Evaluation of representative elementary volume for a vuggy carbonate rock – Part II: Two-phase flow

Abstract

Abstract Large spatial variation of petrophysical properties and heterogeneous fluid displacement behavior are often associated with vuggy carbonate rocks. Because vugular rocks are characterized by large heterogeneity at the core-sample scale, the property measurements vary significantly between samples and the assessment of oil recovery is often unreliable. This paper focuses on variability of two-phase flow properties in a vuggy carbonate material. Drainage and waterflood experiments were performed on 12 different sample sizes, ranging from cm to m scale. The results for oil recovery show that the largest sample is above REV and reflects the average for the smaller volumes. An arithmetic average is found to be an appropriate upscaling scheme for oil recovery, thus the studied vuggy material can be regarded as heterogeneously homogeneous. Based on 2D gamma-ray saturation maps semivariogram functions were calculated. Both initial and residual saturations were spatially correlated. For two larger samples, the drainage and waterflood displacements were performed and monitored by x-ray providing high-resolution images. The results were used to measure the dispersion of the displacement front. The front behavior patterns and dispersion lengths were found to be similar for drainage and waterflood processes within the same sample. The effect of heterogeneities on the drainage capillary pressure curve and on spontaneous imbibition has been investigated. Nuclear Magnetic Resonance (NMR) 1D-saturation monitoring performed on a sample after a centrifuge test was used to investigate the air–water drainage capillary pressure curve. The obtained curve indicated variations along the sample. Imaging of the displacement front for spontaneous imbibition was performed to investigate impact of heterogeneities on the process.