Abstract:
A tool is presented for making density measurements of a formation surrounding a wellbore, comprising a collar housing in a drill string. The housing has at least one first section with a first outer diameter, and at least one sensing section with a second outer diameter located proximate the at least one first section. The second outer diameter is smaller than the first outer diameter. A radioactive source is disposed in the sensing section of the housing. At least two detectors are disposed in the sensing section and spaced from the radioactive source and are positioned to detect radiation resulting from gamma rays emitted by the source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/382,800, filed May 22, 2002. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of logging-while-drilling (LWD) well boreholes, and more particularly relates to an apparatus and methods for minimizing measurement errors in LWD formation density measurements. 
   2. Description of the Related Art 
   The density of formations penetrated by a well borehole is used in many aspects of the petroleum industry. More specifically, formation density is combined with measurements of other formation properties to determine gas saturation, lithology, porosity, the density of hydrocarbons within the formation pore space, properties of shaly sands, and other parameters of interest. 
   Methods and apparatus for determining formation density, comprising an isotopic gamma ray source and two gamma ray detectors are known in the art and are often referred to as dual spaced density logs or gamma-gamma density logs. For examples of wireline tools incorporating the technique see U.S. Pat. Nos. 3,202,822, 3,321,625, 3,846,631 3,858,037, 3,864,569 and 4,628,202. The wireline apparatus is normally configured as a logging tool (sonde) for conveying, preferably with a multiconductor cable, along a borehole thereby “logging” formation density as a function of depth. The source and two detectors are typically mounted in an articulating pad device with a backup arm. The backup arm applies force to the articulating pad to maximize pad contact with the wall of the borehole. The sonde responds primarily to radiation which is emitted by the source and scattered by the formation into the detectors. The scatter reaction is primarily Compton scattering, and the number of Compton scattering collisions within the formation can be related to electron density of materials within the formation. Through sonde calibration means, a measure of electron density of the formation can be related to true bulk density of the formation. 
   Since the dual spaced density measurement technique is based upon a nuclear process, statistical error is associated with the measurement. There is also non-statistical error in the measurement. Although the articulating pad and backup arm tend to position the pad against the borehole wall, the largest source of non-statistical error is generally still associated with the position of the tool within the well borehole, and is generally referred to as standoff error. As used herein, standoff refers to the distance from the outer surface of the sensing section of the tool to the wall of the borehole. The responses of the two detectors are combined in prior art dual spaced density systems using well known algorithms to minimize standoff error. 
   The dual spaced density systems are available as an LWD system. As in the wireline version of the system, the dominant non-statistical error that arises in LWD formation density measurements results from tool standoff. In prior art LWD systems, see  FIG. 1 , the source  201  and two detectors  202 , 203  are mounted in-line on an axial blade  208  having a substantially bit gauge diameter such that the source  201  and detectors  202 , 203  and their associated windows  204 - 206  are in close proximity to the wall  207  of the borehole. For example see U.S. Pat. No. 5,091,644. As the blade wears during drilling, the collimating windows typically associated with such tools also wear thereby changing the response of the tool. There is no known technique that measures and corrects for this tool wear in real time. These errors must be calibrated out in a lab environment. Today&#39;s drilling technology uses high rotational velocities, drills in-gauge hole, and permits very long, continuous drilling periods. Tool wear can no longer be practically calibrated out in the lab, because measurement errors due to wear become excessive during long drilling runs. 
   The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by positioning the source and detector in a tool section substantially protected from such wear. 
   SUMMARY OF THE INVENTION 
   The present invention contemplates a density tool having appropriately located source and detectors to minimize the wear-related error in the density measurement. 
   In one preferred embodiment, a tool is presented for making density measurements of a formation surrounding a wellbore, comprising a collar housing conveyed on a drilling tubular. The housing has at least one first section with a first outer diameter, and at least one sensing section with a second outer diameter located proximate the at least one first section. The second outer diameter is smaller than the first outer diameter. A radioactive source is disposed in the sensing section of the housing. At least two detectors are disposed in the sensing section and spaced from the radioactive source and are positioned to detect radiation resulting from gamma rays emitted by the source. 
   In one aspect of the present invention, a method of minimizing wear related measurement error in a logging-while-drilling density tool in a wellbore, comprises providing a tool having at least one wear-resistant section having a first outer diameter proximate a sensing section with a second outer diameter smaller than the first outer diameter; and taking measurements during drilling with a radioactive source and at least two detectors mounted in the smaller diameter sensing section. 
   Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
       FIG. 1  is a schematic of a prior art density tool; 
       FIG. 2  is a schematic of a drilling system according to one embodiment of the present invention; 
       FIG. 3  is a schematic of a density tool according to one embodiment of the present invention; 
       FIG. 4  is a schematic of a density tool according to another preferred embodiment of the present invention; 
       FIG. 5  is a schematic of a density tool according to another preferred embodiment of the present invention; and 
       FIG. 6  is a schematic of a density tool according to another preferred embodiment of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2  shows a schematic diagram of a drilling system  10  having a downhole assembly containing a downhole sensor system and the surface devices according to one embodiment of present invention. As shown, the system  10  includes a conventional derrick  11  erected on a derrick floor  12  which supports a rotary table  14  that is rotated by a prime mover (not shown) at a desired rotational speed. A drill string  20  that includes a drill pipe section  22  extends downward from the rotary table  14  into a wellbore  26 . A drill bit  50  attached to the drill string downhole end disintegrates the geological formations when it is rotated. The drill string  20  is coupled to a drawworks  30  via a kelly joint  21 , swivel  28  and line  29  through a system of pulleys (not shown). During the drilling operations, the drawworks  30  is operated to control the weight on bit and the rate of penetration of the drill string  20  into the wellbore  26 . The operation of the drawworks is well known in the art and is thus not described in detail herein. Alternatively, a coiled tubing system (not shown), as is known in the art, may be used to convey tools in the wellbore. 
   During drilling operations, a suitable drilling fluid (commonly referred to in the art as “mud”)  31  from a mud pit  32  is circulated under pressure through the drill string  20  by a mud pump  34 . The drilling fluid  31  passes from the mud pump  34  into the drill string  20  via a desurger  36 , fluid line  38  and the kelly joint  21 . The drilling fluid is discharged at the wellbore bottom  51  through an opening in the drill bit  50 . The drilling fluid circulates uphole through the annular space  27  between the drill string  20  and the wellbore  26  and is discharged into the mud pit  32  via a return line  35 . Preferably, a variety of sensors (not shown) are appropriately deployed on the surface according to known methods in the art to provide information about various drilling-related parameters, such as fluid flow rate, weight on bit, hook load, etc. 
   A surface control unit  40  receives signals from the downhole sensors and devices via a sensor  43  placed in the fluid line  38  and processes such signals according to programmed instructions provided to the surface control unit. The surface control unit displays desired drilling parameters and other information on a display/monitor  42  which information is utilized by an operator to control the drilling operations. The surface control unit  40  contains a computer, memory for storing data, data recorder and other peripherals. The surface control unit  40  also includes models and processes data according to programmed instructions and responds to user commands entered through a suitable means, such as a keyboard. The control unit  40  is preferably adapted to activate alarms  44  when certain unsafe or undesirable operating conditions occur. 
   In the preferred embodiment of the system of present invention, the downhole subassembly  59  (also referred to as the bottomhole assembly or “BHA”), which contains the various sensors and MWD devices to provide information about the formation and downhole drilling parameters, is coupled between the drill bit  50  and the drill pipe  22 . The downhole assembly  59  is modular in construction, in that the various devices are interconnected sections. 
   Referring to  FIG. 2 , the BHA  59  also preferably contains downhole sensors and devices in addition to the above-described surface sensors to measure downhole parameters of interest. Such devices include, but are not limited to, a device for measuring the formation resistivity near the drill bit, a gamma ray device for measuring the formation natural gamma ray emission intensity, devices for determining the inclination and azimuth of the drill string, and a nuclear device  125  for measuring formation density. 
   The above-noted devices transmit data to the downhole telemetry system  72 , which in turn transmits the sensor data uphole to the surface control unit  40 . The present invention preferably utilizes a mud pulse telemetry technique to communicate data from downhole sensors and devices during drilling operations. A transducer  43  placed in the mud supply line  38  detects the mud pulses responsive to the data transmitted by the downhole telemetry  72 . Transducer  43  generates electrical signals in response to the mud pressure variations and transmits such signals via a conductor  45  to the surface control unit  40 . Other telemetry techniques such electromagnetic and acoustic techniques or any other suitable technique may be utilized for the purposes of this invention. 
   Referring first to  FIG. 3 , a diagram of the basic components for a gamma-ray density tool  110  in accordance with a preferred embodiment of the present invention is shown. The tool  110  comprises a drill collar housing  105  which contains a gamma-ray source  112  and two spaced gamma-ray detector assemblies  114  and  116 . All three components are placed substantially in-line along a single axis that has been located parallel to the axis of the tool. The detector  114  closest to the gamma-ray source will be referred to as the “short space detector” and the one farthest away  116  is referred to as the “long space detector”. The two gamma ray detector assemblies employ a sodium iodide crystal and glass phototube. Gamma-ray shielding is located between detector assemblies  14 ,  16  and source  12 . Windows  121 ,  122 ,  123  open up to the formation from both the detector assemblies and the source. The windows  121 - 123  have shielding material  120 , such as tungsten, to collimate the radiation as it passes through the windows. 
