WELLOG                                 TOTAL DISSOLVED SOLIDS

 

 

REVISED 06-19-2007

 © WELLOG 2006 - 2007

 All Rights Reserved

 

An initial evaluation of water quality can be done thru measurement of Total Dissolved Solids (TDS). When water quality is important, Total Dissolved Solids contamination should always be a major consideration.

 

 

WHAT IS TDS?

 

Total Dissolved Solids represent the concentration of dissolved minerals including mineral salts in water. 

 

 

HOW IS IT MEASURED?

 

Measuring TDS in tap water:

 

Total Dissolved Solids may be measured directly using commercially available TDS meters from companies like Watts Premier. More information on the Watts Premier TDS monitor (#273001) may be obtained at http://www.premierh2o.com.

 

Measuring TDS in your well:

 

The TDS monitor isn’t an option if you want to obtain reliable water quality information directly from the water producing aquifer in a well. When measurements are desired from the water producing zone, it is possible to estimate water quality indirectly using electric logging techniques.

 

WHAT ARE SOME REFERENCE POINTS FOR TDS AND WATER QUALITY?

 

Distilled water is lowest in TDS – having zero or almost zero TDS measured in parts per million.

 

A common bottled water will have TDS ranging from 0 ppm (Aquafina - highly filtered) to 250 ppm (Arrowhead spring water)

 

The National Groundwater Association refers to TDS levels above 1000 ppm as poor tasting. View chart.

 

Refer to NGWA information at: http://www.ngwa.org/pdf/gwquality/pdf

 

 

Measurement Uncertainties:

 

All measurements have uncertainty. In other words – measurements are never perfect. When a measurement is made, a certain amount of error is possible depending on the person making the measurement and the instrumentation being used, etc.

 

Ask about calibration methods for electrical logging measurements.

 

Electrical instrumentation systems are very good but there are limitations. In salty environments, for example, a focused electrical resistivity tool is required.

 

Another variable is the environment in the well. In an open-hole well with no casing, depending on the fluid over pressure and porosity, mud-cake and drilling fluid invasion into the surrounding formations can affect the accuracy of measurements. Most well logging companies have charts that can be used help correct for known affects using two or more depths of measurement.

 

Natural and unknown affects can still inhibit the ability to obtain an accurate analysis.

 

Comparison with other wells in the immediate area is very helpful in verifying measurements.

 

 

CALCULATIONS:

 

Water containing measurable TDS conducts or resists current flow according to the concentration of dissolved solids and the temperature. The actual dissolved solids are in the form of various mineral ions.  Movement of ions in water is controlled partially by temperature. The relationship is that as temperature increases, conduction increases. Inversely, as temperature increases, resistivity decreases. It is therefore important to measure temperature when measuring conductivity or resistivity and performing conversion to TDS.

 

 

Obtain freshly circulated drilling mud, use mud press to obtain mud filtrate and measure Rmf @ temp.

 

Using SP measurement:                      Obtain ssp from SP log for use in step 3. (see ssp for information)

 

Obtain corrected Rmf;                        Rmf(corr) = [Rmf * (temp + 6.77)/81.770)]

 

Calculate Rwe;                                    Rwe = Rmf(corr) / 10(ssp/-70.7)

 

Calculate Rw(NaCl);                            Rw(NaCl) = Rwe1.227  * 0.825

 

Calculate Rw(NaHCO3);                       Rw(NaHCO3) = Rw(NaCl) / 0.85

 

Calculate conductivity;                       C = 10,000 / Rw(NaCl)            millimhos/centimeter2

 

Calculate conductivity;                       C = 10,000 / Rw(NaCHCO3)     millimhos/centimeter2

 

Calculate TDS;                                     TDS(NaCl) =  5300/Rw(NaCl)                          ppm

 

Calculate TDS;                                     TDS(NaHCO3) =  10000/Rw(NaCHO3)              ppm

 

 

Using LN64:                                          Obtain Resistivity (Rt) from Electric Log - 64 inch measurement. (see elog for information)

 

 

Obtain apparent Rw;                          Deep resistivity Rt in a 100 percent water filled formation = Rw (apparent)

 

Using the value for Rw obtained from Rt

 

Calculate Rw(NaCl);                            Rw(NaCl) = Rwe1.227  * 0.825

 

Calculate Rw(NaHCO3);                       Rw(NaHCO3) = Rw(NaCl) / 0.85

 

Calculate conductivity;                       C = 10,000 / Rw(NaCl)            millimhos/centimeter2

 

Calculate conductivity;                       C = 10,000 / Rw(NaCHCO3)     millimhos/centimeter2

 

Calculate TDS;                                     TDS(NaCl) =  5300/Rw(NaCl)                          ppm

 

Calculate TDS;                                     TDS(NaHCO3) =  10000/Rw(NaCHO3)              ppm

 

 

If you have question, WELLOG has answers.

 

Contact info@wellog.com if you need additional information.