The majority of hydrocarbon reserves are contained in sandstone reservoirs, where sand production is likely to become a problem at some point during the production life of an oil or gas field. When oil or gas is produced from relatively weak reservoir rocks, small particles and sand grains are displaced and enter the fluid flow – up to several tons of sand per day.

Fluid velocities are typically high enough to transport the sand up through the wellbore to the platform topside, accumulating in production separator vessels and production process pipelines.

Produced sand is detrimental for several key reasons. When it reaches the surface, the abrasive nature of sand can damage valves, pipelines, pumps, and separator vessels, and when a significant volume of sand collects in separation vessels, the capacity of the separator to process oil, gas, and water is reduced.

To restore production, wells have previously been shut-in to allow the separation vessels to be opened and the sand manually removed. A critical path shut-down operation will be required, and, as a consequence, deferred production costs are likely for operators and the supply chain.

At Cape Environmental Services Offshore, online sand removal technology is enabling operating companies to continue producing by removing sand from the separation process without the need for a platform shutdown.

Traditional methods for sand removal

Sand jetting systems are commonly installed in the upstream separator to remove solids that settle in gravity separators. They fluidize the solids with pressurized water introduced through specially designed nozzles. The solids are then discharged through sand drains located along the length of the vessel. The system can be arranged to flush the complete length of the vessel at the same time. However, these systems can be problematic as they are prone to blockages, and it is very difficult to clean a blocked nozzle.

Alternative systems using internally installed hydro-cyclone technology are common; however, they are susceptible to unreliability due to changes in sand particle size and density.

Separator vessels with no internal sand management devices can only be manually cleaned. This requires the shutdown of the vessel and, in most cases, the complete cessation of production on the platform. The well will be shut-in, and a critical path shutdown cleaning operation will be required. This type of operation involves opening the vessel and introducing personnel to confined spaces, adding safety risk. In addition to the manual cleaning operation cost, the value of deferred production must be taken into account.

Online vessel sand-removal system

The principle of online vessel cleaning centers on inserting a water pressure hose into the belly drains of a separator to fluidize formation sand so that it is able to flow out of the vessel using the vessel’s own production pressure. A sealing gland system is used to maintain system integrity when the vessel’s in situ valves are opened, and the hose is inserted. The gland is designed to withstand the high axial pressure differential created when the vessel drain isolation valves are opened and the vessel is live.

Sand is transferred through steel conduit pipe work to Cape’s Syphonvac sand management unit where the flow of material is controlled. The material flowing into the Syphonvac is dispensed into an airflow created by the Cape ESO Sludgevac airflow system, with the material being deposited into collection skips by this system for processing that can include chemical treatment, maceration, and overboard disposal.

Using a thermographic camera, the sand accumulation in a separator can be determined. Thermo-imaging is used to identify the areas within the vessel where sand has settled and can indicate relative quantities.

The unit is configured from two systems: one high-flow vacuum and one low-pressure water jetting system. These connect to the vessel discharge point via a common manifold, allowing water to be injected into the vessel via a low-pressure jetting hose while allowing the sand to discharge from the vessel under the vessel’s own pressure. The units are modular and compact, taking up a minimal deck footprint.

Bespoke jetting heads are used that direct the water laterally between the belly nozzles inside the base of the vessel. The jetting head will be inserted 3 in. into the vessel to fluidize the sand. Water is injected at regular intervals in 15-sec bursts at a pressure of around 3,500 psi. As the sand becomes fluidized, it flows into the discharge point and is directed into the common manifold. From the manifold, sand is discharged along steel conduit pipe work connected to the system. This pipe work is rated to 1,434 psi.

Upstream of the system is a two-valve safety isolation unit. A manual choke valve is used to drop pressure and slow the solids flow, and a pneumatic valve is used to automatically isolate the Syphonvac from the vessel being cleaned in case of an emergency.

The manual safety valve is controlled by the operator with hand and foot controls, while the automatic safety valve can be controlled by safety systems on the platform or by a built-in gas detection system should any unforeseen changes take place.

This pressure spillback relief valve is rated to 1.5 times the vessel operating pressure.

The sand management system allows the operator to control the amount of sand discharging from the vessel using the sacrificial control valve and emergency shutoff valve. The sand management system is fitted with gas-sensing equipment. The gas detection system is set at a level that is 50% of the separation module. If gas is detected, all other functions controlled by the system are overridden.

Thermographic checks during the online cleaning operation assist in determining when the sand has been removed from the area around the sand discharge point and prompt the operator to move onto the next available drain isolation point.

Benefits of online cleaning

Making costly platform shutdowns to remove sand from production vessels a thing of the past is the primary focus of online cleaning and already is being proven in the field.

Reducing the risk associated with traditional man-entry to confined spaces techniques is the first immediate benefit of the system.

Operating as a safe and cost-neutral system for removing sand and sludge from production vessels, this system does not interrupt production, enhancing revenues and profitability. It also reduces sand carryover to process systems.

The newest system can be attached to vessels operating at 1,160 psi pressure. The compact system can be moved between discharge points to access sand buildups, without disrupting production.

Its capability to be used prior to enforced shut-down operations to remove sand also reduces the time the asset is required to be out of service.