fluid_properties · water properties
Water Compressibility (cw)
cw = 1 / (7.033·P + 541.5·S − 537.0·T + 403,300) [psi⁻¹, Osif 1988]
click formula to derive ↑
Inputs
psi
g/L NaCl
°F
Description
Computes isothermal water (brine) compressibility from the Osif (1988) correlation, fit to laboratory measurements of NaCl brine compressibility across a wide range of salinity, temperature, and pressure. Water compressibility is a required, if often small, term in total-system compressibility for material balance and well-test analysis.
Variables
| Symbol | Unit | Description |
|---|---|---|
| cw | μpsi⁻¹ | Water Compressibility |
| P | psi | Reservoir pressure at which compressibility is evaluated. |
| S | g/L NaCl | Formation water salinity, expressed as g/L NaCl-equivalent (≈ ppm ÷ 1,000 for the NaCl-equivalent ppm convention used elsewhere in this app). |
| T | °F | Reservoir temperature at which compressibility is evaluated. |
Assumptions
- Water is a NaCl brine — non-NaCl dissolved solids (e.g. CaCl2, KCl) are folded into the salinity value as NaCl-equivalent
- No free gas in solution beyond what Osif's dataset implicitly included (the paper's title includes 'gas' as one of the studied variables, but no separate gas-content input is exposed here — see limitations)
- Salinity input in g/L is a reasonable direct proxy for Osif's original units (his paper cites salinity in g/L NaCl-equivalent)
Limitations
- Osif's stated fit range is 1,000–20,000 psi, 200–270°F, and 0–200 g/L — outside this range (e.g. shallow, cooler reservoirs) the correlation extrapolates and should be used cautiously; the fittedRange warnings reflect exactly this envelope
- Does not expose a separate dissolved-gas-content input; Osif's original work examined gas saturation as a variable but this implementation follows the widely-reproduced pressure/salinity/temperature-only form
- As with any correlation, lab-measured cw is preferred for material-balance-critical work in unusual brine compositions
Use Cases
- → Total compressibility for well testing: Combine cw (weighted by water saturation) with oil and formation compressibility to build ct for pressure-transient analysis.
- → Waterflood material balance: Provide cw for material-balance calculations in reservoirs with active aquifer support or waterflood, where brine expansion contributes measurably to pressure maintenance.
- → Pore volume compressibility comparison: Compare water compressibility against pore_compressibility to judge which term dominates total system compressibility in a given reservoir.
Related Calculations
Region Notes
Permian Basin
Deep Wolfcamp brines (salinity 100–200 g/L, 4,000–8,000 psi, 180–240°F) typically give cw ≈ 2.5–3.2 μpsi⁻¹, near the low end of the typical brine range due to high salinity and pressure.
Global
Fresher, shallower, cooler brines (low salinity, low pressure) trend toward the higher end of the typical 2–5 μpsi⁻¹ window; this correlation's low-pressure extrapolation is less reliable since Osif's data starts at 1,000 psi.
References
Primary source
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