Understanding, innovations continue in shallow water flows

The external flow shut off sleeve is one of several solutions ABB Vetco Gray has designed to overcome shallow water flows. [20,779 bytes] Shallow water flow (SWF) zones continue to be a major headache for operators in deepwater US Gulf of Mexico drilling programs. These zones cause lost circulation and often require an extra casing string. In exploration drilling, there is the additional challenge of not knowing exactly where these zones are located, how many exist, and the relative strength of

Shallow zone solutions simplify completions

William Furlow
Technology Editor
Shallow water flow (SWF) zones continue to be a major headache for operators in deepwater US Gulf of Mexico drilling programs. These zones cause lost circulation and often require an extra casing string. In exploration drilling, there is the additional challenge of not knowing exactly where these zones are located, how many exist, and the relative strength of each.

Often, the drilling crew will think the SWF zone has been cleared, only to encounter another further downhole, forcing them to set an additional casing string. If there is a significant difference in the strength of these flows, then the operator will run into mud problems.

Even in the best cases, SWF zones force drillers to use more casing strings early in their program, so that the wellbore diameter at total depth is at risk of being too small to produce from. In a worst case, the SWF zones flow so strong that they prevent a proper cementing job, causing loss of circulation, and washing out the wellbore. Unidentified zones are especially dangerous because they can wash out cement and prevent a good seal and proper vertical load transfer between the casing and the formation.

Currently, the state of the art in controlling these flows amounts to sealing them off downhole so an operator can drill through them. Depending on the system an operator prefers and the type of project being drilled, the options range from a ball valve shut-off device to an external flow shut-off sleeve. The goal of these devices is to seal off the flow zone between the conductor casing and wellhead, containing it so casing can be set through the zone.

"These systems contain shallow-water flow, but the phenomenon of shallow-water flows will continue to exist," said John Rosso, Product Group Manager of Drilling Systems for ABB Vetco Gray. The fact that the flow still exists means there will still be loss of materials and cementing problems in these zones.

There are three systems ABB Vetco Gray considers when talking with customers about options to contain SWF, according to Rosso.

  • One system incorporates a 38-in., 26-in., 20-in. string. In this system, the operator would run the 38-in. conductor as normally done. Then the 26-in. conductor would be run to a depth just above the SWF zone, hung off in the 38-in. at the mudline, and cemented in place. The 20-in. hole would then be drilled through the SWF zone.
Once complete, the 20-in. casing string would be hung off below mudline, cemented and packed off with a metal-to-metal seal, and subsequently tied back to the wellhead with a seal sub. This system provides SWF zone protection with a metal-to-metal seal at the 20-in. hanger. Therefore, if the field is abandoned and wellhead removed, the SWF zone protection seal would remain in tack.

  • A similar system features an additional conductor housing and consists of a 42-in., 36-in., 24-in., and 20-in. strings. In this system, the 42-in. conductor is run, similar to that of the 38-in. in the previously described system. The 36-in. conductor is then run and cemented just above the SWF zone with the 24-in. run through the SWF zone. In this system, the 24-in. is hung off in the 36-in. conductor with the SWF zone sealed by use of a high integrity seal. The 20-in. is then run to a further depth and cemented as normally done in non-SWF wells. This system also provides sub-mudline SWF zone sealing but provides an additional conductor string and does not use a tieback.
  • The third system is mechanically different in that it uses a sliding sleeve at the mudline to seal the SWF zone. In this system, the operator runs a 38-in. conductor as normally done. He then runs a 26-in. string just above the SWF zone, hang it off with a hanger at the mudline and cement it in place. The operator then runs the 20-in. conductor through the SWF zone and uses the sliding sleeve as a valve to seal the annulus between the high pressure and low pressure housings, therefore securing the SWF zone. The sliding sleeve is activated using a modification to existing tools. In some cases, the sliding sleeve may be substituted by ball valves that can be actuated with a remotely operated vehicle.
The selection of system or system modification is left up to the client. Because deepwater exploration, and especially shallow water flow problems, are geologically based and in the frontier stage of understanding, the new technologies are customer driven.

Rosso said his company has taken a broad approach to solving these problems, branching out from manufacturing equipment into understanding the geology involved in the formations. This broader involvement allows the company to assist clients in the design of specific solutions that address individual problems and desired solutions.

JIP to advance field While these solutions may contain many of the shallow water flows offshore, they do not control these flows. Rosso says the next challenge for the industry - controlling SWF - is taking on momentum. If the operator can prevent or minimize these flows, loss of formation material can be eliminated, minimizing both operational cost and well risk. To this end, ABB has been selected by a joint industry project (JIP) to develop a system for controlling SWF which would operate and compliment any supplier's SWF subsea wellhead system.

The JIP currently consists of Amoco, Chevron, and Conoco. The JIP has been established as an open industry. Companies interested in joining may contact: Amoco- Ken Armagost -281/366-2732; Chevron - Paul Hebert-504/592-7220; or Conoco - Allen Gault -281/293-3338.

While these solutions are able to contain many of the shallow water flows offshore they do not control the flows. Rosso said this is the next challenge the industry is taking on. If an operator can prevent these flows in the first place they would eliminate the lost circulation and washout problems these flows are causing. To this end, ABB has been tapped to manufacture equipment for a system that would operate with existing wellhead designs and prevent SWF from occurring.

Bill Valka, Manager of Subsea Drilling Systems for FMC, said his company has successfully employed a 26-in. hanger with a sealing mechanism between the 26-in. and 20-in. casing strings on top of the conductor housing. This gives FMC the ability to design a pack-off assembly that can be retrieved if necessary. By placing the pack-off in such a configuration, the operator can run the high-pressure housing with the 20-in casing in a single trip, reducing installation costs.

Valka said the FMC design has the advantage of providing a large flow by passage prior to setting the packoff which eliminates the potential for erosion problems in the hardware. Additionally, this design will accommodate full cement returns to the surface. The design also permits rapid setting of the packer mechanism to minimize adverse affects from a flow, and improves cementing effectiveness.

This particular pack-off can be pressure tested after it is set and uses a hydraulic feed back to confirm that the packer has set properly. In addition to this sealing mechanism, Valka said FMC has worked with operators to design systems that allow more casing strings to be run in areas of the Gulf where there are soft soil conditions.

Other designs, driven by customer requests, require that the 20-in. hanger be placed below the mudline and packed off below the mud line. The company is in the conceptual stages of developing alternative ideas for sealing mechanisms that could be installed in one trip down hole. These systems would provide operational advantages for exploratory wells.

"We are in the process of coming up with new ideas and seeking the review of some operators," he said. The problem with these single-trip designs is that the packers are generally not testable, or retrievable. To offset these shortcomings, Valka said FMC is designing in redundancies that would offer a back-up system.

Operator understanding

One example of how closely operators and manufacturers are working together to solve this problem is Valka's assertion that some of what appear to be SWF conditions are actually caused by operator drilling techniques.

He said FMC has been working closely with operators to determine if drilling procedures can produce an affect that resembles SWF. "In some cases the drilling rates the operators strive for are actually over-pressuring the formations," he said.

The operator is pre-charging the formation in the drilling process and the drilling fluid comes back out at a later point in the drilling program in a form that resembles a SWF.

"We are just trying to better understand the whole process so we can contribute something along the way," he said.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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