Shell UK Exploration & Production, an operating unit of Royal Dutch/Shell Group, used an electromagnetic measurement-while-drilling (EM-MWD) tool to transmit downhole-measured data during drilling operations from more than 14,500 ft MD (6,400 ft TVD) in a southern North Sea well.
Calgary-based Precision Drilling Corp. deployed the tool, which it calls the Precision EMpulse EM-MWD system. The companies said, in a press release, that the job sets a new world record for depth range of downhole data transmission using the electromagnetic technology offshore.
The EM-MWD tool transmitted the toolface, inclination, azimuth, gamma ray, and annulus-pressure data while drilling a 6-in. horizontal-hole section below the Zechstein formation in the North Sea's Leman field.
Because the rig's mud system did not interfere with the functioning of the tool, it operated continuously during all drilling activities, even when the rig made drill pipe connections, said the companies.
"We have already witnessed significant time-savings and potentially broader data transmission capabilities that can be realized with the [EM-MWD] tool onshore," said Neil Brown, Precision's technology services group vice-president.
He added, "Offshore, the technology's impact could be far reaching in both conventional and underbalanced drilling applications."
To maximize the technology's performance and potential for success, Precision Drilling incorporated into the well design its patented technology of electrically insulating the casing with an external epoxy coating, according to Precision Drilling's EM product line manager Denis Weisbeck.
The external coating, which insulated the casing strings from the formation, had the effect of minimizing signal attenuation and allowed transmission of the electromagnetic wave from the well depths, without relying on extended-range antennas within the drillstring to move the signal-transmission point closer to surface.
Tool operation
The EM-MWD tool is battery powered and uses low-frequency electromagnetic waves to transmit downhole-measured data to the surface in real time during conventional and underbalanced, horizontal, and directional drilling operations.
Electromagnetic telemetry transmits information through the formation to a surface antenna that receives the data and sends it to the acquisition system for processing and decoding (Fig. 1).
Crews install the surface antenna at the seabed for offshore applications (Fig. 2).
Weisbeck said, "In order for the signal to be decoded at the surface, it first must be injected into the formation and then propagated to the surface. Injecting the signal into the formation is more efficient when the conductivity is high (low resistivity).
"Once the signal is injected into the formation, higher resistivity formations provide a better medium for signal propagation and help minimize attenuation."
Operators using EM-MWD telemetry are able to drill and survey wells independent of rig hydraulics. Bit pressure drop, flow rates, drilling fluid type, and mud losses to the formation are transparent to the technology.
The EM-MWD system can collect and transmit survey data regardless of whether the mud pumps are on or off, as long as the drillstring is not moving.
This is unlike the mud-pulse telemetry that requires a period of time for mud circulation to completely fill the system, resume data retrieval, and then continue drilling operations.
Saving several minutes per connection, EM-MWD capabilities equate to substantial savings in drilling time and total project cost.
Steve Mack, Precision Drilling's logging-while-drilling formation evaluation manager pointed out that EM MWD does not necessarily increase the data-transfer rate over the conventional mud-pulse telemetry.
It does provide two-way communication, however, allowing the EM-MWD engineers to talk to the tool without impacting drilling hydraulics, which is not easily or commonly done with mud-pulse telemetry, he added. Only one application that he was aware of used a mud-modem device to communicate downhole, but again it would impact the rig's hydraulic system.
First use offshore
The first commercial EM-MWD jobs began in 1987 and were performed on land, with more than 3,000 wells drilled using the technology in the past few years.1
Repsol-YPF-Maxus performed the first EM-MWD job offshore, using an extended-range setup, in the company's Krisna D12 well off Indonesia in June 2000.2 3 The company used underbalanced drilling to prevent massive lost circulation and stuck pipe within the reservoir.
To manage underbalanced drilling operations properly and guide the well's trajectory, engineers required bottomhole directional information and annular pressure data.
The well's underbalanced condition and the use of two-phase flow in the mud system precluded the use of mud-pulse telemetry and EM-MWD became the system of choice.
Although the reservoir was relatively shallow at only 4,200 ft TVD, low-resistivity formations above the reservoir would cause high attenuation of the electromagnetic signal.2
To overcome the problem, Repsol employed an extended-range transmitting antenna to keep the point of transmission at about 1,500 ft from surface, which was not the case for Shell's recent Leman field well.
Wireline assemblies with a wet connect and hanger, which the operator had run in two stages, extended the point of transmission to as high as 5,678 ft above the EM-MWD tool.
Repsol achieved its primary goals, avoiding lost circulation, stuck pipe, and other drilling problems during underbalanced operations, with the first extended-range EM-MWD system deployed offshore.2
References
- Hicks, T. and Colvin, S.A., "Leading the Way, EM MWD Tool Proves Effective in Difficult Drilling Environments," The American Oil & Gas Reporter, July 2001, p. 45.
- Weisbeck, D., Blackwell, G., Park, D. and Cheatham, C., "Case History of First Use of Extended-Range EM MWD in Offshore, Underbalanced Drilling," Paper No. SPE 74461, presented at the IADC-SPE Drilling Conference, Dallas, Feb. 26-28, 2001.
- Park, D., Brand, P.R., Allyson, B. and Sodersano, G., "Planning and Implementation of the Repsol-YPF-MAXUS Krisna Underbalanced Drilling Project," Paper No. SPE 67689, presented at the IADC-SPE Drilling Conference, Amsterdam, Feb. 27-Mar. 1, 2001.
