KUWAIT: In terms of methods pertaining to artificial lift in the oil industry, Electrical Submersible Pumps (ESP) play a significant role in the extraction process. In the field, artificial lift is created by lowering the ESP string under tubing during well work-overs. The power cable is connected directly to the motor and clamped externally along the tubing string while the pump runs until failure. Because of rig work-over costs, operators utilize ESPs as sacrificial equipment, which means that no maintenance program can be scheduled.
The method
Wireline Retrievable ESP (WRESP) pertains to a change in ESP deployment. The method consists of first lowering a large bore tubing (e.g. 5 1/2″) with a docking station attached underneath as a permanent part that provides three receptacles for the wet mate electrical connectors. It also is comprised of a power cable connected directly to the connectors of a docking station and clamped along the tubing to the surface.
A retrievable part composed of a standard ESP string (motor, seal protector, gas separator, and pump) utilizes on its bottom side a motor connector provided with a three pin wet mat electrical connector designed to connect the ESP in a docking station once landed. On the top side, a seal assembly is present to separate discharge pressure from intake pressure inside the tubing; it also provides an internal fishing neck profile for lowering or retrieving the ESP string with a slick line method using a 5/16″ non-electrical braided-line. A 5″ GS running/pulling tool is used to engage this internal fishing neck for ESP deployment.
Obstacle
During this first implementation at Kuwait Oil Company (KOC), we faced an obstacle in which the ESP was stuck due to damage of a seal assembly that occurred each time during the ESP retrieval with a wireline method. The issue of seal failure was faced from the beginning of the pilot project in June. First, seal damage occurred to standard seals. After extracting from the hole, seals from the seal assembly were completely damaged and left in the hole.
We posit that rubber pieces had fallen down and might be deposited on the docking station connectors, thus preventing electrical connection. Many attempts were performed with a slick line junk basket to retrieve these small pieces of damaged seals.
Three months after the first trial, Zeitecs (a company that specializes in artificial lift technologies) delivered packer cup seals that were newly designed and used in Guiberson completion packers. These seals were used in the seal assembly to prevent any fluid recirculation through the pump. Unfortunately, during ESP retrieval with the wireline method, a huge over pull was registered on the way up at each tubing collar, even at low speeds (10 feet/min). Once the ESP was at the surface, we found that severe damage occurred to the packer cup seals.
Additionally, due to the continuous up and down movement (attempt to release the ESP string), the GS profile fishing neck was unscrewed and became loose. At this point, it was likely that the ESP string would fall on the permanent part.
After a long discussion with a technology provider, an alternative solution was developed. This solution consisted of lowering the ESP system in two parts that would be assembled with a PBR (Polished Bore Receptacle) as a separation tool. The first part would be a motor connector, seal protector and a pump atop the PBR; the second part would be the seal assembly.
The PBR and seal assembly are designed for two specific applications. As an expansion joint, it provides a stroke length for extreme tubing movement during well treatment and production. As a separation tool, it allows removal of production tubing string while leaving a “polished bore” and “anchor seal assembly” set in the packer.
Results
Unfortunately, after careful evaluation, we found that:
* After lowering the first part, it would be difficult to insert the polished bored in the second part because of the light weight of the wireline tool string.
* During ESP retrieval, there is a high risk that the top part will stick inside the polished bore. Therefore, the entire retrievable ESP part will move together, then most likely on the way up. The first part will be free from the second part (at “separation tool” level) and fall down; consequently, serious damage occur to a completion system.
Due to the repetitive sticking problems faced in the retrieval of the wireline ESP in each retrieval, and after a five-month downtime of the project, the client was at the point of halting the pilot project of this technology. In parallel, a KOC Well Surveillance slick line specialist was conducting a study of the root cause analysis for the problem. The conclusion was as follows:
As designed, packer cup seals create seals and prevent fluid recirculation inside the 5 1/2″ tubing in flowing conditions. During ESP retrieval, seals are not retractable. This means the entire hydrostatic head will be supported by these seals, thus a huge over pull will be created while moving up with a retrievable ESP that leads to serious sticking issues.
After calculation, it was determined that 4,200 feet of fluid above the seals would create a large forces acting downward that is equal to 27,800 lbs. This exceeds the maximum safe working load of the 5/16″ braided line cable. On the other hand, a swabbing effect will happen and a well control issue will be faced. In addition, a force created due to the swabbing effect will be added to dragging forces created by fluid.
Submitted by the Well Services Technical Team, Well Surveillance Group – Kuwait Oil Company
