Apparatus and method for determining a drilling mode to optimize formation evaluation measurements

There are many natural pauses during rotary drilling operations where a portion of the drill string remains stationary. Pauses include drill pipe connections, circulating time, and fishing operations. These pauses are used to obtain formation evaluation measurements that take a long time or that benefit from a quiet environment, as opposed to the naturally noisy drilling environment. Various techniques that are sensitive to the mud flow, weight-on-bit, or motion of the drill string may be used alone or in combination to identify the drilling mode and control the data acquisition sequence.

BACKGROUND OF THE INVENTION
 The present invention relates generally to an apparatus and method for
 measuring properties of an earth formation traversed by a borehole, and
 more particularly, to an apparatus and method for determining a drilling
 mode to optimize formation evaluation measurements.
 To make downhole measurements while a borehole is being drilled,
 measurement-while-drilling (MWD) and/or logging-while-drilling (LWD)
 systems are generally known which measure various useful parameters and
 characteristics such as the formation resistivity and the natural gamma
 ray emissions from the formations. Signals which are representative of
 these measurements made downhole are relayed to the surface with a mud
 pulse telemetry device that controls the mud flow, encoding information in
 pressure pulses inside the drill string. The pulses travel upward through
 the mud to the surface where they are detected and decoded so that the
 downhole measurements are available for observation and interpretation at
 the surface substantially in real time. As an alternative, it has also
 been found useful to provide a downhole computer with sufficient memory
 for temporarily storing these measurements until such time that the drill
 string is removed from the borehole.
 U.S. Pat. Nos. 5,130,950 issued to Orban et al., 5,241,273 issued to Martin
 Luling, 5,017,778 issued to Peter D. Wraight, 5,148,407 issued to
 Haldorsen et al., 5,585,556 issued to Petersen et al., and 5,705,927
 issued to Sezginer et al., describe MWD tools which employ nuclear
 magnetic resonance, sonic, seismic, nuclear, or electromagnetic
 measurements. The tools disclosed in the prior art have disadvantages
 which limit their utility in MWD and/or LWD applications. Sonic,
 resistivity, nuclear, electromagnetic, and seismic measurements are
 directly influenced by the drilling noise. For example, while acoustic
 energy generated at the surface is usually very large, the energy that
 must be detected at the drill bit can be very small due to geometrical
 spreading and attenuation of the acoustic waves in the subsurface
 formation. In many cases, the drilling noise is orders of magnitude larger
 than the acoustic wave energy propagating from the surface to the
 subsurface MWD detector. Also, MWD and LWD nuclear magnetic resonance
 measurements are directly influenced by the vertical and lateral motion of
 the tool. For example, due to the amount of time required to obtain
 T.sub.1 and T.sub.2 measurements, the formation properties may change
 during the measurement cycle. These aforementioned factors adversely
 affect MWD and/or LWD measurements.
 In the presence of a noisy drilling environment, the prior art tools obtain
 nuclear magnetic resonance, sonic, electromagnetic, nuclear, and seismic
 measurements directly influenced by the vertical and lateral tool motion
 and the drilling noise. None of the MWD and/or LWD tools determine the
 drilling mode and accordingly modify the data acquisition sequence to
 optimize formation evaluation measurements.
 SUMMARY OF THE INVENTION
 The above disadvantages of the prior art are overcome by means of the
 subject invention for an apparatus and method for determining a drilling
 mode to optimize formation evaluation measurements. A drill string, which
 includes an MWD tool, an LWD tool, or a plurality of tools, drills a
 borehole into the formation. Standard rotary drilling operations contain
 many natural pauses where the tool remains stationary: connection time as
 a new section of drill pipe is added to the drill string, circulation time
 when mud is circulated and the drill pipe is possibly rotated, and fishing
 or jarring time while the drill string is stuck and has to be freed before
 drilling can resume. These natural pauses, which occur without
 interrupting normal drilling operations, are utilized to make formation
 evaluation measurements of the subsurface formation that take a long time
 or that benefit from a quiet environment, such as nuclear magnetic
 resonance, seismic, sonic, nuclear, or electromagnetic measurements. A
 deliberate pause can be initiated, causing a portion of the drill string
 to remain stationary.
