Abstract: A method for simulating a response of a neutron well logging instrument includes in a computer, defining a function of neutron migration length with respect to expected radiation detector counting rate. The function is defined for selected values of formation porosity. The function is related to neutron slowing down length and neutron diffusion length. The function is weighted for formation density. An expected radiation detector counting rate is calculated in the computer using the defined function based on an initial estimation of formation porosity and density.

Abstract: A method for simulating a response of a neutron well logging instrument includes in a computer, defining a function of neutron migration length with respect to expected radiation detector counting rate. The function is defined for selected values of formation porosity. The function is related to neutron slowing down length and neutron diffusion length. The function is weighted for formation density. An expected radiation detector counting rate is calculated in the computer using the defined function based on an initial estimation of formation porosity and density.

Abstract: A method for a pulsed gamma-gamma density tool to simultaneously compensate for interactions due to the photoelectric effect and density variations caused by standoff enables a more precise determination of bulk formation density. This method includes the steps of providing a source of energetic particles and directing those energetic particles at a formation having a known photoelectric factor and electron density and capturing one or more photons either emitted or deflected from the formation either a first detector or a second detector. The first detector is spaced a first distance from the source, the second detector is spaced a second distance from the detector and a third distance separates the first detector from the second detector.

Abstract: A more precise determination of bulk formation density is attainable using a pulsed ?-? density tool and simultaneously compensating for interactions due to photoelectric effect and density variations caused by standoff. A source directs energetic particles at a formation having a known photoelectric factor and electron density. One or more photons either emitted or deflected from the formation are captured at a first or second detector, respectively spaced at first and second distances from the source, and separated from each other by a third distance. First and second total energies of the photons respectively striking the first and second detectors are measured during a time interval. A first filter is disposed between the first detector and formation effective to cause Pe response to match standoff influence, thereby compensating for both effects simultaneously. In some embodiments, a second filter is provided between the second detector and formation.

Abstract: A method of determining formation density in a cased hole environment using a logging tool having a gamma ray source, a long spacing detector, and a short spacing detector that includes developing one or more cased hole calibration relationships that utilize differences between scattered gamma rays observed by short spacing detectors and scattered gamma rays observed by long spacing detectors to determine corrected formation density values, and using the cased hole calibration relationships and scattered gamma ray measurements obtained by the long spacing detector and the short spacing detector to determine the formation density. An associated article of manufacture and computerized well logging system are also described.

Abstract: A method for determining the diameter of a wellbore, the wellbore being drilled by a drill string immersed in weighted mud, the weighted mud having a significant weight fraction of a heavy component. A well logging instrument having a gamma ray source and energy-sensitive gamma ray detectors rotates within the wellbore to define a transient interface with a facing portion of the wellbore wall. The instrument measures Compton-effect gamma ray scattering and photoelectric-effect gamma ray scattering of gamma rays that cross a first interface, and of later gamma rays that cross an opposite interface, at each of a plurality of locations along the wellbore to produce a group of gamma ray counts at each of a series of wellbore locations. The counts are used to determine standoffs, weight fraction, and wellbore diameter.

Abstract: A method of determining formation density in a cased hole environment using a logging tool having a gamma ray source, a long spacing detector, and a short spacing detector that includes developing one or more cased hole calibration relationships that utilize differences between scattered gamma rays observed by short spacing detectors and scattered gamma rays observed by long spacing detectors to determine corrected formation density values, and using the cased hole calibration relationships and scattered gamma ray measurements obtained by the long spacing detector and the short spacing detector to determine the formation density. An associated article of manufacture and computerized well logging system are also described.

Abstract: Methods are disclosed for determining the standoff of a well logging instrument from the wall of a wellbore and the shape of the wellbore itself. The methods include measuring a compensated gamma-gamma formation density at a rotary orientation proximate a bottom of the wellbore, and measuring an apparent density at at least one other rotary orientation. The apparent density is related to the compensated density, a density of a fluid disposed between the logging instrument and a wall of the wellbore, and the standoff. Repeated measurements at a plurality of axial positions along the wellbore provide a log of the wellbore.

Abstract: Methods are disclosed for determining the standoff of a well logging instrument from the wall of a wellbore and the shape of the wellbore itself. The methods include measuring a compensated gamma-gamma formation density at a rotary orientation proximate a bottom of the wellbore, and measuring an apparent density at at least one other rotary orientation. The apparent density is related to the compensated density, a density of a fluid disposed between the logging instrument and a wall of the wellbore, and the standoff. Repeated measurements at a plurality of axial positions along the wellbore provide a log of the wellbore.

