Senior Technical Editor
In early May, on the eve of the Offshore Technology Conference in Houston, geoscientists and drilling engineers will gather in Galveston for a workshop on seismic while drilling (SWD) jointly presented by the Society of Petroleum Geologists and the Society of Exploration Geophysicists. The last such meeting took place in 2007; since then, deeper wells and a post-Macondo emphasis on well safety have lent new urgency to the effort to develop better methods to see in real time not only what is around the bit during drilling operations, but what lies ahead of it.
|When a sparker is activated, a shock wave radiates from two closely spaced electrodes immersed in fluid.|
SWD systems currently on the market use a surface seismic source and hydrophones deployed downhole in the drillstring to estimate pore fluid pressures ahead of the bit. But the surface source method can slow down the drilling process, and the data gathered can be difficult to interpret. Over the past several years, researchers have devoted much effort to developing SWD technology that uses a downhole source, based on the assumption that the resulting data would provide much greater resolution. But the efforts so far have met with little success in tests, says Robert Radtke, president of Technology International. "The primary reason these efforts were not successful is because of tubular losses. In other words, too much of the energy went up along the drillstring and not enough into the formation. The solution is having greater control over the range of frequencies generated at the source."
Technology International, with support from Apache Corp. and the US Department of Energy, developed and tested a drillstring-deployable sparker tool that demonstrated the feasibility of a source with adjustable frequencies from as low as 2 Hz up to 1,000 Hz. The tool generated pressure waves at frequencies up to 200 Hz that were observed up to 4,500 ft (1,372 m) away from the source. The company is now working with an industry consortium to develop a wireline-deployed reverse vertical seismic profiling (RVSP) tool with extended range and frequencies that can be controlled from the surface. "With the advent of variable frequency bandwidth, we can select the optimal frequency for the range required," Radtke says. "That depends on how deep you are in the hole and how far away your sensors are from your source.
"What we have is a source where we've demonstrated that we can generate frequencies from 2 Hz to a kilohertz," he continues. "That source is what's called a sparker. When operated in water and provided an electrical impulse that discharges across two electrodes, it forms a high pressure bubble in the water. This bubble creates a first pressure pulse and, several milliseconds later, another one due to the bubble collapsing. The time between bubble formation and its collapse determines the center of the frequency band generated. So if you take a conventional sparker and put it in water at depth, the deeper you go, the higher the frequency. We have developed a way to operate a sparker so that we can generate selected frequency bands that are not depth dependant."
The goal, says Werner Heigl, a senior staff geophysicist at Apache Corp., is "to have an artificial source in the drillstring so you do not have to rely on the bit as a source of acoustic energy, and you don't have to have your sensors in the noisy drillstring. The source is designed in such a way that you, one, can operate it in a certain frequency range that does not have much in common with the frequencies of the noise that the drillstring generates, and two, get the frequency low enough so that these waves actually propagate far enough and can be recorded on the earth's surface, or in a nearby well, depending on the application."
Much of the research in SWD since Macondo has been driven by safety concerns, Radtke says. The aim is "to be able to predict pressure ahead of the drill bit, and also to prevent too high of a mud weight, so you don't fracture the formation with the drilling fluid. That is a huge cost factor." The development of RVSP could also improve the detection of salt flanks and other obstructions that are hard to image directly with conventional seismic. Current models, Radtke says, "are not accurate. Typically, they can be off by 500 ft in terms of where productive formations are that (operators) want to access." Seismic data from deep wells can resemble a "snow bank," he says. "What's important to drillers is getting to the target. So the significance of imaging ahead of the bit is that they will be able to reach targets with minimal drilling cost and increased safety."
With backing from the new industry consortium, Radtke is working to double the acoustic energy generated by the sparker in order to produce more uniform seismic signals. "Because we operate at low frequencies, we don't need very high power," he says. "Range is primarily a function of frequency, not power. With our low frequencies, as predicted by acoustic modeling, we could conceivably be at the bottom of a 35,000-ft well, bring a signal to the surface, and see 1,000 to 3,000 ft ahead of bit, depending on the formation."
Making a case for SWD offshore
Offshore spoke recently with Neil Kelsall, seismic domain champion, Europe, Caspian and Africa, at Schlumberger, which launched its seismicVISION seismic while drilling service 10 years ago. Kelsall discusses the advantages and challenges of SWD, and where the technology is headed.
Offshore: In what type of offshore applications is SWD most useful?
Kelsall: Seismic while drilling has been best received in deepwater exploration wells and highly deviated development/exploration wells. In exploration wells, especially wildcats, drillers have found it reassuring to receive reliable updates on the expected target depth, especially if they do not want to drill into it. In the highly deviated/horizontal wells, it is now a routine process to acquire data with SWD using no rig time, which would otherwise not be feasible due to the days of rig time or hole stability issues to convey a wireline tool.