 In order to utilize the pause interval to optimize formation evaluation
 measurements, the subject invention detects downhole conditions,
 determines the drilling process mode of operation, and modifies the data
 acquisition sequence. The detected downhole conditions include mud flow,
 acceleration of the drill string, bending of the drill string,
 weight-on-bit, and rotation of the drill string. The drilling process
 modes include drilling, sliding, tripping, circulating, fishing, a short
 trip (up or down), and drill pipe connections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, a drill string 10, including lengths of drill pipe 12
 and drill collars 14, is disposed in a borehole 16. A drill bit 18 at the
 lower end of the drill string 10 is rotated by the output shaft of a motor
 assembly 20 powered by drilling fluid or mud circulated down through a
 channel of the drill string 10. The drilling fluid exits the drill string
 10 via jets in the drill bit 18 and then circulates upward in the region
 between the outside of the drill string 10 and the periphery of the
 borehole 16. The motor assembly 20 includes a power section 22
 (rotor/stator or turbine) and a bent subassembly 24 that establishes a
 small bend angle, typically 0.5-2 degrees. As is known in the art, when
 the bit 18 is driven by the mud motor 20 only (with the drill string not
 rotating), the bit 18 will deviate in a direction determined by the tool
 face direction in which the drill string 10 is oriented [hereinafter,
 "sliding"]. When it is desired to drill substantially straight, the drill
 string 10 and the mud motor 20 are both rotated at appropriate rates.
 A tool 32 designed for formation evaluation while drilling (LWD), drill
 string characterization while drilling (MWD), or a combination of both
 (LWD/MWD) is connected to the drill string 10. It is within contemplation
 of the subject invention to have a plurality of tools 32 connected to the
 drill string 10. An LWD tool or a tool combining both LWD and MWD
 characteristics operates to measure nuclear magnetic resonance, seismic,
 sonic, electromagnetic, or nuclear properties of the subsurface formation.
 Typical tools with this capability are disclosed, for example, in U. S.
 Pat. Nos. 5,055,787, 5,017,778, 5,448,227, 5,280,243, and 5,148,407. The
 foregoing formation evaluation while drilling technology is generally
 known to those skilled in the art.
 A typical MWD tool 32 may measure such downhole conditions as the
 weight-on-bit, torque acting on the bit, the inclination and azimuthal
 direction of the borehole, mud resistivity, borehole pressure and
 temperature, as well as various other characteristics of the subsurface
 formation penetrated by the drill bit. The MWD tool 32 operates to
 telemeter information to the surface substantially in real time. Drilling
 mud pumped down through the drill string 10 passes through a device that
 modulates the mud flow to produce a stream of pressure pulses that are
 detected by a transducer at the surface. The operation of the valve is
 modulated by a controller in response to electrical signals from a
 cartridge that receives measurement data from sensors within the tool 32.
 Thus, the pressure pulses detected at the surface during a certain time
 period are directly related to particular measurements made downhole. The
 foregoing mud pulse telemetry technology is generally known to those
 skilled in the art. Other types of mud pulse telemetry systems, such as
 those that produce positive pulses, negative pulses, or combination of
 positive and negative pulses also may be used with the subject invention.
 A sensor subassembly 26, which houses items such as sensors, circuit
 boards, batteries, and various other similar items, is included in the
 drill string 10. The sensor sub 26 includes magnetometers and/or
 accelerometers to detect rotational, lateral, and axial motion of the
 drill string 10. The sensor sub 26 may be connected to tool 32 or made an
 integral part thereof. An upper stabilizer 28 is positioned to
 substantially center the tool string in the borehole at this point. A
 lower stabilizer 30 is positioned to stabilize the rotation of the motor
 output shaft and the bit 18.
 Standard rotary drilling operations contain many natural pauses where tool
 32 remains stationary: connection time as a new section of drill pipe 12
 is added to the drill string 10, circulation time when mud is circulated
 and the drill pipe 12 may or may not be rotated, and fishing or jarring
 time while the drill string 10 is stuck and has to be freed before
 drilling can resume. In accordance with the subject invention, these
 natural pauses, which occur without interrupting normal drilling
 operations, are utilized to make formation evaluation measurements, using
 tool 32 or a plurality of tools 32, that take a long time or that benefit
 from a quiet environment such as nuclear magnetic resonance, seismic,
 sonic, nuclear, or electromagnetic measurements. Alternatively, rather
 than waiting for a natural pause during the drilling operation, a
 deliberate pause can be initiated, causing a portion of the drill string
 to remain stationary.
