Abstract:
A logging system for measuring parameters of earth formation penetrated by a well borehole. Measurements made with the system are not adversely affected by varying pressure encountered a borehole environment. This is accomplished by the use of a main compensation element and a detector compensation element to render source and detector geometry invariant to varying pressure. The system is particularly suited for nuclear LWD systems such as back scatter gamma ray density systems. The basic concepts of the system are, however, applicable to other types of nuclear measurement systems that comprise one or more radiation sources, and one or more axially spaced radiation detectors, where system response is a function of source-detector spacing. The basic concepts of the system are also applicable to other types of logging systems, such as electromagnetic and acoustic, where source (transmitter) and sensor (receiver) elements require invariant geometry in order to maximize accuracy of measurements.

Description:
This invention is directed toward a logging system for measuring parameters of earth formation penetrated by a well borehole, and more particularly directed toward a system that is not adversely affected by variations in borehole pressure. The basic concepts of the invention are applicable to nuclear, acoustic, electromagnetic, nuclear magnetic resonance (NMR) and other measurement systems that comprise one or more energy sources, and one or more axially spaced energy sensors or detectors, where system response is a function of the relative geometry of the source and detector. Furthermore, the basic concepts of the invention are applicable to any type of measurement system comprising an energy source and one or more energy detectors or sensors, where system response is a function of the relative geometry of the energy source and energy detector or sensor. 
   BACKGROUND OF THE INVENTION 
   Logging measurement systems have been used for decades to measure various properties of earth formation penetrated by a well borehole. The first systems used downhole instruments or “tools” which were conveyed along the borehole by means of a “wireline” cable. In addition, the wireline served as a means of communication between the downhole tool and equipment at the surface, which typically processes measured data to obtain formation parameters of interest as a function of depth within the borehole. These measurements are commonly referred to as “well logs” or simply “logs”. Logging measurement systems can utilize nuclear, acoustic, electromagnetic, NMR and other types of measurements to obtain formation parameters of interest. For example, nuclear measurements can include measures of formation natural gamma radiation, thermal neutron flux, epithermal neutron flux elastic and inelastically scattered neutron, capture gamma radiation, scattered gamma radiation, and the like. A variety of formation parameters are obtained from these measurements, or combinations of these measurements, such as shale content, porosity, density, lithology and hydrocarbon saturation. 
   Wireline logging is applicable only after the borehole has been drilled. It was recognized in the 1960&#39;s that certain operational and economic advantages could be realized if drilling, borehole directional, and formation properties measurements could be made while the borehole is being drilled. This process is generally referred to as measurement-while-drilling (MWD) for real time drilling parameters such as weight on the drill bit, borehole direction, and the like. Formation property measurements made while drilling, such as formation density and formation porosity, are usually referred to as logging-while-drilling (LWD) measurements. The LWD measurements should conceptually be more accurate than their wireline counterparts. This is because the formation is less perturbed in the immediate vicinity of the borehole by the invasion of drilling fluids into the formation. This invasion alters the virgin state of the formation. This effect is particular detrimental to the more shallow depth of investigation nuclear logging measurements. 
