm NaCl equivalent) to resistivty at reservoir temperature.

The resistivity of a brine sample can be estimated from its chemical composition. Schlumberger chart Gen-8 (see figure-2) can be used to determine the multiplication factors to convert ion concentration to the equivalent NaCl concentration. Henderson Petrophysics has developed an EXCEL spreadsheet program that uses Visual Basic for Applications (VBA) macros to help with these tedious conversion calculations. The program;

• Converts brine analysis from mg/litre to meq/litre using;
  meq/L = mg/L x Valence / Ion molecular weight,
• Converts concentration in mg/L to ppm (mg/Kg) using;
  ppm = mg/L / brine density
• Estimates the ion conversion factor (CF in the spreadsheet) using lookup tables with data
  from Schlumberger chart Gen-8,
• Calculates NaCl equivalent salinity by multiplying concentration in ppm by CF,
• Sums the equivalent ion salinity values to determine the solution NaCl equivalent salinity,
• Uses lookup tables based on Schlumberger chart Gen-9 to estimate brine resistivity at 25 ^oC (77 ^oF
• Creates Stiff diagrams to visually summarise the water data.

Figure 1 shows an example salinity conversion Worksheet. Data is entered in cells with light blue shading.

Figure 1: Example Salinity Conversion Worksheet

Figure 2 shows a portion of Schlumberger chart Gen-8 and tables with interpolated salinity conversion factors. Visual Basic macros interpolate data from these tables.

Figure 2: Salinity Conversion Data

Figure 3 shows a plot of brine resistivity (ohmm at 25 ^o C) vs. salinity (ppm)(mg/Kg). The data were interpolated by visual inspection of Schlumberger chart Gen-9.

Figure 3: Brine Resistivity at 25 oC (77 oF)

Brine resistivity at different temperatures can be calculated using;

• For Celcius use: Rw = Rw25 x 46.5 / (T + 21.5),
• For Fahrenheit use: Rw = Rw25 x 83.77 / (T + 6.77)

Data Visualisation

A Stiff diagram (example shown in figure 4) is a widely used graphical presentation of water analysis data. This type of diagram is especially useful when illustrating data in hydrogeological cross sections. Cations are plotted on the left side of the Stiff diagram and anions are plotted on the right side. The y-axis scaling is only used to separate the parameters. The x-axis scale can be either linear or logartitmic.

The Excel spreadsheet in our Free Download section automatically creates Stiff Diagrams using default scaling. You can modify the scaling to suite your data and save the Excel spreadsheet with your scaling preferences for future use.

Figure 4: Stiff Plot to Visualise Water Analysis Data

Software for Brine Salinity Calculations

The Henderson Petrophysics Free Downloads section contains an example EXCEL Spreadsheet, a Visual Basic Macro file and associated documentation for processing brine salinity data.

The EXCEL Spreadsheet contains Visual Basic for Applications Macros that process data. This Spreadsheet is intented only for advanced EXCEL users who are familiar with using Visual Basic macros. Please look at the ReadMeFirst text file before attempting to open the Spreadsheet. This file discusses issues arising from the use of Spreadsheets with embedded Macros.

Working With Macros in EXCEL

The example EXCEL Spreadsheet available from our Download section contains Visual Basic for Applications Macros that process Petrophysics data. This Spreadsheet is intented only for advanced EXCEL users who are very familiar with Petrophysics data analysis and the use of Visual Basic macros. Please look at the ReadMeFirst text file before attempting to open the Spreadsheet. This file discusses issues arising from the use of Spreadsheets with embedded macros.

Figure 6 shows the warning window displayed by EXCEL when you open a Spreadsheet containing macros. We suggest that you open the Spreadsheet "Petrophysics_Summary_Tables.xls" with the macros disabled. To do this, choose the EXCEL option Tools/Macros/Security and select the High secrutity settingTo examine the macros, type Alt+F11 to open the Visual Basic Project Manager. After convincing yourself that the macros are not harmful to your computer system you can re-open the Spreadsheet with the macros enabled. Alternatively, you can use our macros as a learning resource for developing your own Petrophysics macros.

If you are interested in discovering how the VBA macros work, open the VBA project manager from EXCEL by typing Alt+F11. You can then step through the macros by typing F8 continuaously.

Figure 6: EXCEL Warning window when opening a Spreasheet containing macros

How to Contact Us

In additional to informative articles and useful downloads, our Website contains a lot of information on the services and expertise offered by Henderson Petrophysics.

Henderson Petrophysics Home Page

Henderson Petrophysics offers specialised formation evaluation services to the upstream oil and gas business.

For more information on our services or our fee schedule you can send an e-mail (click on the icon below) or contact us using the following information;

Phone:    +61 7 3300 3980  (61 is the country code for Australia and 7 is the area code for Brisbane)
Fax:        +61 7 3300 9342
Skype:    User ID is DonHenderson4061, or search using my e-mail address don@hendersonpetrophysics.com
E-mail:    don@hendersonpetrophysics.com