A modular system for packaging of sensors and related electronics for an MWD system. A drill collar housing is provided with one or more cavities for receiving sensor modules that are adapted to sense one or more wellbore conditions. The sensor modules are removable and replaceable so that a desired sensor package may be installed within the drill collar housing. The drill collar housing is installed within the drill string, and a desired sensor module or modules are secured within the cavity(ies) of the drill collar housing. Replacement or repair of the sensor portions requires only that the module or modules be removed from the cavity(ies). The drill collar housing need not be removed from the drill string. The replaceable sensor modules may be interchangeably used in drill collar housings of different sizes without resulting in a degradation of sensed information.

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to measurement-while-drilling and logging-while-drilling tools and, more particularly, to arrangements for packaging of the sensor and detector portions of the tools. 
   2. Description of the Related Art 
   Measurement-while-drilling (MWD) and logging-while-drilling (LWD) devices are used to determine wellbore parameters and operating conditions during drilling of a well. These parameters and conditions may include formation density, gamma resistivity, acoustic porosity, and so forth. In a typical drilling run, only some of these parameters and conditions may be of interest, however. MWD and LWD tools generally include a sensor portion that contains the sensors of the type desired and a processor and associated storage medium for retaining the sensed information. Additionally, a telemetry system is often used to transmit the sensed information uphole. The telemetry system may include a mud pulser, acoustic telemetry option, or an electromagnetic transmission system. 
   The sensor portion of MWD or LWD systems is typically housed within a drill collar in a such a manner that the sensor portion cannot be easily removed and replaced. In fact, removal and replacement of the sensor portion typically requires that the drill string be removed from the wellbore, and then the portions above and below the drill collar housing the sensor portion be disassembled from the drill collar. This operation is time-consuming and, therefore, costly. Additionally, the drill collars involved are quite heavy and unwieldy and the process of changing out a sensor section runs the risk of damaging the components. Further, if some of the sensor components malfunction, the entire drill collar must often be removed and shipped off site for repair or replacement. Shipping tools back to a repair center is costly and time consuming. 
   There are several conventional methods for packaging sensor components within a drill collar. In one method, exemplified in U.S. Pat. No. 5,216,242, issued to Perry et al., sensors and detectors are hardwired within the drill collar sub and accessable via removable hatches. Another packaging arrangement is illustrated in U.S. Pat. No. 4,547,833, issued to Sharp. In this arrangement, the sensors and detectors are mounted upon a chassis, which is then retained centrally within an outer cylindrical housing. These components are then secured together with a number of fasteners and integrated into a drill string. Of course, to change out or repair the sensors and detectors, one must first remove the adjacent drill string components, as well as the various fasteners, and then remove the outer housing from the chassis. U.S. Pat. No. 5,613,561 issued to Moriarty illustrates a similar packaging scheme wherein components mounted on the chassis are accessible through ports. 
   MWD and LWD tools have high capital costs and operating costs. Indeed, the high costs associated with LWD tools have made them unattractive for use with land-based wells. Conventional packaging arrangements make it difficult and expensive to design for the three basic hole sizes (8½″, 9½″, and 12¼″). Traditional MWD/LWD tool design has required unique tool for each hole size. Each tool requires many man-years to design and develop. Also, field inventory must be kept on hand for every size, multiplying costs further. To overcome these difficulties, manufacturers often “orphan” one hole size, and adapt a tool from one of the other two hole sizes for the orphaned hole size. For example, a tool designed to be run into an 8½″ hole would be provided with an adapter and run into a 9½″ hole. Unfortunately, the quality of the log of data obtained in this manner is less than satisfactory. LWD tools, in particular, are designed for a particular hole size. The components are integral to the collar. When they are used in a hole size that they were not designed for, the measurement is either lost or seriously degraded. Some tools use a sleeve to improve the measurement by displacing mud away from the measurement sensors. This, however, has limited success because the sensors remain in their original location, yet are now even further displaced from the formation that they are trying to measure the properties of. 
   The present invention addresses the problems of the prior art. 
   SUMMARY OF THE INVENTION 
   The invention provides a modular system for packaging of sensors and related electronics for an MWD system. The system features a drill collar housing with one or more cavities for receiving sensor modules that are adapted to sense one or more wellbore conditions. The sensor modules are removable and replaceable so that a desired sensor package may be installed within the drill collar housing. The drill collar housing is installed within the drill string, and a desired sensor module or modules are secured within the cavity(ies) of the drill collar housing. Replacement or repair of the sensor portions requires only that the module or modules be removed from the cavity(ies). 