   A brief summary of operating concepts of a nuclear density measurement system is presented so that the present invention can be more easily understood. The downhole instrument, or “tool”, comprises typically a source of radiation and one or more radiation sensors or “detectors” axially spaced from the radiation source. For purposes of discussion, it will be assumed that the tool comprises a single source that emits gamma radiation, and two gamma ray detectors that are disposed within the tool at two axial spacings from the source. Gamma radiation is emitted by the source, passes through any material between the tool and the borehole wall, and enters the formation where it interacts with material within the formation. A portion of the radiation is scattered back into the borehole at a reduced energy. A portion of radiation scattered back into the borehole is recorded by the gamma ray detectors. Source gamma ray energy is selected so that the primary mode of reaction is Compton scatter, which is related to the electron density of the composite formation material including the formation matrix material and any fluid filling pore space within the matrix. Electron density is, in turn, related to the “bulk” density of the formation. The count rates measured by each gamma ray detector can, therefore, be related to the formation property of interest, which is bulk density. These relationships are determined by calibrating the tool under known borehole and formation conditions with “fixed” axial source-detector positionings. Stated another way, the tool is calibrated assuming that the spacings between the source and each detector remains constant while operating in a borehole environment. Gamma radiation not only interacts with the formation, but also with any intervening material between the tool and the borehole wall. This intervening material includes borehole fluid and particulate material, known as “mudcake”, which builds up on the borehole wall due to invasion into the formation of borehole fluid. Mudcake and any other intervening material adversely affect the bulk density measurement. Responses of the two gamma ray detectors are combined to minimize the effects of mudcake and tool standoff. The “spine and rib” technique is known in the art as one method for combining the two detector responses. 
   A typical wireline scatter gamma ray density tool is constructed with a gamma ray source and one or more gamma ray detectors disposed within a “pad” which is mechanically forced against the borehole wall to minimize standoff effects. The pad is typically physically robust with minimal void space. Radical changes in pressure, which are typically encountered in a borehole environment, have minimal effect on the dimensions of the pad. As a result, source-detector geometry is relatively invariant to changes in borehole pressure. The response of the system, which is typically calibrated at atmospheric pressure with a fixed source-detector geometry, is typically minimally affected by large changes in borehole pressure. 
   An LWD scatter gamma ray density tool differs from its wireline counterpart in many aspects. One of the main differences stems from the fact that the source and detectors of a LWD scatter density tool are mounted in the drill collar rather than inside a wireline pressure housing, such as a pad. Such layout imposes certain restrictions on the size of the detectors, the length of a pressure housing containing the detectors, the robustness of the pressure housing, and the way the source, the detectors, and intervening radiation shielding are disposed in the collar. A typical layout of an LWD scatter density tool comprises a source shield, made of a heavy material such as tungsten, that is directly affixed to or fabricated as an integral part of the collar. The source shield typically comprises collimator passage openings or “window” covered with a light material relatively transparent to gamma radiation. Axially spaced detectors are typically disposed in a detector pressure housing, which is typically flexibly attached to the source housing. Stated another way, the source and detector housings are not rigidly attached to each other. Components within the detector pressure housing are at an ambient pressure, such as atmospheric pressure, at which the tool is calibrated. Detector shielding components are made of a heavy, gamma radiation absorbing material, such as tungsten. The shielding components also typically comprise collimator passages with windows covered with a light material relatively transparent to gamma radiation. The passages and windows are oriented in the pressure housing to collimate gamma radiation scattered from the borehole and formation environs. In one common embodiment, a stabilizer blade is then assembled over the source housing and the detector pressure housing. Under pressures encountered in the harsh borehole environment, the source remains in its original position since it is directly mounted to the collar. The detector pressure housing, however, compresses under this increased pressure. This compression changes the source-detector axial spacing from that at which the tool was calibrated. This change in source-detector spacings results in non-density related changes in count rate thereby yielding erroneous bulk density measurements. 
