A method is disclosed for generating an areal map of a pre-determined hydrocarbon liquid property of a subsurface kerogen-containing source rock from an electromagnetic resistivity profile. Preferably, the profile is generated by a transient EM method such as a long-offset transient electromagnetic (LOTEM) method. In some embodiments, the areal map is generated by employing resistivity-hydrocarbon liquid-quality relationship data describing a relationship between (i) a property of hydrocarbon liquid generated within the source rock pore space to (ii) an electrical resistivity of the source rock. In some embodiments, it is possible to acquire such data even in the absence of source rock samples where the hydrocarbon liquids within the samples has been preserved. The areal map is useful for determining a target location and/or depth in the source rock to drill for oil. The presently-disclosed techniques are particularly relevant to tight oil formations.

Nuclear magnetic resonance (NMR) logging tools may be configured for situation-dependent NMR logging operations by including two dissimilar coils that may function in four different modes of operation based on logging conditions including: a resistivity of the fluid, a diameter of the wellbore, a depth into the subterranean formation of the volume of investigation, or a combination thereof. For example, an NMR logging tool with a z-coil and a transversal coil may be useful in generating in a volume of investigation of a subterranean formation either (1) a transversal radiofrequency (RF) excitation with the transversal coil or (2) a quadrature RF excitation with both the z-coil and the transversal coil, where the choice of transversal or quadrature RF excitation is based on the logging conditions; and detecting an NMR signal from the subterranean formation with one of: (1) the transversal coil or (2) both the z-coil and the transversal coil.

Systems, devices, and methods for evaluating an earth formation. Methods include conveying a tool on a carrier in a borehole, the tool comprising a conducting tubular having a transmitter and receivers disposed thereon in a spaced-apart relationship; inducing a current in the earth formation using the transmitter; measuring with the first receiver a first time-dependent transient electromagnetic (TEM) signal induced by the formation responsive to the current; measuring with a second receiver a second time-dependent TEM signal induced by the formation responsive to the current; and using at least one processor to estimate a corrected time-dependent TEM signal using the first time-dependent TEM signal, the second time-dependent TEM signal, and a correction coefficient estimated as a function of a non-exponentiated ratio of distances r.sub.b and r.sub.m between the transmitter and the respective receivers.

A downhole tool includes a housing, a permanent magnet in the housing, and magnetic field source that selectively generates a magnetic field which counters a magnetic field generated by the permanent magnet. The downhole tool is deployed in a wellbore that intersects a subterranean formation, and is used for imaging the formation. The magnetic field source can be a coil through which an electrical current is selectively conducted. The electrical current can be continuously applied or pulsed through the coil.

In accordance with embodiments of the present disclosure, a nuclear magnetic resonance (NMR) logging tool for use in wellbore logging operations is provided. The NMR logging tool features a three-part permanent magnet arrangement that reduces the attractive force between the NMR tool and casing, allowing the NMR tool to be lowered through wellbores of any size. Additionally, the disclosed arrangement of permanent magnets in the NMR tool may facilitate an effective pre-polarization of portions of the formation at different logging speeds. Further, the disclosed arrangement of permanent magnets in the NMR tool may provide a uniform or substantially uniform static magnetic field within the antenna window of the radio frequency (RF) antenna used to perform the measurements. The second magnet may be positioned between the first and third magnets in a longitudinal direction along the NMR tool, and a radio frequency (RF) antenna may be disposed adjacent the first magnet.

In accordance with embodiments of the present disclosure, a nuclear magnetic resonance (NMR) logging tool for use in wellbore logging operations is provided. The NMR logging tool features a three-part permanent magnet arrangement that reduces the attractive force between the NMR tool and casing, allowing the NMR tool to be lowered through wellbores of any size. Additionally, the disclosed arrangement of permanent magnets in the NMR tool may facilitate an effective pre-polarization of portions of the formation at different logging speeds. Further, the disclosed arrangement of permanent magnets in the NMR tool may provide a uniform or substantially uniform static magnetic field within the antenna window of the radio frequency (RF) antenna used to perform the measurements. The second magnet may be positioned between the first and third magnets in a longitudinal direction along the NMR tool, and a radio frequency (RF) antenna may be disposed adjacent the first magnet.

A method for well integrity assessment using machine-trained prediction of physical characteristics related to well integrity across a depth interval of a cased wellbore having one or more casing strings. The method includes collecting scattered X-ray signals from a plurality of X-ray detectors arranged within a well logging tool, learning trained weights to predict wellbore characteristics from the scattered X-ray signals, collecting further scattered X-ray signals from the X-ray detectors, using the trained weights to quantitatively predict the wellbore characteristics from the further X-ray signals, and using the predicted wellbore characteristics to assess well integrity.

A sensor assembly for passive detections of downhole well features. Embodiments include a casing collar locator assembly that utilizes fiber optics in combination with a magneto-responsive sensor to detect casing collars and provide real-time location information in a well. The sensor may be configured to work with a poled monolithic structure that is dimensionally responsive to voltage in a way that substantially eliminates noise during detections. Additionally, the sensor may be intentionally imbalanced, utilizing multiple fibers of different lengths and multiple wavelength monitoring so as to provide enhanced directional information as well as allow operators to decipher and address circumstances of polarization fade.

The disclosed embodiments include systems and methods to evaluate formation resistivity. In one embodiment, a method to evaluate formation resistivity proximate a wellbore is provided. The methods includes receiving data indicative of resistivity measurements of a formation proximate a wellbore at different distances within a range of distances from the wellbore. The method also includes analyzing the data to determine approximate formation resistivity of the formation at different distances from the wellbore. The method further includes generating at least one curve, each indicative of an approximate formation resistivity of the formation at the different distances from the wellbore, overlaying the at least one curve on a graph indicative of the formation resistivity of the formation within the range of distances from the wellbore, and providing the at least one curve overlaid on the graph for display on a device.

A magnetic flux receiver assembly includes a collar that defines a central passageway and an axial collar face, and a tool insert disposable within the central passageway and defining an axial insert face. One or more magnetometers are operatively coupled to the tool insert. One or more buttons are coupled to one or both of the axial collar face and the axial insert face such that an axial gap is defined at an axial interface between the axial collar face and the axial insert face to prevent electrical communication across the axial interface.