WATER ANALYSIS METHODS
Laboratory water analysis is an essential measurement required for accurate water saturation calculations from log data. Water samples are collected from drill stem tests or produced fluids. In the case of produced fluids, the water should be captured from the flow line and separated later from the oil. Samples from separators or treaters may not be representative of formation water due to contamination. Samples from drill stem tests are usually taken at the top, middle, and bottom of the test recovery. The bottom sample should have the least contamination from drilling fluid invasion. The top sample will have the most contamination.

Laboratories usually measure from 9 to 15 of the individual ions in a water sample, recorded in milligrams/litre (mg/l) or grams/cubic meter (g/m³). These two sets of units are equivalent: 1 mg/l = 1 g/m^3.  At low to moderate concentrations, one mg/l is very close to 1 part per million (ppm) so mg/l and ppm tend to be used interchangeably. The difference is that ppm = mg/l divided by the density of the water.

In older reports, results were quoted in grains per gallon (gpg)  One grain per US gallon equals 17.1 mg/l or approximately 17.1 ppm. 

The cations (positive ions) are measured by ion chromatograph. These are Sodium (Na), Potassium (K), Calcium (Ca), Magnesium (Mg), Iron (Fe), and sometimes Barium (Ba), Strontium (Sr), or Boron (Bo). The anions (negative ions) cannot be measured by ion chromatograph so Chloride (Cl) and sometimes Iodide (I) and Bromide (B) are still measured by titration. Bicarbonate (HCO₃) and Carbonate (CO₃) are calculated from the volume of acid required to reduce the pH to 8.3 (for CO₃) and 4.5 (for HCO₃). Sulphate (SO₄) is calculated by adding a volume of Barium Chloride and then measuring the turbidity of the solution. The sum of all of these measured ions becomes the calculated total dissolved solids (TDS).

They also measure, in a routine analysis, the pH, relative density, and resistivity (Rw) of the water sample.  The Rw is measured in ohm-m and they will record the temperature at which it was measured. This temperature will be used to adjust the measured Rw to a common "laboratory temperature", usually 25 Celsius or 77 Fahrenheit. Some labs use different standard temperatures.

Some labs also provide mmol/l (or moles/m³).  mg/l (or g/m³) divided by the molar mass of the ion equals mmol/l (or moles/m³). Since mg/l is the same thing as g/m³, mmol/l is the same thing as mol/m³. Molar mass is derived from the atomic weight of the ion on the Periodic Table. For instance, the atomic weight of S is 32 and the atomic weight of O is 16, so the molar mass of SO₄= 32 + 4 * 16 = 96.

Most labs calculate milli-equivalents (MEQ).  MEQ equals mmol/l or moles/m³ multiplied by the valence or charge of the ion.  SO₄ has a negative charge of 2 so 1 mmol/l of SO₄ equals 2 MEQ of SO₄.  Sodium (Na) has a positive charge of 1, so 1 mmol/l of Na equals 1 MEQ of Na. 

The logging company charts that convert ppm to Rw are based on ppm of pure NaCl. This is because the most common formation waters found in the world are NaCl based. These charts will give you the wrong Rw if the fluid is a mud filtrate which is mostly Na₂SO₄ or if the fluid is Sodium Bicarbonate type formation water.

The diagram at the bottom of the water analysis is called a Stiff Diagram. It is a graphical representation of the different ions.  The shape of the Stiff Diagram can become a “fingerprint” which can allow us to distinguish whether the fluid is formation water or an introduced fluid, and can often distinguish the zone from which the formation water was produced.

SAMPLE WATER ANALYSIS REPORT

Water analysis report from a drill stem test recovery, showing chemical analysis, calculated and measured water resistivity, and Stiff diagram of chemical analysis. This is a fairly salty formation water with salinity of 146,000 parts per million (ppm) and a resistivity of 0.066 @ 25C.

WATER SAMPLE CONTAMINATION
Contamination of water samples by drilling fluid invasion is common and the Stiff Diagram helps to spot this problem. For instance, a typical gel-chem type mud filtrate recovery will have Na in MEQ divided by Cl in MEQ of 5 or greater. Most contamination problems require some experience and a supply of Stiff Diagram fingerprints that are reasonably consistent. For more information on fingerprinting water recoveries, please contact Opus Petroleum Engineering Ltd., www.opuspetroleum.com.

The Canadian Water Resistivity Catalog is well screened, but field samples may be contaminated. The following rules of thumb are useful in detecting mud filtrate contamination or meteoric water recharge.

   1. Mg/l and ppm are approximately the same thing except in very saline waters.

   2. Most gel-chem mud filtrates are usually from 3000 to 8000 mg/l TDS.

   3. When the log header says gel-chem mud, they might mean gyp’ed-up mud. Gyp’ed-up mud usually has soda ash added to compensate for drilling through anhydrites. Gyp’ed-up mud filtrates are usually from 10,000 to 25,000 mg/l TDS.

   4. KCl mud filtrates are usually from 30,000 to 50,000 mg/l TDS with lots of K and lots of Cl.

   5. Potassium Sulphate mud filtrates are usually from 50,000 to 80,000 mg/l TDS with lots of K and lots of SO4.

   6. Salt-saturated mud filtrates are usually 300,000 mg/l or higher.

   7. Generally speaking, formation waters increase in salinity with depth but see #12.

   8. Each zone should have a unique formation water “fingerprint” or Stiff Diagram unless it is hydraulically connected to another zone.

   9. This formation water “fingerprint” may change with location in the basin.

   10. Most formation waters have a milli-equivalent Na/Cl ratio of 0.6 to 1.2.

   11. Many formation waters are fresher than expected as they are affected by fresh water recharge from the surface. These recharge waters have a distinctive fingerprint which is high in bicarbonates and the milli-equivalent Na/Cl ratio is usually between 2 and 3. These fresh waters have been found in formations as deep as the Devonian and can under-run more saline formation waters.

   12. Never rely on one water analysis as being representative of the formation water in a particular pool and field. Try to find at least three water samples in your pool that have formation water characteristics, are close to the same TDS and have similar “fingerprints”. Then compare the Rw on your samples to the ones in the catalog.

Agat Lab in Calgary uses a "Smart Chart", incorporating these rules,  to help identify clean water samples from those contaminated by mud filtrate or other chemicals.

This crossplot of TDS versus the Na/Cl ratio helps check for contamination. Data point 1 is in the formation water category. Point 2 is either naturally high in bicarbonates or contaminated by some gyp-based mud invasion. Point 3 is mostly gel-based mud filtrate.