   Pressure related errors of type discussed above are typically more significant in LWD system than in wireline systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are obtained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
       FIG. 1  illustrates a density system embodied as a logging-while-drilling system; 
       FIG. 2  is a partial cross section view perpendicular to the major axis of a collar showing a conduit through which drilling fluid is circulated during the drilling process; 
       FIG. 3   a  is a sectional view of major elements of a prior art dual detector LWD gamma ray density tool, taken along the major axis of the tool, with the tool at atmospheric pressure; 
       FIG. 3   b  is a sectional view of major elements of the prior art dual detector LWD gamma ray density tool, taken along the major axis of the tool, with the tool at elevated pressure; 
       FIG. 4   a  is a sectional view of major elements of a first embodiment of the tool of the present disclosure, taken along the major axis of the tool, with the tool at atmospheric pressure; 
       FIG. 4   b  is a sectional view of major elements of the first embodiment of the tool of the present disclosure, taken along the major axis of the tool, with the tool at elevated pressure; 
       FIG. 5  is a sectional view of major elements of a second embodiment of the tool of the present disclosure; 
       FIG. 6  is a sectional view of major elements of a third embodiment of the tool of the present disclosure; and 
       FIG. 7  is a sectional view of major elements of an embodiment of the tool in which the collar is used as a pressure housing. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   This invention is directed toward a logging system for measuring parameters of earth formation penetrated by a well borehole, wherein the response of the system is not adversely affected by geometric changes induced by variations in borehole pressure. The basic concepts of the invention are applicable to any type of logging system comprising one or more energy sources, and one or more axially spaced energy sensors, where system response is a function of the relative geometry of the source and sensor. These types include nuclear, acoustic, electromagnetic, NMR systems. 
   The concepts of the invention are applicable to both wireline logging systems and LWD systems. As an example, concepts of the invention can be used in a LWD neutron porosity system for measuring formation porosity, wherein the system comprises a neutron source and one or more axially spaced neutron sensors commonly referred to as neutron “detectors”. As an additional example, the concepts of the invention are applicable to electromagnetic systems wherein the geometry of energy source or “transmitter” and sensor or “receiver” array is preferably invariant to borehole pressure. 
     FIG. 1  illustrates an LWD tool, identified as a whole by the numeral  10 , disposed by means of a drill string within a well borehole  18  defined by a borehole wall  24  and penetrating an earth formation  26 . The upper end of the collar element  12  of the tool  10  is operationally attached to the lower end of a string of drill pipe  28 . A stabilizer element of the tool  10  is identified by the numeral  14 . The lower end of logging tool  10  is terminated by a drill bit  16 . It should be understood, however, that other elements can be disposed on either end of the tool  10  between the drill pipe  28  and the drill bit  16 . The upper end of the drill pipe  28  terminates at a rotary drilling rig  20  at the surface of the earth  22 . The drilling rig rotates the drill pipe  28 , the cooperating tool  10 , and drill bit  16  thereby advancing the borehole  18 . Drilling mud is circulated down the drill pipe  28 , through the axial passage in the collar  12 , and exits at the drill bit  16  to return to the surface  22  via the annulus defined by the outer surface of the drill string and the borehole wall  24 . Details of the construction and operation of the drilling rig  20  are well known in the art, and are omitted in this disclosure for brevity. 
   Basic concepts are set forth in this disclosure using the invention embodied as a dual detector LWD gamma ray back scatter density system, wherein elements are configured to minimize errors caused by changes in pressure to which the system is exposed. As discussed above, the invention can also be effectively embodied in a variety of other types of LWD and wireline logging systems. 
     FIG. 2  is a partial cross section view through the major axis of the collar  12  showing a conduit  29  through which drilling fluid is circulated during the drilling process. Also illustrated is a cavity  13  that is sized to receive elements of the tool, as will be discussed and illustrated in detail in subsequent sections of this disclosure. The cavity preferably extends axially along the major axis of the tool  10  with opposing walls  134  defining parallel planes that are normal to an inner surface  231 . 
     FIG. 3   a  is a sectional view of major elements of a prior art dual detector gamma ray density tool along the major axis of the tool  10 , and is used to conceptually illustrate sources of errors induced in density measurements by pressure variations in the borehole environment. Specific design parameters can be varied, but the disposition of these major elements serve to illustrate the borehole pressure problem. 
   Source housing  33  comprises a source of gamma radiation  30 . The source  30  is preferably cesium- 137  ( 137 Cs) which emits gamma radiation with an energy of 0.66 million electron volts (MeV). Alternately, cobalt-60 ( 60 Co) emitting gamma radiation at 1.11 and 1.33 MeV can be used as source material. High Z shielding material  32 , such as tungsten (W), defines a passage  34  and collimates gamma radiation emitted by the source  30  into the borehole environs. At least a portion of the wall of the source collimator  34  (as shown in  FIG. 3   a ) preferably forms an acute angle with the axis of the tool  10  to better focus gamma radiation into the formation. 
