Page 65 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Basic Principles, Definitions, and Data 53

Table 5-12
Summary of Compressibility Values
Compressiblilty, psi-$
Range -pica1 value
Consolidated rock" 2x 10"to 7x 109 3x106
Oil [17, 731 5 x 109 to 100 x 10-9 10 x 10-6
Water (gas-free) [26] 2xlO"to 4x109 3xlO-B
Compressibility, psi-'
At 1,000 psi A1 5,000 psi
Gas [26] 1,000 x 10-6 100 x 10"
Water (with dissolved gas) [26] 15 x 109 5 x 10"
* See Figure 5-35 (for most of samples having porosities of 20 f 10% in Figures 5-35a and 5-35b).

The rock compressibilities in Table 5-12 represent a majority of the con-
solidated 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 [26]:

(5-66)
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 com-
pressibility 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 [20].

Resistivity Index
Since crude oil and natural gas are nonconductors of electricity, their presence
in reservoir rock increases resistivity. The resistivity index or ratio, I, is com-
monly used to characterize reservoir rocks that are partially saturated with water
and also contain oil and/or gas:

(5-6'7)