Wells – Processes – With indicating – testing – measuring or locating
Reexamination Certificate
2002-07-31
2004-07-27
Neuder, William (Department: 3672)
Wells
Processes
With indicating, testing, measuring or locating
C116S254000, C250S262000, C250S269300, C250S269700
Reexamination Certificate
active
06766855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to well logging tools for the acquisition of data in an underground borehole. The present invention relates more specifically to a tool, and a method for using the tool, to determine the dip angle of underground formations traversed by a cased or uncased borehole.
2. Description of the Related Art
The oil and gas industry has devoted much effort to the development of various devices and methods for identifying, recording and analyzing the characteristics of underground formations traversed by boreholes. This information on the characteristics of underground formations is critical to a real time determination of the prospects for locating oil or gas in a drilled location. Extensive studies of geological formations have allowed oil and gas operators to improve the chances of locating and reaching oil and gas bearing formations simply by maintaining accurate information on the composition and structure of the various formations traversed and followed in the process of drilling the borehole.
There are many different well logging techniques and tools available on the market. These tools and techniques generally fall into three or four different categories of investigation. Techniques such as the detection and measurement of resistivity, induction, conductivity, acoustic and electromagnetic field changes to radiation measurements, both active and passive, are utilized in typical well logging systems. These methods seek to identify the nature or composition of the formations surrounding the borehole at any given point, to identify the distance into the borehole that the tool is positioned at, to identify the angle of inclination for the borehole itself, to identify the directional orientation (typically from a reference azimuth or from a magnetic north) of the borehole.
Also of great importance to many well logging techniques, and the primary subject of the present invention, is a determination of the angle at which the underground formations are oriented at the point at which the borehole traverses the formation. This so-called “dip angle” allows the oil and gas operators to not only characterize the nature of the formation surrounding the borehole but additionally to identify the likely direction that the surrounding formations follow or the likely location of other formations of particular interest that are known to be adjacent the referenced formation. Such information has become an aid in determining structure and is particularly relevant to drilling techniques in so called “directional drilling” in angled and horizontal wells where geosteering methods are implemented. This is typically the case in multiwell offshore platforms that necessarily run directional boreholes in order to accommodate a number of wells from a single platform. Rather than working with strictly vertical wells, many current oil and gas explorations incorporate steerable well drilling techniques that allow the oil and gas operators to respond to formation structures and dip angles in a manner that permits a change in the direction of the well drilling, often during the drilling operation.
A “dip log” is the recording of information from which the angle and direction of geological bedding planes may be determined. A dip log device is typically used within an open well borehole without casing primarily because existing techniques rely upon direct contact between the sensors measuring the formation and the borehole wall/surface of the formation. Analysis of the information provided by a dip log device makes possible the identification of reefs, channels and faulting, depth formations around salt domes, and other structural anomalies critical in the analysis of oil field geology.
The standard dip log device has either three or four arms positioned radially about a central cylinder and that extend out to make contact with the borehole wall surface. Attached to each arm is a small flexible rubber pad designed to give good contact with the wall of the borehole. Molded into the face of each pad are several small electrodes that operate in pairs to establish a flow of electrical current in the formation upon contact with the electrodes. The arms track the sides of the borehole as the logging tool moves up or down so that the conductivity or resistivity in each area can be measured. Not only are distinctions made in the conductivity or resistivity according to depth but more specific distinctions are drawn between measurements made by one set of electrodes and those made by a second (or third or fourth) set of electrodes positioned radially distinct from the first. In this manner not only can the depth and “thickness” of the formation be determined but some localized inclination can be measured as well. It is this inclination that can be most useful in geosteering operations and in characterizing an approach to a pay zone formation.
The upper section of a dip log device will typically contain a physical orientation mechanism that continuously establishes the position and directional orientation of the instrument with respect to both the gravitational reference and an azimuthal reference (magnetic north). The dip log is normally run after the other open hole logs have been completed. The instrument is attached to the end of a logging cable (wire line) and is suspended and lowered into the borehole with arms in a closed position (see FIG.
1
). Once the device is at the bottom of a hole or at the point where the deepest interval is to be logged, a calibration of the measuring circuits is made. The arms are then extended placing the face of each measuring pad in direct contact with the borehole wall. (The new device of the present invention does not require surface contact with the formation as do all existing devices. Tool centralizers in the present invention keep the tool in a proper position.) A survey is then made of the variations and changes in the resistivity or conductivity characteristics of the surrounding traversed formations as the assembly is drawn upwards through the borehole.
By tracking both the radial orientation and the conductivity trace of each the four sensor bearing arms (or three as the case may be), detailed information about the compositions and inclinations of the formational structures that intersect the borehole can be determined. In general, however, this information is readily available only by utilizing a wire line well logging system, as the types of sensors required are typically too fragile to withstand the measurement while drilling environment.
Radiation based well logging, as an alternative to contact resistivity based systems, generally involves either the measurement of natural radiation from the geologic structures intersecting the borehole, or a responsive measurement made after a radioactive source is lowered into the borehole. In either case, measurements of radiation are made and an interpretation of the various levels of the different types of radiation is used to indicate the nature of the structures immediately adjacent to the well logging device. A gamma ray log is considered primarily a shale-locating log while neutron logs provide information on the lithology and the porosity of the formation, thereby characterizing the hydrogen richness of the adjacent formations.
Radiation logs are generally nondirectional in the sense that no specific orientation within the borehole is determined and only a depth of the formation measurement is associated with the radiation measurement.
Resistivity and conductivity logging, as discussed above, have serious limitations. Resistivity and conductivity logging are, for example, ineffective in metal cased boreholes. Consequently owners of older, depleted wells lack dependable logs run prior to completion and often lack other adequate drilling records These older wells are therefore at a loss to determine at what depth they could tap potential oil producing strata before abandoning their wells. Radioactive well logging, in part, is intended to addr
Jackson & Walker, LLP
Neuder William
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