LOGGING TECHNOLOGY: Carbon-oxygen logging helps spot reservoir's secret hiding places
For the energy industry, technical innovation has always been critical to the optimization of mature, sometimes declining fields. In the face of recent high oil prices and frequent supply shortages, operators are looking to existing fields as potential sources of increased production.
By locating bypassed hydrocarbon producing intervals, and by better managing and monitoring existing production, operators should be able to economically increase returns on these fields.
As a result of the maturation of the world's oil fields, many are now undergoing waterfloods in order to extend field production life and increase recoverable amounts of hydrocarbons. This extensive use of fresh or sea source waters in the waterfloods has created new opportunities for additional recovery.
Due to the change that floodwaters can make to the original formation water salinity, they have also created a technical challenge when using conventional logging technology for formation reservoir analysis. Historically, using open and cased hole logging tools to evaluate reservoir fluid saturations when the formation salinity is low or unknown has always been a technical challenge. To solve these challenges, operators require new logging technology techniques.
Fluid saturation evaluation
In open hole logging, the use of magnetic resonance imaging (MRI) logs has proved to be an invaluable technology for differentiating oil from water in fresh or unknown formation water salinity. However, in cased hole reservoir monitoring applications, there are several technologies that currently exist.
One method for evaluating saline formation waters in cased hole wells is the pulsed neutron capture log, in existence for more than 30 years. The technology involved in measurement accuracy of this method has improved greatly, but it remains difficult to use in fresh water formations to find oil.
Today, the recognized technology for the evaluation of fresh or unknown water salinity reservoirs, is the carbon-oxygen log, first developed in the early 1970s. Carbon-oxygen tools typically use gamma ray spectroscopy measurements to directly sense the presence of carbon atoms in oil and oxygen atoms associated with water.
The ratio of the carbon to the oxygen measurement, or C/O, allows for the evaluation of differences in water and oil saturations independent of formation water salinity. Historically, the C/O log has experienced limited use due to the poor accuracy and precision of the measurement available to provide valid answers. To achieve the sophisticated answers required by operators, service companies built C/O tools that used large diameter detectors and required excessively slow logging speeds. These large diameter tool designs could only be conveyed into wells where tubing was removed prior to logging.
Over the years, technical advances have yielded tools capable of quantitative saturation evaluation in moderate-to-high porosity and with improved logging speeds that have made C/O logs much more desirable except for their large tool diameters. In the last few years, however, several small diameter tool designs capable of logging through production tubing have become available. Because the C/O measurement has inherent physical limitations, gaining the capability to go through-tubing resulted in a sacrifice of accuracy and logging speed compared to larger diameter tools.
Advances in tools
Halliburton Energy Services developed a through-tubing, carbon-oxygen logging tool, which combines recently developed detector and electronic technologies. The reservoir monitor tool "elite" (RMT Elite), described as a slimhole pulsed neutron logging system, is used in locating bypassed hydrocarbons, as well as for the monitoring and management of hydrocarbon reserve prod-uction. The tool has multiple operating modes and capabilities, including simultaneous inelastic, capture, and water flow measurements.
The engineering effort was centered on the utilization of high-density bismuth germanium oxide (BGO) detectors, following the success of the company's 3 3/8-in. diameter C/O tool, the pulsed spectra gamma tool (PSGT). The high density and high atomic number of BGO makes for a detector that has the ability to improve the detection of gamma rays, particularly high energy ones, that are measured in C/O logging. The goal for the tool was to achieve the resolution and accuracy that had been previously benchmarked in C/O logging by the PSGT. However, the overriding need for a tool diameter capable of allowing its conveyance into a well without pulling tubing, resulted in an extensive engineering effort in electronic design, and packing of detector and photomultiplier tubes. The tool is 2 1/8 in. in diameter (able to fit through 2-7/8 in. or greater tubing).
An accompanying figure presents the three inelastic gamma ray energy spectra measured by the tool in test pit formations. The green spectra was measured in a sandstone formation saturated with 100% oil, while the red spectra was measured in a sandstone formation saturated with fresh water. The difference in the two spectra at the carbon peak represents the difference in 100% oil vs. 100% water. The third spectra was measured in a limestone formation saturated with fresh water.
A modularity design was incorporated, providing the opportunity to combine the tool with a complete string of production logging tool sensors for detailed production analysis. Tool measurement modes that would allow for the simultaneous recording of inelastic and capture spectra, formation sigma and water flow detection measurements were also developed. For logging in salt water saturated formations, the tool also has an operating mode that defaults to pulsed neutron capture measurements.
