Abstract

In March, 2002, a downhole, fiber optic flowmeter was successfully installed in BP's Mahogany A-15 well, offshore Trinidad. The meter was installed at a measured depth of 12,114 ft and a deviation of 71°. The flowmeter is full bore, completely non-intrusive, contains no downhole electronics or moving parts, is electrically passive and was deployed with the production tubing string during completion of the well. It provides high quality measurements of temperature, pressure, gas flow rate, and oil flow rate directly to the platform SCADA system. In addition, data from the instrument is linked to BP's Wide Area Network through a network-accessible hard drive. The installation at Mahogany was a field trial of the flowmeter and represents a number of "firsts" for deployment of fiber optic flow metering technology, including the first application metering a downhole flowing gas phase and the first multiphase application.

Performance of the flowmeter at Mahogany has exceeded expectations and demonstrates the value of real-time downhole production data. Data quality of the fundamental meter measurements of pressure, temperature, bulk velocity, and mixture sound speed from the first nine months of operation has been excellent and within specifications. The high deviation of the well at the meter location, along with a rapid increase in downhole free gas fraction, presented several interpretation challenges, including operation in complex flow regimes and slippage of the oil phase relative to the gas phase at the meter. This was resolved through development and implementation of a pseudo-three-phase slip model. The resulting downhole oil, gas, and water rates from the flowmeter are converted to surface rates using appropriate PVT data before being passed to the SCADA system. Thus far, rates from the downhole optical flowmeter agree with surface well test results to better than ±10%.

Background

BP Trinidad and Tobago's Mahogany Field is located 45 miles off the southeast coast of Trinidad in the Columbus basin in 285 ft of water, Fig. 1. It is the largest oil and gas producer in Trinidad. The field was discovered in 1968, but was not slated for development until the Atlantic LNG (ALNG) project was initiated in 1994. Production commenced in October, 1998.

The primary objective of the Mahogany development is to deliver 475 MMscf/d of gas to the ALNG plant in southwest Trinidad. A secondary objective is to produce as much crude oil as possible from the 60 ft oil rim in the 21 Sand reservoir. Current production from the field is 475 MMscf/d of gas and 38,000 stb/d of crude and condensate. Production from Mahogany A platform wells flows through an 18" pipeline to the Mahogany B platform, were oil and gas separation is achieved. A 40" gas pipeline and a 12" condensate pipeline carries production from the B platform to shore at Galeota Point. The pipelines also carry production from BP's Cassia and Amherstia fields.

The Mahogany field is a faulted anticline structure comprised of Pleistocene-aged sands. There are 15 major pay reservoirs in seven distinct fault blocks. The 21 Sand reservoir is the only oil reservoir in the field and extends laterally across two faults blocks, FBIV and FBV, as shown in Fig. 2. The reservoir trap is a three-way structural closure against the 110 Fault. Initial reservoir conditions were 174°F and 4920 psia. The oil rim underlies a 370 bcf gas cap. PVT analyses of an early oil sample indicate the produced oil has a gravity of 35.8°, a total gas-oil ratio of 979 scf/bbl, and a formation volume factor of 1.44.

Mahogany Alpha 15B01 (MA15B01) was drilled as a horizontal oil well in the 21 Sand reservoir of FBV to a measured depth of 13,463 ft. The structure encountered was 26 ft lower than anticipated, and a decision was made to plug and sidetrack the well, since it was close to the oil-water contact. The sidetrack, MA15B02, was landed in the oil leg at 13,352 ft MD (10,035 ft TVDSS). A 2,273 ft lateral section was drilled and a water-flushed zone was encountered at the toe of the lateral from 15,150 ft to 15,625 ft. An inflatable packer was used to isolate the zone, and the well was completed with an open hole gravel pack.

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