Total Reservolr Compressibility

The total compressibility of oil- or gas-bearing reservoirs represents the combined compressibilities of oil, gas, water, and reservoir rock in terms of volumetric weighting of the phase saturations:

where c, is the total system isothermal compressibility in vol/vol/psi, c,, c,, c and ct are the compressibilities in psi-' of oil, water, gas, and rock (pore volumef respectively, S is fluid saturation, and the subscripts 0, w, and g refer to oil, water, and gas, respectively. Based on the treatment by Martin [72], Ramey [26] has expressed volumes in terms of formation volume factors with consideration for gas solubility effects:

where p is pressure in psi, Rs is the solubility of gas in oil in scf/STB oil, R, is the solubility of gas in water in scf/STB water, and Bg, Bo, and B, are the formation volume factors of gas, oil, and water, respectively. Fluid and rock compressibilities have been discussed in prior sections of this chapter. Table 5-12 provides a summary of these data.

olidated sandstone and limestone data from Newman [71] that have porosities in the range of 10% to 30%. Oil compressibility increases as a function of increasing API gravity, quantity of solution gas, or temperature [17]. As pointed out by Ramey [26], when the magnitude of water compressibility is important, the effect of solution gas in the water will be more important. Clearly, the magnitude of gas compressibility will dominate the total system compressibility if gas saturations are high. In many gas reservoirs, only the gas terms in Equation 5-64 may be significant so that the total system compressibility becomes.

In certain cases of high pressure and high water saturation, rock and water compressibility may be significant so that Equation 565 must be used [26]. In oil reservoirs, gas saturations may be low and, even though gas compressibility is much larger than the other compressibilities, each term in Equation 5-64 or 5-65 should normally be considered [26]. In some cases, not all of the compressibility terms will be important. For example, if reservoir pressure is above the saturation pressure, the gas saturation will be zero [20]. However, if the gas saturation exceeds 2% or 3%, the gas compressibility term dominates the total system compressibility and the other terms become insignificant.