Weatherford’s Alasdair Macneil Fergusson and Wiley Parker discuss how surface electromagnetic transWeatherford’s Alasdair Macneil Fergusson and Wiley Parker discuss how surface electromagnetic transmissions can help control downhole mineral scale deposition.
Controlling mineral scale deposition in oil and gas wells is a long-standing industry challenge. Weatherford’s ClearWELL, based on transmission of an electromagnetic field, is effective in preventing the deposition of scale deposits in a broad range of oilfield production systems.
Scale, foreign materials, and paraffin deposits can all cause maintenances problems in wells. These deposits form on the inside of tubing walls and valve surfaces can restrict flow, reduce control, and change heat transfer characteristics, all of which can lead to compromised production and safety, and contribute to corrosion.
On average, oil and gas fields produce 7 bbls of water for each bbl of oil. This water often contains dissolved minerals that can deposit in the wellbore and its processing system. Calcium carbonate and barium sulfate deposits are typical of these mineral scales, which are often encountered inside tubulars, within surface facilities, and around the wellbore, where pressure drops are greatest.
Much of the effort to mitigate the scale problem has been focused on methods of chemical scale inhibition or by mechanical removal. Chemical methods have been developed that are generally effective but high costs and environmental impacts drive the need for an alternative.
Electromagnetic technology
Weatherford’s ClearWELL technology is a non-chemical control strategy that uses a physical water-treating device to induce pulsed, high-frequency signals into the piping system. This energy causes micro-crystals of scale to form suspended in the produced fluid rather than on the surfaces of downhole and topside equipment.
Developed for industrial water treatment, the technology is based on an understanding that precipitation itself is not the problem; the problem is scale adhering to equipment. If scale can be induced to precipitate and remain in suspension, then the problems associated with mineral scaling are reduced.
Scale chemistry
The materials that form mineral scales have a finite solubility in a fluid, and a system is saturated when this balance is achieved. If more materials can be dissolved, the system is unsaturated. When the solubility limit is reached, a chemical balance is created between the dissolved and solid forms of the substance. At the point material precipitates out of solution, the system is supersaturated. Many oilfield water systems become supersaturated as physical conditions change or dissimilar waters are mixed.
The thermodynamics of solubility determine that in an unsaturated system, overall energy is lowered when a solid dissolves. In a saturated solution, there is no energy advantage to being in a solid or dissolved state. In a supersaturated system, overall energy is lowered when the dissolved material precipitates.
When a material precipitates, not only is a new phase formed, a surface between the oil and new phases must be formed. It takes energy to create a surface, and this energy is not accounted for in the bulk description of solubility.
The energy liberated when forming a precipitate is proportional to the amount of the material undergoing transition. The energy demand to make the new phase is proportional to the size of new surface created: a deposit’s surface area.
In a supersaturated metastable state, the system is waiting for enough energy to begin the phase change. When a particle begins to grow, an insufficient amount of energy is released by the phase change to create the surface. The particle’s large surface area-to-volume ratio means that the energy released from its formation may not be sufficient to form the surface surrounding the particle. If the system cannot acquire this energy from fluctuations in the surroundings, the energy remains trapped. While thermodynamics says the system must form a precipitate eventually, the system can’t begin the process.
If the new phase develops on an existing surface, then fewer boundary surfaces must be created. Less energy is required to make a solid crystal at an existing surface than in solution, so scale forms at surfaces. Unfortunately, when the new phase forms on an existing surface, it adheres strongly to that surface. At the wellsite, this existing surface is provided by pipe, valves, and other internal materials. And deposition on these surfaces causes problems.
The ClearWELL signal creates a timevarying electromagnetic field within the pipe, which acts as both an antenna and a transmission line. As charged particles respond, they absorb energy and move. This movement induces nucleation, but it does not occur at the surface level.
Forming a new phase always requires energy from the environment to start the process. If the only energy source is from the native environment, energy is limited. The systems normally develop according to the principle of least action, i.e., whatever requires the least amount of energy from the environment occurs most rapidly. Thus, deposits form under natural evolution of the system.
