Resistivity logging

Abstract: This paper examines the effects of wettability on the Archie saturation exponent and the formation factor, which are determined experimentally in cores. These parameters are important in the investigation of the hydrocarbon saturation of a formation by use of resistivity data obtained from well logging. The Archie saturation exponent, n, typically has a value of about 2 in water-wet formations and cleaned cores, while in native-state, non-water-wet cores and formations it is generally larger than 2. In uniformly oil-wet cores with low brine saturations, n can reach values of 10 or more. The exponent is higher in oil-wet cores at low saturations because a portion of the brine is trapped or isolated in dendritic fingers where it is unable to contribute to electrical conductivity. If a cleaned water-wet core is used to measure n and the reservoir is actually oil-wet, interstitial water will be underestimated during logging. No definite conclusions can be drawn about the effects of wettability on the formation factor. However, the wettability of clays in a core is likely to affect this parameter.

Abstract: Some developments in the mathematical analysis of resistivity well-logging measurements in anisotropic beds are presented. Treatment of the no-borehole case in which two thick, anisotropic beds meet at a plane interface for bedding planes parallel to the interface is extended to include arbitrary orientation of the bedding planes. Relations are derived for finding the potential distributions in the two media produced by a point current source in one of them. Numerically evaluated apparent-resistivity profiles across the boundary between two anisotropic beds are shown for a normal resistivity-logging device for various conditions of anisotropy and dip. For electrode devices using alternating current, solutions are derived in terms of the vector potential for the case of a borehole penetrating a thick anisotropic bed normal to the bedding planes and for the no-borehole case where the sonde axis is perpendicular to a sequence of beds with all bedding planes parallel to the bed boundaries. For induction-logging devices, the transmitter-coil sources of the electromagnetic (EM) field are treated as (alternating) magnetic dipoles. When the source- and receiver-coil axes both are oriented perpendicularly to the bedding planes, only the component of resistivity parallel to the bedding planes affects the responses. With the addition of coilsmore » oriented parallel to the bedding planes, it is theoreticallypossible to determine formation dip from the out-of-phase (reactive) voltages in the receiver coils. Analyses are outlined for a homogeneous medium, for a thin bed, and for borehole cases usually considered. Values of the horizontal and vertical conductivities (and coefficient of anisotropy) can, in principle, be derived from the measured values of the induction-logging conductivity signal and the out-of-phase signal from the formation. 21 figures, 4 tables.« less

Abstract: Source rock formations generally show a lower density, a lower sonic transit time, and a higher resistivity than other sediments of equal compaction and comparable mineralogy. This phenomenon can be used to identify source rocks on wireline logs provided the source rocks have a minimum thickness within the resolution of the sondes used and are sufficiently rich in organic matter. Classification rules have been established to assist in the recognition of source rocks on a combination of logs. Because of the low density contrast between water and organic matter, the method becomes inaccurate at high water saturations (i.e., low compaction). Within limits, the amount of organic matter contained in a sediment can be estimated from log anomalies. When source rocks become mature, free oil is present in addition to kerogen, and the resistivity increases by a factor of 10 or more.

Abstract: Apparatus is disclosed for determining the resistivity of formations surrounding an earth borehole. An electrically conductive metal body, such as a drill collar on a drill string, is movable through the borehole. A transmitting toroidal coil antenna is disposed on the body and is energized to induce a current which travels in a path that includes the body and the formations. An electrode is disposed on the body, and an electrical signal resulting from said current is measured at the electrode to obtain an indication of the resistivity of the formations. In a disclosed embodiment, one or more electrodes can be mounted on a blade that is mechanically coupled to the body.