1. Field of the Invention
The invention relates to well logging tools, and more specifically to a gamma ray spectral memory tool.
2. Description of the Related Art
A vast array of tools are used for well logging. These tools, which can measure pressure, temperature, and a wide variety of other parameters, are typically lowered into the well at various points in drilling and production operations to determine conditions down hole and to determine the effect of various drilling procedures.
These well logging tools generally process the data they accumulate in two different ways. In the first technique, the tools are lowered on what is known as a wireline, an electric line that allows bidirectional data communication with the surface and allows power to be provided from the surface. In the second technique, the tool is instead lowered on a slick line or non-electrical cable and the tool must provide its own power and data storage.
Wireline systems allow dynamic monitoring of the well logging tools and eliminate the need for large memories within the tool. Wireline, however, is much more expensive than slick line.
A slick line is typically used in conjunction with a tool that has on-board memory of some sort and need not communicate with the surface. A battery powered tool is lowered into the well, the tool records the data, and that data is downloaded when the tool is brought to the surface. Although less expensive to use, slick line tools must be capable of operation independent from surface control.
One parameter that well logging tools measure is the presence of gamma rays. Radioactive materials can be injected into the well and then the location of those materials tracked by the gamma ray output of the radioactive materials. Different radioactive materials output gamma rays having different energies. A well logging tool can monitor either overall gamma ray count per unit time, or a gamma ray spectral tool can further determine not only the gamma ray count, but also the energies of those gamma rays. The former technique, which simply detects gamma rays, has been used in conjunction with both wireline and slick line systems.
Spectral gamma ray tools, however, have historically been used only in wireline systems. Examples of such systems are found in U.S. Pat. No. 4,585,939 to Arnold et al. titled "Multi-Function Natural Gamma Ray Logging System" issued Apr. 29, 1986; U.S. Pat. No. 4,857,729 to Gadeken et al. titled "Method of Radioactive Well Logging" issued Aug. 15, 1989; and U.S. Pat. No. 5,410,152 to Gadeken titled "Low-Noise Method for Performing Downhole Well Logging Using Gamma Ray Spectroscopy to Measure Radioactive Tracer Penetration" issued Apr. 25, 1995, all three of which are incorporated by reference. Because spectral gamma ray tools divide the gamma ray counts based on their energy spectrum, they provide the capability to distinguish between different radioactive substances in or near a well and to determine where each is located. Such radioactive substances are used to trace the location of liquids or solids injected into a well. Solid particles used as tracers are described, for example, in U.S. Pat. No. 5,182,051.
These systems typically use an americium source as a gamma ray energy reference. Americium generates gamma rays of 60 Kev, so the spectral gamma ray tool can use this reference to determine the energy level of other gamma rays in proportion to that source. Gamma ray emissions from the americium source, as well as from other substances, strike a photomultiplier tube (PMT) in the spectral tool, which in turn outputs voltage pulses that are proportional to the energy of the gamma ray that caused the pulse. Electronics within the spectral gamma ray detector convert this pulse to a digital value and then increment a register corresponding to the energy denoted by that digital value, indicating another pulse has occurred within that portion of the energy spectrum. This is repeated over a period of time, for example 1.6 seconds and then this spectral data is then transmitted over the wireline. Typical instruments divide the gamma ray spectrum into approximately 250 energy channels, although the Halliburton TracerScan tool uses 512 energy channels, with approximately 8 bits of count data per channel. After the accumulated data is transmitted, all the registers are zeroed, and then the counting process repeats.
These gamma ray spectral tools provide a great deal of information, but historically these devices have been restricted to use in wireline systems. First, the photomultiplier tube requires a very high voltage source for its grids (around 1500 V), and has historically consumed too much power for extended runs on a slick line. Runs can take as long as eight hours, and it was simply impossible to provide battery power for that length of time.
Second, the volume of data generated by these spectral tools is greater than non-spectral gamma ray tools by over two orders of magnitude. Typically, a gamma ray spectral tool is run through a well zone of interest five times at a logging speed of 10 feet per minute, taking up to 8 hours. Assuming around 640 bytes of data is being generated over a 1.6 second period by the tool (1 byte per energy spectrum per second), in 8 hours an extraordinarily large amount of data is generated--over 11 megabytes. These storage requirements have been excessive because memory chips require power as well as space.
Further, spectral tools sometimes encounter areas of a high gamma ray concentration. In such a case, the reference signal from the americium can be masked by those high gamma ray levels. A stabilizer circuit in the tool typically monitors the digitized PMT output in an attempt to locate the 60 Kev gamma ray spike generated by the americium, which would generally be the strongest gamma ray source found. When this spike drifts off of the register designated for 60 Kev gamma rays, the supply voltage to the PMT is adjusted, forcing the americium spike back into the proper energy channel. (Again, the energy of gamma rays from the americium are constant and known.) In this way, the americium spike provides a reference to compensate for any drift in the PMT output. But when excessive gamma ray radiation is present, this americium spike can be masked, causing this automatic stabilization to fail, leading to uncontrollable misadjustments of the voltage level to the PMT. Gamma ray spectral tools have historically required dynamic monitoring from the surface for these "washout" conditions because otherwise the spectral tool could lose track of the reference signal, possibly causing faulty energy readings of the gamma rays actually being monitored.
For all of these reasons, spectral gamma ray instrumentation has historically been run in wireline systems, rather than slick line systems. This has resulted in increased costs because of the added expense of running wireline as opposed to slick line, which is simple to transport and simple to use.
Therefore, it would be greatly desirable to develop techniques to allow gamma ray spectral tools to be used in slick line systems.