Radiant energy – Geological testing or irradiation – Well testing apparatus and methods
Reexamination Certificate
2001-09-12
2004-06-08
Hannaher, Constantine (Department: 2878)
Radiant energy
Geological testing or irradiation
Well testing apparatus and methods
C250S266000
Reexamination Certificate
active
06747270
ABSTRACT:
This invention relates to a method and apparatuses for logging hydrocarbon reservoirs. More particularly the invention concerns the logging of gas/oil and water/oil contacts in geological formations.
It is highly desirable to be able reliably to distinguish between oil and water in a geological formation penetrated by an oil well. The ability to do so enables one to determine whether a porous formation contains oil or water and the position of the oil/water interface during well production and whether the water driving fluid has broken through to the production well in a water flood secondary production operation. In open-hole, that is a well without a steel liner or casing, it is conventional to distinguish between oil and water by means of a resistivity tool which reads a low resistivity when the formation is saturated with saline water, a good conductor; and a high resistivity when the formation is saturated with oil, an insulator.
It is also conventional to distinguish between oil and water in a cased well by taking advantage of the differences in the macroscopic neutron absorption cross section (Sigma) of oil and the normally saline formation water. Since the saline formation water contains chlorine which has a rather high neutron capture cross section and since oil does not, neutron tools have been developed which essentially measure the macroscopic neutron capture cross section.
For example, U.S. Pat. Nos. 3,566,116 (reissued Jul. 8, 1975 as U.S. Pat. No. 28,477); 3,691,378; and 4,055,763 illustrate variations of one such technique for determining Sigma in which a pulsed neutron source is utilized to irradiate the formation with a repetitive burst of fast neutrons in order to permit a time evaluation of the neutron population in the resultant neutron cloud. Typically, this evaluation is accomplished by detecting capture gamma rays which result when thermalized neutrons of the cloud are captured or absorbed by a nucleus of a constituent element in the formation. In such a time evaluation, advantage is taken of the fact that the neutron cloud density decays exponentially, with the characteristic decay time being a function of the macroscopic neutron absorption cross section of the formation. The macroscopic neutron absorption cross section is the sum of the neutron absorption of the elemental constituents of the formation and of its contained fluids.
While these neutron tools and techniques are quite effective in distinguishing between oil and water under normal circumstances, a number of limitations have been encountered. Once such limitation is the situation in which the non-oil fluid in the formation is fresh water rather than saline (or, more generally, chloride-containing) water. In this circumstance it is not possible using the above described pulsed neutron technique to distinguish between oil and water since the difference in neutron capture cross section between the two formation fluids (oil and fresh water) is not large enough to permit their differentiation.
An additional limitation with the pulsed neutron technique is encountered in wells that have fresh water in the well borehole, even though saline water is present in the formation. In such a circumstance, some neutrons from the neutron burst are thermalized and linger in the fresh water of the borehole, giving rise to an interfering “diffusion” background. This effect of course does not occur in those boreholes having saline water since the chlorine is a strong neutron absorber which rapidly absorbs the diffusing neutrons. The “diffusion” background is a particularly awkward problem in the pulsed neutron technique since the determination of the characteristic decay time following the neutron burst relies on the detection of neutron fluxes whose intensities decrease with time to relatively small values. As a result, the “diffusion” background becomes large relative to the neutron flux of interest so as to obscure the information bearing signal.
In view of the difficulties and limitations inherent in the pulsed neutron technique, other measurements and techniques that might be suitable for distinguishing between oil and water were sought. Another conventional neutron instrument used in logging oil wells is commonly referred to as the neutron-neutron tool since it contains a continuous neutron source for irradiating the formation and neutron detectors for detecting the spatial distribution of neutrons established by the source. It is conventional to utilize this tool to measure porosity of the formation under investigation. U.S. Pat. No. 3,483,376, describes in detail an illustrative embodiment of such a neutron-neutron tool.
Interestingly, in the past, very little has been understood about which parameters of a medium influence porosity response in an investigating instrument. This is indeed the case for neutron-neutron or neutron-gamma porosity tools. Such neutron tools utilize a source for emitting neutrons into the adjacent formations and subsequently or simultaneously detect the spatial distribution of the resultant neutron cloud through either the direct detection of neutrons or through the detection of gamma rays which are created when a neutron is absorbed in the nucleus of an atom of the formation.
Following emission from the source, the neutrons travel through the formation and lose energy by collision with the nuclei of the atoms of the formation. When the energy level of the neutrons is reduced or moderated sufficiently, they may be detected and counted by the investigating instrument. Generally, it is assumed that primarily the hydrogen index (i.e., the number of hydrogen atoms per unit volume of the formation fluid) is responsible for the spatial distribution of the cloud of neutrons. Since hydrogen is the only element whose nuclear mass resembles that of the neutron, hydrogen is the most effective element in reducing the energy level of the neutrons to a level at which they are eventually detected. In general, the formation pore spaces are filled either with water or with liquid hydrocarbons which both contain hydrogen. Thus, this type of neutron log is essentially a record of the hydrogen atom density of the rocks surrounding the borehole. Previously, the neutron log has been considered, therefore, to be a measure of the formation porosity. It is well recognized that gas, on the other hand, will alter this porosity determination since the gas is much less dense than its oil liquid counterpart.
U.S. Pat. No. 4,095,102 compares a value of porosity derived from an epithermal neutron-(gamma) tool with a value of porosity derived directly from a measurement of the thermal neutron absorption characteristic of the formation and the value of the water component of the formation. Where a difference is noted, hydrocarbon may be expected. In a manner similar to those techniques described earlier that utilize pulses of neutrons to determine a characteristic decay time dependent on macroscopic neutron capture cross section and hence a porosity, the disclosed technique requires saline water in the formation.
U.S. Pat. No. 4,416,151 also compares values of porosity derived from two different methods of porosity measurement, and associates differences with the presence of hydrocarbon.
During the life of an oil well the amount of water in the well itself tends to increase, and the above inventions can be sensitive to this, an effect which may mask the reliable detection of water/oil contacts in the reservoir, compromising their value.
According to a first aspect of the invention there is provided a method, of logging a borehole, for use in a cased oil well for detecting the gas/oil or water/oil contact in a hydrocarbon reservoir, using a neutron capture technique to detect one or more properties of fluid by comparing the count rates from two or more radiation detectors spaced at respective first and second distances from an isotopic source of continuous neutron radiation, the method including the steps of, at one or more locations along a said borehole, independently determining the properties of the borehole
Pereira Charles Alexander
Samworth James Roger
Hannaher Constantine
Paul & Paul
Reeves Wireline Technologies Limited
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