A new logging combo takes formation evaluation-quality measurements at much higher speeds.

The latest in high-efficiency premium openhole logging systems will make its debut in Western Canada this summer. The system has been designed to meet the demands of the development drilling market, with many features specifically addressing the western Canadian formation evaluation market. The development drilling market demands two key attributes from any wireline-conveyed, formation evaluation measurement: accurate data and wellsite efficiency. All downhole instruments have been redesigned, incorporating advanced sensor technology, into shorter, lighter, more reliable logging tools capable of logging with the same precision and accuracy as the industry's highest-quality formation evaluation measurements but at much higher logging speeds (up to twice the speed of conventional triple-combo and quad-combo logging tool strings). The logging system, to be commercialized by Baker Atlas under the Focus name, consists of the four major openhole measurements plus auxiliary services.

Design criteria

All sensors have been designed to provide accurate measurements at logging speeds up to 3,600 ft/hr (1,100 m/hr). In a typical 5,000-ft (1,524-m) well with a 4,500-ft (1,372-m) interval to log, this represents a 1 hour and 15 minute savings in logging time. A new verification system enables before- and after-log verifications of all sensors to be carried out while tripping into and out of the well, saving an additional half hour of rig time. The new logging suite is shorter and lighter than conventional tools. The tools are easier for the wellsite crews to handle, enabling faster rig up and rig down, thus reducing the total rig time needed for a logging operation by as much as one hour over conventional tool strings. The shorter instrument design reduces rat hole (logging sump) requirements by as much as 50%, saving rig time needed to prepare the well bore for logging.

Field testing

The tool's reliability has been proven on more than 45 wells, including an extensive field trial in western Canada in early 2003. The system was able to complete logging consistently in the depth range of 3,280 ft (1,000 m) to 4,920 ft (1,500 m) in 3 to 31/2 hours. This equates to an average rig time saving of 2 to 21/4 hours for logging on a typical job compared to times for conventional logging.

A component of maintaining wellsite efficiency stems from reliability of the downhole instrumentation. Part of the field test program has been to prove the design criteria for reliability. Each instrument incorporates sensor experience from Baker Atlas' existing wireline instrument range with logging-while-drilling design and manufacturing knowledge from Baker Hughes Inteq's drilling and evaluation fleet to provide a range of wireline instrumentation that is more rugged and robust than current and previous systems. The field-testing of the system has validated the design target of more than 300 jobs between lost-time failures for the logging instruments. This high mean-time-between-failure design criteria was set to assure confident wellsite efficiencies, borehole conditions permitting.

Promoting accuracy and precision

Data accuracy and precision is also a primary concern with any formation evaluation system. To achieve the accuracy and precision needed, most logging systems require data acquisition at speeds of not more than 1,800 ft/hr (550 m/hr). Acquiring data at speeds higher than the manufacturer's specification leads to degradation of the data quality, resulting in increased uncertainty in the decisions made based on the data. The new system has been specifically designed to acquire logging data at speeds up to 3,600 ft/hr (1,100 m/hr) without compromising the accuracy and precision of the logging measurements.

Accuracy at high logging speeds is ensured in two essential ways. First, the sensors and mechanical systems are designed for logging at higher speeds. Second, all measurements can be corrected for stick and pull, which has a much larger effect on logging measurements at higher logging speeds. The result is data accuracy and precision at the higher speed that is equal, and in many cases superior, to the quality of conventional logging.

The Z-Densilog is an example of the improved designs featured in the instrumentation. The pad is shorter than conventional density pads, with a better mechanical articulation system that provides excellent pad contact with the borehole wall at high logging speeds, even in rugose or washed-out boreholes. The result is a more accurate and reliable density porosity data. The benefits of this new design are apparent in Figure 1.

Figure 2 shows an overlay of the Focus Z-Densilog and Compensated Neutron measurements compared to conventional measurements. Additionally, the tool responses were characterized in multiple test formations at the Baker Hughes Houston Technology Center. Comparisons to conventional measurements were made for all new system tools in a variety of test wells across a spectrum of lithologies and porosities and found to be in excellent agreement.

Depth accuracy is critical at high logging speeds, particularly on wells where reservoir thickness may be limited, and an extra 1 to 2 ft of pay may be commercially relevant. The additional real-time processing step of running a 3-axis accelerometer sensor provides a method to correct all data for true depth before analysis. Even small errors in the depth estimate can corrupt data such as synthetically focused resistivity curves. The assumption that the logging tool is moving smoothly through the borehole is not usually valid due to rugose and sticky borehole conditions and the effect of centralizers and de-centralizers in the tool string. A Kalman filtering method using accelerometer data improves depth estimates by using a state-vector approach to predict the depth of the toolstring given the surface depth estimate and the measurement of the tool acceleration. The powerful Kalman filter approach is iterative, and the error in the depth estimate is computed along with the depth estimate itself.

Real-time log quality indicators provide an efficient means to monitor instrument performance while logging. Easy-to-read quality indicators are presented as part of the standard wellsite deliverable. Figure 3 illustrates the quality control plot typically delivered as part of the high-definition induction log print.

Data flow

Data flow and delivery between logging instrument and data delivery center or client office is another key aspect included in the efficiency design. A satellite communications system is employed that can be linked directly to the Baker Atlas Total Recall well information database system. The log data is typically uploaded within minutes of finishing the data acquisition segment of the logging operation and can immediately be viewed using any computer that is connected to the Internet. This gives clients 24-hour secured access to their data from anywhere in the world.

Conclusion

It has been many years since a comprehensive logging instrumentation sensor and deployment redesign has taken place. The opportunity to redesign a classic quad combo was an opportunity to redesign instruments with an emphasis on today's issues in development well logging: wellsite efficiency and data accuracy.

Field tests have demonstrated a consistent improvement in accurate openhole data, centered around instrument reliability and minimized logging time through logging speed and rig up/down efficiency. Real-time quality control and analysis at the well site accelerates the usability of acquired data, translating to a variety of value-adding products to the operator. From fast and reliable formation evaluation for reserve estimation to accurate delineation of reservoir intervals for improved completion efficiency, measurement quality has much appeal. Adding to that modern use of Web-enabled data transfer and viewing techniques, log data transmitted from the well site directly into a database system and the client laptop enables clients to easily access data when and where they need to.

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