Offshore in depth

In a typical E&P cycle, a period of rising production is followed by a plateau, and then a decline. If it is determined that the decline is not due to reservoir depletion, but downhole problems with one or more wells, an offshore asset team can make a decision to perform a workover, provided there are sufficient reserves to justify the repair cost.

A number of deployment methods have been developed, including wireline/slickline, snubbing and coiled tubing. These can be deployed from a semisubmersible with a workover/completion riser; a specially designed vessel, dynamically positioned or conventionally moored with a workover/completion riser system; with wireline or coiled tubing from a specially designed vessel, either with a riser or riserless; and a TFL/PDT system for subsea wells from a remote surface facility.

For “dry tree” wells from fixed platforms, TLPs or semisubmersible floating production facilities, coiled tubing access would be cost-effective. If the operation can be conducted through the production tubing, the work could be accomplished with wireline/slickline or with coiled tubing, if fluid circulation is required. Both of these methods require a workover/completion riser deployed from an appropriate vessel on the surface.

TFL/PDT. Another workover technology is the TFL/PDT (through-flowline/pump-down-tool) system. As the method indicates, the tools are deployed from the floating surface facility and pumped through the flowline, subsea tree, and down the production tubing. The technology enables routine maintenance operations to be performed through the flowline/production tubing. TFL/PDT provides the means to pump the tools through the flowline and into the well, and thereby, perform minor workovers.

The TFL/PDT has been used successfully for workover operations, such as sand washing, cement squeezing, acid washing and squeezing, perforating, installation or retrieval of gas lift valves in side-pocket mandrels, and even fishing for lost tools in the tubing.

The main components of the TFL/PDT system are locomotive pistons. The tool string is much like a wireline tool string, as it consists of stems, jars and knuckle joints. The pistons serve to transport the tool string through the flowline, and into the point of location in the wellbore. A pump-and-manifold system at the surface provides the means to pump the tools in and out of the wellbore and flowline.

To prove up this system, considerable development testing was performed by BP, Exxon Mobil and Shell. All three operators concluded that it would be a workable, operational workover technology.

There was one development that installed the system in its subsea architecture and downhole completion design. Unfortunately, the multiple subsea hardware and downhole, wellbore completion equipment resulted in the system failing, due to lost tools in the wellbore that could not be “fished out” with the system.

Wireline deployment is restricted by the design limitations of the packing in the lubricator, and the shear-out capacity of the wireline-to-coupling attachment. The TFL/PDT manifold can be based on the well pressure, and since it will be using fluid, the fluid density can be specified accordingly.

Artificial lift via ESP. Electrical submersible pumps (ESPs) are being used in subsea environments to provide a relatively efficient form of artificial lift, across a broad range of flowrates and depths. By decreasing the pressure at the bottom of the well (by lowering bottomhole flowing pressure, or increasing drawdown), significantly more oil can be produced from the well, when compared with natural production.

ESPs can include a water/oil separator that permits the water to be re-injected into the reservoir without the need to lift it to the surface. There are at least 15 brands of oilfield ESPs used throughout the world. Until recently, ESPs had been costly to install, due to the requirement of an electric cable downhole. This cable had to be wrapped around jointed tubing and connected at each joint. Alternatively, the cable could be installed like the electric lines for downhole pressure and temperature instruments, and the hydraulic lines that are connected to SCSSSVs.

Once incorporated into a subsea completion, it may be necessary to replace an ESP, which is typically installed inside the production tubing. Located near the bottom, it can be retrieved through the tubing.

Rigless intervention. The world’s first rigless, fully retrievable (RFR)-ESP was successfully installed recently by Eni at its offshore Congo Foukanda field. The RFR-ESP technology, provided by ZEiTECS, allows the rigless deployment and retrieval of a conventional ESP system through tubing via standard oilfield wireline, rod or coiled tubing technology. The RFR-ESP technology is based around a specially designed oilfield wet connector system. This “plug and play” connector design permits the use of any ESP manufacturer’s equipment, allowing ESP optimization to match changing well conditions and replacement of failed ESPs without rig interventions. This achieves significant cost savings in frontier areas, where workover costs can be high. Furthermore, there is no loss of production while waiting for a workover rig to arrive at a remote location.

As such, a variety of offshore workover options are available to recover production in case of downhole problems. Rigless intervention, wherever possible, is key to reducing costs in the otherwise high-cost offshore environment.