Radiant energy – Geological testing or irradiation – Well testing apparatus and methods
Reexamination Certificate
1998-04-24
2001-03-27
Ham, Seungsook (Department: 2878)
Radiant energy
Geological testing or irradiation
Well testing apparatus and methods
C250S269600, C250S269800
Reexamination Certificate
active
06207953
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is directed toward logging of earth formations penetrated by a borehole, and more particularly directed toward the determination of formation gas saturation and other formation parameters from measures of fast neutron and inelastic scatter gamma radiation induced by a pulsed, fast neutron source.
In the context of this disclosure, “logging” is defined as the measure of a parameter of material penetrated by a borehole, as a function of depth within the borehole. Parameters of interest include density, porosity, and the liquid and gas saturation of the formation.
Density logging systems, which are compensated somewhat for the effects of the borehole, were introduced in the mid 1960s in the paper “The Physical Foundation of Formation Density Logging (Gamma-Gamma)”, J. Tittman and J. S. Wahl, Geophysics, Vol. 30, p. 284, 1965. The system introduced by Tittman et al, commonly referred to as a compensated gamma-gamma density logging system, was designed to operate in boreholes which are “open” and contain no steel casing. An instrument or “tool” is lowered into the well borehole on a cable, and the depth of the tool is determined by the amount of cable deployed at the surface of the earth. This type of tool contains an intense gamma-ray source and preferably two gamma-ray detectors at differing distances from the source. The gamma ray detectors measure gamma rays which are scattered from electrons in the formation, and back into the borehole. Since for most earth formations, the electron density is in constant proportion to mass bulk density, the “backscatter” gamma ray intensity at the detectors provides a measure of formation bulk density. Two detectors are preferably employed to allow the measurement to be compensated for the effect of mudcake that tends to accumulate on the borehole wall from drilling fluid used in the drilling process.
The gamma-gamma density tool has a characteristic shallow depth of investigation into the formation of about 4 inches (“Depth of Investigation of neutrons and Density Sondes for 35% Porosity Sand”, H. Sherman and S. Locke, Proc. 16th Annual SPWLA Symp., Paper T, 1975) and therefore is heavily influenced by the near borehole environment. This tool cannot make quantitative density logs in boreholes which have been cased, where the casing is typically steel and is surrounded by a cement sheath.
One technique for measuring formation porosity utilizes a porosity sensitive tool known in the industry as a “neutron-neutron” porosity system (Dual-Spaced Neutron Logging for Porosity”, L. Allen, C. Tittle, W. Mills and R. Caldwell, Geophysics Vol. 32, pp. 60-68, 1967). The downhole tool portion of the system contains a source of fast neutrons which is typically an isotopic source such as Americium-Beryllium (AmBe). Preferably two detectors sensitive to thermal or epithermal neutrons are axially spaced from the source at different distances. The detectors respond primarily to thermal or epithermal neutrons back-scattered into the borehole by the formation. The measured back-scatter flux is, in turn, primarily a function of the hydrogen content of the formation. If it is assumed that most hydrogen within the formation is contained in water or hydrocarbon in the pore space, the detectors respond to the porosity of the formation. As with the compensated density tool, the two neutron detectors respond to events at differing radial depths in the formation. The ratio of the detector response is formed to minimize the effects of reactions within the borehole, and porosity is determined from this ratio. The radial depth of investigation is about 9 or 10 inches, and the system can be calibrated to operate in both open and cased boreholes.
Cased and/or cemented borehole density logging can presently be done only with the borehole gravity technique. Logging with a gravimetry tool is time consuming and since it responds to a very large spatial volume of formation material, it has very poor depth resolution (“Well Logging for Physical Properties”, J. R. Hearst and P. Nelson, Chapters 6 and 8, McGraw Hill, New York, 1985).
The pore space in formations is fully water saturated in formations below the water table, except when natural gas reservoirs are encountered. In natural gas bearing formations, a fraction of the pore space is partially liquid-saturated with some fraction containing water or possibly oil, and the remainder of the pore space is saturated with natural gas. Sometimes, the pressure of contained natural gas can be quite high. Above the water table, in the so called “vadose” zone, the pore space is also partially liquid saturated typically with water, with the remaining pore space containing air at near atmospheric pressure.
Prior art techniques for determining gas saturation of formations penetrated by an open borehole involve the combination of the responses of the conventional gamma-gamma type density tool and porosity sensitive neutron-neutron tool. When the density and porosity tools are calibrated for the water-saturated pore space condition, and when they log formations that are water-saturated, they will produce values for formation bulk density and formation porosity that are consistent with each other, assuming the tools are logging in a rock matrix that is the same as that used for calibration. However, when a formation zone is encountered where the pore water is replaced by gas, the porosity tool gives an erroneously low porosity indication, while the density tool correctly indicates a decrease in bulk density with corresponding apparent increase in porosity. This results in a “cross-over” of the log response curves from the two tools thereby indicating the presence of gas within the logged formation. This method is problematic in cased boreholes because of the more shallow investigation depth of the density log and its resulting greater sensitivity to variations in borehole conditions, such as variations in the thickness of the cement sheath, immediately behind the casing.
Gas has been detected in cased and cemented boreholes, with limited success, using a combination of a cased hole version of the compensated neutron-neutron logging tool and the gamma-gamma density tool. Cigni and Magrassi (“Gas Detection from Formation Density and Compensated Neutron Log in Cased Hole”, M. Cigni and M. Magrassi, SPWLA 28th Annu. Logg. Symp., paper W, 1987) discuss the cased hole application of the neutron-neutron and gamma-gamma density logging tool combination. This method has been applied to the detection of gas migration in production and monitoring wells at the Prudhoe Bay fields of Alaska. Depth of investigation of the gamma-gamma measurement presents a problem as previously discussed. The neutron log also introduces a problem. Because the present day, commercial neutron-neutron porosity tool responds to changes in the thermal neutron distribution in the formation, which in turn is a function of hydrogen density within the formation, it is not able to distinguish between low porosity water-saturated formations and higher porosity partially gas-saturated formations since the hydrogen density in both formations can be the same. This is a serious disadvantage for the problem of formation gas detection, and is overcome by the density/gas saturation logging system set forth in this disclosure.
Logging for gas in cased, cemented boreholes is also performed using a logging tool containing a pulsed source of fast neutrons (“Examples of Dual Spacing Thermal Neutron Decay Time Logs in Texas Coast Oil and Gas Reservoirs”, Trans. SPWLA 15th Annu. Logging Symp., 1979, and “The Use and Validation of Pulsed Neutron Surveys in Current Drilling Tests” Trans. SPWLA 19th Annu. Logg. Symp., 1978). This “pulsed-neutron decay time” or “pulsed neutron” tool, as it is known in the art, was designed to detect the presence of hydrocarbon liquids (oil) in formations where the water that otherwise fills the pore spaces is normally saline. This sensitivity to fluid type is achieved by measuring the formation thermal neutron cross section. Ho
Felsman Bradley Vaden Gunter & Dillon, LLP
Ham Seungsook
Hanig Richard
LandOfFree
Apparatus and methods for determining gas saturation and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and methods for determining gas saturation and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and methods for determining gas saturation and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2466189