   The drill collar housing need not be removed from the drill string. In some embodiments, the drill collar housing contains power and data transmission means so that power can be supplied to the modules and data transmitted from the modules. In other embodiments, the modules are self-contained and do not require power or data to be supplied to or transmitted from them. In these embodiments, the modules include an internal battery for power and data storage means for storing sensed data. Data is recovered from the modules after the drilling operation is completed and the drilling string removed from the wellbore. Alternatively, the drill collar housing might include or be associated with a mud turbine and pulser for transmission of sensed data to the surface using fluid pulsing techniques that are known in the art. 
   The modular system of the present invention overcomes the problems of the prior art. The replaceable sensor modules may be interchangeably used in drill collar housings of different sizes without resulting in a degradation of sensed information. Further, there is no need to remove the drill collar housing from the drill string in order to repair portions of the sensor arrangement. In addition, the significant costs of transporting entire MWD tool to a remote repair facility or replacing the entire tool. In addition, the costs of maintaining inventory for various hole sizes will be significantly reduced. The concept of modularity permits a low cost alternative by separating the tool hardware from the drill collar. The collar can remain at the wellsite as part of the drilling bottom hole assembly and can be disposed into the wellbore without the modules as a standard component. When a logging job is required, the modules can be secured within the collar and used with surface-based monitoring equipment. In particular aspects, the drill collar may merely be a “dumb” collar having no electronics or power supplies therein and merely serving as a housing for the sensor modules. 
   Drill collar carriers of any size can accept a standard set of modules. In this manner, the drill collar can be optimized for the drilling operation in terms of size and strength. The modules, on the other hand, can be optimized for the measurement of formation and as noted, will fit into any of the drill collar carriers. Since each drill collar carrier is designed for a particular hole size, along with the complete bottom hole assembly, the module will always be in close proximity to the formation and provide a good measurement. An integral stabilizer blade that extends radially outwardly from the drill collar carrier can position a module close to the formation for improved performance. Drill collar carriers can either have radially outwardly extending stabilizer blades for housing the modules or, alternatively, can be integral (slick) to present a generally cylindrical outer surface. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
       FIG. 1  is a schematic side view of an exemplary drill string and bottom hole assembly containing a MWD/LWD drill collar assembly constructed in accordance with the present invention. 
       FIG. 2  is a side view of the exemplary MWD/LWD drill collar assembly shown in FIG.  1 . 
       FIG. 3  is a side, cross-sectional view of the exemplary drill collar assembly taken along lines  3 — 3  in FIG.  2 . 
       FIGS. 4 and 4A  are axial cross-sections of the drill collar assembly taken along lines  4 — 4  and  4 A— 4 A in  FIG. 2 , respectively. 
       FIG. 5  is an isometric, exploded view of an exemplary drill collar assembly constructed in accordance with the present invention. 
       FIG. 6  illustrates the potential alternative placement of a sensor module into drill collar housings of different sizes. 
       FIG. 7  is an isometric, exploded view illustrating use of a hatch cover with a drill collar and sensor module. 
       FIG. 8  is a schematic depiction of a sensor module having an internal power supply and data storage and processing means. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the lower end of an exemplary wellbore  10  that is being drilled into the earth  12  by a drill bit  14  and bottom hole assembly  16  that are suspended by a drill string, indicated generally at  18 . The drill string  18 , as is known, is made up of a plurality of subs and drill pipe sections that are threaded together to form a single tubular string. The drill string  18  defines a central drilling mud conduit  20  therein. During a drilling operation, drilling mud is flowed from the surface of the wellbore  10  downward through the mud conduit  20  and out through the bit  14  in order to lubricate the drilling operation. The drilling mud then returns to the surface of the well via the annulus  22  (as indicated by arrows  24 ) that is defined between the inner surfaces of the wellbore  10  and the outer surfaces of the drill string  18 . 