   Still referring to  FIG. 3   a,  detector pressure housing  31  is disposed within the channel  13 . The detector pressure housing  31  comprises a detector first or “short spaced” gamma ray detector  40  disposed at a first axial distance  46  from the source  30 , and a second or “long spaced” gamma ray detector  50  disposed at a second axial distance  48  from the source, where the second spacing is greater than the first spacing as illustrated. Detectors  40  and  50  are disposed within rigidly affixed shielding materials  74  and  72 , respectively. The shielding materials  74  and  72  are efficient attenuators of gamma radiation, or “high Z” material. The shielding materials  74  and  72  define passages  35  and  52  that collimate the short spaced and long spaced detectors  40  and  50 , respectively, with respect to the borehole environs. Preferably a portion of the passage of at least the short spaced detector collimator  35  (as shown in  FIG. 3   a ) forms an acute angle with the axis of the tool  10  to enhance sensitivity to angular sensitive Compton scattered gamma radiation emanating at preferred scatter angles from the borehole environs. Optionally, the long spaced detector collimator  52  can also be angularly collimated, but angular dependence of detected radiation decreases with source-detector spacing. The short spaced and long spaced detectors  40  and  50  are preferably of the scintillator type to increase detection efficiencies for given detector volumes. Sodium iodide or bismuth germinate are suitable scintillation crystal materials to be used in the scintillation type detectors. Again, W is a suitable high Z material  42 . 
   Still referring to  FIG. 3   a,  the right side of the shielding material  72  is attached to the inside wall of the detector pressure housing  31  by means of a detector housing spring  62 . Right hand side outer wall of the detector pressure housing  31  is rigidly attached to the collar  12 . The detector pressure housing  31  is flexibly attached to the source housing  33  by a main spring  60 . 
   The detector pressure housing  31  comprises “windows”  38  and  54  covering the collimator passages  35  and  52  of the short spaced detector  40 , and the long spaced detector  50 , respectively. The source housing  31  comprises a window  36  covering the collimator passage  34  from the source  30 . The windows are preferably fabricated from low atomic number or “low Z” material which minimizes gamma radiation absorption. Various epoxies, ceramics and the like are suitable low Z material. Axes of the source, long and short spaced detector collimator windows  36 ,  38  and  54 , respectively, are in the plane defined by the major axis of the collar  12  and the center of the surface  231  of the cavity  13  (see  FIG. 2 ). 
   It is noted that additional source and detector shielding material (not shown) can be disposed outside of the pressure housings  33  and  38 , with suitable collimation to allow passage of gamma radiation. 
   An electronics package (not shown), comprising power supplies and electronic circuitry required to power and control the detectors, is located remote from the pressure housing  31 , but preferably located downhole and within the collar  12 . The electronics package is electrically connected to the detectors. The electronics package can also include recording and memory elements to store measured data for subsequent retrieval and processing when the tool  10  is returned to the surface of the earth. 
   Gamma radiation recorded in the long and short spaced detectors  50  and  40 , respectfully, are functions of both the bulk density of the formation material in which the tool is positioned, and axial spacings  46  and  48  of short spaced detector  40  and long spaced detector  50 , respectively, from the source  30 . The tool response is “calibrated” for fixed spacings and under known conditions, as is known in the art. Calibration typically is performed at the surface of the earth at atmospheric pressure. Under these conditions, the length of the detector pressure housing  31  is illustrated by the arrow  44 . The source to short spaced detector X 1  and source to long spaced detector X 2  are illustrated by the arrows  46  and  48 , respectively. 
