Influjo De Agua

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WATER INFLUX

Nearly all hydrocarbon reservoirs are surrounded by water-bearing
rocks called aquifers. These aquifers may be substantially larger than the
oil or gas reservoirs they adjoin as to appear infinite in size, or they may
be so small in size as to be negligible in their effect on reservoir performance.
As reservoir fluids are producedand reservoir pressure declines, a
pressure differential develops from the surrounding aquifer into the reservoir. Following the basic law of fluid flow in porous media, the aquifer
reacts by encroaching across the original hydrocarbon-water contact. In
some cases, water encroachment occurs due to hydrodynamic conditions
and recharge of the formation by surface waters at an outcrop.
In manycases, the pore volume of the aquifer is not significantly larger than the pore volume of the reservoir itself. Thus, the expansion of the
water in the aquifer is negligible relative to the overall energy system,
and the reservoir behaves volumetrically. In this case, the effects of water
influx can be ignored. In other cases, the aquifer permeability may be
sufficiently low such that a verylarge pressure differential is required
before an appreciable amount of water can encroach into the reservoir. In
this instance, the effects of water influx can be ignored as well.
This chapter focuses on those those reservoir-aquifer systems in which
the size of the aquifer is large enough and the permeability of the rock is
high enough that water influx occurs as the reservoir is depleted.This
chapter also provides various water influx calculation models and a
detailed description of the computational steps involved in applying
these models.

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Water Influx

CLASSIFICATION OF AQUIFERS
Many gas and oil reservoirs produced by a mechanism termed water
drive. Often this is called natural water drive to distinguish it from artificial water drive that involves theinjection of water into the formation.
Hydrocarbon production from the reservoir and the subsequent pressure
drop prompt a response from the aquifer to offset the pressure decline.
This response comes in a form of water influx, commonly called water
encroachment, which is attributed to:
• Expansion of the water in the aquifer
• Compressibility of the aquifer rock
• Artesian flow where thewater-bearing formation outcrop is located
structurally higher than the pay zone
Reservoir-aquifer systems are commonly classified on the basis of:
• Degree of pressure maintenance
• Outer boundary conditions

• Flow regimes
• Flow geometries

Degree of Pressure Maintenance

Based on the degree of the reservoir pressure maintenance provided by
the aquifer, the natural water drive is oftenqualitatively described as:
• Active water drive
• Partial water drive
• Limited water drive
The term active water drive refers to the water encroachment mechanism in which the rate of water influx equals the reservoir total production rate. Active water-drive reservoirs are typically characterized by a
gradual and slow reservoir pressure decline. If, during any long period,
the productionrate and reservoir pressure remain reasonably constant,
the reservoir voidage rate must be equal to the water influx rate.
water inf lux  = oil flow +  free gas  + water production 
rate
rate






  rate  flow rate 

or
ew = Qo Bo + Qg Bg + Qw Bw

(10-1)

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Reservoir Engineering Handbook

where ew
Qo
Bo
Qg
Bg
Qw
Bw

= water influx rate,bbl/day
= oil flow rate, STB/day
= oil formation volume factor, bbl/STB
= free gas flow rate, scf/day
= gas formation volume factor, bbl/scf
= water flow rate, STB/day
= water formation volume factor, bbl/STB

Equation 10-1 can be equivalently expressed in terms of cumulative
production by introducing the following derivative terms:
ew =

dWp
dN p
dN p
dWe
= Bo
+ (GOR – R s )
Bg +...