Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2000-07-14
2003-01-21
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
Reexamination Certificate
active
06509738
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of well logging, and more particularly to measurements of electrical conductivity of earth formations penetrated by a wellbore. More particularly, the invention relates to conductivity measuring instruments which are azimuthally sensitive.
2. Background of the Art
Measurements of selected properties of earth formations penetrated by a wellbore are typically recorded with respect to the depth within the borehole at which they are made. The records are commonly referred to as. “well logs”. Properties of interest of the earth formations are derived from acoustic, electromagnetic, nuclear and other types of measurements. The measurements are typically obtained by conveying a measuring instrument or “sonde” along the wellbore by attaching the instrument to one end of an electrical cable (“wireline”) and extending and retracting the cable by means of a winch or other spooling device located at the surface of the earth. Using the electrical cable as the means for instrument conveyance, the measurements of the formation properties are obtained subsequent to the drilling of the wellbore.
Various drilling parameters such as weight and torque on a drill bit used to drill the wellbore are typically measured during the actual drilling of the wellbore. In addition, formation properties, such as those previously described that are made by wireline conveyed instruments, are also measured during the drilling process rather than subsequent to drilling as in wireline logging operations. These techniques are usually referred to as measurement-while-drilling (MWD) and logging-while-drilling (LWD), respectively.
Drilling techniques known in the art include drilling wellbores from a selected geographic position at the earth's surface, along a selected trajectory. The trajectory may extend to other selected geographic positions at particular depths within the wellbore. These techniques are known collectively as “directional drilling” techniques. One application of directional drilling is the drilling of highly deviated (with respect to vertical), or even horizontal, wellbores within and along relatively thin hydrocarbon-bearing earth formations (called “pay zones”) over extended distances. These highly deviated wellbores are intended to greatly increase the hydrocarbon drainage from the pay zone as compared to “conventional” wellbores which “vertically” (substantially perpendicularly to the layering of the formation) penetrate the pay zone.
In highly deviated or horizontal wellbore drilling within a pay zone, it is important to maintain the trajectory of the wellbore so that it remains within a particular position in the pay zone. Directional drilling systems are well known in the art which use “mud motors” and “bent subs” as means for controlling the trajectory of a wellbore with respect to geographic references, such as magnetic north and earth's gravity (vertical). Layering of the formations, however, may be such that the pay zone does not lie along a predictable trajectory at geographic positions distant from the surface location of the wellbore. Typically the wellbore operator uses information (such as LWD logs) obtained during wellbore drilling to maintain the trajectory of the wellbore within the pay zone, and to further verify that the wellbore is, in fact, being drilled within the pay zone. Techniques known in the art for maintaining trajectory are described for example in, R. Fagin et al,
MWD resistivity tool guides bit horizontally in thin bed
, Oil and Gas Journal, Dec. 9, 1991. The technique described in this reference is based upon LWD conductivity sensor responses. If, as an example, the conductivity of the pay zone is known prior to penetration by the wellbore, and if the conductivities of overlying and underlying zones provide a significant contrast with respect to the pay zone, a measure of formation conductivity made while drilling can be used as a criterion for “steering” the wellbore to remain within the pay zone. More specifically, if the measured conductivity deviates significantly from the conductivity of the pay zone, this is an indication that the wellbore is approaching, or has even penetrated, the interface of the overlying or underlying earth formation. As an example, the conductivity of an oil-saturated sand may be significantly lower than that of a typical overlying and underlying shale. An indication that the conductivity adjacent the wellbore is increasing can be interpreted to mean that the wellbore is approaching the overlying or the underlying formation layer (shale in this example). The technique of directional drilling using a formation property measurement as a guide to trajectory adjustment is generally referred to as “geosteering”.
In addition to electromagnetic measurements, acoustic and radioactive measurements are also used as means for geosteering. Again using the example of an oil producing zone with overlying and underlying shale, natural gamma radioactivity in the pay zone is generally considerably less than the natural gamma ray activity of the shale formations above and below the pay zone. As a result, an increase in the measured natural gamma ray activity from a LWD gamma ray sensor will indicate that the wellbore is deviating from the center of the pay zone and is approaching or even penetrating either the upper or lower shale interface.
If, as in the prior examples, the conductivity and natural radioactivity of the overlying and underlying shale formations are similar to each other, the previously described geosteering techniques indicate only that the wellbore is leaving the pay zone, but do not indicate whether the wellbore is diverting out of the pay zone through the top of the zone or through the bottom of the zone. This presents a problem to the wellbore operator, who must correct the wellbore trajectory to maintain the selected position in the pay zone.
Electromagnetic induction logging instruments used in wireline logging techniques are well known in the art for determining conductivity of formations surrounding the wellbore. See for example, U.S. Pat. No. 4,651,101 issued to Barber et al, U.S. Pat. No. 4,873,488 issued to Barber et al and U.S. Pat. No. 5,688,475 issued to Orban et al. The instruments described in these patents, generally speaking, include a transmitter coil and an array of receiver coil pairs disposed at selected positions along the instrument. Each receiver coil pair includes a main receiver coil and a “bucking” coil electrically connected to the main receiver coil. In general, the transmitter and receiver coils are in the form of magnetic dipoles having their axes substantially coaxial with the instrument axis (referred hereinafter as axial magnetic dipoles “AMD”).
Electromagnetic induction well logging instruments are well suited for geosteering applications because their lateral (radial) depth of investigation into the formations surrounding the wellbore is relatively large, especially when compared to nuclear and acoustic instruments. The deeper radial investigation enables induction instruments to “see” a significant lateral (or radial) distance from axis of the wellbore. In geosteering applications, this larger depth of investigation would make possible detection of approaching formation layer boundaries at greater lateral distances from the wellbore, which would provide the wellbore operator additional time to make any necessary trajectory corrections. However, induction logging instruments are capable of resolving axial and lateral (radial) variations in conductivity of the formations surrounding the instrument, but the response of these instruments generally cannot resolve azimuthal variations in the conductivity of the formations surrounding the instrument. A limitation on geosteering ability which results from this limitation of induction logging instruments will be further explained.
A well logging instrument which provides directionally sensitive measurements would give valuable information in directional dril
Chang Shu-Kong (Steve)
Clark Brian
Minerbo Gerald N.
Aurora Reena
Jeffery Brigitte L.
Lefkowitz Edward
Schlumberger Technology Corporation
Segura Victor H.
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