Monitoring and containing water production in Egypt’s West Delta Deep subsea wells
Ingar Tyssen, Roxar
Mohamed Baydoun, Burullus/Rashpetco
Low recovery rates from subsea wells are not acceptable. Use of water production profile monitoring systems allows Burullus Gas Co. to run each of its West Delta Deep wells in the Mediterranean off Egypt aggressively at the limit of its water production. The results are that fewer wells are needed in the reservoir; there is greater efficiency from those wells already there; and maximum reservoir performance from wet gas fields. The installations also have helped Burullus monitor water production profiles in real time.
Over the past few years, the installation of Roxar Wetgas meters has helped Burullus monitor water production profiles in numerous West Delta Deep Marine (WDDM) wells in real time. Through providing early warnings of water produced, the system also has helped optimize performance of these wells and has saved at least three from water breakthrough.
Burullus Gas Co., a joint venture established in 1999, has developed various fields in Egypt’s WDDM concession. Partners in the venture are British Gas International, Petronas Carigali, and EGAS.
WDDM is in the Mediterranean Sea, about 130 km (80.7 mi) northwest of Alexandria. It is Egypt’s largest gas-bearing block offshore, and produces gas for domestic consumption and the country’s growing LNG industry. There are three main fields in the concession:
- Simian/Sienna, a gas discovery with little condensate which is tied back to Scarab/Saffron subsea development. The latter incorporates single open-hole gravel-packed completions
- Sapphire, a gas condensate field developed with subsea completions, also tied back to Scarab/Saffron. The stacked sands are developed via intelligent wells, and completed as two zones operated hydraulically with smart completions and sand control
- Scarab/Saffron, containing high grade methane gas but low levels of condensate. Commingled gas from the various fields is transferred to Burullus’ onshore processing facilities at Idku, near Alexandria.
Production characteristics include high gas rate wells with low water gas ratios (0.3-10 bbl/MMcf). Formation water production is expected to increase over the field life, with Simian/Sienna and Sapphire also likely to produce condensed water.
The nature of the development and the fluid composition raises technical challenges for production monitoring, production control, and flow assurance in the reservoirs. One main concern in subsea production is unchecked water, particularly in condensate and gas wells. This is the case in subsea tiebacks such as WDDM, where it takes longer to detect water breakthrough in a well, leading potentially to severe consequences including pipeline damage.
Water, formation water in particular, can cause scaling and corrosion of pipelines and chokes to thereby reduce well production. Formation water also carries solids that tend to plug the formation for less production efficiency.
By measuring the early onset of formation-water production in real time, Burullus can take preventive or remedial action, such as adjusting the pH in the MEG/water mix and controlling the hydrate inhibitor added to the produced fluids. This optimizes the inhibitor dosing rate and reduces inhibitor recovery costs while attaining the required level of hydrates protection. Other, more extreme options include choking the well or instigating zonal isolation.
Burullus also has to ensure high availability, to fulfill commitments to Egyptian Liquefied Natural Gas to provide LNG for the domestic market. This demands accurate estimates of the timing of future developments, including infill wells and production facility needs. Poor estimates could lead to premature advances of development phases to avoid shortfalls in production, causing significant financial losses.
Additionally, Burullus must address reservoir management challenges such as optimizing future development phases and focusing on the timing and locations of the next drilling programs.
Optimization goals
It was essential for Burullus to measure all produced fluids from the WDDM fields accurately well-by-well for production optimization and allocation. The joint venture also had the reservoir management goal of creating a dynamic simulation model of the reservoir that can be updated continually with production and pressure data.
Results then could be matched to historical pressure and daily production data by adjusting any number of reservoir parameters. This would reduce uncertainty, improve understanding of reservoir behavior, and create a more robust predictive tool.
Burullus evaluated numerous metering options, including inference systems, differential pressure meters, multiphase meters, and wet gas meters. All were assessed under the following criteria: number/location of meters, accuracy, reliability, and ability to measure water.
All systems also were evaluated on the basis of the project design for Simian/Sienna and Sapphire, and were designed within the constraints imposed by the project schedule. This meant the only design possible was a single meter on each well. Although costly, this allows each well to be monitored continuously and avoids wear on flow-switching valves.
The WDDM fields are likely to produce some water at low levels, and this is extremely important for history matching and to understand reservoir performance. Burullus concluded that wet gas meters offered the only solution to water flow rate measurement of high gas void fraction well streams in remote subsea locations. It considered differential pressure meters unsuitable because of changing composition in the well stream, especially as a result of water breakthrough. Multiphase flow and inference metering systems were omitted because of high liquid flow uncertainty.
Wet gas meters are essentially venturi meters designed to measure gas flow. A sufficient number of pressure or pressure differential measurement stations need to be included so that both gas and liquid flow rates can be determined. To obtain more accurate water measurements, a way to measure water cut needs to be added. This can be done using a gamma densitometer or through the use of microwave technology.