   A drill collar assembly  26  is schematically illustrated in FIG.  1  and shown integrated within the drill string  18  just above the BHA  16 . The drill collar  26  is an exemplary sensor sub constructed in accordance with the present invention and which features an improved packaging arrangement for the sensor and detector components of an MWD system Above the drill collar  26  is a tubular sub (the lower end of which is shown at  28  in  FIG. 1 ) that carries additional MWD or LWD system components, including a processor and storage medium. As such components are known in the art and, thus, will not be described further herein. The sub  28  also includes a turbine (not shown), of a type known in the art that is powered by flow of drilling mud through the mud channel  20 . The turbine is used to provide electrical power to the drill collar assembly  26  for actuation of sensor components therewithin. Suitable turbines for this application are available commercially from Baker Hughes, Inteq Division at 2001 Rankin Rd., Houston, Tex. 77267. It is noted as well that the present invention is not limited to use of a turbine and that other power sources known in the art could as easily be used to supply power to components within the drill collar assembly  26 . Such power sources include, but are not limited to batteries and cables that extend from the surface of the wellbore  10 . The sub  28  may also include a telemetry device, such as a pulser that is capable of transmitting data via a fluid column using encoded pulses. 
   An exemplary drill collar assembly  26  is shown in greater detail in  FIGS. 2 ,  3 ,  4 , and  5 . The drill collar assembly  26  includes a generally cylindrical drill collar housing, or body,  30  with a first, upper end  32  having a box-type threaded connection  34  and a second, lower end  36  having a pin-type threaded connection  38 . The upper end of the drill collar housing  30  presents three radially outwardly extending stabilizer blades  39 . The drill collar housing  30  defines a central mud flow channel  40  along its length. When the drill collar assembly  26  is integrated into a drill string, the mud flow channel  40  aligns with and become a portion of the mud conduit  20 . 
   A pair of sensor module cavities  42 ,  44  are defined within the drill collar housing  30 . One module cavity  42  is located upon the outer radial surface of the drill collar assembly  26 , while the other module cavity  44  is located on the outer radial surface of a stabilizer blade  39 . Both module cavities  42 ,  44  are open to the radial exterior of the drill collar assembly  26 , essentially providing recesses therewithin. While two cavities  42 ,  44  are shown in  FIGS. 2-5 , it should be understood that there might be more or fewer, depending upon the needs of the user and the desired number of sensor modules. It is also noted that, although the cavities  42 ,  44  are shown disposed upon one side of the drill collar housing  30 , in practice these cavities might be spaced from one another angularly about the circumference of the drill collar housing  30 . For example, it might be desirable to house a module in each of the three stabilizer blades  39  to ensure that the modules are positioned in close proximity to the wall of the borehole  10  during use. Sensor modules  46 ,  48  are releasably secured within the cavities  42 ,  44 , respectively. Clamps  50  are disposed over the modules  46  or  48 , as illustrated, and screws  52  are used to secure the clamps against the body  30 . As an alternative to the clamps  50 , a unitary hatch cover might be used to enclose the modules  46 ,  48  within the cavities  42 ,  44 .  FIG. 7  illustrates use of an exemplary hatch cover  51  to secure a module  48  within cavity  44 . The hatch cover  51  is secured to the body  30  using suitable connectors, in the same manner as the clamps  50  described previously, but may be more desirable when, for example, the wellbore  10  contains extremely corrosive fluids and it is desired to protect the modules from such fluid. The hatch cover  51  includes a window  53  that allows formation signals to more easily be transmitted to the module  48  through the hatch  51 . The window  53  may comprise an opening in the hatch cover  51 , but more preferably is a solid material that permits passage of energy and signals. An example is a beryllium metal window that allows low energy gamma rays to pass through and reach the module  48 . The window  53  is located upon the hatch cover  51  so that it will be aligned with the sensor  60  of the module  48  when affixed to the housing body  30 . 
   The drill collar housing  30  further includes a data and power transmission line  54  (visible in  FIG. 3 ) that provides electrical power to the sensor modules  46 ,  48 . The transmission line  54  also provides a means for data that is obtained by the sensor modules  46 ,  48  to be transmitted to a processor and storage medium, which is contained within a neighboring sub. A suitable current data and power transmission line for this application is that which is ordinarily referred to in the industry as the “M-30” arrangement, meaning “modem and 30 volts.” Additionally, a power and data transmission cable  56  (see  FIG. 3 ) is disposed within the body  30  to permit transmission of power and data between the two cavities  42  and  44 . Electrical plug receptacles, schematically indicated at  58  are located on the upper portion of each sensor module cavity  42  and  44 . 
