Logging-while-drilling challenges in deepwater
Editor's Note: This column continues the July Drilling & Production discussion of topics presented at the recent MWD/LWD Advanced Technology Workshop for Geoscientists and Engineers (Austin, Texas).
Anisotropy
Discussions about anisotropy were not unlike those of the early 1990s, when the industry talked about it in fundamental terms, which is surprising considering the gains in measurement and processing technologies over the past decade.
Anisotropy, an old foe of log analysts and geologists around the world, still raises its ugly head in the logging industry. Present in a lot of older wells, it was either misinterpreted or ignored completely as useless information. Since the early 1990s, the industry has recognized the peculiar log responses to this unique geologic scenario, and dealt with it in various ways:
- Acknowledge that it was present and accept it as a necessary evil
- Recognize the effect and use it to assist in real-time drilling and steering decisions
- More accurately correct for it, and return a corrected, true formation resistivity for evaluation purposes.
Operators, at the above-mentioned workshop, indicated that they wanted a true resistivity output, correcting for the anisotropic effect(s). Although a corrected true resistivity is needed, we must not forget what the apparent (raw measured) curves are telling us. The actual tool response is correct, but showing erroneously high readings because of rock physical characteristics within the tool's measurement volume.
This response speaks volumes about what is actually present geologically. Thin lenticular beds, or more importantly, large beds with significantly different resistive responses, are indicated by this tool response. So, while we strive for more accuracy, don't forget the fundamentals of the interpretation process.
This led to another discussion about what exactly is the measurement volume of an induction tool device. Strong argument was presented on the idea that current flow is not necessarily spherical, yet biased in the direction of highest conductivity. This means that the more-conductive element within the volume of measurement will dominate the current flow, and thus the tool response. This will weigh the overall true resistivity value, should one be computed, to the more conductive element. Many papers have been presented in the past decade on this issue and can be referenced for further information.
Although we have the ability to correct (basically ignore) anisotropy and its associated effects on logging, it is not going away and will continue to be an area of debate for years to come.
Geotechnical/geological solutions
Drilling mechanics, geotechnical issues, and the real-time measurements involved in these particular solutions continue to increase in popularity. Wellbore stability problems in competent rock and shallow water sediment problems are driving the interest.
The most prominent tool measurement currently considered the best solution option for these problems is the PWD (pressure while drilling) measurement. It is a fundamental, wellbore dynamics measurement, easy to reliably measure, and highly effective in areas where fracture and drilling pressure gradients are in close proximity.
More imaging products
More real-time imaging capability was mentioned as an area of improvement. Currently, real-time wellbore imaging is available from nuclear and resistivity tools. Of course, real-time image quality will be a function of the data transmission rate and the drilling rate of penetration (ROP).
Recorded mode data, collected and stored while drilling and later downloaded to surface processing computers, can be used for more detailed wellbore images. Enhanced downhole processing capability has improved the nuclear imaging capabilities substantially, and allowed for very reliable real-time resistivity imaging. Look for commercialized 16-quadrant neutron processing soon.
Drilling tool selection on log quality
The effects of rotary steerable drilling systems and bi-center bits on hole and log quality were briefly debated. It is well documented that rotary steerable systems drill a more gun barrel hole than a conventional steerable motor. However, there are some drawbacks. Slightly enlarged and spiraled holes have been observed from their use - a result of the bit side forces generated by the systems' designs.
The spiraled nature of the rotary steered holes makes measurement errors cyclical, repeating in intervals. This implies a need for borehole correction algorithms to incorporate a cyclic-recognition logic to render more consistent corrections.
The jury is still out on which rotary steerable tool design inflicts the most hole geometry damage - pushing or pointing the bit. Arguments currently split along company lines. Adding to the problem is the selection of bit type used with the rotary steerable system - long or short gauge bits.
Another bit issue of some concern to log analysts is the increased use of bi-center bits and the poor log quality they generate. This bit type has been receiving impressive reviews in the last year. Improved drilling efficiency is the popular reason for running these bits. Unfortunately for log analysts, the extremely cyclic boreholes they generate are causing log interpretation problems. The above mentioned cyclic-recognition logic to downhole and surface processing software will improve this situation.
Reliability
Operators noted that MWD/LWD reliability has fallen in the past couple of years. It appears that improvements in electrical components have not substantially decreased the failure rates of MWD and LWD tools across the industry. This may be deceiving however. It was noted that in the past couple of years, operators have pushed the limits with the number of measurements run per tool string. The shortcoming in perceived reliability is most likely communication problems between all of the measurement subs within the tool strings. Hard sensor failures are probably down, but masked by these real-time communication problems.
Communications
Improved communications capabilities continue to optimize decision-making processes. Serial data transfer is a thing of the past. Advances in microwave, wireless, and satellite communications have allowed transmission of larger packets of information and made it possible for operators to actually work in real-time with a wellsite MWD/LWD engineer from their offices on land.
Operators are expressing an interest in having more interface with the engineer at the wellsite. Taking that a step further, you may see a MWD/LWD engineer in the operator's office controlling and interpreting data flow from the rig, and spending more time in a collaborative, interpretive role with geologists and engineers.
More data - faster
The current maximum for real-time data transmission seems to be in the 30-parameter range. Discussions produce different answers, but if you specify a given rate of penetration and an acceptable data density per foot value, one can normalize all providers to the same situation and find out that the current industry maximum is about 30 parameters.
The transmission data rates for these parameters are 0.5-6.0 bits per second (bps). Higher rates are possible, but not often used due to reduced signal strength at higher transmission speeds and deeper drilling depths. The most common transmission speed is in the 1.5-3.0 bps range. One major irony in the MWD/LWD logging business is that the zones requiring the most measurements - more variables per unit time - are in the producing reservoirs, usually the farthest depths in the well. This happens to be where signal strength is the weakest and requires a stronger pressure signal - a reduction in transmission speed - to accomplish data transmission to the surface.