   As the tool is conveyed into the well borehole, pressure increases. The increase in pressure compresses the detector pressure housing  31  axially. Referring to  FIG. 3   b,  the detector pressure housing  31  compresses by an amount  65  to an axial dimension  144 . Since the right hand side of the detector pressure housing is affixed to the collar  12 , shielding elements  74  and  72  move to the right thereby compressing the detector assembly spring  62 . The left hand side of source housing  33  is rigidly affixed to the collar  12 . The holder and source  30  therein do not move. The main spring  60  expands to compensate for the compression of the detector pressure housing  31 . Under pressure, the spacing X 1,p  identified at  46  between the source and the detector increases. Likewise, under pressure the spacing X 2,p  identified at  48  increases. An increase in pressure changes the source-detector spacings. Mathematically, X 1,p &gt;X 1  and X 2,p &gt;X 2 . These changes in spacing induce detector count rate changes that are not related to formation bulk density. Stated another way, changes in pressure to which the tool is exposed introduces error in formation density measurements. A change of as small as 0.01 inches (0.254 millimeters) of the source-detector spacing can result in a large change in the measured density, especially for the short spaced detector spacing  46 . For a typical short spaced detector spacing  46  of 6 inches (152.4 millimeters), a 0.05 inch compression in the pressure housing can result in as much as 0.2 grams per cubic centimeter error in density measured by the short spaced detector  40 . Such an error deems a density log unusable, as can be seen by processing count rate data using the previously referenced spine-and-rib method. 
   The previously discussed adverse effects of pressure are virtually eliminated by the manner in which the major elements of the tool are operationally connected. Several embodiments can be used to obtain this borehole pressure invariance. Furthermore, elements for compensation for compression and expansion are not limited to a spring. These compensation elements can comprise any material that can be reversibly distorted In addition to springs, compensation elements can comprises an elastic material such as an elastomer, a gas filled cylinder, or any material or assembly that reversibly distorts under pressure. 
     FIG. 4   a  illustrates a LWD dual detector density source-detector array disposed in a pressure housing  31 , with some identifying numbers shown in  FIGS. 3   a  and  3   b  being omitted for clarity. It is assumed that the array shown in  FIG. 4   a  is at atmospheric pressure. The source housing  33  containing the source  30  is disposed within the channel  13 . The detector pressure housing  31  is likewise disposed in the channel  13 , and rigidly attached to the source housing  33  as illustrated. The left side of the source housing  32  is flexibly attached to collar  12  by means of a main compensation element  80 . Hereafter, the shielding materials  74  and  72  with associated collimators  35  and  52 , respectively, will be referred to simply as “shields”  74  and  72 . Shields  74  and  72 , with short spaced and long spaced detectors  40  and  50 . respectively, disposed therein, are rigidly attached to each other. The right hand side of the shield  72  is flexibly attached to the inner wall of the detector pressure housing  31  by means of a detector compensation element  70 . The right hand end of the detector pressure housing  31  is rigidly attached to the collar  12 . Dimensions  44 ,  46  and  48  represent the axial length of the detector pressure housing, short and long detector spacings, respectively. 
   Attention is directed to  FIG. 4   b,  which illustrates the effects of increased borehole pressure. As discussed preciously, an increase in borehole pressure compresses the detector pressure housing  31  thereby reducing the axial dimension  44 . The detector compensation element  70  compresses thereby allowing the short and long spaced detectors (and associated shields  74  and  72 , respectively) to slide to the right within the detector pressure housing  31 . Since the source housing  33  is rigidly attached to the detector housing  31 , the source  30  is moved the same axial distance to the right. The main compensation element  80  expands to account for the displacement of the source housing. Since the source housing  32  and rigidly attached detector pressure housing  31  are equally displaced axially within the channel  13 , the original short space and long spaced detector spacings  46  and  48 , respectively (at which the tool was calibrated) is preserved. Resulting density determinations are not, therefore, adversely affected by increases in borehole pressure. The axial extents of the low Z windows  38  and  54  are greater than the maximum openings of the collimation channels  35  and  52 , respectfully. This allows the detectors housings  74  and  72  to move axially within the detector pressure housing  31 , and still maintain full coverage of the respective collimators with low Z material. As pressure is reduced, as is the case when the tool is moved up the well borehole, the pressure housing  31  expands, and the detector compensation element  70  expands accordingly, and the main compensation element  80  compresses thereby maintaining the desired source-detector spacings  46  and  48 . It is noted that any external shielding is preferably rigidly attached to the detector housing and hence moves in the same direction and with the same magnitude as pressure induced variations in the pressure housing. 
