This invention relates generally to oil and gas well logging tools. More particularly, this invention relates tools for measuring hydrocarbon saturation of earth formations through the use of acoustic velocities measured through casing
In petroleum and hydrocarbon production, there is considerable commercial value in the recovery of gas from reservoirs. Over the course of production of gas, there is an increasing influx of water into the reservoir. This may be due to natural causes or it may be, in the case of secondary recovery operations, the result of injection of water into the reservoir. The production of gas thus leads to a decrease in gas saturation of the reservoir. In addition, due to the fact that reservoirs by their very nature comprise permeable earth formations within impermeable strata, production of gas leads to a decrease of gas pressure. The decrease of gas pressure in turn affects the flow pattern of reservoir fluids. Decreases in gas pressure also allow gas that was in solution within liquid phases (e.g., water or oil) to come out of solution. This changes the properties of those liquids. Knowledge of the gas pressure is also very helpful in reservoir development. Knowledge of gas saturation is also important in enhanced oil recovery programs (EOR) where a gas is injected into an injection well and used to direct the flow of oil from the reservoir into a production well. Similar phenomena exist with oil and gas condensate.
A basic methodology underlying the determination of gas saturation and/or gas pressure is that of density determination. One approach involves detection of gamma radiation produced in the formation in response to a high-energy neutron source, referred to as induced gamma ray logging. When the neutron source is pulsed, gamma rays are produced by one of two reactions. The first is inelastic scattering of fast neutrons (neutrons with energies above about one MeV or within about one order of magnitude). The second mechanism is from capture of epithermal neutrons (neutrons with energy of about one eV). The third is from capture of thermal neutrons (neutrons with energy of about 0.025 eV). The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons slow down to a thermal energy level and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters). The number of capture gamma rays is strongly influenced by the amount of hydrogen and the thermal neutron capture cross section of the formation. The number of gamma rays produced from inelastic scattering is less dependent on these quantities, and a measurement of such gamma rays is more directly related to the formation density. Use of a pulsed neutron source allows capture gamma rays to be separated from inelastic gamma rays, giving a better estimate of density.
U.S. Pat. No. 3,780,301 to Smith Jr. et al. discloses a method and apparatus for determination of gas saturation using a logging tool deployed in an open borehole. A pulsed neutron source produces pulses of neutrons with energy of about 14 MeV. A single gamma ray detector measures counts of inelastic gamma rays resulting from interaction of the neutrons with nuclei in the formation. Specifically, counts are made in energy bands corresponding to C, O, Si and Ca. By comparing the Si/Ca and C/O ratios in these regions to the Si/Ca and C/O ratios for a known water sand, the relative abundance of limestone in the low hydrogen content formations may be estimated thus distinguishing gas zones from water saturated low porosity limestone.
An alternative method which could be used, but has not hitherto been used in cased boreholes for the determination of hydrocarbon saturation is to measure acoustic velocities (compressional- and shear-wave). Acoustic measurements are deterministic by nature and are not subject to the inherent statistical fluctuations associated with nuclear measurements. The present invention uses acoustic measurements through casing for the determination of hydrocarbon saturation of earth formations.
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