Patent Publication Number: US-2005139759-A1

Title: Lifetime pulsed neutron/chlorine combination logging tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/949,596 filed Sep. 10, 2001, now U.S. Pat. No. 6,825,459, issued on Nov. 30, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 09/225,029, filed on Jan. 4, 1999, which is now abandoned, which are all herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to the field of radioactive well logging to evaluate sub-surface conditions surrounding boreholes for producing hydrocarbons, thereby determining a likelihood of oil and/or gas zones.  
      2. Description of the Related Art  
      The production of hydrocarbons from sub-surface locations typically includes the drilling of a borehole into the earth in a location where hydrocarbons are likely to be found, physically isolating the borehole from the earth surrounding it by the placement of casing therein, cementing the casing in place, and penetrating the casing at zones known or suspected to have producible quantities of hydrocarbons. This enables the hydrocarbons to flow into the casing and thence be pumped or otherwise flowed to the surface.  
      The location of zones likely to produce hydrocarbons is often determined by passing a tool, commonly known as a sonde, along the length of the borehole. Such sondes typically emit radiation, and in response, receive signals from the formation indicative of the geological structure adjacent to the borehole into which the radiation penetrated and the likelihood that such formations include hydrocarbons therein. Where neutrons are released from the sonde, the formation returns a signature gamma ray or gamma radiation released when a formation atom “captures” the emitted neutron.  
      Nuclear logging techniques may be considered to be in two classes that include thermal neutron lifetime logging (“lifetime logging”) and hydrogen and chlorine logging (“chlorine logging”). U.S. Pat. No. 3,564,248 to Hopkins, et al. describes the lifetime logging while U.S. Pat. No. 3,772,513 to Hall, et al. describes the chlorine logging. With the chlorine logging, gating a detector for discrete energy ranges enables independent sensitivity for hydrogen and chlorine when the detector is shielded by a neutron-absorbing material such as samarium. In contrast, the lifetime logging detects the total amount of neutron capture per unit time regardless of energy levels in order to determine a thermal neutron capture cross section of the formation that is based on a slope of a decay curve.  
      In operation, both logs provide information relating to the formation. For example, the chlorine logging enables distinguishing between the presence of salt water and hydrocarbons in the formation based on differences detected between hydrogen and chlorine in the formation. Further, the value of the thermal neutron capture cross section determined with the lifetime logging provides an indication of the chlorine content, which indicates salt water presence.  
      For highly accurate quantitative interpretation of the lifetime log, it is often necessary to know accurately certain parameters, such as formation porosity, fluid salinity, and shale fraction of the formation. For example, boron within shale of the formation affects the slope of the decay curve from the lifetime log. Thus, quantitative interpretation solely utilizing lifetime logging often proves unsatisfactory where certain of these parameters can only be roughly estimated. In contrast, the chlorine log when used alone requires an estimate of salinity and porosity and is subject to errors caused by variations of borehole effects or parameters, such as those resulting from washouts, poor cementation, borehole size variations and the like. However, boron within a shale formation surrounding the borehole provides a similar response as chlorine when using this chlorine log such that the shale formation appears as salt water. Accordingly, the two types of logs compliment each other in several ways when run together and benefit from more accurate porosity estimates.  
      Interpreting the results from the logs remains vulnerable to formation unfamiliarity since the results depend in part on a background understanding of the formations traversed by the borehole. Without such information, the results can return false positive results for the presence of hydrocarbon and false negative results which overlook the presence of hydrocarbon. Additionally, the effect of the borehole due to the presence or absence of fluids therein can affect the reliability of the data produced. Thus, the sonde may require operation in a laboratory or test borehole of known constituents and use of the test results to interpret the results obtained when used in an actual borehole.  
      Therefore, there exists a need for improved methods and apparatus for analyzing formations adjacent to a borehole with a minimum of calibration and a minimum of borehole effect.  
