Method for determining wettability index of rock from T
A method for rapid wettability evaluation during exploratory drilling of a hydrocarbon. The method include pre-saturation of the sample by a brine, measuring the bulk brine's T 2 NMR relaxation parameter, expelling the brine by oil in the sample, measuring the oil's bulk and pore T 2 NMR relaxation parameter, measuring the brine's non-reducible content and T 2 NMR relaxation parameter of water in the presence of dominant oil content, expelling the oil by the brine and measuring the T 2 NMR relaxation parameter of the irreducible oil content in the dominant brine. The measurements are combined in the index: I =[( T 2WB −T 2WIOIRR )/ T 2WB ]−[( T 2OB −T 2OIWIRR )/ T 2OB ], where WB is water bulk, OB is oil bulk, WIOIRR—injected pore water at the irreducible oil content, OIWIRR—injected pore oil at the irreducible water content.
1. A method of determining wettability of a porous rock, comprising:
a) providing an oil and a water-based brine fluid comprising one or more isotopes selected from the group consisting of 1 H, 2 H, 13 C and 14 C;
b) saturating the porous rock with the brine by pressurizing the porous rock with the brine at a pressure sufficient to overcome the capillary pressure in the pores of the porous rock and reach full saturation with the brine (S w =1);
c) measuring a T 2, WB signal of the brine fluid by NMR outside the porous rock;
d) displacing the brine fluid with an oil by pressurizing the porous rock with the oil at a pressure sufficient to overcome the capillary pressure in the pores of the porous rock and reach a final non-decreasing content of the brine fluid, wherein the irreducible amount corresponds to pore wall-bound water;
e) measuring a T 2,sWR signal in the porous rock saturated with the brine fluid;
f) measuring a T 2,OB signal in the oil outside the porous rock by 1 H or 13 C NMR;
g) saturating the porous rock with the brine until reaching an irreducible oil content;
h) measuring a T 2,SOR signal in the oil saturated porous rock, wherein the T 2, SOR signal corresponds to the brine fluid coexisting with the residual irreducible oil in the porous rock;
i) calculating a value
WI
w
=
T
2
WB
-
T
2
,
Sor
T
2
,
WB
representing the difference between the signal of the brine fluid outside the porous rock and the signal of the brine fluid inside the pores of the porous rock;
j) calculating the value
WI
o
=
T
2
,
OB
-
T
2
,
Swr
T
2
,
OB
representing the difference between the signal of the oil outside the pores of the porous rock and the signal of the oil inside the pores of the porous rock;
k) calculating the wettability index of the porous rock I NMR =WI w −WI o .
2. The method of claim 1 , further comprising, between (d) and (e), exposing the porous rock to a reduced pressure and an elevated temperature until the brine fluid evaporates to the level of <1% of a content in (d) to increase the oil-wettability of the porous rock.
3. The method of claim 1 , wherein the NMR measurement includes a T 2 estimated by Carr-Purcell-Meiboom-Gill (CPMG) sequence.
4. The method of claim 1 , wherein the NMR is measured on a plurality of porous rock samples obtained at different depths of a geological formation.
5. The method of claim 1 , further comprising one or more of:
measuring a relative displacement of the oil in the porous rock in response to the increase in saturating brine pressure;
measuring a relative change in conductance or resistivity of the oil in the porous rock in response to the increase in saturating brine pressure;
measuring a relative redistribution of the oil and the brine fluid in the pores of the porous rock at different diameter categories of the pores.
6. The method of claim 1 , further comprising:
setting a bench for a wettability-prediction.
7. The method of claim 1 , further comprising:
comparing at least two geological formations with a down hole NMR loggin tool and/or a downhole resistivity logging tool.
8. The method of claim 1 , further comprising:
modeling a wettability index W according to the expression:
A 1 ×[τ NMR B1 ]×[ R B2 ]×[Φ NMR B3 ]×(τ NMRZ ) B4 /(τ NMRXY ) B4 ×S w B5 ×S wir B6 ×(Δ H/H ) B7 ×[ D ] B8 =W
Where:
W—is dimensionless wettability index in the range (−1, 1),
A 1 —is an empirical proportionality factor,
[τ NMR B1 ]—is a diffusional tortuosity determined by NMR,
[R B2 ]—is a relative resistivity at a brine fluid saturation level,
[Φ NMR B3 ]—is an overall porosity determined by NMR,
[(τ NMRZ ) B4 /(τ NMRXY ) B4 ]—is a ratio of diffusional tortuosity in the direction Z to one measured in the plane XY,
S w B5 —is a water saturation fraction,
S wir B6 —is an irreducible water content,
(ΔH/H) B7 —is a fraction of the oil expelled from the porous rock at S w ,
(D) B8 —is a weight-averaged diameter of the pores;
wherein the values are represent water flooding conditions in an innermost 5 mm layer of a borehole wall in the geological formation.