Abstract
A geochemical investigation of oils in sandstone core plugs and drill stem test oils was carried out on samples from a North Sea reservoir. A sample of diesel used as a constituent of the drilling fluids was also analysed. The aliphatic and aromatic hydrocarbons and polar non-hydrocarbons were isolated using solid phase extraction methods. GC analysis of the hydrocarbon fraction of the core extract indicated that contamination may be diesel derived. From analysis of diesel some compound classes are less likely to be affected by contamination from diesel itself including: steranes, hopanes, aromatic steroid hydrocarbons, benzocarbazoles and C0–C3-alkylphenols.Large quantities of sterols (ca. 30 mg g-1 total soluble extract) were identified in the polar non-hydrocarbon fractions of the core extract petroleum, presumably resulting from contamination. The origin of sterols is likely to be due to an additive introduced into the drilling fluid. Sterols are surface active compounds and in significant quantities may affect engineering core property measurements including wettability determinations. In addition, bulk petroleum composition screening methods, such as Iatroscan, used for determining saturated and aromatic hydrocarbons, resins and asphaltenes (SARA) content of core extract petroleum may also be affected.
Highlights
One of the fundamental threats confronting geochemists during analysis of reservoir cores and produced petroleum is contamination by drilling mud components.[1]
Early recognition of drilling mud contamination may be achieved by GC analysis of the aliphatic hydrocarbons
This may provide an indication of potential problems during analysis of the polar non-hydrocarbons
Summary
One of the fundamental threats confronting geochemists during analysis of reservoir cores and produced petroleum is contamination by drilling mud components.[1] Drillinguids are made up of a number of components which may include diesel, emulsifying agents, wetting agents, surfactants, scale inhibitors, biocides, kerosene and even crude oils from other producing wells. Wettability measurements on reservoir cores may be seriously affected by oil-based mud contamination where extreme changes from water-wet cores to strongly oil-wet conditions have been observed.[3] Drillinguid invasion is more pervasive in permeable sandstones and could result in large scale displacement of in-situuids. Obtaining cores with lowuid invasion is dif®cult and while methods have been designed for both oil and water based muds,[4] contamination is still a common problem
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