     SUMMARY OF THE INVENTION  
      Embodiments of the invention generally relate to methods and apparatus for logging a wellbore and determining a presence or absence of a hydrocarbon bearing formation. A sonde includes at least two gamma radiation detectors that are utilized for chlorine logging and lifetime logging in combination. Additionally, two detectors of the sonde are spaced axially from each other at different distances from the source to enable determination and compensation for various other parameters such as porosity and water flow velocity. Appropriate gating of the detectors enables sensing total counts of radiation emitted from adjacent formations and sensing of specific energy ranges of radiation when the formation is bombarded with energy. Signals from the lifetime logging enable adjustment of the chlorine logging for a borehole effect and background radiation. The detectors can include a sheath of a high capture cross-section material that interacts with neutrons to produce gamma radiation to shield the detectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  is a sectional view of an embodiment of a sonde located in a borehole.  
       FIG. 2  is a graph of the relationship between relative intensity and gamma ray energy returned using a first pair of detectors of the sonde.  
       FIG. 3  is a graph of the relationship between relative count rates of gamma ray energy returned versus time using a second pair of detectors of the sonde.  
       FIG. 4  is a sectional view of an alternative embodiment of a sonde located in a borehole. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Embodiments of the invention generally provide methods and apparatus for performing logging of boreholes for the detection of hydrocarbon producing zones. Generally, well logging according to embodiments of the invention includes placement of a sonde at various locations along the span of a borehole such that the sonde generates signals and receives return signals for processing in order to generate data indicative of the features of the formation surrounding the borehole.  
       FIG. 1  illustrates a cross section of a sonde  100  suspended by a cable  104  into a borehole  102 . As shown, casing  101  held in place by cement  103  isolates the borehole  102  from the formation  105  that the borehole  102  traverses. A housing  106  of the sonde  100  attaches to the cable  104  to provide a secure and waterproof environment within the interior of the sonde  100 . The sonde  100  includes a neutron generator or radiation source  110 , a first pair of detectors  112 ,  114  for chlorine logging, a second pair of detectors  116 ,  118  for lifetime logging, and a control section  120  all located within the housing  106 . To prevent and/or reduce the effect of direct neutron bombardment of the detectors  112 - 118  by the neutron generator  110 , the sonde  100  includes shielding  108  composed of lead, tungsten, boron or another similar element or combination thereof disposed between the neutron generator  110  and each pair of detectors. Additionally, the sonde  100  may include a centralizer device such as a leaf spring (not shown) that extends from the outer surface of the sonde  100  to cause the sonde  100  to push against the side of the borehole  102 .  
      The control section  120  powers the neutron generator  110 , manipulates and/or stores signals generated by the detectors  112 - 118 , and may establish the position of the sonde  100  in the borehole  102 . Accordingly, the control section  120  of the sonde  100  may include a power supply  122  such as a battery, a controller  124 , a telemetry section  126 , and a gamma ray and casing collar locator (CCL) detector  127 . The controller  124  initiates powering of the neutron generator  110  in response to operator input or a depth signal. The telemetry section  126  receives the signals generated in the detectors  112 - 118  in response to the receipt of gamma radiation. Alternatively, the cable  104  supporting the sonde  100  may provide power to the sonde  100  and may provide a signal pathway for transmission of data from the sonde  100  to a surface location for analysis. The optional gamma ray and CCL detector  127  detects naturally occurring gamma radiation for correlation to the original depth log run in the borehole  102  and detects collars in the string of casing  101  for depth reference. Thus, the gamma ray and CCL detector  127  can send a signal indicative of depth to the telemetry section  126  for recording the depth at which the gamma radiation is recorded. For some embodiments, monitoring a length of the cable  104  lowered into the borehole  102  establishes the depth of the sonde  100  during the logging operation.  
