Method for evaluating properties of a fluid medium

1. A method for determining the fluid characteristics in a well, wherein said method comprises:

/a/ receiving a mass ratio by Downhole Fluid Analysis (DFA) measurements, each mass ratio being associated with a set of hydrocarbon cuts, and receiving a molecular ratio, said molecular ratio being associated with a set of hydrocarbon cuts;

/b/ converting received mass ratios into molecular ratios based on predetermined molecular weights, each predetermined molecular weights being associated with an element in the sets of hydrocarbon cuts;

/c/ normalizing converted molecular ratios with the received molecular ratio;

/d/ determining parameters (α,β) of a sequence defined by

x

n

=

α

⁡

(

1

-

β

n

)

⁢

x

n

-

1

,

 each normalized molecular ratio being associated with a member of the sequence or a sum of members of the sequence, at least a difference between said normalized molecular ratio and the associated member or the associated sum of members being minimized;

/e/ computing an estimated molecular weight (Mw 6+ ) function of

(

k

+

1

1

-

α

)

⁢

Δ

⁢

⁢

M

,

 where k is an integer and ΔM is a predetermined value representing a molecular weight increment between two hydrocarbon cuts;

/f/ if a stabilization criteria is met, the steps /b/ to /f/ are iterated with the estimated molecular weight as one of the predetermined molecular weights in step /b/;

/g/ outputting the values of the parameters determined in step /d/ to predict the concentration of hydrocarbon cuts of a rank n, n being strictly higher than 6, and

h/ quantify properties for the hydrocarbon cuts heavier than 6 to determine at least one in situ fluid characteristic chosen among API gravity, downhole density, gas oil ratio and formation volume factor.

2. A method according to claim 1 , wherein the estimated molecular weight (Mw 6+ ) represents hydrocarbon cuts having more than k carbons.

3. A method according to claim 1 , wherein the stabilization criteria of step /f/ comprises at least one following condition:

an absolute difference of a value of α between two iterations of steps /b/-/f/ is lower than a predetermined threshold;

an absolute difference of a value of the estimated molecular weight between two iterations of steps /b/-/f/ is lower than a predetermined threshold;

a number of iteration of steps /b/-/f/ exceeds a predetermined value.

4. A method according to claim 1 , wherein ΔM is initially set to a value between 12 and 14.

5. A method according to claim 1 , wherein ΔM is initially set a mean value for various fluids compositions examined in laboratory conditions.

6. A non-transitory computer readable storage medium, having stored thereon a computer program comprising program instructions, the computer program being loadable into a data-processing unit and adapted to cause the data-processing unit to carry out the steps of claim 1 when the computer program is run by the data-processing unit.

7. A method for determining a fluid characteristic for a plurality of elevation values (z 1 , z 2 , z 3 ) in a well, wherein said method comprises:

/i/ determining a plurality of values Δ by executing for each of the elevation values (z 1 , z 2 , z 3 ) the steps of:

/a/ receiving mass ratios by Downhole Fluid Analysis (DFA) measurements, each mass ratio being associated with a set of hydrocarbon cuts, and receiving a molecular ratio, said molecular ratio being associated with a set of hydrocarbon cuts;

/b/ converting received mass ratios into molecular ratios based on predetermined molecular weights, each predetermined molecular weights being associated an element in the sets of hydrocarbon cuts;

/c/ normalizing converted molecular ratios with the received molecular ratio;

/d/ determining parameters (α,β) of a sequence defined by x n =α(1−βnxn−1, each normalized molecular ratio being associated with a member of the sequence or a sum of members of the sequence, at least a difference between said normalized molecular ratio and the associated member or the associated sum of members being minimized;

/e/ computing an estimated molecular weight (Mw 6+ ) function of

(

k

+

1

1

-

α

)

⁢

Δ

⁢

⁢

M

,

 where k being an integer and ΔM being a predetermined value representing a molecular weight increment between two hydrocarbon cuts;

/f/ if a stabilization criteria is met, the steps /b/ to /f/ are iterated with the estimated molecular weight as one of the predetermined molecular weights in step /b/;

/g/ outputting the values of the parameters determined in step /d/ to predict the concentration of a hydrocarbon cuts of rank n, n being strictly higher than 6, and

/h/ quantify properties for the hydrocarbon cuts heavier than 6 to determine at least one in situ fluid characteristic chosen among API gravity, downhole density, gas oil ratio and formation volume factor;

/ii/ determining a mean molecular weight increment value by computing a plurality of values

RT

g

⁡

(

z

i

-

z

j

)

⁢

ln

⁡

(

α

⁡

(

z

i

)

α

⁡

(

z

j

)

)

,

 z i ) being an elevation value in the plurality of elevation values and α(z i ) being the determined value α in step /i/ for the elevation value z i , z j being an elevation value in the plurality of elevation values different from z i and α(z j ) being the determined value α in step /i/ for the elevation value z j ;

/iii/ if a stabilization criteria is met, the steps /i/ to /iii/ are iterated with the mean molecular weight increment value as ΔM in step /d/;

/iv/ outputting the values of the parameters determined in step /i/ and the mean molecular weight increment value determined in step /ii/.

8. A method according to claim 7 , wherein the stabilization criteria of step /iii/comprises at least one following condition:

an absolute difference of a value of the mean molecular weight increment value between two iterations of steps /i/-/iii/ is lower than a predetermined threshold;

a number of iteration of steps /i/-/iii/ exceeds a predetermined value.