Porosity and Permeability Changes in Sandstones and Carbonates Under Stress and Their Correlation to Rock Texture

摘要

The porosity and permeability of carbonate and sandstone cores were measured using a CMS-300 permeameter under a variety of stresses.Stress-permeability correlations showed significant variations with rock type.Porosity decay was similar between the rock types.The difference in the stress-permeability behaviors is explained by the difference in the pore structure.Permeability loss during reservoir depressurization has important consequences on production.Laboratory quantification of stress-dependent permeability conducted in this study provided useful data for reservoir models.In addition,the correlation between stress-permeability behavior and rock type is helpful in establishing a bridge between microscopic structure and geomechanics.In sandstone cores,the permeability decayed by 10-20% with increasing stress while the porosity decreased by about 3%,similar to what is expected from a Carman-Kozeny-type correlation.In some carbonate cores tested,the permeability was altered by an order of magnitude with the same porosity reduction.Cores with artificial fractures behaved similarly to un-fractured carbonates.These results indicate that the porosity in carbonates can be divided into a conductive portion that controls the permeability through the Carman-Kozeny equation,and a non-conductive(vuggy)portion that has limited effect on fluid flow,similar to the division between fracture porosity and matrix porosity in fractured rocks.The porosity-permeability relationship of carbonates was used to separate the conductive and non-conductive(vuggy)porosities.The vugginess of carbonate samples were projected to be 25-90% of the total effective porosity.Microscopic imaging shows favorable initial agreement with model projections.Reservoir rock properties are not static when they are being produced.Instead,pores change their shapes and sizes with varying stress conditions.Understanding those changes are valuable in their own right but this geomechanical response also indicates important features of the rock.This research not only characterized the stress-permeability behaviors of sandstone and carbonates,but also successfully correlated them to their characteristic texture.The method of characterization of vugginess through a geomechanical test is new,and this principle can be applied to fractured rock samples to find not only the compressibility of rock but also the fracture and matrix porosities.