Experimental and numerical investigation of high temperature imbibition into preferential oil-wet chalk

Spontaneous imbibition (SI) of water into the matrix blocks is believed to be a very important drive mechanism for oil recovery from naturally fractured reservoirs, including the Ekofisk field in the North Sea. This work presents SI of artificial seawater containing sulfate ions at 130 °C on outcrop chalk saturated with crude oil (plus connate water) of high acid number to render the wettability of the porous medium preferential oil-wet. The results for the SI tests show that artificial seawater containing sulfate ions is able to spontaneously imbibe into preferential oil-wet chalk (at 40 °C) due to a wettability modification process towards more water-wet conditions. No imbibibition of any water took place at 40 °C. After raising the temperature to 130 °C, a water saturation change of 28% was observed during 15 days of imbibition time on two duplicated core samples (from Swi = 24% to 52%).The SI taking place at elevated temperature is interpreted as a wettability modification process, where the sulfate ions in the imbibing water phase are assumed to play a very important role. One core sample was re-saturated with crude oil after the SI process had ceased and the relative permeability of oil and water was measured using the steady-state method. The high end-point relative permeability of water krw⁎ = 0.761 indicated core surface to be close to neutral wettability. The measured relative permeability curves were then utilized to generate a capillary pressure curve by history matching oil production vs. time to the oil production measured experimentally using numerical simulations. The result showed that weak capillary forces were generated as a result of the SI process. Further fine-grid simulations after magnifying the size of the core sample up to 100 times (diameter D = 350 cm and height H = 691 cm) showed that weak induced capillary forces contributed significantly to the initial oil production rate even for the largest sample investigated. This result indicated that the induced capillary forces after a wettability modifying process may have effect on field oil production rates, as the matrix blocks investigated are representative for those blocks sizes (D = 350 cm and H = 691 cm) usually applied, when modeling fractured reservoirs by the dual-porosity scheme.