Method for characterizing the porosity of rock
The present invention provides a method for determining the porosity of rock from a digital image of the rock. A three-dimensional image of a rock is obtained and segmented, and an image porosity is determined from the segmented image of the rock. A porosity correction parameter is obtained from a non-wetting liquid capillary pressure curve derived from the segmented image, and the porosity correction parameter is applied to the image porosity to obtain a corrected porosity of the rock.
1. A method for estimating the porosity of rock, comprising:
obtaining a three-dimensional image of a rock wherein the image is comprised of a plurality of voxels and the image has a resolution;
processing the three-dimensional image of the rock to segment the image by selecting each voxel of the image to represent either pore space in the rock or solid material in the rock;
determining an image porosity of the rock from the segmented three-dimensional image of the rock;
deriving a non-wetting liquid capillary pressure curve from the segmented three-dimensional image of the rock;
determining a pore geometric factor from the non-wetting liquid capillary pressure curve;
determining a pore throat resolution factor from the non-wetting liquid capillary pressure curve and the resolution of the image;
utilizing the pore geometric factor and the pore throat resolution parameter to determine a porosity correction parameter;
applying the porosity correction parameter to the image porosity of the rock to obtain a corrected porosity of the rock.
2. The method of claim 1 , wherein the three-dimensional image of the rock is obtained by x-ray computer tomography.
3. The method of claim 1 , wherein the rock is obtained from a hydrocarbon-bearing formation and is comprised of sandstone.
4. The method of claim 1 , wherein the rock is obtained from a hydrocarbon-bearing formation and is comprised of carbonate.
5. The method of claim 1 , wherein the rock is obtained from a hydrocarbon-bearing formation and is comprised of shale.
6. The method of claim 1 , wherein the non-wetting liquid is mercury.
7. The method of claim 1 , wherein the non-wetting liquid capillary pressure curve is derived from the segmented three-dimensional image of the rock at pressures of up to an image-limited pressure, where the image-limited pressure is the minimum pressure that can be applied on the non-wetting liquid to overcome the capillary pressure of the narrowest pore throat distinguishable from the segmented three-dimensional image of the rock.
8. The method of claim 1 , wherein the pore geometric factor is determined from the non-wetting liquid capillary pressure curve by plotting a best fit curve to the non-wetting liquid capillary pressure curve and determining the pore geometric factor from the shape of the curve.
9. The method of claim 1 , wherein the pore throat resolution factor is determined by determining a pore throat size of pores entered by the non-wetting liquid at an entry pressure from the non-wetting liquid capillary pressure curve and calculating a ratio between the pore throat size of pores entered by the non-wetting liquid at the entry pressure and the resolution of the image.