 An improved formation evaluation measurement is realized during the pause
 interval since noise and vibration caused by the drilling operation are
 eliminated and the tool 32 remains stationary with respect to the
 formation so that formation properties are not changing during the
 measurement cycle. The regular spacing between drill pipe connections
 enables quality control and calibration operations to be performed at
 regular depth intervals. In order to utilize the pause interval to
 optimize formation evaluation measurements, it is necessary to detect
 downhole conditions, determine the mode of operation which includes, but
 is not limited to, drilling, sliding, tripping, circulating, connections,
 short trips, and fishing, and modify the data acquisition sequence to make
 stationary measurements during the pause interval. Items included within
 the drill string 10, such as sensors, circuit boards, batteries, and
 magnetometers and/or accelerometers, are used to determine the drilling
 mode. These items may be within tool 32 or sensor subassembly 26. However,
 these items may be located anywhere within the drill string 10.
 In the sensor sub 26 or tool 32, a circuit board containing digital logic
 uses the downhole condition measurements singly or in various combinations
 to determine automatically the drilling process mode:
 TABLE 1
 Drilling
 Process Acceleration Weight-
 Mode Flow Axial Transverse On-Bit Rotation Bending
 Drilling Yes Yes Yes Yes Yes Yes
 Sliding Yes Yes Yes Yes No Yes
 Tripping.sup.1 No Yes No No No No
 Circulating Yes No No No N/A No
 (pipe
 stationary)
 Connections.sup.1 No No No No No No
 Short Trips.sup.1 No Yes No No No No
 Fishing No N/A No N/A No N/A
 In Table I, a "yes" indicates a drilling process mode detected by the
 captioned downhole condition, "no" indicates a drilling process mode
 undetected by the captioned downhole condition, and "N/A" indicates a
 drilling process mode inconclusively determined by the captioned downhole
 condition. Drilling process modes denoted with a superscript require
 further measurements and/or the history of the flow, acceleration,
 weight-on-bit, rotation, or bending measurements to distinguish between
 these modes.
 FIG. 2 graphically illustrates a representative flow chart for determining
 a drilling mode. Tool 32 detects downhole conditions, for example mud
 flow, acceleration (axial and transverse), motion (rotation and lateral),
 weight-on-bit, and bending. One skilled in the art recognizes that the
 method of the subject invention determines the drilling mode given one or
 a plurality of downhole condition measurements. The flow chart will vary
 based upon the number and type of downhole condition measurements that are
 factored into determining the drilling mode.
 By way of example, the flow chart of FIG. 2 reflects a method for
 determining the drilling mode given the following downhole condition
 measurements: mud flow, rotation, and acceleration. At step 110, a
 detection item senses mud flow. A circuit board connected to the intertool
 power and communications bus may be used to detect power-down or data
 traffic on the bus thereby signaling a pause caused by a mode of operation
 where a portion of the drill string remains stationary, i.e., drill pipe
 connections, short trips (up or down), tripping, or a fishing operation.
 Pressure sensors for probing the mud flow or the mud motor shaft rotation
 may be used to detect a mode of continuous operation, i.e., drilling,
 sliding, or mud flow circulation.
 If mud flow is detected, a detection item senses rotational motion of the
 drill string 10 (step 120). The tool face direction is measured by
 magnetometers and/or accelerometers in the sensor sub 26 or MWD tool 32.
 In the presence of mud flow, the presence of rotational motion indicates a
 mode of continuous operation, i.e., drilling or mud flow circulation. On
 the other hand, in the presence of mud flow, the absence of rotational
 motion is attributed to sliding or mud flow circulation. Detecting
 rotational motion of the drill string does not conclusively determine some
 modes of operation, such as mud flow circulation, where the drill pipe
 remains stationary.
