Abstract:
The present invention provides an apparatus and method for minimizing movement of a drill string during MWD with vibration-sensitive instruments comprising conveying a drill string into a borehole with a sensor mounted on the drill string for sensing a parameter of interest of a formation. A clocked controller is disposed on the drill string for controlling timing of the sensor; and a second clocked controller is disposed at a surface location. During drilling operations, the clocked controllers are synchronized such that the surface controller is performing certain tasks in timed sequence with the sensor even though the surface controller is not connected to the sensor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to oil well tools, and more particularly to down-hole measurement tools. 
     2. Description of the Related Art 
     In the oil and gas industry, hydrocarbons are recovered from formations containing oil and gas by drilling a well borehole into the formation using a drilling system. The system typically comprises a drill bit carried at an end of a drill string. The drill string is comprised of a tubing which may be drill pipe made of jointed sections or a continuous coiled tubing and a drilling assembly that has a drill bit at its bottom end. The drilling assembly is attached to the bottom end of the tubing. To drill a borehole, a mud motor carried by the drilling assembly rotates the drill bit, or the bit is coupled to drill pipe, which is rotated by surface motors. A drilling fluid, also referred to as mud, is pumped under pressure from a source at the surface (mud pit) through the tubing. The mud serves a variety of purposes. It is designed to provide the hydrostatic pressure that is greater than the formation pressure to avoid blowouts. The mud drives the drilling motor (when used) and it also provides lubrication to various elements of the drill string. The mud is also used in many systems as a signal transmission medium using a transmission method known as mud-pulse telemetry. 
     It is often desirable to gather information of a specific formation once the borehole reaches an area known in the art as the zone of interest. At the zone of interest, down-hole instruments and/or sampling devices are utilized to gather data regarding various parameters of interest including pressure, temperature and other physical and chemical properties of the formation fluid and or mud. The down-hole operations are known as measurement while drilling (MWD) or logging while drilling (LWD). 
     One MWD method used to determine characteristics of formation fluid is known as nuclear magnetic resonance or NMR well logging. NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations. 
     In NMR tools, a magnet is used to produce a static magnetic field in the formation. The static field aligns the nuclear spins within the formation. An RF field is applied to realign nuclear spins generally perpendicular to the static field. At the end of the RF pulse, the nuclear spins precess back towards alignment with the static field. Signals generated from the precessing spins are picked up by a receiver. The tools use an antenna for creating the RF field and for receiving the echo signal from the formation fluid being analyzed. High gain amplifiers are utilized to amplify the received echo prior to processing the signal, but it is very important that the echo is distinguishable over other signals known as noise. 
     A major problem with NMR testing relates to tool movement. A typical pulse NMR measurement is sensitive to movement such as vibration, axial, horizontal and rotational displacements with respect to the formation. These types of movement during a test may induce noise in the system, and will deteriorate the NMR result, sometimes to the extent that test data is invalidated. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems encountered when testing formations with movement-sensitive instruments during drilling operations. The invention provides an apparatus and method to minimize vibrations caused by drilling operations at during periods of testing. 
     The present invention provides an apparatus for MWD comprising drill string with a sensor mounted on the drill string for sensing a parameter of interest of a formation. A clocked controller is disposed on the drill string for controlling timing of the NMR data acquisition; and a second clocked controller is disposed at a surface location. During drilling operations, the two clocked controllers are initially synchronized such that the surface controller is performing certain tasks in timed sequence with the sensor even though the surface controller is not connected to the sensor. 
     The present invention also provides a method for MWD comprising conveying a drill string into a well borehole and sensing a parameter of interest of a formation traversed by the borehole with a sensor mounted on the drill string. The method also includes controlling timing of the sensor with a first clocked controller disposed on the drill string, and synchronizing a second clocked controller disposed at a surface location with the first clocked controller. 
     The benefits accorded by the present invention are, among others, cost savings and drilling efficiency. The cost savings are realized when expensive testing is not repeated due to data corruption by unnecessary movement. The drilling operations are more efficient, because drilling is only halted for the brief amount of time a test is performed. Drilling is resumed at the precise moment a test is complete. 
    
    
     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 and wherein: 
     FIG. 1 is an elevation view of an overall simultaneous drilling and logging system that incorporates an embodiment of the present invention; 
     FIG. 2A is a cross section view of a portion of the present invention showing a BHA disposed in a borehole; 
     FIG. 2B is a timing graph of a typical test sequence using the embodiment of FIG. 2A; 
     FIG. 3A shows a surface controller according to an embodiment of the present invention connected to the downhole tool of FIG. 2A while the tool is at the surface; and 
     FIG. 3B shows the surface display of FIG. 3A operating in synchronization with the down-hole tool. 