     FIG. 5  illustrates another embodiment of an LWD dual detector density source-detector array disposed in a pressure housing  31 , with some identifying numbers again being omitted for clarity. The source housing  33  containing the source  30  is disposed within the channel  13 . The detector pressure housing  31  is likewise disposed within the channel  13 , and rigidly attached to the source housing  33  as illustrated. In this embodiment, the left side of the source housing  32  is rigidly attached to the inner wall of the collar  12 . Shields  74  and  72 , with short spaced and long spaced detectors disposed therein, are rigidly attached to each other. The right hand side of the shield  72  is flexibly attached to the inner wall of the detector pressure housing  31  by means of a detector compensation element  70 . The left hand side of the shield  74  is rigidly attached to the source housing  33 . The right side of the detector pressure housing  31  is flexibly attached to the collar  12  through the main compensation element  80 . Comparing the embodiment shown in  FIG. 4   a  with the embodiment shown in  FIG. 5 , it is apparent that the short and long detector spacings  48  and  46 , respectfully, are invariant to changes in the axial length  44  of the detector pressure housing  31 . Compensation mechanics are essentially the same as discussed in connection with  FIGS. 4   a  and  4   b,  but with the main compensation element  80  being disposed on the opposite end of the source-detector array. 
     FIG. 6  shows a LWD dual detector density source-detector array disposed in a common pressure housing  131 . The right hand end of the common pressure housing  131  is rigidly attached to the collar  12 . The left hand end of the common pressure housing is flexibly attached to the collar via the main compensation element  80 . For purposes of discussion, it is assumed that the common pressure housing is exposed to atmospheric pressure. The source shield  32  with the source  30  therein, the short spaced detector shield  74  with the short spaced detector  40  disposed therein, and the long spaced detector shield  72  with the long spaced detector  50  disposed therein, are all rigidly attached. The right hand side of the shield  72  is flexibly attached to the inner wall of the common pressure housing, as shown in  FIG. 6 . As pressure increases, the common pressure housing  131  compresses, and the dimension  151  is reduced. The source shield  32  with the source  30  therein, the short spaced detector shield  74  with the short spaced detector  40  disposed therein, and the long spaced detector shield  72  with the long spaced detector  50  disposed therein all move to the right with common pressure housing compression. The main compensation element  80  expands, and the detector compensation element  70  compresses. The source-detector dimensions  46  and  48  remain constant, thereby yielding density measurements invariant to borehole pressure. 
     FIG. 7  shows a LWD dual detector density source-detector array disposed within a cavity in the wall of the collar  12 , which serves as a pressure housing. The source shield  32 , the short spaced detector shield  74 , and the long spaced detector shield  72  are all rigidly attached to one another as illustrated. The long spaced detector shield  72  is flexibly attached to the collar  12  via the detector compensation element  70 . The source shield  32  is flexibly attached to the collar  12  via the main compensation element  80 . The compensation elements  70  and  80  are optionally used as mounting elements. These compensation elements are not required for spacing compensation since all elements within the collar remains at atmospheric pressure. 
   In the embodiments of the invention discussed above and illustrated in  FIGS. 4   a,    4   b,    5 ,  6  and  7 , the main compensation element  80  and the detector compensation element  70  are preferably springs. 
   While the foregoing disclosure is directed toward the preferred embodiments of the invention, the scope of the invention is defined by the claims, which follow.