Methods and systems for determining gas permeability of a subsurface formation

Patent metadata

Field	Value
Publication number	US-10416064-B2
Application number	US-201916381310-A
Country	US
Kind code	B2
Filing date	Apr 11, 2019
Priority date	Dec 14, 2015
Publication date	Sep 17, 2019
Grant date	Sep 17, 2019

Abstract

Methods and systems disclosed here include conducting two pressure-dependent permeability tests having the same range of effective stress but two different values of pore pressure. For the test with the higher pore pressure, the permeability is only impacted by the mechanical deformation of the rock, while for the one with lower pore pressure the permeability is impacted by both mechanical deformation of the rock and the Knudsen diffusion. By using the same range of effective stress, the contribution from the mechanical deformation of the rock should be the same. Therefore, by subtracting the permeability with higher pore pressure from the one with lower pore pressure, the impact of Knudsen diffusion and the mechanical deformation of the rock can be determined.

First claim

The invention claimed is:

1. A transient flow method for determining gas permeability of a subsurface formation, comprising:
acquiring a sample of the subsurface formation;
positioning the sample in a pressure vessel comprising a fluid and a pressure gauge and applying a constant confining pressure, P c (1);
running a first test by equilibrating the sample at a predetermined first pore gas pressure, p i ;
applying a predetermined constant second gas pressure, p o , to an inlet of the sample, the second gas pressure being greater than the first pore gas pressure;
measuring a third pore gas pressure, p, as a function of time, t, at a plurality of locations along the axis of the sample in the pressure vessel;
in a computer, determining a gas density or total gas mass per unit volume of the subsurface formation, m; and
in the computer, determining the gas permeability of the subsurface formation as a function of pore pressure, k(p), based at least in part on the first pore gas pressure, the second gas pressure, the third pore gas pressure as a function of time, and the gas density as a function of pore pressure.

2. The method according to claim 1 , further comprising:
determining a transport parameter of the subsurface formation, D(p), using a first formula:

D
⁡

(
p
)

=

-

∫

p
i

p

⁢

λ
2

⁢

d
⁢

⁢
m

dp

⁢
dp

dp

d
⁢

⁢
λ

where λ is an independent variable calculated using the formula k=xt −1/2 ; and
determining gas permeability k of the subsurface formation from D(p) using

D
⁡

(
p
)

=

k
⁢

⁢
ρ

μ

where μ stands for gas viscosity, and ρ for gas density.

3. The method according to claim 2 , further comprising:
determining the total gas mass per unit volume of the subsurface formation, m, using a second formula:
m =ϕρ+(1−ϕ)ρ a

where ϕ is porosity of the subsurface formation, ρ is gas density of the gas, and ρ a is adsorbed gas mass per unit volume of the subsurface formation.

4. The method according to claim 3 , further comprising:
determining the porosity ϕ of the subsurface formation using a third formula:

ϕ
=

B
-

A
⁢

∫

p
i

p
0

⁢

λ
⁢

⁢

d
⁢

⁢

ρ
a

dp

⁢
dp

A
⁢

∫

p
i

p
0

⁢

λ
⁢

⁢

d
⁡

(

ρ
-

ρ
a

)

dp

⁢
dp

where A is a cross-sectional area of the sample, and B is a slope of a curve of the cumulative gas flow into the sample at x=0 versus t 1/2 .

5. The method according to claim 4 , further comprising:
determining the slope of the curve, B, using a fourth formula:

B
=

A
⁢

∫

p
i

p
0

⁢

λ
⁢

⁢

d
⁢

⁢
m

Assignees

• Saudi Arabian Oil Co

Inventors

• Chen Huangye
• Liu Hui-Hai
• Zhang Jilin Jay

Classifications

• E21B49/08 Primary

  Obtaining fluid samples or testing fluids, in boreholes or wells · CPC title

• G01N15/0826 Primary

  and measuring fluid flow rate, i.e. permeation rate or pressure change · CPC title

• E21B49/02

  by mechanically taking samples of the soil · CPC title