Mechanical and hydraulic properties of carbonate rock:The critical role of porosity

Carbonate rocks are extensively used in civil infrastructure and play a critical role in geoenergy geoengineering,either as hydrocarbon reservoirs or potential repositories for CO_(2)geological storage.Carbonate genesis and diagenetic overprint determine the properties of carbonate rocks.This study combines recent data gathered from Madison Limestone and an extensive dataset compiled from published sources to analyze the hydraulic and mechanical properties of limestone carbonate rocks.Physical models and data analyses recognize the inherently granular genesis of carbonate rocks and explain the strong dependency of physical properties on porosity.The asymptotically-correct power model in terms of(1-Ф/Ф*)a is a good approximation to global trends of unconfined stiffness E and unconfined compressive strength UCS,cohesive intercept in Mohr-Coulomb failure envelopes,and the brittle-toductile transition stress.This power model is the analytical solution for the mechanical properties of percolating granular structures.We adopted a limiting granular porosityФ*=0.5 for all models,which was consistent with the loosest packing of monosize spheres.The fitted power model has exponent(α=2)in agreement with percolation theory and highlights the sensitivity of mechanical properties to porosity.Data and models confirm a porosity-independent ratio between unconfined stiffness and strength,and the ratio follows a log-normal distribution with mean(E/UCS)≈300.The high angle of internal shear strength measured for carbonate rocks reflects delayed contact failure with increased confinement,and it is not sensitive to porosity.Permeability spans more than six orders of magnitude.Grain size controls pore size and determines the reference permeability k^(*)at the limiting porosityФ*=0.5.For a given grain size from fine to coarse-grained dominant carbonates,permeability is very sensitive to changes in porosity,suggesting preferential changes in the internal pore network during compaction.

Influence of strong preformed particle gels on low permeable formations in mature reservoirs

In mature reservoirs,the success of preformed particle gel（PPG） treatment rests primarily on the ability of the PPG to reduce and/or plug the high permeable formations,but not damage the low permeable formations.Static test models（filtration test model and pressure test model）were used to determine the effect of PPG on low permeable formations.This work used a strong preformed particle gel,Daqing（DQ） gel made by a Chinese company.The particle gel sizes were ranged from 30 to 120 mesh for this work.PPGs are sized in a millimeter or micrometer,which can absorb over a hundred times their weight in liquids.The gel strength was approximately 6500 Pa for a completely swollen PPG with 1 %（weight percentage） NaCl solution（brine）.0.05 %,1 %,and 10 % NaCl solutions were used in experiments.Sandstone core permeability was measured before and after PPG treatments.The relationship between cumulative filtration volumes versus filtration times was determined.The results indicate that DQ gels of a particle size of 30–80 mesh did not damage the cores of a low permeability of 3–25 m D.The DQ gels of a smaller particle size ranging from 100 to 120 mesh damaged the core and a cake was formed on the core surface.The results also indicate that more damage occurred when a high load pressure（400 psi） was applied on the high permeability cores（290–310 m D）.The penetration of the particle gelsinto the low permeable formations can be decreased by the best selection of gel types,particle sizes,and brine concentrations.