Abstract
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 focused on tight carbonate formations, the world may gain a major source of lower-rate power over several decades. Here, we overview the Arab D formation in the largest oil field on earth, the Ghawar. We investigate the occurrence of microporosity of different origins and sizes using scanning electron microscopy (SEM) and pore casting techniques. Then, we present a robust calculation of the probability of invasion and oil saturation distribution in the nested micropores using mercury injection capillary pressure data available in the literature. 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 sufficiently 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 waterflooding until further measures are taken to modify the system’s surface chemistry and/or create substantial local pore pressure gradients.
Highlights
Viable improved oil recovery (IOR) from a microporous carbonate formation can be successful only with the thorough understanding of pore architecture, oil distribution after primary drainage, and wettability changes
If oil did not displace a majority of brine in the small pores, a microporous carbonate formation may not be an IOR target
We used high-resolution scanning electron microscopy (SEM) images of carbonates pore casts of Arab-D samples to identify microporosity types summarized in Table 1 [29,30]
Summary
Viable improved oil recovery (IOR) from a microporous carbonate formation can be successful only with the thorough understanding of pore architecture (geometry, topology, and other petrophysical characteristics), oil distribution after primary drainage, and wettability changes. If oil did not displace a majority of brine in the small pores, a microporous carbonate formation may not be an IOR target. Applications of our approach to fractured carbonate and sandstone formations, shales, coal seams, and to determination of remaining oil saturation abound, see e.g., [1,2,3,4,5,6,7]. The occluded microporous micritic carbonates [8] (solidified and diagenetically altered muds) remain largely inaccessible to waterflooding, especially when the crude oil is asphaltenic and rock mixed-wet. The carbonate mudrocks and limestones in the Permian Basin [11,12,13], Bakken [14,15], and Eagle Ford [16]
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