Method for logging an earth formation using recycled alpha data

METHOD FOR LOGGING AN EARTH FORMATION USING RECYCLED ALPHA DATA

Cross-References

This présent application claims the benefit of U.S. Provisional Application No. 60/017528 filed May 10, 1996 (attorney docket number 20.2646).

Background of the Invention

A major goal of well logging is to maximize the amount of hydrocarbons recovered from an earth formation. By continuously monitoring oil saturation (S_o) in the earth formation, secondary and tertiary techniques may be employed to enhance recovery of hydrocarbons. Three methods hâve been developed for monitoring oil saturation during production of a well. One method, electrical resistivity, measures the water saturation, S_w, and uses the différence, 1-S_W, to dérivé oil saturation, S_o. This method dépends upon the presence of salts dissolved in the water and is less effective in fresh water than in sait water environments.

The second method involves drilling and coring a new well and analyzing the core to obtain oil saturation. This method is expensive and may not accurately reflect changes in oil saturation because the core samples represent different areas of the formation.

The third method is based on the fact that hydrocarbons contain carbon and water contains oxygen. A carbon/oxygen ratio (COR) is used to compute oil j i u + 8 2 saturation. The COR is derived by applying a spectral fitting technique to an inelastic gamma ray spectrum to compute carbon, oxygen, and other éléments présent in the formation. This approach provides an accurate means for computing the COR, however, due to the large amount of data required to obtain 5 a précisé S_o measurement, the statistical précision is worse and results in a substantially decreased logging speed. Alternatively, the COR is derived using counts from broad energy Windows in the inelastic gamma ray spectrum across the région of the prédominant carbon and oxygen gamma ray energies. These broad Windows contain a large number of counts allowing for faster logging 10 speeds, however, the window counts are strongly influenced by other formation éléments, the casing and the tubing configuration in the borehole, thereby, resulting in a less accurate oil saturation computation.

The Réservoir Saturation Tool (RST), a mark of Schlumberger, uses a method known as “alpha processing” to combine the accuracy of spectral 15 fitting with the précision of Windows processing (see U.S. Pat. Nos. 4,794,792, 4,768,796, and 4,909,075 issued to Flaum et al., assigned to Schlumberger Technology Corporation) to détermine the oil saturation in a formation. While this technique provides a précisé computation of the oil saturation, each logging run which utilizes alpha processing greatly increases the logging time needed to 20 obtain the accuracy of spectral fitting. When relogging and calculating the COR for a well over a period of months, particulariy in a field under water flood, steam flood, or any other secondary or tertiary recovery system, the RST does not î b 2 precisely detect changes in oil saturation. An efficient use of these recovery systems will translate into a substantial savings in the cost for producing a well.

For the foregoing reasons, there is a need for a method which precisely détermines the oil saturation of a formation producing hydrocarbons over a period of time.

Summary of the Invention

The above disadvantages of the prior art are overcome by a method and apparatus for determining a characteristic of an earth formation traversed by a borehole. A logging tool having a neutron source and at least one detector longitudinally spaced from the source. The logging tool is lowered into the borehole and the source irradiâtes the formation using sufficient energy to interact with atoms of the formation and the borehole. At least one detector detects the gamma rays resulting from the interaction of neutrons with atoms of the formation and borehole. An inelastic energy spectrum is formed and used to dérivé a first attribute of the formation. Also, count rate signais are generated based on the detected gamma rays and used to dérivé a second attribute of the formation. A value for the linear corrélation between the first attribute and second attribute is determined. After irradiating the formation on a subséquent logging pass, count rate signais are generated based on the detected gamma rays and used to dérivé a subsequet attribute of the formation. The linear corrélation value is combined with the subséquent attribute of the formation to dérivé a formation characteristic.

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Brief Description of the Drawings

The advantages of the présent invention will become apparent from the foilowing description of the accompanying drawings. It is to be understood that 5 the drawings are to be used for the purpose of illustration only, and not as a définition ofthe invention.