   The sensor modules  46 ,  48  each include a plurality of sensors, schematically indicated at  60  in FIG.  3 . The modules  46 ,  48  also include an electrical plug member  62  that is complimentary to the electrical plug receptacle  58  within the respective cavity  42  or  44 . While the sensors  60  are shown in  FIG. 3  to be a point source, in fact, the sensors  60  may be of any configuration and may actually cover a large portion of the surface area of the sensor module  46  or  48 . The sensors  60  of each module  46 ,  48  are of a type known in the art for sensing a variety of wellbore or logging conditions (hereinafter, merely referred to as “wellbore conditions”), such as, principally, resistivity or porosity. Other wellbore conditions might also be detected in addition to or instead of these parameters, including velocity, imaging, photoelectric effect, acoustics, temperature, pressure, gamma radiation, position, and density. The modules  46 ,  48  each feature a housing, or sensor body,  64  that is shaped and sized to fit within one of the cavities  42 ,  44  of the drill collar housing  30  in a complimentary fashion. In the exemplary embodiment depicted in  FIGS. 2-5 , the sensor body  64  is cylindrical. However, other shapes and configurations may be used as well. 
   As best illustrated by  FIG. 4 , the outer diameter of the drill collar assembly  26  is not affected by insertion of the modules  46 ,  48 , thereby not restricting the ability of the drill collar assembly  26  to be inserted into a borehole.  FIG. 4A  illustrates that the module  48  will reside within a stabilizer blade  39  of the drill collar housing body  30 . This placement is desirable where the sensor must be positioned very close to the wall of the wellbore  10  during use in order to properly collect data. The use of standardized sizes and plugs for the sensor modules  46 ,  48  greatly improves the logistics associated with MWD and LWD tools. 
   Standardized modules are usable with drill collar housings of all hole sizes. For example, the modules  46 ,  48  might be removed from the first drill collar housing  26 , which for purposes of example, is a 9½″ diameter drill collar housing and then placed into a second larger drill collar housing  26   b  (a 12¼″ housing) or, alternatively, a smaller drill collar housing  26   a  (an 8½″ housing), as illustrated in FIG.  6 . In this case, the size of the receptacle  44  remains the same among the various drill collar sizes despite the fact that the diameter of the drill collars does change. In addition, each of the various sizes of drill collars,  26 ,  26   a , and  26   b , preferably accommodates a common size of clamp  50  and connector  52  without requiring changes in the spacing or sizes of these components. 
   In operation, the sensor modules  46 ,  48  are inserted into the cavities  42 ,  44  of a properly sized drill collar  26 ,  26   a , or  26   b . That drill collar is then integrated into the drill string  18 . The drill string  18  is disposed into the wellbore  10  until the drill collar assembly  26 ,  26   a , or  26   b  is located proximate a desired zone of interest within the wellbore, which may be the bottom of the hole  10 . Electrical power is transmitted via the data and power transmission line  54  to the sensor modules  46 ,  48 , which then detect one or more wellbore conditions, depending upon the particular type of sensors that are incorporated into them. Data representative of the sensed wellbore conditions is then transmitted from the modules  46 ,  48  via the data and power transmission line  54  to a neighboring sub, which transmits the data uphole, in a manner known in the art. 
   In an alternative embodiment, the sensor modules  44 ,  48  are self contained so that they do not require an external power source or communication of data to portions of the drill collar housing.  FIG. 8  schematically depicts an exemplary self-contained sensor module  80  of this type. The module  80  includes a body  82  that carries a sensor  84  upon the outside surface. The sensor  84  is operably interconnected with a data storage and processing means  86 , of a type known in the art. An internal power source  88 , such as a battery, provides power to the data storage and processing means  86 . When a self-contained module, such as module  80  is used, there is no need for an electrical plug member  62  to be included on the module or for the electrical plug receptacle  58  or for a data and power transmission line  54  or a power and data transmission cable  56  to be included in the body  30  of the drill collar housing. In this instance, the drill collar housing is merely “dumb” iron and serves only as a carrier for the module  80 . In operation, the module  80  senses wellbore information with the sensor  84  and transmits the sensed data to the internal data storage and processing means  86  where the data resides until after the drilling operation is completed and the drill string removed from the wellbore  10 . The module  80  may then be removed from the drill collar housing and the information retrieved from the data storage and processing means  84 . 
   Other variations of the above-described constructions are possible utilizing the modular concepts described herein. For example, the drill collar housing  26  might, itself, have incorporated therein a bus wire, mud turbine power generator and mud telemetry pulser for transmitting sensed data to the surface. Additionally, the drill collar housings might be formed with or without stabilizer blades, such as blades  39  described previously. 
   The present invention improves log quality since there is no need to adapt a tool that is principally designed to operate in a different size hole for an orphaned hole size. The invention also improves utilization of the capital cost of a tool. Sensor components may be easily changed out or repaired without the necessity and cost of shipping the drill collar off-site for repair work. 
   Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.