Abstract: A gamma-gamma sonde for use in determining the bulk density of an earth formation adjacent a borehole comprises a source of gamma radiation and three attenuation-type gamma ray detectors DN, DI and DF spaced apart from the source, respectively, at successively greater distances. The count rate of each detector is converted into an apparent formation density .rho. based on data derived from calibration of the sonde in calibration media having known uniform density, thereby to obtain apparent formation densities .rho..sub.F, .rho..sub.I and .rho..sub.N. The ratio(.rho..sub.F -.rho..sub.I) to (.rho..sub.F -.rho..sub.N)is substantially independent of mudcake density and provides an estimate of the mudcake thickness. The calibration data involving mudcakes of known density and thickness enable the use of the mudcake thickness estimate to determine the value of k in the algorithm.rho..sub.apparent =.rho..sub.F +k(.rho..sub.F -.rho..sub.N),from which formation density is determined.

Abstract: A method for determining the porosity of a subsurface geological formation traversed by a borehole is provided. The method generally comprises obtaining a means which relates ranges of apparent formation porosity (.phi..sub.a) as determined by a neutron-neutron log, formation fluid salinities, formation total capture cross sections (.SIGMA.), formation matrix and fluid constituents, and true formation porosity (.phi..sub.T) according to a predetermined equation. The predetermined equation relates the apparent porosity to a function of a modified migration length which is obtained in a semi-empirical manner (i.e. physics modifed by data). The modified migration length includes a slowing down length and a diffusion length, but causes at least the diffusion length to be a function of the slowing down length. The means which relates the apparent porosity to salinity, .SIGMA., matrix and fluid constituents, and .phi..sub.T solves a forward problem.

Abstract: Logging apparatus and methods for detecting first signals indicative of the absolute concentrations of the first category of elements in the rock matrix, for irradiating the formation with a pulse of high energy neutrons and for detecting a second signal indicative of the relative concentrations of a second category of elements in a rock matrix. The absolute and relative concentrations of a plurality of elements are determined, and this information is used to transform at least one of the relative concentrations into an absolute concentration. One of the measured absolute concentrations is for aluminum, from which correlations are used to determine the absolute concentrations of other, non-measured elements. A tool system for measuring the absolute aluminum concentration includes a californium-252 source and a gamma ray detector having a plurality of windows from which the aluminum count rate can be compensated for interference by manganese activation.

Abstract: Methods for the determination of clay types and/or clay volumes of an earth formation are disclosed. Information as to the hydroxyl type of the clay minerals is obtained from the difference between an epithermal (or thermal) neutron porosity measurement and a density porosity measurement, and information as to the relative volume fractions of the clays is obtained by cross plotting this porosity difference against the thermal neutron macroscopic capture cross section of the formation. Additional indicators of clay type, e.g., the potassium, iron and boron content of the minerals, may be used to further distinguish between clays, particularly where the intrinsic capture cross sections of different clays are similar.

Abstract: Disclosed are a method and a system for natural gamma radiation well logging in which the radiation detected in five energy windows is converted into a log of thorium, uranium and potassium (Th,U,K) which is corrected for the presence of gamma radiation emitting materials (e.g., potassium) and strong gamma radiation attenuators (e.g., barite and/or hematite) in the borehole fluid.

Abstract: A method for determining porosity or other formation characteristic of a sub-surface geological formation traversed by a borehole is claimed. A neutron logging tool is passed through the borehole while irradiating the formation with neutrons. The neutron logging tool includes a neutron source and first and second detectors spaced from the source by different distances. Neutrons exiting the formation are detected with the neutron detectors and count rate signals are generated. In response to these signals, an indication of porosity, substantially independent of error due to tool standoff from said borehole wall, is produced. In addition, values of tool standoff are also generated. These standoff values are then filtered by a filter to reduce statistical variations and are used to generate improved indications of porosity.

Abstract: A technique for determining the density of an earth formation with a logging sonde including a gamma ray source and two gamma ray detectors spaced at different distances from the source is disclosed. The count rate of the short-spaced detector is measured in two energy ranges covering back-scattered gamma rays which have undergone relatively low and high attenuations respectively. A first density correction is determined from the difference between the apparent density derived from the count rate of the long-spaced detector and the density derived from the count rate of the short-spaced detector in the energy range covering gamma rays with relatively low attenuation. A second density correction is determined from the difference between the densities derived from the two short-spaced detectors' count rates. These two density corrections are added to the apparent density to give the true formation density.