Page 86 - Introduction to Petroleum Engineering

70 PROPERTIES OF RESERVOIR ROCK
reduction in producing formations. Starting in the mid‐1960s, oil producers were
required to inject about 1.05 barrels of water into the Wilmington Field for every
barrel of oil and water produced. The water injection stopped surface subsidence.
Most of the lost surface elevation has since been recovered.
Subsidence is a common problem in oil and gas operations, but it is usually much
less than the subsidence seen in the Long Beach area. The amount of subsidence
depends on rock strength and thickness of the producing formations. The strength of
the rock can be measured in terms of pore volume compressibility

1 ∆ V
c = p (4.4)
f
V ∆ p
p
where V is the pore volume and p is the pressure in the pore volume. Equation 4.4
p
can be rewritten as
∆
∆V = c Vp (4.5)
f
p
p
Pore volume compressibility is positive because the pore volume increases as
pressure in the pore volume increases. Alternatively, as pressure in the pore volume
decreases, the pore volume also decreases. We see from Equation 4.3 that a decrease
in pore volume leads to a corresponding decrease in bulk volume. Typically, pore
volume compressibility c , which is also referred to as formation compressibility,
f
6 –
6 –
×
×
ranges from about 10 10 psi to 6010 psi .
–1
–1
4.1.2 Saturation
Pore spaces in reservoir rock are occupied by fluid phases, including oil, water, and
gas phases. The fraction of the pore volume that is occupied by any phase is called
the saturation of that phase. For example, S , S , and S are the saturations of oil,
g
o
w
water, and gas phases, respectively. Saturation can be expressed as a percentage or as
a fraction. For a reservoir with oil, water, and gas phases, the sum of the saturations
must satisfy the constraint
1 = S o + S w + S g (4.6)

since the sum of the pore space occupied by each phase must equal the total pore
space. If just oil and water are present in a reservoir, then gas saturation S = 0 and
g
1 = S o + S w (4.7)

Example 4.2 Core Saturation
A sandstone core is completely saturated with 2.95 ml water. Oil is injected
into the core and 2.10 ml of water is collected from the core. What are the oil
and water saturations in the core?