Core Cleaning for Restoration of Native Wettability

Abstract

Certain additives used in drilling and completion fluids may alter the wettability of core. For example, the surfactants in invert-oil-emulsion drilling muds are known to al,ter the wettability of core. Because routine cleaning methods do not remove these surfactants, core that is to be used for special laboratory analyses, such as relative permeability, capillary pressure, and saturation exponent, is left with an altered wettability. Experiments with such core will obviously produce erroneous data. To obtain reliable results, cleaning methods must be developed to remove these surfactants, allowing restoration of plugs to their natural wettability. Once the core is cleaned, the wettability of the cleaned core can be restored by saturating with synthetic formation brine and uncontaminated reservoir crude, then aging at the reservoir temperature.This report ranks the efficiency of solvents in removing surfactants from plugs contaminated with invert-oil-emulsion drillingmud filtrate. Eight solvents for Berea sandstone and seven solvents for Guelph dolomite have been ranked after Dean-Stark extraction of mud surfactants using the U.S. Bureau of Mines (USBM) wettability index. The best solvent for both rock types was a 50/50 mixture of toluene/methanol, or the equivalent, containing 1 % ammonium hydroxide. Toluene, the most commonly used solvent for core cleaning, was one of the poorest solvents tested. The solvents that are effective in removing surfactants can be used to clean contaminated core.

Wettability and Prediction of Oil Recovery From Reservoirs Developed With Modern Drilling and Completion Fluids

“…The efficacy of cleaning processes has been studied for WBM and OBM-contaminated cores (e.g., Anderson, 1986;Cuiec, 1989), but studies of cleaning SBM-contaminated cores are limited (McCaffery et al, 2002). Gant and Anderson (1988) tested many common cleaning solvents and combinations of solvents for cleaning cores contaminated with invert emulsion oil-based muds. The best results in both sandstone and carbonate cores were achieved using a Dean-Stark extraction method with either a 1:1 mixture of toluene and methanol or the same 1:1 mixture to which 1% NH 4 OH was added.…”

“…An even more rigorous cleaning was attempted using a five-step or six-step sequence including 1% wt/vol NaOH in isopropanol. Some single-step tests using the mixture recommended by Gant and Anderson (1988) of 49.5% toluene, 49.5% methanol, and 1% wt/vol NH 4 OH, were included in the extraction series for comparison to the multi-step processes introduced in this work. The solvent sequence details, identified by the number of steps involved are summarized in Table 3.8.2-2.…”

Wettability and Prediction of Oil Recovery From Reservoirs Developed With Modern Drilling and Completion Fluids

“…An even more rigorous cleaning was attempted using a five-step or six-step sequence including 1% wt/vol NaOH in isopropanol. Some single-step tests using the mixture recommended by Gant and Anderson (1988) of 49.5% toluene, 49.5% methanol, and 1% wt/vol NH 4 OH, were included in the extraction series for comparison to the multi-step processes introduced in this work. The solvent sequence details, identified by the number of steps involved are summarized in Table 4.…”

“…The efficacy of cleaning processes has been studied for WBM and OBM-contaminated cores (e.g., Anderson, 1986;Cuiec, 1989), but studies of cleaning SBM-contaminated cores are limited (McCaffery et al, 2002;Skalli et al, 2004;Zhang et al, 2004). Gant and Anderson (1988) tested many common cleaning solvents and combinations of solvents for cleaning cores contaminated with invert emulsion oil-based muds. The best results in both sandstone and carbonate cores were achieved using a Dean-Stark extraction method with either a 1:1 mixture of toluene and methanol or the same 1:1 mixture to which 1% NH 4 OH was added.…”

Wettability and Oil Recovery by Imbibition and Viscous Displacement from Fractured and Heterogeneous Carbonates

“…Reservoir carbonates, which are commonly oil-wet or intermediate-wet, however, are almost impossible to clean to a strongly water-wet condition without causing change in pore structure (Cuiec et al, 1979;Gant and Anderson, 1988). One major advantage of working with outcrop rocks is that they are typically, although not always, very strongly water-wet.…”