Imaging Oil Recovery from Mixed-Wet Microporous Carbonates

This paper summarizes some of our efforts in comprehensive imaging of a complex limestone-water-asphaltenic crude oil reservoir system at scales ranging from a fraction of a micron to centimeters. These types of imaging and analyses that follow are necessary if one were to understand fully the fundamentals of improved oil recovery in mixed wet rock. Perhaps as much as 50% of the oil-in-place in carbonate formations around the world is locked away in the easy to bypass microporosity. If some of this oil is unlocked by the improved recovery processes designed speci cally for tight carbonate formations, the world may gain a major source of lower-rate power over several decades. Here, we overview our work on the Arab D limestones and Indiana limestones. We investigate the occurrence of microporosity of different origins and sizes using scanning electron microscopy (SEM) and pore casting techniques. We show that large portions of the micropores in Arab D formation would have been bypassed during primary drainage unless the invading crude oil ganglia were su ciently long. We also show that, under prevailing conditions of primary drainage of the strongly water-wet Arab formations in the Ghawar, the microporosity there was invaded and the porosity-weighted initial oil saturations of 60-85% are expected. Considering the asphaltenic nature of crude oil in the Ghawar, we expect the invaded portions of the pores to turn mixed-wet, thus becoming inaccessible to water ooding until further measures are taken to modify the system's surface chemistry and/or create substantial local pore pressure gradients. All types of imaging and experiments described in this paper guide our spontaneous counter-current imbibition in Amott cell experiments, a convenient laboratory method of studying oil recovery from oil-saturated rock samples in secondary or tertiary oil recovery by water ood of tunable composition. Classical Amott cell experiment estimates ultimate oil recovery. It is not designed, however, for studying the dynamics of oil recovery. In this work we identify and x a aw in the classical Amott cell imbibition experiments that hinders the development of predictive recovery models for mixed-wet carbonates. We then follow with a statistical analysis and scaling of the imbibition. We apply Generalized Extreme Value distribution to model the cumulative oil production. Here, we start with the Amott imbibition experiments and scaling analysis for Indiana limestone core plugs saturated with mineral oil. The knowledge gained from this study will allow us to develop a predictive model of water-oil displacement for reservoir carbonate rock and crude oil recovery systems.