   Stabilizer  138  is attached to the collar housing  105  on one side of sensing section  150 . Stabilizer  138  has a larger diameter than that of sensing section  150  and provides a contact wear surface against the wall of wellbore  26 . The diameter of stabilizer  138  may be from approximately {fraction (1/16)}″ to approximately ½″ larger than the diameter of sensing section  150 . The stabilizer has multiple blades, common in the art, arranged for allowing mud to pass upwards in the annulus. The blades may be straight in the axial direction, or, alternatively, they may spiral around the diameter of the collar housing. The blades are surfaced with an enhanced wear-resistant material such as tungsten carbide or any other suitable wear-resistant material. A wear pad  140  of wear resistant material is placed on the other side of the sensing section  150  away from stabilizer  138  and also is larger in diameter than sensing section  150 . The combination of larger diameters on stabilizer  138  and wear pad  140  act to substantially prevent contact between sensing section  150  and formation  101 . This prevents wear of the source and detector windows and shielding and substantially eliminates errors caused by these factors. As a result, a layer of drilling fluid (mud) is present in the standoff region between the formation and the detector assemblies and source. 
   The tool  110  is placed into service by loading it with a sealed chemical source (typically cesium  137 ) and lowering it into a formation. Gamma-rays are continuously emitted by the source and these propagate out into the formation  101 . 
   Two physical processes dominate the scattering and absorption of gamma rays at the energies used in density tools. They are Compton scattering and photoelectric absorption. The macroscopic Compton scattering cross section (i.e., probability of scattering while passing through a set thickness of material) is proportional to the electron density in the formation and is weakly dependent on the energy of the incident gamma ray (it falls fairly slowly with increasing energy). Since the electron density is, for most formations, approximately proportional to the bulk density, the Compton cross section is proportional to the density of the formation. Unlike the Compton cross section, the photoelectric cross section is strongly dependent on the energy of the incident gamma rays and on the materials in the formation (the lithology). 
   Formation density is determined by measuring the attenuation of gamma rays through the formation. Shielding in the tool minimizes the flux of gamma rays straight through the tool. This flux can be viewed as background noise for the formation signal. The windows  121 - 123  increase the number of gamma rays going from the source to the formation and from the formation to the detectors. The layer of mud  130  between the sensing section  150  diameter and the formation is compensated for by using a “rib” algorithm, known in the art. 
   The compensation for the mud standoff  130  is usually accomplished through the use of two detectors: a short space and a long space detector. Since gamma rays travel through more of the formation to reach the long space detector than they do to reach the short space detector, the long space detector shows a significantly larger count rate change for a given change in formation density. This allows for the compensation using the two detector responses and a “rib” algorithm known in the art. The rib function, allows for the calculation of compensation (which should be equal to the difference between the true and the measured long space density), as a function of the difference between the short and long space densities. Any wear on the source and detector windows or any reduction in shielding thickness due to wear causes additional error that can not be accounted for by the known techniques. 
   In one preferred embodiment, see  FIG. 4 , a formation density comprises a collar housing  405  having a stabilizer  438  with multiple blades  439  mounted on one end of an sensing section  450  where the outer diameter of the stabilizer blades  439  are larger than the diameter of the sensing section  450  by the same range as mentioned previously. The stabilizer  438  is locked onto the housing  405  by lock nut  437 . Alternatively, the stabilizer may be integrally machined on the housing, a press-fit sleeve, a shrink-fit sleeve, or a sleeve welded on the housing  405  using techniques common in the art. A wear pad  440  is located at a distal end of sensing section  450 . The stabilizer  438  and the wear pad  440  act to prevent contact with a wall of a borehole (not shown) to prevent wear on cover  445 . A cavity (not shown) is formed in housing  405  for mounting the source and detectors previously described. Cover  445  covers and sealed the source and detectors and contains source window  421 , short-space detector window  422 , and long-space detector window  423  along with suitable collimating shielding as previously described. An acoustic sensor  460  is mounted substantially in-line with the gamma source and detectors and measures the distance to the borehole wall. The distance measurement provides an indication of the standoff distance from the cover  445  to the borehole wall for use in standoff compensation. Suitable circuitry, power and processing capability, common in the art, are contained in the tool  400  for processing the gamma density detection measurements. A processor (not shown) acts according to programmed instructions downloaded in the tool to make the proper corrections. 
   In another preferred embodiment, see  FIG. 5 , a gamma source and sensors (not shown) having sensor windows  521 - 523  are located between stabilizer blades  538 . The blades  538  have an outer diameter larger than the sensing section diameter  505  by the previously described range. Locating the sensors between the blades  538  provides protection from wear on the sensor section and the extra advantage of reducing tool length. 
   In yet another preferred embodiment, see  FIG. 6 , a gamma source and sensors (not shown) having sensor windows  621 - 623  are mounted proximate a stabilizer  638  having blades  639 . The blades  639  have an outer diameter larger than the sensing diameter section  605  by the previously described range. The location of the sensors near the stabilizer provides protection for the sensor section. 
   The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible. It is intended that the following claims be interpreted to embrace all such modifications and changes.