Temperature gradient and pH effects on Sor estimates from SWCT tests - The no buffer case

Tom Pedersen

doi:10.1016/j.petrol.2020.107652

Abstract

Residual oil saturation (Sor) is the fraction of immobile oil that remains after a water flood. This information is important for calculating recoverable reserves and evaluating EOR campaigns. Single Well Chemical Tracer (SWCT) tests yield a near-well average Sor based on a large rock volume. Correctly performed and analyzed, SWCT tests yield reliable Sor estimates. There are, however, two important effects that commonly are ignored. One is cooling of the reservoir during injection of the cold brine resulting in decreased hydrolysis rate of the primary tracer. The other is the hydrolysis rate's pH dependence. To study these phenomena, we have developed a numerical model of an ethyl acetate SWCT test and applied it to a generic case with test and formation data based on published values. An analytical model is used to calculate how the brine injection temperature varies with time. The ethyl acetate hydrolyses into ethanol and acetic acid that lowers the pH and changes the hydrolysis rate. Any buffer capacity in the injected fluid or formation is neglected. Our model is the first ever to combine realistic temperature calculations with pH dependent ethyl acetate hydrolysis rate during a SWCT test. Sor is estimated from synthetic tracer production curves using the direct chromatographic separation equation as well as the mean residence time. A large number of simulations were performed to investigate how the Sor estimates vary with different model assumptions and input data. In all cases, both methods underestimate the true Sor, typically by 2–4% - in some cases significantly more. We demonstrate that neglecting dispersion in the wellbore during injection and production is warranted with the rock and test data used in this study.

Highlights

Residual oil saturation (Sor) refers to the immobile oil that remains after a water flood
Since the formation and test data used in the calculations are averages from a rather large number of various Single Well Chemical Tracer (SWCT) tests, we would argue that the common assumption of a constant hydrolysis rate used in most SWCT test models (e.g., Deans and Majoros, 1980; Jerauld et al, 2010; Jin et al, 2015) is an over simplification
The tracer curves do differ, the Sor estimates changed by no more than 1%. This indicates that perhaps the use of more realistic SWCT simulators could reduce the volume of primary tracers required for a successful test and minimize its environ mental impact as well as any changes in Sor caused by the SWCT test itself (e.g., Gadgil, 1979)

Summary

Introduction

Residual oil saturation (Sor) refers to the immobile oil that remains after a water flood. It is defined as the fraction of pore space occupied by immobile oil. Reliable estimates of Sor are important for calculating recoverable re serves, in decisions on whether Enhanced Oil Recovery (EOR) opera tions are suitable for further exploitation of a reservoir as well as in assessments of their effect (e.g., Al-Mutairi et al, 2015; Jin et al, 2015; Khaledialidusti and Kleppe, 2015; Al-Shalabi et al, 2017). Reliable estimates of Sor are important for calculating recoverable re serves, in decisions on whether Enhanced Oil Recovery (EOR) opera tions are suitable for further exploitation of a reservoir as well as in assessments of their effect (e.g., Al-Mutairi et al, 2015; Jin et al, 2015; Khaledialidusti and Kleppe, 2015; Al-Shalabi et al, 2017). Teklu et al (2013) give a review of various methods to estimate Sor

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