Relative permeability as a function of temperature, initial water saturation and flooding fluid compositions for modified oil-wet chalk

Abstract

This paper addresses the effect of brines, containing sulphate and magnesium (found in sea water) and distilled water as initial saturating and flooding fluids, on relative permeability of modified chalk cores by fatty acids to more oil-wet.

A model oil system (n-decane) containing different fatty acids (present in oil) such as 18-phenyloctadecanoic acid (PODA—long chain fatty acid with unsaturated ring), Stearic acid (SA—saturated straight long chain), and brines containing sulphate and magnesium ions (0.03 M SO₄²⁻ and 0.06 M Mg²⁺) dissolved in distilled water are used.

Fatty acids alter the wettability of chalk to more oil-wet. PODA shows higher tendency compared to SA, at the same concentration, to alter the chalk to more oil-wet. Relative permeability curves are used as indication of the modified chalk behaviour.

It also addressed the effect of the temperature on the relative permeability. A shift in the relative permeability to the right side indicates more water-wet with temperature up to ≤ 80 °C. At a higher temperature of 130 °C, the relative permeability curves indicate more oil-wet tendency.

Wettability indicated by the relative permeability curves shows the influence of initially saturated fluid composition and the flooding fluid composition, where the modified cores initially saturated with ion free water (distilled water) and flooded by fluids containing Mg²⁺ or SO₄²⁻ shows a shift indicating more water-wet compared to reference core (initially saturated and flooded by ion free water). However, if the initial saturating and flooding fluids contain 0.06 M Mg²⁺ ions, the wettability tends to be more oil-wet compared to that if the modified cores are saturated and flooded with 0.03 M SO₄²⁻ and distilled water.

The calculated pore size distribution index (λ) using Huang and Honarpour correlation, is used to determine the corrected (actual) relative permeability for capillary end effect. A good agreement between experimental and simulated relative permeability data was also obtained by using the calculated λ.