Having selected its approach, Burullus decided that the Simian/Sienna and Sapphire flow regimes all would be within the operating envelope of Roxar Wetgas meters, except at the highest production rates. The gas/water flow measurement accuracy of the Roxar Wetgas meter swayed the decision in its favor.
System capabilities
The Wetgas meter, recommended for gas/condensate wells with gas void fractions greater than 95%, was developed in partnership with major operators and is qualified for high-pressure/high-temperature applications in water depths of up to 3,048 m (10,000 ft).
Externally, the systems comprise a meter body installed directly onto the subsea pipeline or manifold either by flanges or welded directly. Pipe dimensions can be between 2 in. and 12 in. (5.1 and 30.5 cm), and the meter body is constructed using UNS S31803 duplex stainless steel.
The electronic canister consists of two parts (top and base) made from the same duplex grade. The canister top is the container protecting the electronics and housing the interface with the system control module (SCM). Two power connectors can be mounted on the top.
The canister base allows for mounting of the electronic canister to the meter body and also forms the electronics platform. When the canister is assembled, the electronics are surrounded by 1 atm of nitrogen and the connectors are pressure compensated to 3,048 m (10,000 ft) water depth.
A pressure transmitter (PT) mounted on the meter body measures pressure and temperature in the process flow. The PT housing is constructed using duplex steel and is qualified to operate at up to 10,000 ft subsea and in a process temperature range of -40 to 150º C (-40 to 302º F), pressures of 0-700 bar (70 MPa), and maximum line pressure of 10,000 psi (69 MPa).
A delta pressure transmitter (DP) also is mounted on the meter body to measure the pressure change over the V-cone. Measurements are performed through impulse lines between the process and the membranes in the DP cell. Measurements are used to calculate the process flow.
The Wetgas meter uses microwave-based dielectric measurements to generate accurate gas and condensate flow rates based on standard DP devices. It detects the resonant frequency in a microwave resonance cavity according to the dielectric properties of the fluid mixture in the cavity.
Since the permittivity of water is much higher than that of gas or oil/condensate, the dielectric properties of the wet gas mixture are consequently very sensitive to the water content. Tests have shown that the meter can detect changes in water production with a sensitivity better than +0.005% volume, while the absolute accuracy was +0.1% volume in high GVF (>99%) cases.
A PVT software package calculates individual liquid (condensate) and gas densities, and the actual gas/oil volume ratio at meter conditions. The calculated GOR is used subsequently to discriminate between gas and oil and hence to determine the oil and gas fraction, once the water fraction has been found through the meter.
The Wetgas meter provides online and direct measurement of water in a wet gas flow. Direct measurement of water is feasible as soon as it starts to be produced from the well, and the meter also can measure continuously the liquid fraction to correct the gas flow measurement and to determine the liquid flow rate. The technology has been refined further for accurate detection of salt water in the wet-gas stream, allowing the user to distinguish between condensed and formation water.
Wetgas configurations
After evaluating five different wet gas metering options, Burullus deployed the following configurations:
- Simian/Sienna: Eight venturi gauges, eight wet gas meters, eight dual wellhead gauges, eight dual flowline gauges, and eight sand monitor probes
- Scarab/Saffron: Eight venturi gauges, eight dual wellhead gauges, eight dual flowline gauges, and eight sand monitor probes
- Sapphire: Eight wet gas meters, four smart dual downhole gauges, four single downhole gauges, eight dual wellhead gauges, eight dual flowline gauges, and eight sand monitor probes.
Water production monitoring started almost immediately. Each meter was equipped with the water detection functionality to distinguish between fresh and saline water, and to identify potential water breakthrough.
On May 25, 2006, a water breakthrough was detected. Burullus opted to reduce the choke size in order to lower the amount of water produced and to generate better production profiles.
After three weeks producing at lower rates, Burullus decided to open the upper zone of the smart completion. Immediately water production started to increase. To avoid catastrophic water breakthrough, the well was shut in for a week. When production resumed, however, water started to increase to dangerous levels.
The lower inflow control valve position then was lowered, causing water output to decrease. Burullus subsequently increased the lower inflow control valve position in order to find the optimum gas production at the lowest water rate.
By providing early warnings of the water produced, the wet gas meters helped save three wells from water breakthrough. This also gave Burullus information needed to optimize the performance of its wells, leading to increased availability.
In the future, Burullus hopes to create a dynamic simulation model of the reservoir to be updated continually with production data. Thus, production and pressure data from the simulation model can be matched to historical pressure and daily production data by adjusting any number of reservoir parameters. This should reduce uncertainty and improve understanding of reservoir behavior. •
Editor’s note: This is an edited version of a paper delivered at PennWell’s Deep Offshore Technology International Conference in New Orleans in February 2009.