      The first pair of detectors  112 ,  114  for the chlorine logging are located above the neutron generator  110  and spaced such that a first near detector  112  of the first pair of detectors is closer to the neutron generator  110  than a first far detector  114  of the first pair. Likewise, a second near detector  116  of the second pair of detectors for the lifetime logging is closer to the neutron generator  110  than a second far detector  118  of the second pair. Preferably, the near detectors  112 ,  116  of each pair are on opposed sides of the radiation source  110  and spaced an equal distance from the radiation source  110 , and the far detectors  114 ,  118  of each pair are on opposed sides of the radiation source  110  and spaced an equal distance from the radiation source  110  that is greater than the distance between the source  110  and the near detectors  112 ,  114 .  
      Each of the radiation detectors  112 - 118  include, for example, a scintillation detector in the form of an optically transparent thallium-activated crystal of sodium iodide or the like with an end-window photomultiplier tube optically coupled to the crystal. A suitable amplifier receives output electrical pulses generated in the photomultiplier tube and linearly amplifies such pulses. Thus, the detectors  112 - 118  emit signals in response to the receipt of gamma radiation. Additionally, gating circuits associated with respective ones of the detectors  112 - 118  receive the electrical pulses representing the detection of gamma rays and gate the detectors  112 - 118  such that the first pair of detectors  112 ,  114  for the chlorine logging are configured to be responsive to gamma radiation in specific energy ranges and the second pair of detectors  116 ,  118  for the lifetime logging are configured to detect the quantity of gamma radiation reaching the detectors  116 ,  118  during specific time periods.  
      A sheath  109  of preferably samarium oxide (Sm 2 O 3 ) optionally surrounds the first pair of detectors  112 ,  114  for chlorine logging and can additionally surround any of the detectors disclosed herein. For this embodiment, the sheath  109  provides a samarium shield as one type of suitable material which has a characteristic neutron capture gamma radiation emission spectrum predominantly within an energy band that includes a significant part of the neutron capture gamma spectrum of certain elements (i.e., hydrogen) in the formation. The sheath  109  further includes a characteristic neutron capture gamma radiation emission spectrum substantially outside another energy band (i.e., a spectrum associated with chlorine). Accordingly, the sheath  109  enables a shale compensating effect when used for chlorine logging. While samarium is preferred as the material for the sheath  109 , other materials may be employed rather than or together with samarium. For example, europium or gadolinium may also be employed as the material for the sheath  109  since these materials possess a generally similar characteristic neutron capture gamma radiation emission spectra to samarium.  
      As the sonde  100  lowers into the borehole  102 , the power supply  122  initiates firing or operation of the neutron generator  110  upon reaching an appropriate depth. In response to the supply of power, the neutron generator  110  emits burst of neutrons in the 14 MeV (million electron volts) range that last an appropriate length of time such as approximately 60 μs (microseconds). In this manner, the neutron generator  110  generates pulses of neutrons in discrete time periods. As each burst is emitted, the neutrons travel from the generator  110 , through the housing  106 , the borehole constituents, if any are present, the casing  101 , the cement  103  and thence into the adjacent formation  105 . The neutrons interact with constituents of the materials that they pass through and generate thermal neutrons, gamma radiation and other side effects.  
      At each location where the neutrons emitted by the neutron generator  110  pass, a portion of the neutrons collide with atoms of the constituents of the materials and result in a slowing of the neutrons with each collision. Eventually the neutrons are captured by the nucleus of atoms with which they collide, thereby causing excited atoms to be formed. Each excited atom has a half life that depends upon the element and can last from milliseconds to a half of an hour. The excited atom decays into the original unexcited state and simultaneously emits a gamma ray or capture gamma radiation when it returns to its steady state. The energy of the gamma radiation has an energy characteristic of the atomic species that produced the gamma radiation.  