 If rotational motion is detected, an accelerometer senses transverse
 acceleration of the drill string motion of the drill string 10 (step 130).
 In the presence of transverse acceleration, the mode of operation is
 determined to be drilling (step 140). In the absence of transverse
 acceleration, the mode of operation is determined to be mud flow
 circulation (step 150).
 If rotational motion is not detected at step 120, an accelerometer senses
 transverse acceleration of the drill string motion of the drill string 10
 (step 160). In the presence of transverse acceleration, the mode of
 operation is determined to be sliding (step 170). In the absence of
 transverse acceleration, the mode of operation is determined to be mud
 flow circulation (step 180).
 If mud flow is not detected at step 110, a detection item, such as an
 accelerometer, senses axial acceleration of the drill string 10 (step
 190). Even when the mud flow is stopped, under certain conditions, the
 drill pipe undergoes axial movement, e.g., pulling out the kelly at the
 upper end of the drill string. In the absence of mud flow, the presence of
 axial acceleration at step 200 indicates a mode of continuous operation,
 i.e., tripping or short trips (up or down). On the other hand, in the
 absence of mud flow, the absence of axial acceleration at step 210 is
 attributed to a drill pipe connection or fishing mode of operation.
 Further measurements and/or a history of the flow, acceleration, and
 rotation measurements are necessary to further distinguish the modes of
 operation identified at steps 200 and 210.
 In the present invention, a detection item, such as strain gauges in the
 drill collar, senses weight-on-bit, bending, or torsion. The gauges may
 signal either a continuous operation, i.e., drilling or sliding, or a
 pause interval caused by drill pipe connections, mud flow circulation,
 short tips (up or down), or tripping (step 210 or 240). Detecting
 weight-on-bit, torsion, or bending does not conclusively determine some
 modes of operation, such as a fishing operation where the drill pipe
 remains stationary.
 After automatically determining a drilling mode, for example a pause during
 the drilling operation, tool 32 provides nuclear magnetic resonance,
 seismic, sonic, nuclear, or electromagnetic measurements utilizing an
 acquisition mode suitable for the detected drilling mode. There are a
 number of useful formation measurements that can be made during a pause
 interval. If tool 32 provides NMR measurements, the pause interval can be
 utilized for optimizing the NMR measurement of the tool. Preferably, the
 pause interval is used for tuning the tool 32. The B.sub.0 field might
 change in an unpredictable manner due to the collection of magnetic debris
 on the tool 32 or more predictably due to changes in the temperature.
 Tuning the tool 32 is easier when the tool is stationary and is
 accomplished differently depending on whether a saddle point or a gradient
 geometry is used.
 For a saddle point geometry, the NMR signal has a maximum for .omega..sub.0
 =.gamma.B.sub.0 at the saddle point. By sweeping through frequencies in
 search of the Larmor frequency, the same geometry and volume is ensured
 for the sensitive region. For a gradient tool, no Larmor frequency search
 is necessary or possible. The measurement is always made at the resonance
 frequency and the lengthy search for the Larmor frequency is avoided. The
 disadvantage to the gradient measurement is that a correction may be
 necessary if the volume of the sensitive region changes as B.sub.0
 changes. One possibility to solve this problem in a gradient tool consists
 of the following steps:
 1. Optimizing the pulse duration for a given frequency .omega..sub.0 by
 varying the 90.degree. and/or 180.degree. pulse durations, t.sub.90 and
 t.sub.180 respectively, to obtain the maximum signal. For the 180.degree.
 pulse, for example,
 ##EQU1##
 are known. This provides B.sub.1 in the sensitive volume.
 2. Determine the quality factor Q of the antenna either by using a small
 injection loop which induces a signal in the antenna and determining the
 overall gain of the receiver path, correcting for the known gain of the
 receiver electronics, or by measuring the strength of the signal of the
 antenna in a small pickup loop.
 3. Since B.sub.1 .alpha.QI.phi.(r) where .phi.(r) describes the known
 radial dependence of the B.sub.1 field, one can find .phi.(r) and invert
 it to obtain the effective radius and hence the volume of the sensitive
 region.
