The Effect of Rock Properties on Remaining and Residual Oil Saturation in Heterogeneous Carbonate Rocks
Authors: El Din Samy Serag , Dernaika Moustafa , Hannon Loay , Kalam Zubair 
Published: December 2013

Reliable evaluation of residual and remaining oil saturations is a critical challenge in deciding if a reservoir has reached the limit of economic production. Residual and remaining oil saturations could be dependent upon rock properties and depth above Free Water Level (FWL). However, such dependence may be influenced by reservoir heterogeneities and complex fluid flow history that might have contributed to the formation of oil saturations in the reservoir. Reservoir heterogeneity in carbonates presents complex petrophysical challenges to accurately simulate storage and flow capacities in the reservoir. Oil recovery from waterflood injections is strongly dependent on capillary pressure characteristics and the variation of the capillary pressure curves with reservoir rock properties. Establishing the relationship between oil saturations and rock properties is an on-going challenge in heterogeneous reservoirs and therefore detailed understanding of this relation is crucial for predicting reservoir performance.

Porous Plate (PP) capillary pressure and resistivity index measurements were performed on carbonate reservoir rocks to study the effect of rock properties on capillarity and residual oil saturation. The rock samples were selected to represent main reservoir rock types based on poroperm characteristics, X-ray CT images, Thin-Section (TS) photomicrographs and mercury injection experiments (MICP). The porous plate tests were carried out at reservoir conditions and the obtained results suggest mixed-wet behavior. The residual oil saturations (Sor) were determined at the end of forced imbibition. The remaining oil saturations (ROS) could be inferred from lower (forced imbibition) capillary pressures and from spontaneous imbibition. While residual oil saturations do not depend on the level of capillary pressure, remaining oil saturations are function of the imposed capillary pressures.

Variations in capillary pressure behavior and oil saturations (Pc-Sw) were observed among the different rock types. Those variations were analysed by accurate analysis of both poroperm characteristics and pore throat size distributions. Reservoir cores mainly composed of macro pores with high permeability and moderate porosity gave low residual oil saturations (i.e. ~10%) regardless of irreducible water saturations (Swi). Macro to meso range pore sized samples with lower poroperm values could be drained to the same irreducible water saturation but gave higher residual oil saturations due to less pore accessibility and hence more oil trapping in the meso pores during forced imbibition. Little spontaneous imbibition (i.e. 4%–7% Sw units) was detected in those two rock types which must have occurred in the smaller water-wet meso pores. Rocks with more uniform pore throat size distributions (meso to micro pores) with high porosities and moderate permeabilities were drained to similar Swi values and showed large spontaneous imbibition (i.e. 20%–30% Sw units) and moderate residual oil saturations (i.e. 20%–30%). The spontaneous imbibition behavior was closely examined and confirmed by associated decrease in resistivity on many samples of these rock types. This behavior was absent in previously modeled dynamic data and hence would introduce new challenges in the modeling scenario for similar mixed wet rocks. Those variations clearly demonstrate the complex effects of the relative percentages of pores present and their distribution on the dynamic data, and would provide better understanding on the variability of reservoir behavior with different rock types which would lead to enhanced predictions of the reservoir performance.