      The first pair of detectors  112 ,  114  for the chlorine logging count radiation in the energy range of gamma radiation emitted by hydrogen and chlorine, as shown in  FIG. 2 . Specifically, gamma radiation emitted by the decay of hydrogen is in an energy range of about 1.2 MeV to about 2.2 MeV, whereas that indicative of decay by chlorine is in the energy range of about 2.2 MeV to about 8 MeV. The detectors  112 ,  114  detect gamma radiation caused by neutron capture of the constituents of the borehole  102 , the casing  101 , the cement  103  and the formation  105  as the sonde  100  moves in the borehole  102  and the neutron generator  110  generates the pulses of neutrons. The quantity of gamma radiation detected at the far detector  114  has a smaller amplitude or a smaller count rate than the quantity of radiation detected at the near detector  112  since a substantial quantity of the neutrons emitted by the source  110  are captured immediately adjacent to the source  110 . Thus, fewer neutrons emitted by the source  110  reach a location where subsequent gamma radiation from the constituents can reach the far detector  114 .  FIG. 2  demonstrates the lower gamma radiation detected at the far detector  114  as shown by curve  130  that represents the counts from the far detector  114  compared to curve  132  that represents the counts from the near detector  112 . Additionally, the timing of the receipt of the counts of hydrogen and chlorine gamma radiation can be recorded verses the number of counts received for further data manipulation.  
      Simultaneously as the first pair of detectors  112 ,  114  for the chlorine logging count gamma radiation in specific energy gates, the second pair of detectors  116 ,  118  for the lifetime logging are likewise bombarded with gamma rays resulting from the collision of neutrons emitted by the neutron generator  110  with the constituents of the borehole  102 , the casing  101 , the cement  103  and the formation  105 . Since the second pair of detectors  116 ,  118  are time gated, the second pair of detectors  116 ,  118  count all gamma rays reaching the detectors  116 ,  118  during a specified time period regardless of their energy.  
       FIG. 3  shows a representative plot of total gamma radiation count versus time as detected by the detectors  116 ,  118  during the lifetime logging. Line  134  of the plot represents counts from the far detector  118  while line  136  represents counts from the near detector  116 . During the period that the neutron source  110  emits neutrons, each detector of the second pair of detectors  116 ,  118  is gated closed so as to not send a signal indicative of gamma radiation detected. Following the emission of the burst of neutrons from the source, the detectors are gated open and counts of gamma radiation received at the detectors  116 ,  118  are generated and sent for recording in a time verses counts manner. As seen in  FIG. 3 , the initial count rate of gamma radiation detected by the detectors increases and then falls off into a decay mode. As labeled in  FIG. 3 , the initial portion of the detected gamma radiation is indicative of the interaction of the emitted neutrons with the constituents of the borehole  102  and the casing  101 . Thereafter, the count rate versus time relationship shows a decay relationship where the number of counts decreases over time as the total quantity of neutrons emitted by the source are captured by the formation and are no longer available to subsequently create gamma radiation.  
      As labeled in  FIG. 3 , the second pair of detectors  116 ,  118  detect residual background radiation of the borehole  102  and the formation  105  during a background interval portion of each operating cycle defined by a background circuit subsequent to the measurement portion of the cycle. The residual background radiation provides a compensating factor during the derivation of both the thermal neutron capture cross section for the lifetime logging and the determination of the relative presence of hydrogen and chlorine in the formation for the chlorine logging.  
      Using the data returned by both pairs of detectors  112 - 118  enables determination of a delay period after which the counting of hydrogen and chlorine counts provided by the first pair of detectors  112 ,  114  is cumulated. The hydrogen and chlorine counts obtained by the first pair of detectors  112 ,  114  may be selected in time from the region of formation decay illustrated in  FIG. 3  using the results of the time gated analysis provided by the second pair of detectors  116 ,  118 . Thus, the first set of detectors  112 ,  114  may be time gated in addition to being energy gated. This eliminates or substantially reduces any localized borehole  102 , casing  101  and/or cement  103  effect (the borehole effect) upon the data returned from the first pair of detectors  112 ,  114  during the chlorine logging.  
