Acoustic Velocity Measurements during Fluid-induced Pore Geometry Alterations in Carbonate Rocks
Description
Overview: Different rock samples were selected for precipitation (cementation) and dissolution experiments based on initial porosity and permeability. Loose Bahamian ooid sand (35–37% porosity, 300–400 µm grains) was used for precipitation, washed to remove organics and packed to prevent grain loss. For dissolution, well-cemented, permeable carbonates—Pleistocene Bahamian grainstone and Cretaceous Italian rudist grainstone (18–24% porosity)—were chosen to observe porosity increases and velocity declines. The experimental study investigates the effects of calcite cement precipitation and dissolution on rock acoustic properties and pore structure. A modified NER Autolab1000 system integrates a continuous-flow filtration setup to percolate fluids (supersaturated or undersaturated with CaCO₃) through rock samples while monitoring fluid chemistry and acoustic velocity. Key components include a peristaltic pump (50 mL/min flow rate), sensors for pH and temperature, and automated fluid adjustments to maintain constant geochemical conditions. The Autolab1000 measures acoustic velocities (P- and S-waves), electrical resistivity, and permeability under controlled pressures. Acoustic Velocity Measurement: Samples are dried (60°C for 72 hours), equilibrated to ambient conditions, and subjected to ultrasonic testing. A 1 MHz transducer propagates waves along the core axis, with arrival times determined by a 3% amplitude threshold. Velocities are calculated from travel time and sample length, achieving ~3% precision. Measurements are performed under 10 MPa confining pressure, with pore pressure regulated by flow rate. Samples remain in the pressure chamber throughout experiments to enable repeated velocity assessments. Cementation (Precipitation) Setup: In open-system flow-through experiments, calcium carbonate precipitation is induced by circulating a solution (24 mg Ca, 36 mg CO₃ per liter, pH 7.7) through the rock. A dosing pump administers NaHCO₃, CaCl₂, and NaOH (0.25M) to maintain fluid chemistry and pH. Continuous flow (≤50 mL/min) over days/weeks simulates natural cementation processes. Closed-system experiments (stationary fluid) halt reactions once equilibrium is reached. Dissolution Setup: Dissolution experiments use acidic DI water (pH ≤6, adjusted with HCl) in a non-recirculating system. Pore fluids are collected post-filtration for analysis. These shorter-duration tests (3 days) prioritize maintaining undersaturation and stable pH. Both setups preserve in situ saturation and pressure conditions without sample removal. Pore Geometry Visualization: Optical Light Microscopy (OLM) and Scanning Electron Microscopy (SEM) identify calcite crystal distribution and morphology. CT scans (pre/post-experiment) visualize pore structure changes. CT data are segmented into pore space, rock matrix, and microporous regions using density thresholds. Digital Image Analysis (DIA) parameters link pore geometry to acoustic/permeability trends.