     FIG. 4 shows a tool according to the present invention wherein a selectively extendable probe is used to extract fluid from a formation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referencing FIG. 1, an elevation view of a simultaneous drilling and logging system that incorporates an embodiment of the present invention is shown wherein measurements are taken on a formation  101 . 
     A well borehole  102  is being drilled into the earth under control of surface equipment including a rotary drilling rig  104 . In accordance with a conventional arrangement, rig  104  includes a derrick  106 , derrick floor  108 , draw works  110 , hook  112 , kelly joint  114 , rotary table  116 , and drill string  118  that includes drill pipe  120  secured to the lower end of kelly joint  114  and to the upper end of a section of drill collars including, but not separately shown, an upper drill collar, an intermediate drill collar, and a lower drill collar bottom hole assembly (BHA)  121  immediately below the intermediate sub. The lower end of the BHA  121  carries a downhole tool  122  of the present invention and a drill bit  124 . 
     Drilling mud  126  is circulated from a mud pit  128  through a mud pump  130 , past a desurger  132 , through a mud supply line  134 , and into a swivel  136 . The drilling mud  126  flows down through-the kelly joint  114  and an axial central bore in the drill string, and through jets (not shown) in the lower face of the drill bit. Borehole fluid  138  containing drilling mud, cuttings and formation fluid flows back up through the annular space between the outer surface of the drill string and the inner surface of the borehole to be circulated to the surface where it is returned to the mud pit through a mud return line  142 . A shaker screen (not shown) separates formation cuttings from the drilling mud before the mud is returned to the mud pit. 
     The system in FIG. 1 uses mud pulse telemetry techniques to communicate data from down hole to the surface during drilling operations. To receive data at the surface, there is a transducer  144  in mud supply line  132 . This transducer generates electrical signals in response to drilling mud pressure variations, and a surface conductor  146  transmits the electrical signals to a surface controller  148 . 
     Optionally, the drill string  118  can have a downhole drill motor  150  for rotating the drill bit  124 . Incorporated in the drill string  118  above the drill bit  124  is the downhole tool  122  of the present invention, which will be described in greater detail hereinafter. A telemetry system  152  is located in a suitable location on the drill string  118  such as above the tool  122 . The telemetry system  152  is used to receive commands from, and send data to, the surface via the mud-pulse telemetry described above. 
     Reference will now be made to FIGS. 2A and 2B for further description of downhole components associated with the present invention. FIG. 2A is a cross section view of a portion of a BHA  121  disposed in a borehole  102 . Borehole fluid  138  containing drilling mud, formation fluid and cuttings is shown flowing toward the surface in an annulus between the BHA  121  and borehole wall  102 . Mounted in the BHA  121  is a formation testing tool  122  including a movement-sensitive component such as a NMR instrument  202 . The instrument  202  is connected to a downhole controller  204  via a suitable interface such as electrical conductors  206 . Inside the controller  204  are a processor  208  and a synchronizing clock  210 . The processor  208  and clock  210  operate together to control the instrument  202  such that the instrument performs tests on the formation  101  at a predetermined periodic rate as shown in FIG.  2 B. 
     The controller  204  also interfaces with a communication unit  212  via electrical conductors  214 . The communications unit  212  transmits signals to a complementary surface unit ( 148  in FIG.  1 ). The transmission method is typically mud-pulse telemetry, but may be any other suitable method. 
     FIG. 2B is a timing graph showing a typical test sequence using the embodiment of FIG.  2 A. The downhole controller  204  activates the NMR instrument  202  automatically at a predetermined time  211 . The instruments tests for a period of time controlled by the controller  204  and clock  210 . The instrument is deactivated at another predetermined time  214  when the test is complete. The tool remains deactivated until the next test time period  216  begins. 
     FIG. 3A shows a surface controller according to an embodiment of the present invention connected to the downhole tool  122  of FIG. 2A while the tool  122  is at the surface. A detachable cable  302  connects the tool clock  210  to a surface processor  304  disposed in the surface controller  148 . Another similar cable  306  connects the processor  304  to a surface display unit  308 . The display unit  308  is a standard display graduated in units of time, preferably seconds. An independent surface clock  310  mounted in the controller  148  drives the display  308 . The surface processor  304  synchronizes the independent downhole clock  210  with the independent surface clock  310  such that the display  308  will show the status of the downhole tool  122  even though the downhole clock  210  operates autonomously once the detachable cable  302  is removed. 
     Other useful components may be integrated into or connected to the surface controller  148 . A monitor  312  and keyboard  314  may be used for operator interface with the controller. A storage device  316  such as a disk or hard drive may be associated with the surface processor  304  for storing commands and data. Commands and data signals can be transmitted to and from downhole via a surface communications unit  318 , which is complementary to the downhole communications unit  212 . 
