Hybrid Drainage Technique Application on Bimodal Limestone

Representative relative permeability and capillary pressure must be provided for reliable reservoir production forecasts. These petrophysical parameters are exclusively obtained through laboratory experimental work and numerical simulation. Thus, appropriate techniques are necessary for restoring initial water saturation (Swi) and wettability. In a previous paper (Fernandes et al., 2023), we presented the Hybrid Drainage Technique (HDT), which is a primary drainage method used in water/oil systems. It generates a homogeneous water saturation profile at a targeted Swi value in reduced experimental duration compared to the porous plate method. The proof-of-concept study was successfully applied to simple rocks, such as Bentheimer and Richemont outcrop samples. In this paper, we tested a more complex bimodal system, represented by the Estaillades limestone rock, with the aim of comparing the HDT with the viscous oilflood method (VOF). Primary drainage by VOF is commonly used on full-size cores, as it is very time consuming to desaturate large rocks to Swi using the porous plate method and impossible to use the centrifuge technique. The points of interest for this comparison are as follows: reaching the targeted Swi value, homogeneity of the saturation profile at the end of primary drainage, wettability alteration, and imbibition interpretation via a history-matching process of differential pressure, oil production, and saturation profiles when available. During VOF experiments, we noticed a difficulty in obtaining low values of Swi (below 35%). This could be due to the dual-porosity nature of the Estaillades limestone and the poor connectivity between large and small pore networks. On the other hand, the capillary-pressure-controlled step of the HDT provides access to both porosity networks, thus reaching lower Swi values. Furthermore, the invasion process is different for the two techniques. Therefore, even if the same Swi value is obtained, pore occupancy at the end of primary drainage is different on such a bimodal pore network. This will also impact wettability alteration and, thus, imbibition results. Monitoring using both nuclear magnetic resonance (NMR) and microtomography (μ-CT ) imaging allowed visualization of fluid saturation and distribution at pore and core scales. These were not only valuable input data for interpreting flooding experiments through numerical simulations but also for phenomenological understanding.