Digital rock analysis systems and methods with reliable multiphase permeability determination

1. A multiphase permeability determination method performed by one or more computers, the method comprising:

obtaining a three-dimensional pore and matrix model of a rock formation sample based on digital imaging of the sample using a scanning microscope;

determining an axis of flow passing through said sample, wherein the determination is based on the orientation of the sample, formation pressure gradiens, or user specifications;

verifying that the dimension of the model along said axis exceeds that of a representative elementary volume (REV);

selecting a direction of the flow;

extending said model by mirroring if pore statistics at a given saturation are mismatched for different percolating phases;

increasing a resolution of digital imaging of the sample and repeating said obtaining, determining, verifying, selecting, extending, and increasing, if a smallest non-percolating sphere dimension is below a predetermined threshold;

measuring relative permeability of said sample using the model; and

providing the relative permeability measurements to a user interface for display to a user.

2. The method of claim 1 , further comprising: determining absolute permeability along each axis before said determining the axis of flow.

3. The method of claim 1 , wherein said selecting a direction of the flow includes:

examining opposing faces perpendicular to the axis of flow to eliminate any face having large pores near a boundary; and

if two faces remain, selecting the face having better pore homogeniety as a preferred inlet face.

4. The method of claim 3 , further comprising:

determining a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and

verifying that a strong directional dependence does not exist.

5. The method of claim 1 , wherein said selecting a direction of the flow includes:

determining a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and

verifying that a strong directional dependence does not exist.

6. The method of claim 1 , wherein the predetermined threshold is a diameter of 3 voxels.

7. The method of claim 1 , wherein said obtaining includes:

scanning a physical rock sample using the scanning microscope to obtain a three-dimensional digital image; and

deriving the pore and matrix model from the three-dimensional image.

8. The method of claim 1 , wherein said pore statistics include a distribution of standard deviation of porosity.

9. The method of claim 8 , wherein said pore statistics further include a distribution of standard deviation of pore surface to volume ratio.

10. A multiphase permeability determination system that comprises:

a user interface;

a memory having software; and

one or more processors coupled to the memory to execute the software, the software causing the one or more processors to:

obtain a three-dimensional pore and matrix model of a rock formation sample based on digital imaging of the sample using a scanning microscope;

determine an axis of flow passing through said sample, wherein the determination is based on the orientation of the sample, formation pressure gradiens, or user specifications;

verify that the dimension of the model along said axis exceeds that of a representative elementary volume (REV);

select a direction of the flow;

extend said model by mirroring if pore statistics at a given saturation are mismatched for different percolating phases;

increase the resolution of digital imaging of the sample and repeat said obtain, determine, verify, select, extend, and increase steps if a smallest non-percolating sphere dimension is below a predetermined threshold;

measure relative permeability of said sample using the model; and

provide the relative permeability measurements to the user interface for display to a user.

11. The system of claim 10 , wherein the software further causes the one or more processors to determine absolute permeability along each axis before said determining the axis of flow.

12. The system of claim 10 , wherein as part of selecting a direction of the flow, the software causes the one or more processors to:

examine opposing faces perpendicular to the axis of flow to eliminate any face having large pores near a boundary; and

if two faces remain, select the face having better pore homogeniety as a preferred inlet face.

13. The system of claim 12 , wherein the software further causes the one or more processors to:

determine a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and

verify that a strong directional dependence does not exist.

14. The system of claim 10 , wherein said as part of selecting a direction of the flow, the software causes the one or more processors to:

determine a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and

verify that a strong directional dependence does not exist.

15. The system of claim 10 , wherein the predetermined threshold is a diameter of 3 voxels.

16. The system of claim 10 , wherein as part of obtaining a three-dimensional pore and matrix model, the software causes the one or more processors to:

retrieve a three-dimensional image of a physical rock sample generated using the scanning microscope; and

derive the pore and matrix model from the three-dimensional image.

17. The system of claim 10 , wherein said pore statistics include a distribution of standard deviation of porosity.

18. The system of claim 17 , wherein said pore statistics further include a distribution of standard deviation of pore surface to volume ratio.