Evaluation of carbonate pore system under texture control for prediction of microporosity aspect ratio and shear wave velocity

• Estimative of the macro-mesopore, and microporosity aspect ratio of carbonate rocks
• Prediction of the S-wave velocity using microporosity aspect ratio
• Relative pore volume reduction (RPVR) estimative and effects on elastic properties
• Estimative of carbonate properties under texture control and effective pressure
• Polynomial curves are fitted between elastic velocities and porosity.

This work evaluates a suite of carbonate rocks from Albian age in the Campos Basin — Brazil, complemented by data from the literature, totaling 472 samples with detailed description of diagenetic features, quantitative mineralogy analyses, and P- and S-wave velocities (Vp and Vs) measured at three ranges of effective pressure loading: low (5–7.5 MPa), moderate (20 MPa) and high (40–50 MPa) values. Digital image analysis (DIA) was applied on microtomography (μCT) images to quantitatively describe the macro-mesopore system of the Albian carbonates, and was extended to characterize different textures from literature data to estimate reference values for carbonates. The methodology utilized to predict the aspect ratio of microporosity assumes three pore-space scales in two representative scenarios: 1) measured macro-mesopore aspect ratio from DIA, and 2) predicted microporosity aspect ratio, using Vp measurement as the main input parameter. The differential effective medium model (DEMM) is combined with analytical theories of data analysis to characterize microporosity. Shear modulus and microporosity aspect ratio calibrated by this methodology were used to predict Vs, which was compared to experimental data, resulting in a good match for all samples. Polynomial curves are fitted with a variety of carbonate textures by velocities at effective pressure and bulk porosity crossplots, establishing important relationships for velocity prediction. The effects of effective pressure on the pore system within dry plugs of Albian samples were evaluated by combining triaxial measurements at 0–10 MPa, relative pore volume reduction (RPVR) and microporosity aspect ratio prediction. According to the results, micropores that exhibit low aspect ratio tend to close with stress and cause an increase on Vp and Vs. A wide textural heterogeneity of data base and different digital image analysis and resolutions were employed successfully, combining rock physics methodologies and concepts to characterize carbonate pore system as microporosity and pressure effects.