Fluid Saturations

Introduction

Fluid Saturations• Definition - The fluid saturation for a particular fluid is the

fraction of pore volume occupied by that fluid; H2O, oil or gas

• Fluid saturation = total volume of the fluid phase

pore volume

– Saturation is an intensive property

– Equation Form: So=Vo/Vp, Sw=Vw/Vp, Sg=Vg/Vp

• These fluid volumes are measured under specific conditions of pressure and temperature (e.g. reservoir, or laboratory)

– reservoir conditions are often noted as “in situ”

Fluid Saturations• Fundamental Relationships

– Pore volume is occupied by fluids (water, oil, and/or gas)

• for the two phase case, only one of the two saturations is independent, the other must make the sum of the saturations equal to unity (1)

• similarly, for the three phase case, only two saturations are independent

gowp

gow

gowp

SSSV

VVV1

VVVV

++=++

=

++=

Fluid Saturations• Fundamental Relationships (continued)

– Mass of fluids in the pore volume is comprised of: water, oil, and/or gas

• at laboratory conditions it is often assumed that gas density is negligible

[ ]ggoowwp

ggooww

gow

SρSρSρV

VρVρVρ

mmmmassfluid

++=++=

++=

Initial Fluid Saturations in Reservoir

• Concepts: typical petroleum accumulation scenario

– pores are initially saturated with water (Sw=1)

– hydrocarbons migrate up dip into traps due to having density less than water density (gravity force)

– hydrocarbons (oil and/or gas) is distributed such that gravity and capillary forces are in equilibrium

• minimum interstitial water saturation remains in hydrocarbon zone, even after accumulation occurs

– water wet, drainage accumulation process» irreducible wetting phase saturation

– oil wet, imbibition accumulation process

» residual non-wetting phase saturation

Initial Fluid Saturations in Reservoir

• Methods for determination of reservoir fluid saturations– Direct Measurement

• Core Analysis of samples obtained from the formation of interest in their original state and measure saturations directly is ideal.

– Indirect Measurement

• Capillary Pressure Measurement

• Well Log Analysis

– electrical conductivity depends primarily on water saturation

Fluid Saturations - Core Analysis

• Factors affecting fluid saturations in cores

– flushing of core by filtrate from drilling fluids (especially for overbalanced drilling)

• water filtrate– water based mud

– oil emulsion mud

• oil filtrate– oil based mud– inverted oil emulsion mud

• gas filtrate– air drilling

– foam drilling

Fluid Saturations - Core Analysis

• Factors affecting fluid saturations in cores (continued)– Changes in pressure and temperature as core sample is

brought from bottomhole conditions to surface conditions

– Example: Oil zone at minimum interstitial water saturation, water based drilling mud

Application of Core Saturations

• Application of Core Saturations: Water Based Mud– presence of oil zone– original oil/gas contact– original oil/water contact

• Application of Core Saturations: Oil Based Mud– fairly accurate minimum interstitial water saturation– original oil/water contact

• Other Applications of Core Saturations– correlation of indirect methods

Estimating Fluid Contact Depths from Core Saturations

So

Gas

Water

Oil

500

Depth

So ≈ 0 in gas zone

So > ≈0.15 in oil production zone

0 < So < ≈0.15 in water production zone

Maximum Water Saturationfor Oil and Gas Production

The trend shown here continues for even lower permeability, with productive reservoirs existing with Sw>0.60 for k<0.01 md

Commentary on Core Saturations

• Qualitative Value: “The saturation values obtained directly from rock samples [cores] are usually not reliable for determining the quantity of each fluid in the rock [reservoirs]. Other uses exist for fluid-saturation determinations from core samples.”

• Overall Value: “Thus, in summary, it is seen that although fluid-saturation determinations made on core samples at the surface may not give a direct indication of the saturations within the reservoir, they are of value and do yield very useful and necessary information”

Fluid Saturation

Connate water (Swc): water entrapped in the interstices of the rock (either sedimentary or extrusive igneous) at the time the rock was deposited.

Interstitial water: Water that occurs naturally within the pores of rock. Water from fluids introduced to a formation through drilling or other interference, such as mud and seawater, does not constitute interstitial water. Interstitial water, or formation water, might not have been the water present when the rock originally formed. In contrast, connate water is the water trapped in the pores of a rock during its formation, also called fossil water.

Fluid Saturation

• Irreducible water saturation (Swir): the fraction of pore volume occupied by water in a reservoir at maximum hydrocarbon saturation. In water-wet rock, it represents the layer of adsorbed water coating solid surfaces and the pendular grain contacts and at pore throats.

• The irreducible saturation of a fluid is the minimum saturation of that fluid attainable when that fluid is displaced from a porous medium by another fluid immiscible with the first.

• Residual oil (Sor): Oil remaining in the reservoir rock after the flushing or invasion process, or at the end of a specific recovery process or escape process.