1. Field of the Invention
The invention relates to well logging with nuclear tools. In particular, it relates to apparatus and methods for the determination of formation properties using neutron and gamma-ray measurements.
2. Background Art
The characteristics of geological formations are of significant interest in the exploration and production of subsurface water and mineral deposits, such as oil and gas. Many characteristics, such as the hydrocarbon volume, porosity, lithology, reservoir location, and permeability of a formation, may be deduced from certain measurable quantities. Among these quantities are: density, porosity, photoelectric factor (Pe), hydrogen index, salinity, and thermal neutron capture cross section (Sigma). These quantities are typically measured by logging-while-drilling (LWD) or wireline tools that include gamma-ray and/or neutron detectors.
Nuclear logging tools often carry nuclear energy sources that radiate or emit energy into the formation. One or more detectors on these tools then detect signals that result from interactions between formation materials and the radiation. Detected signal data are typically transmitted uphole, temporarily stored downhole for later processing, or combined in both techniques, to evaluate the geological formation from which the data was gathered. Typically, once uphole, these data are used in one or more formation evaluation models to derive the desired formation properties. Formation models are typically software programs used to evaluate the geological formation from which the data was gathered. The petroleum industry uses various tools to obtain measurements for estimating earth formation properties. These measurements are often used in combination to derive the formation properties. For example, the formation density is often combined with other measurements (e.g., neutron porosity measurements and resistivity measurements) to determine gas saturation, lithology, porosity, the density of hydrocarbons within the formation pore space, properties of shaly sands, and other parameters of interest.
Gamma-ray tools for formation density measurements are based on detecting Compton scattered gamma-rays in one or more gamma-ray detectors installed at a suitable distance from a neutron, gamma-ray, or x-ray source. The number of Compton scattering collisions within the formation and the resulting attenuation of the radiation is related to electron density of materials within the formation. Thus, the signals detected by such tools can be analyzed to derive formation electron density. Through calibration, the electron density of the formation can be related to true bulk density of the formation.
The first density measurements were made with single-detector tools. However, these tools had no capability to compensate for borehole effects. The limitations inherent in the single-detector approach have led to the development of modern dual-detector density tools, in which compensation is based on a short-spacing (SS) and a long-spacing (LS) detectors. See, e.g., U.S. Pat. Nos. 5,390,115, 5,596,142, 6,376,838, 5,528,029, and 4,691,102
Gamma rays may also be recorded with their energies (frequencies) to provide gamma-ray spectra. Such spectral measurements can be used to correct the apparent formation density for the formation Pe in each detector. The idea that spectral measurements from a single detector can be used to correct undesired interference in principle can also be applied to a borehole-compensated density. While this theoretical possibility has been around for at least fifteen years, currently there is no working borehole-compensated, single-detector density tool available.
As with density tools, the first neutron tools were single-detector tools without borehole compensation. Dual-spacing tools arose some time later, giving rise to the possibility of removing some of the sensitivity of the measurement to environmental effects. Standard techniques for accomplishing the compensation are the ratio-based method and a spine-and-ribs approach borrowed from the density tools. More recently, an improved ratio-based method referred to as borehole-invariant porosity has been developed, as disclosed in U.S. Pat. No. 5,767,510. However, unlike most gamma-ray detectors, present-day neutron detectors used in the oilfield service business do not measure the energy deposited by the incoming neutron. The current state of the art does not permit a borehole-compensated, single-detector neutron measurement.
Because gamma-ray tools and neutron tools measurement different formation properties, these tools are often used together in the same logging operations. When gamma-ray detectors are used together with neutron sources, care must be taken to avoid or minimize neutron-induced interference in the gamma-ray measurements. Currently, there are no methods available to correct for this kind of interference. Therefore, these detectors are often deployed on separate tool sections with a great distance between the neutron source and the gamma-ray detectors. This makes the tool string unnecessarily long.