Logging while drilling - deeper (LWD-"D")
The oil and gas industry is full of "three-speak and four-speak," the language of technology identification, classification, and application. They are acronyms that help disseminate what, where, when, and how current technologies are used. Logging while drilling is no exception. As the push into deeper waters quickens, maybe a new acronym should be considered: LWD-"D" (logging while drilling - deeper).
Operating in deeper drilling environments means finding more ways of cutting costs. In the well construction phase of a well, few trips in and out of the hole help keep costs low. The use of LWD technologies can reduce the number of bit trips, and thus lower costs. To address this market, service providers are developing more while-drilling capability everyday. Conventional direc- tional drilling and formation evaluation data have been supplemented with drilling mechanics/efficiency data. Leaps in battery power, downhole processing, memory capabilities, and ruggedized electronics are all helping to prolong downhole tool life, yielding bit runs of considerable length and time. It is commonplace to stay in the hole seven days with a MWD/LWD drilling assembly (MWD = measurement while drilling).
Pushing the technology envelope, however, creates a need for industry collaboration periodically to assess where we are as an industry, and where we are going in the future.
The Society of Petroleum Engineer's (SPE) recent "Developments in MWD/LWD for Drilling and Geoscience Professionals - Adva-nced Technology Workshop" convened to identify new challenges and define better practices and procedures for while-drilling measurements. The dominant theme was deepwater and its effects on while-drilling measurement operations. Four key areas involving current LWD technologies and applications were identified and discussed in detail.
Depth control
The time is quickly approaching when the industry may finally be ready to accept a one true depth - be it wireline, LWD, or pipe tally. The question of redundancy has always been stated as the reason for not making a decision on one measurement or the other, but maintaining multiple depth measurements for decision-making peace of mind. Conventional pipe tally has always been the benchmark for correcting LWD data at the well site. Wireline data is then used to correct the LWD data - again. Note that this process corrects LWD data twice, whether it is correct or not.
In deepwater environments, a reduction in conventional wireline logging may result, leaving LWD data and pipe tally depth measurements as the only reference. With this possibility, methods to improve reliability and build operator confidence in the measurement are needed. Deepwater drilling contractors and equipment providers are outfitting rigs with digital depth sensors and multiple signal outputs for service providers on the rig at any one time.
"The pipe tally must go" was a frequent comment made at the workshop. Simple pipe tallies, although subject to human error, provide a good check and balance function for critical measurements. As a side issue, the industry may not be ready to totally abandon pipe tallies altogether. Automated pipe handling and digital drilling control systems are making pipe tallies a lot less time consuming and less susceptible to human error. The one solution left is the LWD depth. This is an opportunity for industry guideline organizations such as API, IADC, SPE, and others, to step forward and help construct an industry standard.
Log quality
Increasing emphasis on LWD data and more measurement sensors in the drillstring demand a greater need for a standardized, industry-wide, log-quality control manual. The manuals/guidelines exist now in the form of internal or operator provided guidelines. In the case of service companies, each adds a little bit to the previous competitor's offering to the industry. Unfortunately, in a lot of cases it caters to the specific data processing and output capabilities of the respective data provider. Again, this is an opportunity for industry guideline organizations and/or a consortium of both service and operating companies to deal with this problem.
It is sometimes necessary to depth shift data - for example, shifting real-time LWD resistivity data to wireline porosity and density data. Or, in a much worse situation, depth shifting LWD data for a miscounted pipe tally. A potentially costly mistake in real-time data can appear in casing point selection and other critical well construction decisions. Depth shifting implies uncertainty and a lack of confidence in measurement methods. The question we should ask ourselves is not "Why are we having to depth shift?" but "Are we allowing two completely different measurement methods to confuse the decision-making process?" Two different methods will always deliver two different answers.
Data management eventually comes into play. The increased amount of LWD and real-time collected data has to be in a highly flexible, readily available format. Real-time monitoring, and post well analysis of the well data should be easily facilitated by the operating company.
BHA design
Bottom-hole assembly (BHA) design is determined by the directional and logging measurement requirements for a particular bit run. The decision on what wellbore and formation parameters to log is made before the bit run is started, and even as far ahead as the well planning stage. Advances in electronics and circuitry have resulted in an assortment of tool alternatives for operators to choose from. For deeper water depths more while-drilling measurements will be required, and with current technologies, longer bottom hole assemblies. This may not be acceptable because of lost-in-hole costs or directional drilling concerns.
The subject of more consolidation in measurement was discussed at the workshop. A demand for more measurements by operators will work against the modular approach currently taken by some service providers. This results in too many different subs in the BHA. A re-think and subsequent redesign may be required to accommodate this desire for shorter, and more functional BHA's.
Future applications
Future fundamental requirements of MWD/LWD services will be more accurate placement, correlation, and pore pressure estimation capabilities. All technology development should revolve around these critical points. Elaborate tool strings dominated workshop discussion on future needs and applications. A wish-list of measurements in a single drill string was debated, including traditional directional and basic formation evaluation measurements, and other future technologies:
- An NMR tool (available soon)
- Formation testing dipole shear sonic
- Newer nuclear measurements
- LWD sonic measurement for shallow seafloor elastic constants
- MWD gyro.
Of course, the next demand was to have all of the measurements as close as possible to the bit. A disposable logging tool was mentioned in one discussion. Directional motors are already being sacrificed in some projects around the world. A cheaper, expendable version of a tool string, good enough to get you from point A to point B, could have application in the future. There was interest in a more reliable sonic measurement for porosity evaluation. This may indicate remaining skepticism by operators' for the while-drilling measurement.
Geotechnical topics, also discussed, were a shift in focus for LWD measurements. Problems with shallow water flows and wellbore stability situations are driving this change in thinking. Improved well-to-seismic and LWD-to-core data will yield better synthetic (modeled) porosity, density, velocity, and seismogram information for use in forward-looking, real-time predictive techniques.
The workshop discussions and question/ answer sessions indicate that LWD-related issues are key areas for improving well construction performance and thus reducing project costs. Operators are likely to focus more attention on this technology area in the coming year (This Drilling & Production Column in August will continue the discussion on LWD-"D").
