Abstract:
Methods for density logging utilizes gamma-rays above a pair-production threshold so as to determine lithology information of formations whereby to correct a measured density data.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of borehole logging. Borehole logging is the science dedicated to the measurements of rock or reservoir properties in subsurface wells. 
     BACKGROUND OF THE INVENTION 
     By exposing the earth formation surrounding a well-bore to a gamma-ray (photon) source and observing the amount of radiation that returns or scatters, one can measure the average electronic density of the formation. This average or bulk density is in turn used to estimate the formation porosity, i.e., that volume of the rock formation that is potentially available to fluids or hydro-carbons. Accurate porosity estimates depend on the knowledge of the density of the underlying matrix rock (i.e. of the formation without the pore volume). As such one must identify also the rock type in the formation. This is done via a lithology or chemical composition measurement. The bulk density of rock matrix can be accurately parameterized in terms of its average atomic number. 
     Currently, most nuclear density tools rely on the measurement of scattered photons. Typically, photons travelling in the formation may scatter (Compton scattering) or be absorbed via the photo-electric effect. This approach is used both in “logging while drilling” and in wireline configurations for open-hole wells. In either case, the source of gamma-radiation employed is typically a high activity  137 Cs cartridge that yields characteristic single-energy photons of 0.662 MeV. 
     The gamma-ray density measurement provides the best available measurement of formation porosity. Given their success,  137 Cs sources are ubiquitously found in oil &amp; gas fields around the world, and they have been used for decades in the E&amp;P business. However, given the very high activity of the sources employed (1.5-2.0 Ci each), the continued use of  137 Cs sources poses great safety and security risks, that service companies must manage with continued significant costs in terms of resources and energy. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the method of the invention, the method includes a technique to determine lithology information of rock formations using high energy gamma-ray scattering. The method exploits the process of pair production. In pair production reactions gamma-rays of energy above 1.02 MeV are absorbed by the electric field of atoms in a subsurface formation. The rate at which this conversion occurs approximately depends on the square of the formation average atomic number Z ave  that in turn is related to the formation&#39;s lithology composition (e.g. sandstone, limestone and dolomite). 
     The lithology information (e.g. an estimate of Z ave ) can be obtained by comparing the rate of elastic Compton scattering to that of pair-production reactions. Whereas the former depend on the formation bulk density alone (□ b ), the latter is approximately proportional to the product (□ b * Z ave ). This difference can be inferred by looking at the relative count rate of scattered gamma-rays of different energies, as measured in one or more photon detectors in a borehole tool. Events that fall in different regions of the scattered energy spectrum (such as in a “medium” and a “high” energy window) carry information that is largely dominated by Compton or pair-production reactions, respectively. Knowing the bulk density as determined from Compton scattering events, we can determine the matrix-rock type from the yield of pair-production events. This Lithology information can be utilized to independently correct the measured bulk density and thus more accurately determine the overall average formation porosity. 
     We define a lithology-sensitive pair-production formation factor PPF (PPF=Z ave /10) in analogy to the photo-electric formation factor PEF (PEF=(Z ave /10) 3.6 ). Similarly to the PEF, the PPF is a measure of the sensitivity to different lithological compositions, relative to bulk density effects. 
     Whereas the PEF or photo-electric absorption signal can in practice be determined only from gamma-rays of energies below approximately 0.2 MeV, the PPF or pair-production signal is carried by photons over an “open-ended” range of energies above the pair-production threshold at 1.02 MeV. 
     The lithology information obtained with the pair-production technique has the advantage of being insensitive to those effects due to the presence of barite-rich muds, well-casing, tool housing that significantly limit the quality and depth of investigation of photo-electric measurements. In addition, the PPF signal at the detector is relatively smooth and can be integrated over large energy windows with improved signal-to-noise characteristics relative to a PEF measurement at low energies. 
     PPF measurements preferably require a photon source with energy up to several MeV or more such as an accelerator based source. Accelerator based photon sources are intrinsically safe as they can be controlled and turned on or off by the user. The use of high-energy photon sources also offers the advantage of a greater depth of investigation, and thus of sampling the formation at greater distances from the borehole with less contribution from perturbations due to the borehole system itself. 
     One unambiguous signature of pair-production reactions is the measurement of positrons produced in the formation and scattering to the detectors. Such positrons can be uniquely identified via the characteristic 0.511 MeV peaks, e.g., annihilation signal, they produce in the detector response. 
     In summary, the pair-production technique offers significant advantages that increase the range of applicability of nuclear density tools and ultimately yield to more accurate and safer density measurements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  illustrates an inset in the high-energy region of pair production showing the spectrum enhancement caused by different rock matrix types, wherein accurate measurements in this region (and/or at higher energies yet) allow one to identify the rock type for a given formation bulk density, according to at least one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Given the different ways gamma-rays interact with matter, gamma-density measurements are sensitive to the energy of the incoming radiation, e. g., the energy emitted by the available nuclear source. 
