--> Abstract: Application of Geochemistry to Reservoir and Well Problems: a Case Study From Nigeria, by P. Sundararaman and O. T. Udo; #90933 (1998).

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Abstract: Application of Geochemistry to Reservoir and Well Problems: a Case Study From Nigeria

Sundararaman, P. and O. T. Udo - Chevron Nigeria Limited

For the past thirty years Chevron has successfully applied geochemistry to solve reservoir-related problems. The main analytical tool has been gas chromatography (fingerprinting) of oils and reservoir rock extracts. Application of this technique is based on the general observation that oils from the same reservoir exhibit nearly identical fingerprints; whereas oils from separate reservoirs usually show measurable chromatographic differences. In recent years, we have successfully applied this technology to solve various reservoir related problems in Chevron Nigeria Limited (CNL).

In 1994, a development well (OB-9) was drilled in the Opuekeba field situated in the northern part of Niger Delta. This well encountered oils in three sands (CD-01, H-02 and J-01) and was completed dually in the CD-01 (short string) and the J-01 (long string) sands. Chromatographic fingerprints of the oil from the CD-01 sand of OB-9 well did not match the fingerprints of oils from other wells producing from the same sand. This suggests lateral reservoir discontinuity of CD-01 sand between the OB-9 well and the other wells producing from the same sand, a conclusion which did not agree with seismic and other geological data.

With time the well developed high casing pressure. Fluids from the casing and the two strings were collected and analyzed to determine the origin of the fluid in the casing. Chromatographic analyses showed that the casing fluid did not match the oils from the two strings. In addition the composition of the oil from short string (CD-01) was different from the oil collected when the well was originally completed. The oil from the short string was collected periodically and its fingerprint was compared with the original oil. The composition of the oil kept changing with time and after several months was identical to the composition of oils from the CD-01 in other wells.

Further investigation showed a leak in the packer that was used to separate the CD-01 and the H-02 sand. Due to the higher pressure of the H-02 sand, the short string started producing oil from the H-02 rather than the CD-01 sand through the leak in the packer. This explains the differences in chromatographic fingerprints of oil from the CD-01 sand of OB-9 well and the rest of the wells producing from the same sand. With time the H-02 sand was depleted and the short string started producing oil from the CD-01 sand. This was reflected in the chromatographic fingerprint of the oil from the CD-01 sand of OB-9 well, which changed with time. Eventually the chromatographic fingerprints of oil producing from the CD-01 sand of all the wells including the OB-9 well were identical indicating the lateral continuity of this sand.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil