Method for ascertaining optimum location for well stimulation and/or perforation

Abstract

A method is disclosed for the in situ examination of earth formations penetrated by a borehole to ascertain the optimum location along the length thereof for instituting well stimulation operations or, where the well is cased, the optimum location for perforating the casing. The method utilizes natural gamma ray logging to determine the optimum locations with the natural gamma ray radiation of the earth formation surrounding the borehole measured and the total measurement then separated into potassium-40, uranium, and thorium energy-band signals. A differential value is derived by subtracting the energy-band signal for either potassium or thorium from the energy-band signal for uranium, with the differential thereafter compared to a energy-level standard having a preselected magnitude. The optimum locations for perforating and/or instituting well stimulation operations will be those zones in which the differential exceeds the preselected energy level standard.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radioactivity well logging and, more particularly, to a method utilizing natural ray gamma ray logging to determine the optimum locations along the length of a borehole penetrating earth formations to initiate well stimulation operations or, if the borehole is cased, the optimum location for perforation operations.

2. Description of the Prior Art

Various models and apparatus have been utilized in the well logging art to study the radioactive properties of subsurface formations, both where the radioactivity is natural and where it is artificially induced. In general, such methods and apparatus when applied to the field of natural gamma ray well logging having utilized three channels of spectra centered on the 1.46 MeV potassium-40, the 1.76 MeV uranium, and the 2.62 MeV thorium energies. Further, such prior art techniques have been directed to utilizing the logs to locate so-called "source rocks" for oil production in regions being explored.

One such technique utilizes natural gamma ray logs to determine the characteristics of shale formations as described in U.S. Pat. No. .Badd.gama gamma radiation in the earth formations, are coupled into a multichannel analyzer 20 which sorts gamma radiation, occurring naturally in the earth formations, as a function of energy, separating the energy into at least three energy channels or bands to separate gamma radiation occurring from the radioactive decay of isotopes of potassium, uranium and thorium. Additionally, a fourth energy channel containing the total measured spectrum is provided. Signals from total radiation, potassium, uranium and thorium channels are each coupled into a count rate meter, 22, 24, 26 and 28 respectively. Each meter 24, 26 and 28 accumulates a background-connected count rate for the particular isotope associated therewith, with count rate meter 22 accumulating the total number of gamma rays detected by crystal 12 to provide an indication of the total gamma ray count rate.

Accordingly, the multi-channel analyzer, acting through the count rate meters, provides output signals representative of the number of counts occurring in each energy channel. Each count number is characteristic of the respective radioactive decay of the isotopic potassium, uranium and thorium atoms in earth formations. These output signals are coupled into a spectrum stripper 30. As is known in the art, spectrum stripper 30 may comprise a small general purpose digital computer.

Spectrum stripping refers to the process whereby background count rates are electronically subtracted in a mathematical process from the potassium and uranium channels in the stripper 30. As a result of having the highest energy level, the thorium count rate is not stripped and may be used for further processing or forming a log directly. Thus, the stripping process is only necessary in the potassium and uranium channels as a result of the addition therein of energy-degraded thorium and, in the case of the potassium channel, uranium gammas. Count rates in the potassium and uranium channels that are obtained solely from energy-degraded thorium gammas are subtracted from the count rates due to the radio-isotopes themselves. A similar procedure for stripping energy-degraded uranium gammas from the potassium channel count rate is also performed. In this way, accurate concentrations of potassium, uranium and thorium are determined. Techniques for determining the amount of stripping required is well known in the art and will not be discussed in detail here. It will suffice to state that the spectrum measured by instrument 10 during a traverse of subsurface borehole is compared against spectral standards supplied from a standard spectrum data source (not shown) in which the gamma spectrum of known standard elements may be quantatively compared with that of the unknown earth formation penetrated by the borehole. Accordingly, coefficient representative of the fraction of the gamma ray spectrum caused by the standards as an estimate of the borehole radiation may be derived for use in the stripping process.

The total gamma energy spectrum signal along with the stripped energy spectrum signals for potassium, uranium and thorium are coupled into an interface unit 32. Unit 32 provides the interface necessary to couple the signals to various processing and/or display equipment such as a computer 34 or a logging camera 36 for the subsequent processing which comprises the method of applicant's invention as will be hereinafter described with reference to the pictorial representation of the energy spectrum log 38 depicted in FIG. 2. Although depicted as a visual representation or plot, it is to be understood that the signals developed by the apparatus of FIG. 1 may be utilized in digital computer 36 to develop the optimum location between depth-points along the borehole length indicative of the preferred location for conducting well stimulation and/or perforating operations.

