Fluid/Rock Interactions in Porous Carbonate Rocks: An Integrated Mechanical, Ultrasonic and Micro-Structural Study

Abstract

The mechanical and physical properties of reservoir rocks are affected by the nature of the fluids saturating their pore space. Water flooding is a commonly adopted practice for enhanced oil recovery, however saturating the reservoir with water can result in a reduction in rock strength (i.e. water weakening) leading to enhanced compaction which in turn can impact reservoir productivity and management. This study is concerned with the laboratory measurement of mechanical, elastic and fluid transport properties of samples of synthetic limestones with the aim of investigating the mechanisms involved in water weakening. Blocks of synthetic limestone were fabricated using the Calcite In situ Precipitation System (CIPS), a proprietary mineral cementation grouting technology (Lithic Technology Pty Ltd), and sub-samples from these were experimentally tested to assess the sensitivity of their mechanical and elastic properties to water saturation. To this end, high pressure geomechanical tests were conducted to characterize the behaviour of the samples under dry and water saturated conditions while monitoring elastic wave velocities (compressional and shear waves) at ultrasonic frequencies. Three types of geomechanical tests were performed covering the brittle to ductile range of rock responses: i) unconfined compressive strength (UCS); ii) multistage triaxial (MTXL); and iii) hydrostatic (isotropic) compaction. The saturated hydrostatic compaction and MTXL tests were run with a pore pressure of 1MPa and maintaining the same effective stresses as used for the dry tests. The laboratory geomechanical tests were complemented by a series of petrophysical and microstructural analyses aimed at characterizing the porosity, permeability, pore and grain size distributions of the rock samples and track their evolution as functions of the applied stress. Experimental results show that in all tested stress configurations water-saturated CIPS-cemented samples are weaker and more compliant than those with empty pore spaces (i.e. dry); therefore the mechanical and elastic properties of the CIPS calcite cement are affected by the presence of water. The results provide new insights into the micromechanical mechanisms leading to water weakening, strain localization and failure in high porosity carbonate rocks as well as links between their elastic wave velocities, water saturation and degree of deformation.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015