In the drawings:

Fig. 1 is a schematic illustration of a well logging tool within a borehole;

Fig. 2 illustrâtes a flow diagram for logging an earth formation using standard alpha processing;

Fig. 3 illustrâtes a flow diagram for logging an earth formation using recycled alpha data;

Fig. 4 illustrâtes an oil saturation curve using the RST and standard alpha processing; and,

Fig. 5 illustrâtes an oil saturation curve using the RST and the method ofthe subject invention.

Detàiled Description ofthe Preferred Embodiment

Fig. 1 schematically illustrâtes logging sonde 10 in a borehole 12 during logging operations. The borehole 12 is typically lined with steel casing cemented in place to the formation and may further include production tubing. The sonde comprises a pressure résistant housing 14 suspended by an armored cable

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16. A winch (not shown) is located at the surface and is used to lower and raise the housing 14 in the borehole 12. An accelerator or pulsed neutron source 18 is mounted in the sonde 10 with near detector 20 and far detector 22. Signais from the detectors 20, 22 are passed to the electronics 24 for transmission to the surface. Computer 26 receives gamma ray spectral data and count rates from detectors 20, 22 and processes the information according to the method of this invention to produce a measurement représentative of formation hydrocarbon saturation, S_o.

Fig. 2 illustrâtes a flow diagram of oil saturation derived from alpha processing the inelastic energy spectra and gamma ray counts detected by the sonde 10. After acquiring the gamma ray spectrum at step 110, a carbon-oxygen yields ratio is computed at step 112 and a carbon-oxygen Windows ratio is computed at step 114. The volume of oil determined from the carbon-oxygen yields ratio is calculated at step 116 and the volume of oil determined from the carbon-oxygen Windows ratio is calculated at step 118. Next, at step 120, the linear corrélation between the volume of oil determined at steps 116 and 118 is used to dérivé a. This corrélation may be the same over the entire length of the borehole. If the corrélation varies, a variable length filter is used to obtain the corrélation as a function of depth. Finally, at step 122 the alpha processed volume of oil is computed.

Referring to Fig. 3, a flow diagram for logging an earth formation using recycled alpha data is illustrated. For an initial logging pass (step 210), an cc_baseline is obtained at step 210. In a preferred embodiment of the invention, the a_baseline is derived according to steps 110, 112, 114, 116, 118, and 120. For each subséquent logging pass (step 210), a gamma ray spectrum is acquired (step 230). Next, the carbon-oxygen Windows ratio is computed (step 240). At 250, the oil volume from the carbon-oxygen Windows ratio is calculated. At 260, the «baseline ^is added to the oil volume calculated at step 250. A linear constant may be added to the oil volume calculated at step 260 to compensate for variations between logging tools. Since a_base,_ine is a function of the borehole tubulars and independent of oil saturation, a_base|_ine remains constant for each subséquent logging pass. By recycling the a_basei_ine and omitting the steps of determining the carbon-oxygen yields ratio, determining the volume of oil from the carbon-oxygen yields ratio, and determining a linear relationship between the volume of oil from carbon-oxygen yields ratio and the volume of oil from carbon-oxygen Windows ratio, the logging speed for subséquent runs is increased without diminishing the accuracy of the oil volume calculated for each subséquent run.

Fig. 4 illustrâtes an oil saturation curve for five logging passes obtained during a single time period using the RST and alpha processing. For purposes of monitoring changes in oil saturation over a predetermined period of time, these five passes would be difficult to utilize due to the imprécision of the resulting signal. Fig. 5 illustrâtes an oil saturation curve for five logging passes obtained during a single time period using the RST and the method of the subject invention. Recycling the a_base|_ine improves the précision of the measured d i u <fc82 oil volume by approximately 47.5%. The method of the subject invention also increases the logging speed by a factor of 2.18.

The foregoing description of the preferred and alternate embodiments of the présent invention hâve been presented for purposes of illustration and description. It is not intended to be exhaustive or limit the invention to the précisé form disclosed. Obviously, many modifications and variations will be apparent to those skilled in the art. As can be seen, Fig. 4 and Fig. 5 illustrâtes the practice of the invention with the RST. While so illustrated, it should be évident that the invention may be practiced with any logging tool that generates an inelastic îo gamma ray spectrum. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the accompanying ciaims and their équivalents.