Heterogeneity, petro-fabric and petrophysical subdivision of the El Garia Formation (Ypresian, Tunisia): Implications for CO2 storage

Assessing and evaluating each reservoir suitability for carbon dioxide storage (CCS) prior to injection is crucial to ensure safe and long-term geologic carbon storage. Rock physics is a key technique in determining the CO 2 storage potential of rock formations. Each assessment should start by understanding the microstructural heterogeneity of each reservoir. Microstructural and petrophysical studies of El Garia nummulitic limestone reservoir were carried out in order to: a) understand the heterogeneity of this complex reservoir and b) subdivide this formation into petrophysical levels in order to define the suitable levels for CO 2 storage and c) define factors controlling the heterogeneity of microstructural and petrophysical properties and determine its interrelation as a neecessary step to assess suitability of the formation to host CO 2 . Microstructural observations, made on 24 thin sections under optical and scanning electron microscopy (SEM), identifies seven deposit facies. Porosity has been measured on seven samples by the triple weight method and estimated from the optical microscopy and SEM observations using JMicrovision software. After microstructural and reservoir characterization studies focusing on sedimentology, diagenesis, types of cement and porosity values and types, a petro-fabric zonation of El Garia reservoir was carried out identifying around four distinct levels based on these cited parameters. In the light of these results, a rock physics study was established. Electrical conductivity, P wave velocities, and anisotropy of magnetic susceptibility have been measured in five samples corresponding to these different levels of El Garia reservoir in order to delves into the microstructural and petrophysical subdivision, to determine the factor controlling the formation heterogeneity and its interrelation. Four parameters have been determined when studying the electrical conductivity (surface conductivity, formation factor, tortuosity and cementation exponent). Petrographical characteristics and petrophysical properties of the sample plugs indicate a correlation between these parameters, showing a microstructural and petrophysical heterogeneity of this reservoir. The results obtained for these electrical parameters show heterogeneity in the pore connectivity and in the arrangement of mineral crystals, particles and pores from one facies to another. The acoustic measurement results show that the vertical evolution of the acoustic velocity varies as a function of the diagenetic evolution and the internal microstructure of each facies of the El Garia Formation. The anisotropy study allows the definition of four acoustic fabric levels into the formation.