Formation water density

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Formation fluid pressure and its application
Author	Edward A. Beaumont, Forrest Fiedler
Link	Web page
Store	AAPG Store

Formation water density is a function of three variables:

• Temperature
• Pressure
• Total dissolved solids (TDS)

It is the mass of the formation water per unit volume of the formation water and is given in metric units (g/cm³). For reservoir engineering calculations, it is considered equivalent to specific gravity.

Estimating density from TDS

If TDS is known from a chemical analysis of formation water, then the formula below can be used to estimate formation water density (ρ_w):

$\rho _{\rm {w}}=1+{\mbox{TDS}}\times 0.695\times 10^{-6}{\mbox{ g/cm}}^{3}$

Procedure: estimating density from r_w

Figure 1 Determining NaCl concentration. Copyright: Schlumberger.

Figure 2 Use to estimate formation water density from ppm NaCl and temperature. Copyright: Gearhart-Owens.

Use the procedure outlined in the table below to estimate formation water density at reservoir conditions using R_w. The approximate error is ±10% (after ).

1. Gather data: formation temperature (T_f ), water resistivity (R_w ), and formation pressure. Estimate pressure by multiplying depth by 0.433 psi/ft or other appropriate gradient. Check for T_f errors.
2. Estimate sodium chloride (NaCl) concentration from R using Figure 1.
3. Estimate density from wt % NaCl and temperature using Figure 2.

Gather data

To estimate formation water density, collect estimates of the following:

• Formation temperature
• Formation pressure
• Formation water resistivity

1. Estimate formation temperature T_f using the following formula:

   • ${\displaystyle {\mbox{T}}_{\rm {f}}={\mbox{T}}_{\rm {s}}+{\mbox{D}}_{\rm {f}}\left({\mbox{$BHT}}-{\frac {{\mbox{T}}_{\rm {s}}}{\mbox{TD}}}\right)}

     where:

     T_s = average surface temperature (°F)

     D_f = depth to the formation (ft)

     BHT = bottom-hole temperature (found on log header) (°F)

     TD = total depth (BHT and TD must be from the same log run) (ft)

2. Estimate formation pressure (psi) by multiplying 0.433 (freshwater gradient) by formation depth.
3. Obtain formation water resistivity R_w [ohm.m] in one of three ways:
   • From a sample of water from the formation of interest measured for R_w
   • Using a water catalog
   • Calculating it from an SP log

Determine NaCl concentration from r_w

The predominant solute in most formation water is sodium chloride (NaCl). Its concentration determines formation water density and R_w. When only R_w is available, we can use NaCl concentration to determine density.

Use Figure 1 to determine NaCl concentration. At the intersection of formation temperature (from Y-axis) and R_w (from X-axis), find the NaCl concentration (in ppm) by reading diagonal line labels and interpolating.

Estimate density

Estimate formation water density from ppm NaCl and temperature using the chart in Figure 2. The following table describes the procedure to use with the chart.

1. Enter the chart along the X-axis using formation temperature.
2. Proceed vertically to the appropriate salt concentration expected in the zone.
3. Proceed horizontally to read the liquid density at atmospheric pressure.
4. Using the “Effects of Pressure” segment of the chart, add a density increment to the above-computed density to correct for pressure effects.

See also

• Determining hydrostatic pressure gradient

External links

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bottom hole temperature

total depth