The latest issue
A new generation of gamma ray tools promises far greater reservoir resolution
Unconventional plays create unconventional challenges. Not knowing the precise mineral makeup and concentration of hydrocarbons in a shale play, for instance, can have serious economic ramifications. Schlumberger Limited has just launched Litho Scanner high-definition spectroscopy service, a new gamma ray tool that allows explorers an unprecedented scale of precision and efficiency.
“It is one of the most important new technologies that wireline has launched in the past decade,” says Maria Lorente, wireline product champion for Litho Scanner. “It is a new generation of spectroscopy tools that measures the elements in the formation to get an accurate mineralogy and total organic carbon [TOC]. This is very important in unconventional plays.”
Traditional spectroscopy tools rely on bismuth germanate (BGO) gamma ray detector designs that have been around for several decades. While adequate for most uses, they can lead to insufficient spectral resolution and loss of sensitivity when exposed to downhole temperatures for an extended period of time.
Moving away from BGO generation tools, Schlumberger over the past few years set out to develop and incorporate state-of-the-art technology in Litho Scanner. “The improvements are due to several factors,” says Lorente. “We moved away from a chemical source and incorporated a pulsed neutron generator [PNG] that delivers a high neutron output, which means higher precision and faster logging speed. Under ideal conditions, we are able to record up to 3,600 feet per hour, which is double the speed of previous generations.”
What really makes the Litho Scanner tool stand out from others, however, is the cerium-doped lanthanum bromide detector. “The lanthanum bromide gamma ray detector provides the industry’s most precise and accurate mineralogy computations, and the high-performance pulsed neutron generator eliminates the need for a chemical source,” says Lorente.
The changes now allow the tool to operate at high temperatures for longer periods. “The previous-generation tools needed cooling systems, or the spectra would degrade over time,” says Lorente. “Now, the PNG and the detector can operate over long intervals like those encountered in unconventional plays without deterioration of performance.”
HOW IT WORKS
The downhole device is only 4.5 inches (114 millimetres) in diameter, so it can easily be conveyed on wireline through most wellbores at high speed. As the tool is drawn through the hole, the PNG pulses neutrons in a controlled, measured amount. According to Schlumberger, the gamma rays produced are then captured and processed by a pulse-height analyzer. Spectra are acquired during and after each neutron burst, which enables clear separation of the inelastic and capture gamma rays. Each spectrum is decomposed into a linear combination of standard spectra from individual elements.
The coefficients of the linear combination of the standard spectra are converted to elemental weight fractions via a modified geochemical oxides closure model or by using an inversion approach. The analyzer then generates mineralogy and lithology from the elemental concentration logs. By subtracting the amount of inorganic carbon associated with carbonate minerals from the total inelastic measurement of carbon, it can also determine TOC. Lithology and TOC output are then presented as a continuous well log.
Schlumberger has been field testing the tool for over a year, performing 80 jobs for more than 35 customers. “The Litho Scanner tool has performed very well in all complex lithologies,” says Rob Badry, petrophysics domain champion in Canada. “The focus in Canada has been on shale oil and gas, heavy oil and bitumen-filled reservoirs, where TOC is especially important. We’ve had excellent results overall, including complex bitumen-saturated carbonates reservoirs.”
Northern Cross (Yukon) Ltd. is one of the customers that tested out the new device. The Calgary-based company is conducting exploration of a series of unconventional Carboniferous and Devonian shales in a Canadian frontier basin. “We were primarily focused on understanding free-carbon weight percentages so that we could estimate TOCs,” says Don Stachiw, vice-president, exploration, for Northern Cross.
“The estimated TOCs from this tool were used in conjunction with other open-hole logs, mud gas response, cutting descriptions and TerraTek HRA [heterogeneous rock analysis] modelling to choose sidewall core points. We chose the sidewall core points and successfully executed the coring program. We will now compare the measured TOC values to the Litho Scanner–estimated values to determine the quality of the prediction; this has yet to be completed.”
After the commercial launch in October at the Society of Professional Engineers’ Annual Technical Conference and Exhibition in San Antonio, Texas, Schlumberger will be looking at expanding the applications for the tool. “One application is called sCore lithofacies classification, which takes the mineralogy data and maps it into ternary diagrams together with reservoir and completion quality indicators to allow selecting the optimal completion intervals in unconventional reservoirs,” says Lorente.
“In the future, we will continue to work with customers to answer their specific questions, and to generate applications for individual areas and particular cases,” says Badry.
So far, Northern Cross has been impressed with the new tool. “The turnaround time from gathering the data, transmitting the data from a remote frontier-basin location and interpreting the data resulting in a TOC estimation was less than 24 hours, thus allowing real-time operational decisions to be made,” says Stachiw.