A field trial was conducted in June, 2001, of a downhole fiber optic two-phase flowmeter in two wells at PDO's Nimr field in Oman. The fiber optic flowmeter provides real-time measurements of volumetric flow rate and water cut. The purpose of the field trial was to evaluate the performance of the flowmeter for potential application at Nimr. Specifically, objectives were to determine the deployability of the flowmeter at Nimr, to determine the ability to operate with heavy, viscous Nimr crude, and to determine the ability of the flowmeter to distinguish between high water cut legs in dual lateral completions with artificial lift. Accuracy goals for the flowmeter were ±3% on water cut and ±10% on total volumetric flow rate.
The fiber optic flowmeter was successfully installed in two wells at Nimr. In one of the wells the meter was installed in the completion string just below an electrical submersible pump (ESP) and in the other well it was installed just below a beam pump. A surface Coriolis mass flow meter served as a reference for the two-week field trial. In the beam-pumped well it was found the noise environment is not compatible with the physical measurement principle of the optical flowmeter, and a comparison against the surface-measured rates was not possible. In the ESP-pumped well, comparison of the flow rates and water cut measured by the fiber optic flowmeter and the Coriolis meter were better than expected, -1% to +2% on water cut and -1% to +3% on total rate, and well within specifications.
The Nimr field is located in southern Oman in the South Oman Salt Basin, Fig. 1. It was discovered in 1980 and is the second largest oil field in Oman, with current production of about 27,000 m3/d of oil from over 300 wells. The field produces from hydrostatically pressured sandstone reservoirs at a depth of about 900 m. Reservoir temperatures and pressures are approximately 51°C and 9,500 kPa. Artificial lift for the fluids at Nimr is provided by ESP's and beam pumps.
Nimr produces a viscous (200–400 cp), heavy (21°API) crude oil that contains no appreciable solution gas, with a gas-oil ratio of about 0.6 sm3/m3. production is supported by a bottom water drive. An unfavorable mobility contrast between the oil and formation water results in rapid water breakthrough, and a large portion of a well's reserves are produced at high water cuts. The average economic limit of Nimr wells is about 98% water cut. Thus, water management plays a key role in well economics.
Initial field development used a grid pattern of vertical producers. This was switched to a grid of horizontal wells in the 1990's, with wells located at the crest of the structure. Each horizontal producer develops a "tent" of oil beneath it, as shown in Fig. 2, with the optimum spacing of wells driven by viscosity contrast between the oil and water, the thickness of the oil column, and the vertical-to-horizontal permeability ratio.
In 1997, a Field Development Plan was implemented which involved drilling dual lateral wells to connect two horizontal grid locations to a common vertical mother bore and surface hookup. The advantage of using dual laterals is a reduction in the number of wells required to drain the field and a corresponding reduction in capital expense. This is balanced by a decreased ability to monitor and control water production from individual horizontal legs at the surface. Heterogeneities in the reservoir can result in water breakthrough in one leg of the well before the other. This will impact the ultimate expected recovery and the required capacity of facilities for processing and disposing of the produced water.