      A ratio of radiation counts detected between the first near detector  112  and the first far detector  116  indicates the porosity and formation matrix of the formation  105 . Alternatively or in combination, a ratio of radiation counts detected between the second near detector  116  and the second far detector  118  also indicates the porosity of the formation  105 . Thus, either pair of the detectors  112 - 118  for either chlorine logging or lifetime logging can measure porosity of the formation  105 . Preferably, the porosity measurement is not split between different pairs of the detectors  112 - 118 . The basis for these porosity measurements derives from the fact that where the porosity is high, the hydrogen filling the pore space captures the neutron quickly and close to the source  110  such that measurements at the near detectors  112 ,  116  are high. In contrast, a lower porosity enables the neutrons to diffuse further into the formation  105  and hence increase the measurements at the far detectors  114 ,  118  due to the fact that the pore space therein limits the hydrogen in close proximity to the source  110  for capturing the neutrons. Advantageously for the porosity measurements, hydrogen located within the pore space of the formation  105  does not depend on whether oil or water is present in the formation  105 .  
      The sonde  100  may also be used to track a water flow velocity in or behind the casing  101  by using one or more of the detectors  112 - 118  and gating them to detect gamma radiation at a specific gamma radiation energy emitted by oxygen as the result of capture of a thermalized neutron by the oxygen. Specifically, the detectors  112 - 118  detect energy at all energy levels and configuration of detection circuitry for tracking the velocity of water flow gates the signal from selected ones of the detectors  112 - 118  to discriminate energy detected in certain ranges or specific energies associated with the gamma radiation energy released by oxygen atoms after capture of the thermal neutrons. This information enables flow rate within the borehole  102  to be determined if it can be established or known that the flow is inside the casing  101 .  
      In an operation to detect the water flow velocity, the source  110  emits a burst of high energy neutrons from the sonde  100  that can be stationary in the borehole  102 . Subsequent pulses from the source  110  can be slowed down or shut off for a period of time while measurements are made in order to determine the water flow velocity. Frequency of the pulses can be changed from the surface via the cable  104 . If the sonde  100  is moving in the borehole  102 , this movement must be compensated for in determining the water flow velocity. As discussed previously, the neutrons emitted by the source  110  travel into the borehole  102 , casing  101 , cement  103  and formation  105 . The neutrons emitted by the source  110  subsequently result in the emission of about a 6.13 MeV gamma ray if thermalized and captured by an oxygen atom, which indicates water presence. Since the half-life of the excited oxygen atom is approximately 7.35 seconds, the moving water molecule that releases the gamma ray will likely have moved between the time of capture proximate the source  110  and the time of release of the gamma ray.  
      Accordingly, the gamma radiation emits from a different location with respect to the location where the neutron was emitted from the source  110 . One or both detectors of the first pair of detectors  112 ,  114  sense the upward flow of water while one or both detectors of the second pair of detectors  116 ,  118  identify the downward flow of water. Monitoring which ones of the detectors  112 - 118  receives the majority of the gamma radiation provides an indication of the water flow direction that can be horizontal and/or upwards or downwards. The water moving in a direction away from one of the detectors  112 - 118 , either vertically and/or horizontally, causes an observed decrease in activity due to an expected exponential decay characteristic plus an additional decrease caused by the induced radioactivity in the water being swept away from the vicinity of the detector by the movement thereof. The observed decrease in the induced activity above the expected exponential decay characteristic can then be used to determine the velocity of the moving water. For detecting horizontal flow, a directional shield may be used.  
      Further, every slug of activated flowing water generates an increase of counts as it gets closer and closer to one of the detectors  112 - 118  and causes a decrease of counts as it moves away from the detector. After a time dependent upon the spacing of the source  110  and the detectors  112 - 118  and the flow velocity, the slug of water does not contribute anymore to the counts. Thus, the total counts curve comprises a characteristic peak representative of the flow and an exponential decay curve representative of stationary oxygen. This peak includes information about the flowing oxygen and thence about the water flow velocity. The peak detected with a single one of the detectors  112 - 118  indicates velocity based on the distance between the source  110  and the detector and the time between activation and detection of the peak. Where the peak is detected with one of the pairs of detectors, the peak appears in time at a respective one of the near detectors  112 ,  116  depending on the direction of water flow before the peak is detected with the respective one of the far detectors  114 ,  118  since the slug passes by the near detector before the far detector. Accordingly, the distance between the near and far detectors and the time difference between respective peaks sensed with the near and far detectors enables calculation of the water flow velocity.  
