Canadian Nexen Petroleum Yemen (CNPY) produces two million barrels of fluid per day at 95 percent watercut from the Masila Block (Block 14) in Yemen. Asset development decisions are based on numerous factors, one of the most important being the amount of oil produced from a well. With increasing oil price, it is becoming economically feasible to produce wells at significantly higher watercuts. The analytical variability of volumetric methods used in Block 14 to measure watercut restricts the accuracy and precision of these measurements. The authors developed a robust and reproducible water cut determination method for this mature petroleum asset based on gravimetric measurement of oil and water phases.

The gravimetric measurement approach follows the volumetric method procedures but uses a balance to weigh the oil or water fractions. To keep consistent with historical reporting of oil production, the density of each phase is then used to correct the values to a volume basis.

The methodology outperforms the volumetric determination of water cut by being more reproducible (better precision) and allowing for the accurate measurement of water cuts up to 99.5 percent. In side by side comparison of the gravimetric and volumetric methods at 99.5 percent water cut, the gravimetric methodology showed a five fold increase in accuracy. In addition, the gravimetric method provides six to ten times better precision in the measurement of watercut on representative field samples compared to the volumetric methodology. The program at Block 14 investigated the use of a reference calibration matrix standard as a data quality system to establish the detection limits of the methodology and to institute a testing procedure to monitor laboratory performance.

The increased accuracy and precision in watercut measurement gives reservoir engineers the confidence to make economic decisions on well viability to maximize overall project NPV.

Introduction and Economic Justification

Block 14 is located in Yemen approximately 130km inland from the southern coast (Figure 1). Oil was discovered in Block 14 in 1990 and production commenced in 1993. Since then, CNPY has been producing oil from this region and transporting it to the Export Terminal on the Gulf of Aden.

CNPY is currently producing 100,000 BOPD and two million BWPD from Block 14. Oil is produced from several unique and independent reservoirs. Crude densities vary from 16° to 42° API across the Block depending on the reservoir. Water cuts have increased significantly as production has matured to the point where the current average water cut in the field is greater than 95%. The field is produced using ESP artificial lift with individual well fluid rates varying from 150 to 20,000 BFPD. Production is gathered through an extensive network of pipelines and collected for processing at the central processing facility (CPF). Produced water is re-injected into disposal zones and into producing reservoirs for pressure maintenance.

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