 As another example, the pause interval can be utilized to provide T.sub.2
 measurements with an NMR tool 32. The noisy drilling environment, and
 particularly the lateral motion of the drill pipe, makes NMR measurements
 difficult. NMR measurements are inherently slow and may consist of a
 rather long sequence of pulses and echoes. With the subject invention, the
 pause interval during the drilling operation provides an excellent
 opportunity to acquire formation data.
 One appropriate measurement during the pause interval is hydrocarbon
 typing, where a measurement is made that responds to the bulk hydrocarbon
 properties. This can often involve T.sub.1 and T.sub.2 on the order of a
 few seconds, and the measurement time is very long, approximately several
 tens of seconds or more. The following examples of T.sub.1 and T.sub.2
 measurements with NMR devices is given in R. Akkurt, H. J. Vinegar, P. N.
 Tutunjian, and A. J. Guillory, NMR Logging of Natural Gas Reservoirs, THE
 LOG ANALYST (November-December 1996).
 TABLE 2
 T.sub.1 (msec) T.sub.2 (msec) HI Dox10.sup.-5 (cm.sup.2 /s)
 Oil 5000 460 1 7.9
 Gas 4400 40 .38 100
 TABLE 2
 T.sub.1 (msec) T.sub.2 (msec) HI Dox10.sup.-5 (cm.sup.2 /s)
 Oil 5000 460 1 7.9
 Gas 4400 40 .38 100
 In Table 2, the T.sub.2 properties are based on a high gradient (17
 Gauss/cm) wireline device with the inter-echo time, T.sub.E, approximately
 equal to 1.2 milliseconds. The reference does not include the total
 acquisition time or number of echoes. In Table 3, a total number of 1200
 echoes is assumed which corresponds to 1.44 seconds and represents
 approximately 3*T.sub.2 (oil) worth of data.
 TABLE 4
 T.sub.1 (msec) T.sub.2 (msec) HI Dox10.sup.-5 (cm.sup.2 /s)
 Oil 5000 1450 1 7.9
 Gas 4400 156 .38 100
 TABLE 4
 T.sub.1 (msec) T.sub.2 (msec) HI Dox10.sup.-5 (cm.sup.2 /s)
 Oil 5000 1450 1 7.9
 Gas 4400 156 .38 100
 In Table 4, the T.sub.2 properties are based on a low gradient (1.7
 Gauss/cm) logging tool with the inter-echo time, T.sub.E, approximately
 equal to 6 milliseconds. The inter-echo time is increased to provide a
 good separation between oil and gas. For a gradient of 1.7 Gauss/cm, a
 separation of about 1 decade is obtained. Due to reduced diffusion, the
 T.sub.2 properties are different for a low gradient tool. Referring to
 Table 5, a total number of 750 echoes corresponds to about 4.5 seconds and
 represents approximately 3*T.sub.2 (oil) worth of data. Since
 signal-to-noise varies as a function of the square root of the total
 number of measurements averaged together, it may be desirable to repeat
 these measurements several times and stack the results. For either a high
 gradient or low gradient NMR device, the drilling environment is possibly
 too noisy to make a long T.sub.2 measurement. A pause interval triggering
 event such as drill pipe connections, a fishing operation, or mud
 circulation affords the opportunity to obtain an improved T.sub.2
 measurement because noise and vibration caused by the drilling operation
 are absent during the pause interval.
 As another example, the pause interval can be utilized to provide T.sub.1
 measurements with an NMR tool 32. T.sub.1 is generally controlled by
 surface relaxation mechanisms and not affected by diffusion. Therefore,
 the knowledge of the T.sub.1 distribution of the formation is desirable
 and may be easier to interpret than the corresponding T.sub.2 spectrum.
 The two T.sub.2 measurements with varying wait times discussed with
 reference to Table 2 and Table 4 can be inverted to provide an indication
 of the T.sub.1 of the formation. These measurements could either be made
 sequentially using a single NMR experiment, e.g., multi-wait station
 logging, or could be made simultaneously on two different volumes of the
 formation. In either case, the long measurement times and need for high
 signal-to-noise suggests the need for a stationary measurement.