      The sonde  100  having two pairs of detectors  112 - 118  can be used to determine formation parameters such as porosity and formation matrix and water flow direction and velocity. By combining the capabilities of the chlorine logging with the lifetime logging, the invention enables locating likely reserves of hydrocarbons adjacent to boreholes without the need for complex calibration paradigms and without the need to know the likely porosity, resistivity, etc. of the formations before using the sonde  100  or evaluating the results. Further, the data from the lifetime logging can be used in accurately eliminating or reducing the borehole effect during the chlorine logging.  
      Although a specific embodiment of the sonde  100  for logging has been disclosed with reference to  FIG. 1 , departures therefrom are well within the scope of one skilled in the art. As apparent from this disclosure, some embodiments can utilize a single detector for the chlorine logging and a single detector for the lifetime logging, which are each spaced different distances from the source  110 . For other embodiments, the sonde  100  can utilize a single detector for either the chlorine logging and a pair of detectors for the lifetime logging or a pair of detectors for the chlorine logging and a single detector for the lifetime logging. According to yet other embodiments, a single set of detectors (e.g., the detectors  112 ,  114 ) spaced from the source can be gated for both the chlorine logging and the lifetime logging to combine functions of the detectors  112 ,  114  with the detectors  116 ,  118 . Additionally, U.S. Pat. No. 6,825,459, which is herein incorporated by reference in its entirety, describes applications utilizing near and far detectors during chlorine logging that can be implemented when both the near and far detectors  112 ,  114  for chlorine logging are utilized.  
      As an example,  FIG. 4  illustrates another embodiment of a sonde  400  utilizing an alternate arrangement of detectors to perform the lifetime logging, the chlorine logging, the porosity and formation matrix determinations and the water flow determinations as discussed above. Similar to the sonde  100  shown in  FIG. 1 , the sonde  400  includes a neutron generator or radiation source  410 , shielding  408  and a control section  420 . A single set of chlorine and lifetime logging detectors  412 ,  414  are spaced from the source  410  at near and far distances, which can be approximately eighteen and twenty-three inches, respectively. The detectors  412 ,  414  are each gated for both the chlorine logging and the lifetime logging and can be used together for the porosity and formation matrix determinations. Preferably, the chlorine and lifetime logging detectors  412 ,  414  are disposed on an opposite side of the source  410  from the control section  420  due to spacing requirements for the control section  420 .  
      The sonde  400  additionally includes a first pair of water flow detectors  450 ,  452  located above the source  410  and an optional second pair of water flow detectors  454 ,  456  located below the source  410  and the control section  420 . Near water flow detectors  452 ,  454  are preferably spaced approximately twenty-three inches from the source  410  while far water flow detectors  450 ,  456  are preferably spaced about forty-two inches from the source  410 . Due to their close proximity to one another, the near water flow detector  452  can be combined with the far chlorine and lifetime logging detector  414  by incorporating the appropriate gating in a single combined detector.  
      Similar to the detection of water flow as described above, the first pair of water flow detectors  450 ,  452  detect water flowing past the source  410  in a direction toward the first pair of water flow detectors  450 ,  452  while the second pair of water flow detectors  454 ,  456  detect water flowing past the source  410  toward the second pair of water flow detectors  454 ,  456 . In other operations to detect water flow, the source  410  emits a burst of high energy neutrons from the sonde  400  at a first location and the sonde  400  is then moved a distance such that the first pair of water flow detectors  450 ,  452  are positioned below the first location by a predetermined amount prior to taking measurements. In this manner, the far water flow detector  450  measures a near distance from the first location where the source  410  emitted and the near water flow detector  452  measures a far distance from the first location in order to detect water flowing past the source  410  away from the first pair of detectors  450 ,  452 .  
      While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.