Borehole caliper derived from neutron porosity measurements

Details Borehole caliper derived from neutron porosity measurements Borehole caliper derived from neutron porosity measurements

1999-03-30

2001-09-04

Hannaher, Constantine (Department: 2878)

Radiant energy

Geological testing or irradiation

Well testing apparatus and methods

C250S265000, C250S269100

Reexamination Certificate

active

06285026

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed toward the determination of radial dimensions or “caliper” of a borehole penetrating earth formation, and more particularly directed toward determining caliper by irradiating formation with neutrons and measuring neutron flux within the borehole. The invention can be embodied to measure caliper while the borehole is being drilled, or alternately embodied to measure caliper as a wireline logging system used after borehole drilling has been completed.
2. Description of the Art
Accurate borehole caliper data is important for both the drilling of a well borehole, in the measurement of earth formation parameters penetrated by the borehole, and in completing the well after drilling.
In drilling a typical borehole for hydrocarbon production, the drill string is formed from sections or “joints” of drill pipe which are added to the drill string by threaded collars, and which is terminated by a drill bit. The drill string is rotated my means well known in the art, and the borehole is advanced by the cutting action of the drill bit. Drill bits must be periodically replaced as they become dulled by the drilling action. Bit replacement requires that the drill string be pulled or “tripped” from the borehole by sequentially removing joints of drill pipe. Borehole caliper data from successive trips in the borehole can be used to monitor wellbore conditions such as early indications of borehole washout and impending wellbore instability. Caliper information can allow a driller to take remedial actions during the drilling operation to prevent damage or catastrophic loss of the borehole, of drilling equipment, and even the loss of life of drilling personnel.
Formation parameter measurements as a function of depth, commonly referred to as formation “logging”, can be made subsequent to the drilling of the borehole by instruments conveyed by wireline, or can be made while drilling the borehole by instrumentation conveyed by a drill string. These techniques are commonly referred to as “wireline logging” and “logging-while-drilling” or “LWD”, respectively. Wireline and LWD measurements use borehole caliper data to correct measured parameters for various effects related to the radial dimensions of the well borehole. As examples, responses of most prior art neutron porosity, scatter gamma ray density, and resistivity type logging systems are functions of borehole size and must be corrected for borehole size effects to obtain optimum measurements of the desired formation parameters.
Once the drilling of a borehole is drilled to the desired depth, it is “completed” typically with a string of steel casing around which cement is pumped thereby filling the casing-borehole annulus. Caliper information is very useful in determining completion requirements, such as the amount of cement required to properly cement casing.
Many wireline and LWD systems are designed to minimize the effects of borehole size. The basic methodology utilizes two or more axially spaced sensors in the downhole “tool” portion of the system. Each sensor responds in a different degree to borehole size, and the responses are combined to minimize borehole effects. As an example, a dual detector neutron porosity wireline system was introduced in the 1960's in an attempt to minimize the effects of the borehole upon the measurement of formation porosity. Such a system is described in U.S. Pat. No. 3,483,376 to S. Locke issued Dec. 3, 1963. Two thermal neutron detectors are spaced axially at different distances from the source of fast neutrons. The ratio of the responses of the two detectors varies with formation porosity, yet is somewhat less sensitive to borehole parameters than the count rate from either of the two individual detectors. The ratio is, therefore, the measured parameter used to compute porosity. Corrections are made to the porosity value computed from the ratio in order to improve accuracy. Although much smaller than for single detector systems, borehole diameter corrections for dual detector systems are significant and can be quantified if an effective borehole caliper is available. More sophisticated algorithms have been used to combine sensor responses. Again using a dual detector neutron porosity system as an example, U.S. Pat. No. 4,423,323 to Darwin V. Ellis and Charles Flaum, issued Dec. 27, 1983, applies what is commonly known as the “spine and rib” interpretation to the count rate of each neutron detector in order to obtain a borehole size invariant porosity measurement without using an independent borehole caliper signal. The algorithm is relatively complex, and the range of borehole diameter variation over which reliable compensation can be obtained is relatively limited.
Various types of wireline borehole calipering devices were, and today still are, run in conjunction with borehole size sensitive wireline logs to provide a measure of borehole diameter from which borehole size corrections are computed. One type of caliper is obtained from an articulating arm of a pad type tool such as a pad mounted scattered gamma ray density tool, which was introduced commercially in the 1960's and is well known in the art. This type of caliper measures only one radial dimension, which is typically the major radial axis in a non-round borehole. Other prior art wireline calipers utilize measurements from multiple arm devices. These devices can be “stand-alone” caliper tools. Alternately, borehole caliper information can be obtained from arm positions of other types of logging tools such as multiple-arm formation dip tools. Although yielding a more representative measure of borehole size than a single arm device, multiple-arm devices are notoriously complex mechanically, difficult to operate effectively in harsh borehole conditions, difficult to maintain calibrated, and expensive to fabricate.
Prior art LWD systems, like their wireline counterparts, are sensitive to borehole size. Accurate caliper information is required to properly correct parametric measurements from these systems. It is readily apparent that arm type wireline calipers are not applicable to LWD since the drill string is typically rotating, and the arms engaging the penetrated formation would be quickly severed by this rotational movement. Other basic approaches must, therefore, be applied to LWD calipering.
Various methods have been used to obtain borehole size in LWD systems. Estimates can be obtained from the drill bit diameter, the drilling fluid pumping pressure, and the mechanical properties of the formation being penetrated. This method, at best, provides only a rough estimate of a borehole caliper in the vicinity of the drill bit since formation and drilling mechanical conditions can change rapidly. Other methods have been employed in an attempt to reliably caliper the borehole without using a specifically dedicated LWD caliper system. Generally speaking, these methods combine data from a plurality of LWD devices which exhibit different sensitivities to borehole geometric parameters. Such additional LWD devices might include well known scattered gamma ray density devices and resistivity devices which respond to varying radial depths of the borehole and formation environs. Borehole information is extracted by combining responses of these devices, and borehole corrections are derived from these responses. Again, generally speaking, this method of calipering a borehole and correcting measurements for borehole effects is not reliable. In addition, a relatively complex suite of LWD devices must be employed in order to practice this method.
U.S. Pat. No. 5,175,429 to Hugh E. Hall. Jr. et al, issued Dec. 29, 1992, addresses borehole calipering as a tool stand-off compensation method for nuclear LWD measurements. No independent borehole caliper or any other subsystem is required to obtain the desired tool stand-off or borehole size compensation. Count rates from a plurality of nuclear detectors are sorted and stored in “bins” as a function of apparent instrument stand-off. De

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