Offshore: What are the chief benefits of SWD technology?
Kelsall: SWD technology can quickly reduce significant depth uncertainty, save rig time, and reduce well costs. It is used to measure seismic velocity to answer operational problems in wells where wireline deployment is too risky or is very expensive in terms of rig time. Acquiring the seismic checkshots in real-time mode during the drilling phase gives the ability to update the driller with the target depth, increasing accuracy while drilling closer to the target.
In exploration, that means the ± 150-m (500-ft) safety margin to set casing can be reduced to ± 20 m (50 ft). This impacts well design, potentially removing the need for side tracks, contingency casing, and smaller hole sizes.
SWD has already played a role in presalt and subsalt plays, providing real-time prediction of the depth of targets ahead of the bit. Over the last 12 months, we have seen increasing interest from operators planning these types of wells.
Offshore: What kind of equipment and procedures are needed in an offshore SWD program?
Kelsall: We take offshore a seismic air gun array with compressed air supply, a small case with programming and clock synchronization system, and the seismic logging-while-drilling tool. For some applications, the gun equipment will be on a boat along with a navigation system to accurately position the seismic source.
In terms of procedures, there is almost no impact on the drilling program, as the measurement is made during pipe connection. We ask for the riser boost pump to be shut off when we acquire data at the drillpipe connections, and in case of pipe movement caused by rig heave, we may ask for the heave compensation to be activated to keep the pipe still for a few minutes.
Walk-above surveys in highly deviated wells involve recording data at stationary pipe connections when pulling the drillpipe out of the hole after drilling is complete. The boat captain and navigator keep the guns vertically above the downhole seismic tool as it moves toward the surface. It is usually possible for the boat to stay in position above the tool without impacting the speed at which the pipe is pulled out of hole.
Offshore: What are the technology challenges involved in SWD?
Kelsall: In the beginning, the main hurdle for SWD was having a downhole clock with enough accuracy for borehole seismic measurements and the ability to survive incredibly harsh drilling environments. Once this was successfully accomplished, further challenges remained. There is also some difficulty in ensuring that the SWD tool detects seismic signal rather than noise. SWD sometimes operates in an often less-than-ideal environment for acoustic measurements, and there are challenges for obtaining real-time data that can be processed downhole without human intervention. For those cases, and when processing for seismic reflection events deeper than the current tool depth, the whole recording needs to be transmitted to surface before the next acquisition. This creates some data conditioning and transmission challenges.
Additionally, seismic airgun deployment for offshore and onshore wells is a challenge that can limit the feasibility for SWD. For onshore wells, gun pits are not always feasible or robust enough for a SWD run lasting several days. Offshore rigs may be limited by crane availability for airgun deployment during drilling.
One of the development goals for seismicVISION services is for it to be transparent to the drilling operation such that the driller's procedures and operating time are the same with or without the tool in the drilling bottomhole assembly (BHA).
Beyond these technology challenges, the main difficulty for a SWD job is the coordination of the complex SWD equipment and the specialized crew for downhole, surface, and navigation equipment, as well as the interpretation, data deliverables, and decision making often taking place in distant and remote locations. Experience is the key to getting this right.
Offshore: Operators have sometimes been slow to embrace SWD. Is that still the case?
Kelsall: The SWD service is experiencing a broad lifecycle. We have seen early adopters who are keen to assess new technology, along with operators with a unique problem for which SWD has provided a solution. There has been steady growth over the last few years, and the technology is beginning to mature, with SWD appearing in increasingly more logging while drilling and directional drilling tenders from a wide range of operators.
I believe that we will continue to see more SWD applications across the globe, especially in deepwater exploration, as operators are keen to reduce the risk exposure and costs on these large projects.
Offshore: Is cost a factor? Or a lack of case studies or cost/benefit analyses?
Kelsall: The cost of the SWD service is usually not an issue as it is much lower than the issue it is run to address - for example, saving time for high spread rate rigs or remedial actions for drilling risks like side tracks or extra casing strings.
SWD and borehole seismic in general have not been a "one size fits all" service and need to be considered on a well-by-well basis to ensure the technology can provide a solution to the problem. In many cases where borehole seismic can help, a wireline deployed tool may make more sense.
There have been many cases where the operator has drilled an exploration well without SWD and would need a compelling reason to justify using it on the next well. It may be that they have an established working practice to reduce depth uncertainty using the correlation between a combination of surface seismic, basin modeling, LWD logs, synthetic seismogram, mud logging, and biostratigraphy. If none of these are conclusive, the operator may acquire a single wireline checkshot for the answer. However, SWD can save rig time and cost in such instances.
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