The ClearWELL system works by channeling energy from an external source into the materials that will form the new phase, providing sufficient energy for the new phase to form in suspension rather than on an existing surface. This channeling of energy to alter the natural evolution of a metastable system is illustrated with a carbonated beverage and an ultrasonic cleaning bath. When energized, the bath generates high-intensity sonic waves that impinge on a partially emptied soda bottle. The field induces nucleation of gas bubbles and the fluid spews from the bottle. The surface-installed electromagnetic technology does not alter the thermodynamic state of the system or its evolution, as with chemical treatments. Instead, it controls where and how that thermodynamic evolution occurs by transmitting an electromagnetic field that interacts with the ions present in the aqueous solution to generate nucleation sites. ClearWELL works as a threshold nucleation device.
The technology uses a magnetic loop antenna to induce a radio frequency signal in the metallic piping and aqueous conductive path provided by the water system. The conductive medium acts both as an antenna and as a transmission line for the current.
There are several advantages to introducing an interaction that moves throughout the system. By delocalizing, the system can be installed and applied without detailed knowledge of the location where conditions exist for the formation of mineral deposits. Because the current moves throughout the system, the device can be surface-mounted, without intrusion into the wellbore.
Application examples
Offshore scale control is a complicated endeavor. There are logistical issues associated with timely chemical delivery as well as environmental concerns associated with any chemical residues which may be released into the environment. Environmental concerns have been especially problematic in North Sea applications. ClearWELL has been tested by an international operator off Denmark in the North Sea, on a platform well with a dry tree producing from a chalk reservoir. Treated seawater was used for reservoir pressure support. Historical operation of the well resulted in severe calcium carbonate deposition throughout wellbore and surface equipment. Chemical control is impractical and the deposit problem has been historically managed by reducing the choke setting to minimize pressure drop. This severely impacts the rate at which oil can be produced. Even with the reduced choke setting, acid treatments were required on a nine-month schedule to maintain production. Historical data showed that at full choke well scale up occurred in approximately two weeks.
ClearWELL-R was installed in March 2013. Production strategy was to maintain original choke setting for two weeks.
Result: no change in production. At the end of the two-week period the choke setting was changed to that where the maximum CaCO3 deposition had been observed. The well continued to operate at high efficiency and production levels indicate no scale deposits after 10 months of steady operation.
ClearWELL was applied to control carbonate scale in wells offshore Africa. In this instance well operation was plagued by scale build-up, which restricted and eventually blocked oil production. The operator wanted a method to control scale as determined by bottomhole pressure without acid treatment or installation of capillary tubing for continuous injection of scale inhibiting chemicals. Fig. 1 shows a graph of bottomhole pressure as determined during a trial period and prior to the trial. BHP steadily increased prior to the ClearWELL program and has remained near-constant since its introduction.
The technology can also be applied onshore. In the Ketchum Mountain area of West Texas, water from several sources is sent to a pumping system for reinjection. Mineral content of the waters produces significant BaSO4 that reduces pump effectiveness and lifetime (Figs. 2-3). The water injection system uses polyethylene piping for the distribution headers to electric submersible pumps (ESP). The mineral content of the waters in the distribution system is very concentrated and thus the water conductivity is high.
This circumstance allows the aqueous medium to serve as both the antenna and transmission line for the electromagnetic signal. The impedance of the transmission line is much higher per linear foot than those seen in metal piping schemes. This limits the propagation of the signal. To partially account for this, two ClearWELL units were installed—one on the water input and one on the output distribution header.
The units were placed on the system after one day of operation. In a 34-day observation period, the installation saw no net deposit formation, and deposits that had formed in the unit prior to installation were cleaned up by the system (Fig. 4).
A major solution
Scale is arguably one of the most difficult and expensive problems facing production operations. Much of the mitigation effort has been centered on chemically inhibiting its formation or by mechanically removing it. But the steps are often unsatisfactory.
A new approach to the problem is focused on preventing the deposition of scale rather than treating it when it appears. The success of this system in global applications suggests a major step forward in production operations.
Mineral deposits had reduced pump effectiveness and lifetime in a reinjection system. No deposits occurred after installation of the ClearWELL system: (A) shows the pump condition at startup; and (B) shows it after three weeks.
Alasdair Macneil Fergusson is the Director of engineered chemistry for Weatherford Oil Tool Middle East Ltd., based in United Arab Emirates. He has a degree in Chemistry from University of Aberdeen.
Wiley Parker is a Technical Specialist in Weatherford’s Business Development division, based in Houston. He earned a PhD in Chemical Physics from Washington State University.
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