     FIG. 3B shows the surface display  308  operating in synchronization with the downhole tool  122 . The display  308  includes a start point  211 A and end point  214 A along with predetermined graduated intervals between the start and end points  211 A and  214 A. The timing graph  320  indicates that the instrument  202  begins a test at a start point  211 , ends a test at an end point  214  and is off for a predetermined duration. As shown the downhole tool is in testing mode at a given point in time  322  i.e. 45 seconds into the test. The surface display  308  indicates 45 seconds  322 A at the same point during the test with a shaded bar  324  or other indicator, even though the surface clock  310  and downhole clock  210  are not connected. 
     Another embodiment according to the present invention includes downhole synchronization rather than surface synchronization as described above. Referring to FIGS. 3A and 3B, this alternate embodiment does not include the detachable cable  302 . The downhole clock  210  includes an activation mechanism  326 . The mechanism can be any suitable known electronic switch capable of remote activation. The switch  326  is activated by a command transmitted from the surface to the downhole communications unit  212 . The communications unit  212  sends the command via the interface conductor  214  to the switch  326  disposed in the controller  204 . The switch  326  controls the downhole clock  210  by providing power to define the start point  211 . The predetermined test routine as described above can then commence. 
     The apparatus described above and shown in FIGS. 1 through 3B may be modified using FTWD elements known to those skilled in the art without deviating from the spirit and scope of the present invention. FIG. 4 shows an exemplary tool according to an embodiment of the present invention wherein a selectively extendable probe  402  well known in the art is used to extract formation fluid for analysis in the tool  122 . A pad seal  404  seals a portion of the borehole wall when the probe  402  is extended. The probe may be extended by a power source  406  such as a hydraulic system, mud motor or electric motor. A biasing element  408  such as a spring is preferably used to bias the probe in a retracted position, but biasing in the extended position is also used in some cases. A pump  410  housed in the drill string and connected to a port  412  in the probe  402  is used to extract fluid from the formation by reducing pressure at the port  412 . A sample chamber  414  in the drill string receives the extracted fluid, and a sensor  416  senses characteristics of the extracted fluid to determine a parameter of interest for the formation. 
     Various sensors may be used in conjunction with or in lieu of the NMR sensor described above for sensing the characteristics of the extracted fluid. Any of several well-known sensors may be incorporated in the tool for formation fluid investigations by sensing porosity, pore-size distribution, hydrocarbon identification, acoustic velocity, and formation pressure. 
     A BHA is synchronized above ground in one method according to the present invention. The method can be described in three phases. In the first phase, the downhole tool is synchronized with a surface display by connecting the tool clock  210  and display clock  310  to a surface controller  148 . The controller is used to synchronize the two clocks. The tool is disconnected from the surface controller once the two clocks are synchronized. 
     Phase two begins by conveying the tool into the borehole on a drill string  118 . The tool remains in an off state for a predetermined amount of time as the drill string progress through the formation. The surface clock is driving the surface display  308  at the same time as the tool clock continues to run. 
     Phase  3  is a monitoring phase. A drilling operator uses the surface display to monitor the status of the downhole tool. When the surface display indicates a test is about to begin, the operator signals appropriate personnel to stop all drilling. The drilling systems are shut down before the time  211  that the downhole tool begins a test routine as indicated  211 A on the display unit. There is minimum vibration during the test routine, because no machinery is running. During this operating interval, the sensor receives signals that are analyzed by the downhole processor to obtain transverse and/or longitudinal relaxation times of nuclei in the formation and/or other parameters such as porosity, pore-size distribution, hydrocarbon identification, acoustic velocity, formation pressure, and dielectric constant of formation. These parameters may fluid may be obtained using the above sensor if applicable or other appropriate sensor known in the art. When the display indicates the test is complete  214 A, the operator signals the appropriate personnel to resume drilling. The drilling is resumed at the moment testing is complete  214 . 
     Sending a start pulse downhole via the surface communication unit  318  in another method according to the invention synchronizes the tool  122 . A mud-pulse telemetry signal carrying the start command is received at the downhole communications unit. The switch  326  is activated thereby providing power to initiate a predetermined test sequence. The surface clock  310  is synchronized with the downhole clock by initiating the surface clock when the start signal is transmitted. In this method there is a known lag time between the sending and receiving of the start signal. The lag is taken into consideration when setting the surface clock. 
     Surface operations are halted when the start signal is transmitted, or at a short predetermined time thereafter. When the surface display indicates that a test is complete, drilling operations are resumed until another start command is transmitted. 
     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 without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.