 Another T.sub.1 measurement technique is inversion recovery. Inversion
 recovery requires a time consuming series of experiments. In each
 experiment, the equilibrium magnetization is inverted by a 180.degree.
 pulse. After a varying recovery time, the magnetization is read out by a
 90.degree. pulse, which rotates the partially recovered magnetization into
 the transversal plane and produces a measurable signal. Since between
 these experiments the magnetization needs to be relaxed and a wait time
 long in comparison to T.sub.1 needs to be introduced, these measurements
 are very time consuming and usually only performed in laboratory
 experiments. For example, a series of 30 experiments with a wait time of
 five seconds between each experiment requires at least 150 seconds. A
 pause interval triggering event such as drill pipe connections, a fishing
 operation, or mud circulation affords the opportunity to obtain an
 improved T.sub.1 measurement because the tool remains stationary during
 the pause interval.
 If tool 32 provides acoustic measurements, the pause interval can be
 utilized to obtain a vertical seismic profiling while drilling measurement
 (VSPWD). The drilling process generates a wide spectrum of acoustic noise
 as the drill bit destroys the rock to create a borehole. The noise levels
 at seismic frequencies are high enough for detection by geophones or
 hydrophones, thousands of feet away, at the surface. Noise levels rapidly
 decrease at high frequencies but significant levels still exist in the
 sonic frequency band. While-drilling acoustic measurements are directly
 influenced by the drilling noise. This is particularly the case for the
 measurements that require recording of small arrivals or echoes. VSPWD is
 the technique where a powerful acoustic source, such as an air gun, is
 fired at the surface, and the travel time of seismic waves from the
 surface to the drill bit is measured by recording the seismic waves by a
 downhole acoustic receiver, such as a hydrophone or geophone. Although the
 acoustic energy generated at the surface is usually very large, the energy
 that must be detected at the bit can be very small due to geometrical
 spreading and attenuation of the acoustic waves in the subsurface. In many
 cases, the drill noise is expected to be orders of magnitude larger than
 this signal sent from the surface. Therefore, in these cases, the VSPWD
 type measurement is possible only when the drilling process is stopped.
 Another acoustic measurement that requires recording of possibly very small
 levels of energy is the borehole sonar. In this measurement, an acoustic
 source located downhole on the bottom hole assembly sends sound waves into
 the formation. These waves reflect back from layer boundaries and echoes
 are recorded by an acoustic receiver also located on the bottom hole
 assembly. The strength of the echo depends on the distance of the
 reflector to the borehole, the reflectivity of the reflector, the strength
 of the source, and the attenuation properties of the medium. Since
 downhole sources have limited power, the strength of the reflections in
 many cases could be lower than the drilling noise levels, and the sonar
 echoes could only be detected when the drilling is stopped.
 If tool 32 provides electromagnetic measurements, the pause interval can be
 utilized to obtain deep reading electromagnetic measurements. Potential
 applications range from traditional two megahertz resistivity measurements
 to lower frequency resistivity measurements to ground penetrating radar.
 These techniques require high power and result in fairly low signals. The
 subject invention provides three significant benefits. First, during
 drilling, a portion of the power from the downhole turbine and/or
 batteries is used to charge an electrical storage device such as a
 capacitor bank. During the pause interval, the stored energy is used to
 power the deep reading EM device or other devices, such as an acoustic
 transmitter. In this way, high power is provided without the need for
 deploying either a high power turbine or high power battery systems.
 Second, when the electromagnetic measurement is made, receivers placed at
 a relatively long spacing are used to make the deep measurement. The low
 signal level recorded at these receivers require the low noise environment
 found during drilling pauses. Third, near borehole electromagnetic
 processing techniques require fixed source to receiver positions. Making
 the measurements during pauses insures that the source to receiver
 position is fixed and well known.
 The foregoing description of the preferred embodiment of the present
 invention has been presented for purposes of illustration and description.
 It is not intended to be exhaustive nor to limit the invention to the
 precise form disclosed. Obviously, many modifications and variations will
 be apparent to those skilled in the art. The embodiments were chosen and
 described in order to best explain the principles of the invention and its
 practical application thereby enabling others skilled in the art to
 understand the invention for various embodiments and with various
 modifications as are suited to the particular use contemplated. It is
 intended that the scope of the invention be defined by the accompanying
 claims and their equivalents.