As shown on log 38, the signals developed in the apparatus of FIG. 1 are depicted in graphical form of a total counts line, 40 a potassium counts line, 42, a uranium counts line, 44, and a thorium counts line, 46. To practice the method of applicant's invention, and again recognizing the signal processing may be performed within digital computer 34, a differential count valve value is derived by algebraically adding or otherwise comparing either the negative of potassium log line 42 or thorium log line 46 with uranium log line 44 at each given depth-point along the chart 38. The differential value thus obtained is then compared with a preselected reference having a constant count magnitude measured against the uranium differential count and as indicated by line 48. The count magnitude of reference line 48 is predetermined by the lithology of the earth formation. The selection of the magnitude of reference value 48 is within the ability of one skilled in the art. In accordance with the invention, those areas 50 where the magnitude of the differential value is greater than the magnitude of line 48 are indicative of the optimum locations along the length of the borehole for performing well stimulation operations. Further, in the cased borehole, the same area 50 is indicative of the optimum location for perforating the casing in preparation of performing the well stimulation operation.

Accordingly, while particular embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects.

Claims

1. A method for determining the optimum location along the length of a borehole penetrating an earth formation for instituting well stimulation operations, comprising the steps of:

transversing the borehole penetrating the earth formation with a logging instrument having a gamma ray detector;

detecting natural gamma radiation occurring in the earth formation in the vicinity of the borehole and generating signal functionally and depth-point related to the energy and frequency of occurrence of such gamma radiation;

separating said signals into at least three energy regions corresponding to gamma radiation produced by naturally occurring radioactive isotopes of potassium, uranium and thorium occurring in said earth formations at said related depth-point.

processing said separated signals for producing individual count signals each representative of radiation occurring in each of said at least three energy regions at said relative depth-point;

combining the count signals representative of at least one of such energy regions corresponding to gamma radiation produced by radioactive isotopes of potassium and thorium with said remaining energy region count signals corresponding to gamma radiation produced by radioactive isotopes of uranium for deriving a differential signal; and

comparing said differential with a preselected reference signal for determining optimum stimulation locations and at said depth-point location as indicated by said differential signal exceeding said preselected reference signal.

2. The method of claim 1, including repeating the steps therein through said step of comparing said differential signal with said reference signal for a plurality of depth-point locations in the borehole and recording all optimum locations for performing well stimulation operations as a function of borehole depth.

3. The method of claim 1, further including the step of traversing a cased borehole and perforating the casing at said locations where said differential signal exceeds preselected magnitude of said arbitrary reference signal.

4. The method of claim 1 wherein the step of deriving said differential signal, comparing said differential signal with said reference signal and displaying, as a function of depth, those areas wherein said differential signal exceeds said reference signal is performed in a digital computer.

5. The method of claim 1 wherein said step by developing said optimum location for performing well stimulation operations includes the steps of:

generating a log tape displaying the count signals of the measurement of the radioactive isotopes of potassium, uranium and thorium as a function of depth;

generating a reference cutoff value positioned at a preselected location along said log tape comparing said count signals with said reference line on said log tape; and

deriving depth related locations for performing said well stimulation operations from said comparison.

6. A method for determining the optimum location along the length of a borehole penetrating an earth formation for instituting well stimulation operations, comprising the steps of:

traversing the borehole penetrating the earth formation with a logging instrument having a gamma ray detector;

detecting natural gamma radiation occurring in the earth formation in the vicinity of the borehole and generating signal functionally and depth-point related to the energy and frequency of occurrence of such gamma radiation;

separating said signals into at least three energy regions corresponding to gamma radiation produced by the decay of the naturally occurring radioactive isotopes of potassium, uranium and thorium occurring in said earth formations at said related depth-point;

processing said separated signals for producing individual count signals each representative of radiation occurring in each of said at least three energy regions at said relative depth-point;

combining the count signals representative of at least one of such energy regions corresponding to gamma radiation produced by the decay of the radioactive isotopes of potassium and thorium with said remaining energy region count signals corresponding to gamma radiation produced by the decay of the radioactive isotopes of uranium for deriving a differential signal; and

comparing said differential with a preselected reference signal level for determining optimum stimulation locations and at said depth-point location as indicated by said differential signal exceeding said preselected

reference signal.

7. The method of claim 6, including repeating the steps therein through said step of comparing said differential signal with said reference signal for a plurality of depth-point locations in the borehole and recording all optimum locations for performing well stimulation operations as a function of borehole depth.

8. The method of claim 6, further including the step of traversing a cased borehole and perforating the casing at said locations where said differential signal exceeds the preselected magnitude of said reference signal.

9. The method of claim 6 wherein the step of deriving said differential signal, comparing said differential signal with said reference signal and displaying, as a function of depth, those areas wherein said differential signal exceeds said reference signal is performed in a digital computer.

10. The method of claim 6 wherein said step by developing said optimum location for performing well stimulation operations includes the steps of:

generating a log tape displaying the count signals of the measurement of the decay of the radioactive isotopes of potassium, uranium and thorium as a function of depth;

generating a reference cutoff value positioned at a preselected location along said log tape;

comparing said count signals with said reference line on said log tape; and

deriving depth related locations for performing said well stimulation operations from said comparison.