     At low energies, gamma-ray interactions are dominated by the photoelectric reactions, where the incoming photon is absorbed by an atom resulting in a low-energy electron being knocked out. In this case gamma-rays are lost as well as information from the formation. 
     At the intermediate energy provided by  137 Cs sources (0.662 MeV), gamma-rays interact by both photoelectric and Compton scattering processes. The Compton reaction dominates only for photon energies above 0.5 MeV. In Compton reactions photons do not disappear but scatter elastically off atomic electrons, including scattering in the backward direction (e.g. back towards the borehole). The rate at which Compton scattering occurs is directly proportional to the atomic electron density in the medium, which in turn can be shown to be proportional to the average mass density of the formation (in g/cc). With information about the rock matrix type, formation density can be accurately translated into porosity, i.e. that fraction of formation volume (pore space) that is available to fluids such as oil and gas. 
     The well-known gamma-ray density measurement widely employed in the E&amp;P industry is indeed based on the Compton process. A variety of compensation schemes for borehole effects have also been disclosed. In these prior art implementations one typically utilizes an axis-symmetric tool string consisting of one nuclear source of gamma rays and two or more detectors at different distances along the length of the tool. These techniques are for instance described in U.S. Pat. No. 3,321,625 by Wahl, U.S. Pat. No. 4,048,495 by Ellis and U.S. Pat. No. 5,912,460 by Stoller et al. 
     These commonly used techniques present two different types of problems. First, incoming photons of 0.662 MeV penetrate at most only up to a few inches into the formation, thus the volume probed for density information is limited compared to the scale of the reservoir. In order to probe a greater fraction of the reservoir one would have to drill additional exploration holes, which entails additional operational costs, time and risks, or increase the intensity of the source, which entails additional safety and security costs and risks. 
     Secondly scattered photons are inevitably degraded on their way back to the borehole and the detector system. Indeed the probability of undergoing photoelectric reactions increases very rapidly for decreasing photon energies and therefore a significant fraction of the scattered photons may be absorbed by photoelectric reactions on their return path. This results in some of the density and lithology signal being lost and/or obscured and/or in the signal being dominated by effects taking place at short distance from the borehole wall or in the borehole itself such as those due to drilling muds and other borehole fluids, the presence of mudcakes and/or contaminated invasion zones, and tool and or borehole casings. Indeed, photo-electric absorption effects are strongly dependent on the material encountered right before entering the detectors, when the scattered photon energy is at its lowest. Environmental corrections to the gamma-density log data for these effects are quite common and at times become quite large. 
     These problems can be mitigated with higher energy photons. At higher energies photons are more penetrating and thus probe deeper into the formation. At high energies, gamma-rays can also interact in a new way, e. g., through the pair-production mechanism. In these reactions, photons annihilate into electron-positron pairs and thus are absorbed by the formation. 
     By measuring events at the detector in a medium and high energy window one can extract the formation porosity via measurements of the average (bulk) formation density and the properties of the formation matrix (lithology). Events that fall in the first region are dominated by Compton effects, and their count rate increases with increasing formation density. Events that fall in the high energy region are dominated by pair production, and their count rate decreases with increasing formation density. This is contrast with prior-art techniques where one utilizes a medium and a low energy window, dominated respectively by the Compton and the photo-electric effects. 
     According to an embodiment of a method of the invention, the method includes that the reliance on photoelectric effects can be eliminated, while the essential Compton mechanism is still in place. The situation is illustrated in  FIG. 1  where we show a photon energy spectrum at equilibrium in a rock formation. The curves are for typical down-hole formations such as sandstone, dolomite and limestone (top to bottom), and are normalized to a given formation density. The inset in the high-energy region of pair production shows the spectrum enhancement caused by the different rock matrix types. Accurate measurements in this region (and/or at higher energies yet) allow one to identify the rock type independently from the overall formation density. This is conceptually similar to how it is done for the low-energy part of the spectrum (photoelectric region) but without the problems associated with low-energy photons that were described in the text. Note in  FIG. 1 , we assumed a source energy of several or possibly 10 MeV but similar results can be obtained for other source energies, including continuous bremstrahlung-sources where the end-point energy is above the pair-production threshold. 
     With a gamma-ray source in the range of a few to several MeV and beyond one can significantly improve the accuracy of the gamma-density porosity determination by operating in a regime were the underlying physics of gamma-ray interactions is optimal. At the same time, the use of an accelerator or electronic source of photons also provides the added benefit of significantly lowering the risks and liabilities involved in field operations. Accelerators are intrinsically safe as their operation relies on sophisticated controls that can be easily disassembled or locked-out. High-energy gamma-ray logging is thus a safer, more secure and scientifically improved approach to formation evaluation with the potential of significantly advancing the industry&#39;s current oil-exploration capabilities.