Schlumberger Technology Corporation

Darwin V Ellis

Charles R Case

Charles Flaum

Hubert D Scott

Peter D Wraight

Bradley Albert Roscoe

Michael Lynn Evans

James A Grau

Tong Zhou

Chanh Cao Minh

Benoit Couet

Felix K Chen

Roger Griffiths

Jeffrey S Schweitzer

Tancredi Botto

Russel C Hertzog

Françoise Allioli

Joseph M Chiaramonte

Mohammed Doghmi

Marvin E Markley

Charles Fulton

Marie Laure Mauborgne

Dhanoolal Kandhai

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.

Formation characterization for fast forward neutron models

The present disclosure relates to a method to determine the capture cross-section of a subsurface formation at a desired depth in the formation. A database of Sigma values for known lithologies, porosities, and salinities is provided, and multiple Sigma measurements are obtained from a downhole logging tool. Within the database, Sigma values are interpolated to determine the respective depths of investigation of the multiple Sigma measurements. A monotonic function is fitted to the multiple Sigma measurements at the determined depths of investigation, and the capture cross-section of the subsurface formation at any desired depth in the formation is determined using the fitted function. Similarly, a system to determine the capture cross-section of a subsurface formation at a desired depth in the formation and/or a depth of invasion of drilling fluids is also disclosed.

Estimating sigma log beyond the measurements points

A more precise determination of bulk formation density is attainable using a pulsed &#x3b3;-&#x3b3; 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.

Method of extracting formation density and Pe using a pulsed accelerator based litho-density tool

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.

Method of determining petro-physical information with high energy gamma rays

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.

Measuring formation density through casing

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.

Measuring wellbore diameter with an LWD instrument using compton and photoelectric effects

The invention concerns a neutron measurement method for determining porosity of an earth formation surrounding a borehole comprising:

Neutron measurement method for determining porosity of a formation surrounding a borehole

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.

Wellbore caliper measurement method using measurements from a gamma-gamma density

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

Compensated gamma-gamma density sonde using three detectors

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. Thus, the inverse problem of determining true porosity may be solved by obtaining determinations of salinity, formation matrix and fluid constituents, .SIGMA., and .phi..sub.a, and using the relating means to provide the true porosity. The provided method properly and separately accounts for the effects of capture cross section and salinity, and can be used to correct the apparent porosity determination of a neutron-neutron tool, regardless of the formation matrix or formation fluid constituents.

Determining a formation porosity from neutron logging information

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. A further correction to the aluminum count rate has a functional dependence on the neutron slowing down length in the formation, the formation macroscopic cross section, the borehole microscopic cross section, the bulk density of the formation, and the borehole diameter. When used in conjunction with a density tool sensitve to the effects of photoelectric absorption of gamma rays in the formation, the techniques of the invention allow the magnesium concentration to be determined.

Logging apparatus and method for determining absolute elemental

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.

Clay type and volume evaluation

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.

Correction of natural gamma radiation logs for the effects of gamma ray

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.

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Ridgefield, CT, United States of America

Darwin V Ellis