Better Characterization of Capillary Desaturation Curves in Carbonate Reservoirs

Abstract

Most of the oil remains trapped in the reservoir after both primary and secondary recovery stages. Enhanced oil recovery (EOR) techniques are usually targeted after secondary recovery to mobilize the trapped oil. Meanwhile, the EOR techniques involve a considerable amount of investment and carry uncertainty depending on the amount of the remaining oil. Very few studies are conducted on carbonate rocks for the accurate Sorw determination. Thus, the concept of capillary desaturation curve (CDC) has been introduced as guidance to estimate the mobilized residual oil. In this study, the true residual oil saturation to water (Sorw) was determined for carbonate outcrops using ultracentrifuge and coreflood methods under harsh conditions, then investigating the effect of trapping number, initial oil saturation, and permeability affecting Sorw, and finally characterizing the CDCs for carbonate rocks by further reducing the Sorw using surfactant flooding.

For this purpose, nine carbonate Indiana limestone outcrop samples with different permeability range (4-100 mD) and fluid samples i.e., field-representative oil, formation water, seawater, and surfactant solutions were used. The drainage was performed systematically using a coreflooding system to establish initial water saturation by injecting heavy oil followed by crude oil and aging for two weeks. Afterwards, all the nine core samples were subjected to spontaneous imbibition using Amott cell. This was further followed by forced imbibition using both centrifuge and coreflooding systems for comparison purposes and achieving Sorw condition. Finally, surfactant flooding was performed on all the cores using coreflood to generate CDC using three different surfactants IFT values.

The results showed that all rock samples achieved irreducible water saturation in the range of 18-32%, indicating no correlation between irreducible water saturation and rock permeability. No correlation was found between the initial water saturation and porosity. Also, no strong correlation was observed between initial oil saturation and porosity with Sorw. While Sorw values in low, medium, and high permeability range samples were found to be 20-30%, 22-26%, and 10-19%, respectively; highlighting no correlation exist between rock permeability with Sorw. However, a very strong correlation was found between trapping number and Sorw. While comparing Sorw from coreflood and centrifuge, it was observed that the centrifuge samples were already at true Sorw range during seawater injection and did not produce any oil, compared to the case of coreflood in which slight oil production was observed. The critical trapping number for water injection in all samples was found to be in the range of 10−5 to 10−4 which was higher than the trapping number reported in literature (10−8 to 10−6 ). The highest trapping number achieved for this under case study was 10−3 which had only reduced 15-45% of residual oil after waterflooding. Based on the CDC studies, it can be concluded that reducing oil after waterflooding would be challenging using chemical EOR and the selected surfactants as it would only be able to reduce 15-45% of residual oil after waterflooding with trapping number five times higher than field trapping number (10−8 ). Thus, Sorw determination and CDC characterization will aid in EOR screening of future surfactant flooding pilot projects in middle east carbonate rocks before their implementation.

Date of Award	Dec 2022
Original language	American English
Supervisor	WALEED ALAMERI (Supervisor)