Method and apparatus for water-based mud filtrate contamination monitoring in real time downhole water sampling

1. A method for contamination monitoring, comprising: measuring a water based mud filtrate density and a water based mud filtrate resistivity at surface conditions;

converting via a downhole apparatus the measured water based mud filtrate density and resistivity to downhole conditions;

logging via the downhole apparatus properties of a downhole fluid to find at least a resistivity and a density of a fluid sample at the downhole conditions;

establishing via the downhole apparatus a linear relationship between a water based mud filtrate conductivity at downhole conditions and the water based mud filtrate density at the downhole conditions;

determining via the downhole apparatus an existence of a high quality pressure gradient at the downhole conditions;

fitting via the downhole apparatus at least one of a density and a resistivity data for native formation water using a power function and obtaining at least one of a density and a resistivity for native formation water when no high quality pressure gradient exists;

estimating via the downhole apparatus the density for native formation water from the pressure from the high quality gradient when the high quality pressure gradient exists;

estimating via the downhole apparatus the resistivity for native formation water, using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density;

estimating via the downhole apparatus a density for water based mud filtrate using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density; and

estimating via the downhole apparatus a water based mud filtrate contamination based on the water based mud filtrate density.

2. The method according to claim 1 , wherein the estimating the resistivity for native formation water is performed using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density.

3. The method according to claim 1 , wherein the estimating the resistivity for native formation water is performed using Archie's expression

R

w

⁢

⁢

f

=

1

a

⁢

φ

m

⁢

S

w

n

⁢

R

t

≈

1

.8

⁢

φ

2

1

⁢

R

⁢

t

where

R wf represents resistivity of native formation water;

a represents a constant;

φ represents a formation porosity;

m represents a cementation factor;

S w represents water saturation;

n represents a saturation exponent;

R t represents a formation resistivity.

4. The method according to claim 1 , further comprising:

estimating a time required for sampling to reach a water based mud filtrate contamination level.

5. The method according to claim 1 , wherein the estimating the water based mud filtrate contamination is performed through an equation:

η

=

1

R

n

-

1

R

fw

n

1

R

wbm

n

-

1

R

fw

n

=

ρ

fw

-

ρ

ρ

fw

-

ρ

wbm

where

η represents the water based mud filtrate contamination;

R represents the resistivity of the fluid sample;

R fw represents the resistivity of native formation water;

R wbm represents the water based mud filtrate resistivity;

n represents an adjustable parameter;

ρ fw represents the density of native formation water;

ρ represents the density of the fluid sample;

ρ wbm represents the water based mud filtrate density.

6. The method according to claim 1 , wherein the determining the existence of the high quality pressure gradient at the downhole conditions is performed through a downhole pretest.

7. A method for contamination monitoring, comprising:

measuring via a downhole apparatus water based mud filtrate density and resistivity at downhole conditions;

logging via the downhole apparatus properties of a downhole fluid to find at least a resistivity and a density of a fluid sample at the downhole conditions;

establishing via the downhole apparatus a linear relationship between a water based mud filtrate conductivity at downhole conditions and the water based mud filtrate density at the downhole conditions;

determining via the downhole apparatus an existence of a high quality pressure gradient at the downhole conditions;

fitting via the downhole apparatus at least one of a density and a resistivity data for native formation water using a power function and obtaining at least one of a density and a resistivity for native formation water when no high quality pressure gradient exists;

estimating via the downhole apparatus the density for native formation water from the pressure from the high quality gradient when the high quality pressure gradient exists;

estimating via the downhole apparatus the resistivity for native formation water, using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density;

estimating via the downhole apparatus a density for water based mud filtrate using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density; and

estimating via the downhole apparatus a water based mud filtrate contamination based on the water based mud filtrate density.

8. The method according to claim 7 , wherein the estimating the resistivity for native formation water is performed using the linear relationship between the water based mud filtrate conductivity and the water based mud filtrate density.

9. The method according to claim 7 , wherein the estimating the resistivity for native formation water is performed using Archie's expression

R

w

⁢

⁢

f

=

1

a

⁢

φ

m

⁢

S

w

n

⁢

R

t

≈

1

.8

⁢

φ

2

1

⁢

R

⁢

t

where

R wf represents resistivity of native formation water;

a represents a constant;

φ represents a formation porosity;

m represents a cementation factor;

S w represents water saturation;

n represents a saturation exponent;

R t represents a formation resistivity.

10. The method according to claim 7 , further comprising:

estimating a time required for sampling to reach a water based mud filtrate contamination level.

11. The method according to claim 7 , wherein the estimating the water based mud filtrate contamination is performed through an equation:

η

=

1

R

n

-

1

R

fw

n

1

R

wbm

n

-

1

R

fw

n

=

ρ

fw

-

ρ

ρ

fw

-

ρ

wbm

where

η represents the water based mud filtrate contamination;

R represents the resistivity of the fluid sample;

R fw represents the resistivity of native formation water;

R wbm represents the water based mud filtrate resistivity;

n represents an adjustable parameter;

ρ fw represents the density of native formation water;

ρ represents the density of the fluid sample;

ρ wbm represents the water based mud filtrate density.

12. The method according to claim 7 , wherein the determining the existence of the high quality pressure gradient at the downhole conditions is performed through a downhole pretest.