Abstract

In October of 2000, a downhole fiber optic flowmeter was installed in Shell's Mars A-18 well in deepwater Gulf of Mexico. The production tubing-deployed system was installed to a measured depth of 21,138 feet in a highly deviated section of the well, immediately above the producing zones, in 2940 feet of water. This installation represents the first all-fiber optic, multiphase flowmeter system deployed in a commercial well.

The fiber optic flowmeter delivers real-time measurements of downhole pressure, temperature, flow rate, and phase fraction. It is completely non-intrusive and contains no downhole electronics or moving parts. The meter requires only one wellhead penetration and is deployed on a single fiber optic cable with the production tubing string during well completion, in a manner similar to conventional, electronic downhole monitoring systems. Installation of the flowmeter at Mars was achieved as planned with no additional rig time required. In fact, the entire well completion operation was finished ahead of the budgeted schedule. This in large part is attributable to pre-job planning and preparations for the entire operation.

Performance of the flowmeter at Mars exceeded expectations and demonstrates the value of real-time downhole production data. Along with providing the production engineer with downhole pressure and flow rate data to control draw down while the well was being ramped up, data from the flowmeter provided other valuable information such as a temperature and pressure profile of the well during run-in-hole, well behavior during cleanup, and well productivity data.

Mars Field Overview

The Mars field was discovered by Shell in 1989 on Mississippi Canyon Blocks 763 and 807 in the Gulf of Mexico, about 130 southeast of New Orleans, Fig. 1. The Mars tension leg platform (TLP) was installed in May, 1996 in 2940 feet of water, Fig. 2. There are 24 well slots, and additionally, a subsea well is tied back to the TLP. Shell Deepwater Production, Inc. is operator of the field and has a 71.5% interest, and BP has the remaining 28.5% interest.

The production facilities on the TLP are designed to recover about 500 million barrels of oil equivalent. First production from the TLP was in July, 1996. Current design capacity is 220,000 barrels of oil per day and 220 million cubic feet of gas per day. The oil is transported 116 miles via an 18/24-inch pipeline to shore and the gas is transported 55 miles via a 14-inch pipeline to West Delta 143.

Fiber Optic Flowmeter Description

The design, development and testing of the downhole fiber optic multiphase flowmeter are described in detail in the companion to this paper and elsewhere.1,2,3 The meter contains no in-well electronics, is non-intrusive, and is capable of robust, reliable operation in harsh downhole environments. It utilizes well established methods to measure the speed of sound of the bulk fluid and the bulk fluid velocity. The speed of sound measurement is combined with a knowledge of the densities and speeds of sound of the individual phases to determine phase fraction (water cut or gas fraction), which is used together with the velocity measurement to determine individual phase flow rates. The novel manner in which these methods are implemented enable full-bore, non-intrusive measurement of pressure, temperature and flow with no downhole electronics.

The flowmeter installed in the Mars A-18 well is constructed of Inconel 718. It is compatible with 3 1/2-inch production tubing, with an internal bore of 2.992 inches. The maximum outside diameter of the sensing tube is 5.6 inches. The meter consists of two sub-sections, a pressure section and a flow fraction section, as shown in Fig. 3. The pressure section is about 5 feet in length and contains a 150°C, 15,000 psia fiber optic pressure and temperature transducer. The flow section is about 12 feet in length and contains the fiber optic velocity and speed of sound sensors. Maximum operating conditions of the meter are 125 C and 15,000 psia.

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