This article was originally published in the Offshore Engineer magazine in May 2015.
Desanding to stop wellbore tubing clogging-up is nothing new, but FourPhase has developed their own technology to keep the flow going.
Conventional oil and gas accumulations are found in sedimentary sandstone and carbonate rock formations. Unconsolidated sandstone reservoirs are susceptible to sand/fines production when permeability is 0.5-8 Darcies.
These sand/fines particles become dislodged during the production process, giving rise to solids production. The debris then accompanies the water and oil flowing through the tubing to surface. Historically, without intervention, the wellbore eventually becomes partially or completely blocked.
Remedial measures include reducing – or choking back – the flow rate, gravel packing, installing downhole sand screens or injecting chemicals into the formation to strengthen artificially. However, these processes reduce production and could create threshold velocity.
The transported sand may then accumulate as a stationary dune, which can cause partial or complete blockage of the wellbore and potentially a complete shutdown of the production separator.
There are two methods to prevent this happening. First, produce at the maximum flow rate to get the sand, oil, gas, and water to topside where the separator is mounted, and second, use barriers downhole in order to prevent the sand entering the wellbore.
Gravel packing, chemical cementing and sand screens provide this initial downhole sand barrier. However, over time, the sand erodes these barriers and moves into the tubing. Typically, sand is heavy, which means it is not very mobile and eventually starts collecting in the tubing, restricting flow and eventually clogging up. So you need something to clean it out.
FourPhase has developed a dual cyclone solids removal system called DualFlow. The unit, which comprises two cyclone pressure vessels comprised of 1.4507 (APM2327), used in parallel so one can be isolated and flushed while production continues through the second. The unit removes solids from the production flow, by using centrifugal forces internally to accelerate and separate the sand, allowing a higher production flowrate, stopping solids accumulating either in the wellbore or production separator(s). It includes real-time information logging, recording flow rates, pressure, temperature, and solids removed. Its dynamic range is 20 micron to 5mm solids. Standard DualFlow units are rated to either 5000psi or 10,000psi as required.
The system, which weighs 8500kg and has a 200 x 200 x 328 cm footprint, has a smaller footprint, at one third of the size of existing units on the market; uses Super Duplex piping and a carbon steel frame; and is 99.8% efficient. FourPhase also supplies its own X-Flow choke manifold, which has directional control and a reversible flowpath. It can be used for well clean-up or kick start applications or permanently installed in the production facility.
Production from the well is routed to the DualFlow unit and the flow rate increased so as to allow enough velocity and energy in the fluid in order to lift the accumulated sand/fines particles from the wellbore. The weight of the accumulated solids in the unit is typically 20-200 kilos/hr – flowrate dependent.
Norwegian energy giant Statoil chose FourPhase’s solution for the Gullfaks C offshore installation, in the Norwegian Sea. Gullfaks started production in 1986. Initial recoverable reserves were estimated to be 2.1 billion bbl.
Production peaked at 605,000 b/d in 1994 and is now about 110,000 b/d, with a 62% recovery rate. Intended production recovery is aimed at 74%, with part of this being achieved by using sand control technology.
Gullfaks C is known for its challenging solids production, which has resulted in reduced production rates due to wellbores having to be choked back and even shut in. Choking back means having to reduce wellbore flowrate to reduce the amount of sand produced, while shut-in means no flowrate because downhole tubing is plugged with sand.
Historically, sand is handled by locating a sand trap at the entrance to the test separator. This trap usually handles 1-5% of total sand production, corresponding to 0.3-1.5 kg/hr of sand per well.
Statoil’s well 34/10-C-19 at Gullfaks was drilled and completed in 1993/94, and recompleted in 2002. It was found that the sand rate rapidly exceeded the acceptable solids rate (ASR) limit after clean-up, with well flow being choked back. This occurred because the flow was too low to lift the sand up to the surface, to the topside-mounted separator, which would result in sand accumulating in the wellbore over time and a sand plug being formed.
A sand clean-up is required once this happens. Conventionally, coiled tubing (CT), or snubbing, is used to establish flow into a test separator.
CT is run into the hole with a circulating nozzle, going to the bottom and circulating the solids. The CT is tied-in to the riser and connected into the DualFlow, to separate out the solids, and returning either through the redundant Xmas Tree or tied directly into the test separator if available.
The test separator is not always accessible at the tie-in points. So FourPhase has used the wellhead service wing with both master valves closed and the production wing open. Without coiled tubing, pressure support is needed in the reservoir to lift the solids.
But, using CT is a relatively expensive process, costing some NOK 20-30 million (approx. US$3-5 million), equivalent to having a solids removal system on a rig for a whole year. Snubbing involves running the drill string on a pipe string using a hydraulic workover rig at even greater cost.
A sand clean-up in Gullfaks’ C-19 had been carried out almost every second year since it came online. The process also has disadvantages, including sand separation possibly being insufficient, a new CT rig having to be mounted for each well, having to send the sand to shore for handling, and the clean-up operation conflicting with other operations in the area.
Up to 2012, Statoil had used a de-sander during CT cleanout on C-19 that took out about 50% of the sediments, but the well was sending more sand, from the wellbore, than the production facility could handle. In 2013, DualFlow was introduced, which routed the production from the well to the DualFlow unit. The flowrate was then increased to allow enough velocity and energy in the fluid to lift the accumulated sediments from the wellbore. The DualFlow unit measured the accumulated solids, which were 20- 200 kilos/hr, depending on flowrates. Online flushing of alternate cyclone vessels ensures the well remains in constant production while being cleaned.
Over the last two years, a total 15,000kg of solids has been removed from C-19 alone. Two well bores were also being treated going to press. In total, FourPhase has removed 37,000kg of solids from 14 different wells from the start of the operation in Gullfaks.
In 2013, Wintershall took over the operatorship of Brage from Statoil. However, a Brage well suffered badly from solids production and was shut-in.
FourPhase mobilized early to kick-start the well and potentially be hooked-up for further solids removal. The well was successfully kick-started and some 7-tonnes of solids was removed from the wellbore without CT or snubbing.
FourPhase’s solids removal system kick-started the blocked well without downhole intervention. The DualFlow took out the solids upstream of any topside process units. The well is now the second-best producer on the whole field, contributing to 10-11% of Brage’s total production.