簡介：現(xiàn)在已經(jīng)進入了隨鉆地震的時代。本文主要介紹了斯倫貝謝公司和道達爾公司在隨鉆地震技術(shù)方面的先進之處和經(jīng)驗。
Real-time seismic-while-drilling
Seismic-while-drilling has finally come of age.
Time-lapse seismic results for the Angolan Xikomba field show how water has moved from injection wells and along the original oil-water contact. Gas has been injected at the crest and on the northern flank of the structure.
You hear the saying constantly: “The easy oil is gone.” But let’s face it ? the really easy oil was gone once oil companies stopped drilling into oil seeps. Since then, it’s required an increasingly sophisticated understanding of geology and geophysics to find reservoirs that leave no trace of their existence on the surface.
Wireline logging has been one of the key advancements in evaluating the earth close to the reservoir once a well has been drilled, and more recently logging-while-drilling (LWD) has put those measurements on or near the drill bit, meaning that formation measurements can be taken in real time while the well is being drilled. One by one, all of the standard wireline measurements have found their way into an LWD setting. But the king of all look-ahead tools —seismic has had a hard time finding its niche within this series of advancements.
Technical roadblocks have been the main hurdle. These roadblocks have included the required accuracy of the measurement, timing issues, sensors, automation, and the lack of an electrical connection between the downhole tools and the surface. Early seismic-while-drilling (SWD) tools relied on the noise generated by the drill bit as a source, but this technique had limitations ? it didn’t work in deep wells, it was difficult to deploy offshore, and it didn’t work well with PDC bits.
So one service company, Schlumberger, went back to the drawing board. The company spent 15 years perfecting synchronized downhole clocks accurate enough to measure milliseconds (or ms, standard timing in seismic surveys), but also robust enough to survive in the drilling environment.
Commercial since 2003, seismicVISION has been used in countless applications and is proving its worth as a useful well placement technique as well as a cost-saving device.
Benefits seismicVISION offers two key advantages: it helps reduce rig time, and it can often be the only way to collect data in very deviated holes or holes with stability issues where wireline tools are difficult or non-economical to run. Total EP has used the tool on several occasions and has had some eye-opening experiences with it.
In one case, the company had prestack depth-migrated data over a complex field in West Africa. The tool was used in real time for the first leg of a deviated well to verify the possibility of driving the bit on the seismic. The velocity field measured by the tool was different from the velocity model used to process the surface seismic data. This time shift could be explained by the difference of acquisition geometry. Given this conundrum, the company was hesitant to rely on the SWD measurements. However, on the second leg a different tool was run, and it corroborated the evidence given by the SWD tool run in the first leg.
“When we ran the seismic-while-drilling tool on the first leg, we had a shift of 50 ms between the borehole seismic and the prestack depth migration,” said Jean-Christian Perrin, caracterisation des reservoirs-Sismique de Puits for Total. “The company decided at that point that we had an uncertainty. But we knew that we’d need the data for calibration after the second leg. The second run proved the veracity of the downhole measurement.
Added Andy Hawthorn, domain champion for Schlumberger, “Total has used this tool in a variety of ways in both deviated development and vertical exploration wells.” In the Gulf of Mexico wells, the real-time data, including real-time waveforms, was compared to the surface seismic while drilling the well. This showed exactly where the well was in relation to the seismic section, and the drilling program was cut short by 500 ft (152.5 m) with significant cost savings to the operator.
In other situations, Total has used the tool to gain information in complex fields where there is very little well control and where wireline logging is impractical or too expensive. For instance, in many deepwater wells the tool can provide the same information without requiring extra rig time to run the logs. In extremely deviated wells, the tool is run while drilling, meaning that wireline tools do not have to snake through the difficult and often unstable deviated wellbores.
Total’s conclusions about the tool on the wells mentioned include:
• Good data quality in open holes;
• Adequate data quality to solve operational problems through
single casing;
• Safe and transparent operation (no interference with the drilling process);
• No need for an additional wireline run;
• Real-time waveform capability, allowing the drill bit position to be plotted and used to confirm the depths of the upcoming targets; and
• Possible reduction or optimization of drilling time (due to early calibration of the target).
The process
The system provides direct measurements of seismic travel times from the surface to the survey locations along the wellbore. Data are used to track the bit’s position on the original surface seismic image used to plan the well. In addition, sections of the stacked waveforms used for the checkshot measurement can be sent uphole by mud-pulse telemetry. This improves quality control on the checkshot and in certain circumstances can also provide a limited image many hundreds of feet ahead of the bit.
The information gathered is used to steer the well, set casing points, and avoid drilling hazards.
The tool has a processor and memory and receives its seismic energy from a surface seismic source, an airgun array located on either the rig or a source vessel offshore or a vibrator or dynamite shot on land. After acquisition, the signals are stored and processed, and checkshot data and quality indicators are transmitted uphole in real time by mud-pulse telemetry. The time-depth data are used to position the well on the seismic map, and waveforms can now also be sent uphole in real time. All of the raw recorded waveforms are stored in memory for processing after the tool gets back to surface.
One of the key advantages of the tool is that it does not interfere with the drilling process, and it doesn’t require any extra rig time.
“The tool operates a little differently to most standard LWD tools in that it will only operate during pauses in the drilling process, such as connection times,” Hawthorn said. “During this time, six to 12 shots are fired. The downhole logic stacks these acquired waveforms, calculates the checkshot, and sends the data uphole as soon as drilling commences.”
A team effort
Like many of the Schlumberger LWD tools, seismicVISION requires expertise to run correctly. “It’s not a standard LWD tool,” Hawthorn said. “It requires experience to do a job, and it requires close collaboration between Schlumberger and the client to decide whether or not this is the right tool for the application.”
Usually, he added, seismic is a pre-drill or post-drill operation, and geophysicists are not used to the timeframe constraints of the actual drilling process. Plus, drilling engineers typically don’t speak the same language as the geophysicists. “Most organizations are organized vertically,” he said. “This is something that requires horizontal integration. Geophysics tends to be outside the normal realm of expertise of most people who are drilling wells.”
But in cases like Total’s, new experiences can be good for us. “They’re using this tool where they’ve got a real-time issue that they need to resolve, and they’re using it where it’s difficult or impossible to run a wireline string,” Hawthorn said. “They need that information.”
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