METHOD FOR FORMATION EVALUATION WHILE DRILLING

Abstract: A logging while drilling (LWD) and measuring while drilling (MWD) method and device (16) are disclosed for reducing the sensitivity of NMR measurements to tool (16) motions. The invention is based on NMR relaxation measurements determining longitudinal relaxation times T1 instead of standard T2 measurements, and involves saturating a relatively wide sensitive region of the formation (10) and processing NMR echo signals which originate approximately from the center of the sensitive region.

Description: METHOD FOR FORMATION EVALUATION WHILE DRILLING This application is a continuation of provisional application No. 60/033,986 filed December 30, 1996. FIELD OF THE INVENTION This invention is directed to a logging while drilling (LWD) and measuring while drilling (MWD) approach for obtaining nuclear magnetic resonance (NMR) data concerning petrophysical properties of a formation. More specifically, the invention is directed to a method and device for reducing the sensitivity of NMR measurements to tool motions. BACKGROUND OF THE INVENTION U.S. Pat. No. 5,280,243 to Miller discloses an NMR apparatus and method of use for geophysical examination of a bore hole as it is being drilled. The patented apparatus is connected to the drill bit and follows it through the bore hole as it is being formed. In operation, the apparatus generates a gradient static magnetic field in a region of the bore hole adjacent the apparatus. This static field extends radially with respect to the longitudinal axis of the apparatus and has a generally uniform amplitude along the azimuth with respect to that axis. Next, a pulsed radio frequency magnetic field is generated to excite nuclei in a substantially cylindrical shell around the tool that defines in the formation a sensitive region extending along the length of the tool and having thickness of about 1 mm. Due to this relatively narrow sensitive region, standard wireline NMR relaxation time measurements are difficult to perform with this tool because lateral vibrations during the measurement time would reduce the accuracy of the measurement . U.S. Pat. 5,557,201 to Kleinberg et al . discloses a pulsed NMR device in which the accuracy of the measurement with respect to lateral tool vibrations is enhanced by providing a larger sensitive region. This is achieved by a special tool architecture shown in Figs. 2A-B, using two tubular permanent magnets 22 with same poles facing each other, and an antenna 26 positioned in the recess between the two magnets. In operation, this tool architecture provides a sensitive region in the formation which is larger laterally, but is greatly reduced along the borehole axis, because of the presence of a single stationary point in the formation. It is expected therefore that vertical tool motions would affect the accuracy of the tool measurements. Accordingly, it is perceived that there is a need for improved sensitivity of pulsed NMR measurements using pulsed NMR tools with respect to tool motions. SUMMARY OF THE INVENTION The present invention concerns a novel method and device for formation evaluation while drilling a borehole using pulsed NMR tools with magnetic fields that are rotationally symmetric about the longitudinal axis of the borehole. In a preferred embodiment, the method of the present invention is based on NMR relaxation time measurements determining longitudinal relaxation times Tl . In particular, the method comprises the steps of generating at least one radio frequency pulse covering a relatively wide range of frequencies to saturate the nuclear magnetization in a cylindrical volume around the tool; transmitting a readout pulse at a frequency near the center of the range of covered frequencies, the readout pulse following a predetermined wait time; applying at least one refocusing pulse following the readout pulse; receiving at least one NMR echo corresponding to the readout pulse; repeating the above steps for a different wait time to produce a plurality of data points on a Tl relaxation curve; and processing the produced Tl relaxation curve to derive petrophysical properties of the formation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A shows a side elevational view, partly in section, of the lower end of the apparatus disclosed in U.S. Pat. No. 5,280,243. Fig. IB is an enlarged sectional view taken along the line 2-2 in Fig. 1A. Fig. 2A shows a cross-section of a tool for pulsed NMR formation evaluation disclosed in U.S. Pat. No. 5,557,201. Fig. 2B shows a static field in a vertical plane of the same tool. DETAILED DESCRIPTION The description of the preferred embodiment of the method of the present invention is made with reference to the tool disclosed in U.S. Pat. No. 5,280,243 to Miller, owned by the assignee of the present application. The content of the Miller patent is expressly incorporated herein for all purposes. Figs. 1A and IB respectively show a side elevational view, partly in section, of the lower end of the Miller tool and an enlarged sectional view taken along the line 2-2 in Fig. 1A. It should be noted, however, that the method of the present invention can also be used with any tool that generates a rotationally symmetric magnetic field including, for example, the tool disclosed in U.S. Pat. No. 5,557,201 to Kleinberg, illustrated in Figs 2A and 2B . The method of the present invention is based on NMR relaxation time measurements determining longitudinal relaxation times Tl , instead of transversal relaxation times T2 that are typically used by a wireline tool. In particular, the method takes advantage of the magnetic field gradient which can be approximated in the proximity of the sensitive volume as a linear fall-off of the magnetic field strength (and also of NMR resonance frequency) in the radial direction . In a preferred embodiment, at the start of a measurement, one or more radio frequency pulses covering a relatively wide range of frequencies, or using one or more pulses which are frequency swept, are transmitted to saturate the nuclear magnetization in a cylindrical volume around the tool. The range of frequencies can be, for example, 50-100kHz and is covered in a specific embodiment using a rapid succession of short radio frequency pulses similar to the first pulse in a standard CPMG pulse sequence, or using a single long pulse in a frequency sweep. Changing the range of frequencies used in this step varies the position and the width of the sensitive region in the formation. In a specific embodiment using the Miller tool, a frequency range between 50 and 100kHz saturates the nuclear magnetization in a cylindrical volume around the tool, where the cylinder has a typical diameter of 14", a height of 24", and thickness of between about 1/2" to 1". Following the step of saturation, which typically takes about 1 ms, in accordance with the present invention a readout pulse is transmitted at a frequency near the center of the range of covered frequencies. In alternative embodiments one or more subsequent readout pulses can also be used. In accordance with the present invention, a readout pulse sequence is comprised of a 90° pulse followed by data acquisition, or of a 90° pulse followed by a 180° pulse, followed by data acquisition, where the steps of applying a 180° pulse and data acquisition can be repeated. The readout pulse sequence generally follows a predetermined wait time, as explained in more detail below. In a specific embodiment the readout pulse sequence is transmitted at a center frequency of about 500kHz, and is followed by one or more refocusing pulses. Following the readout pulse (s), corresponding NMR echo signals are received, amplified and stored for further processing. Preferably, only the first, the second echo or a combination thereof is retained. In accordance with a preferred embodiment, the amplitude of the retained echo signal is interpreted as the level of nuclear magnetization present after the particular wait time. In the particular example considered above, the center frequency of the NMR echo signals corresponds to about 14" diameter of investigation. The measurement process described above is repeated for a series of increasing wait times the values of which can, for example, be equally distributed on a logarithmic scale. In a specific embodiment, wait times are stepped through the values 1ms, 3ms, 10ms, 30ms, 100ms, 300ms, 1000ms and 3000ms, and the measurement results are stacked to produce several data points on a multi-component Tl relaxation curve. A data point corresponding to the longest wait time is obtained by a readout pulse sequence which is not preceded by a saturation pulse . Finally, in accordance with the present invention the produced Tl relaxation curve is used to derive petrophysical properties of the formation, as known in the art. In particular, the resultant Tl relaxation curve is processed to extract the dominant Tl relaxation modes, from which amounts of bound water, free water and hydrocarbons are estimated. The characteristic Tl times of the surface-wetting phase can also be used to estimate formation pore size distributions and formation permeability. It should be noted that since the readout pulse and the signal acquisition have a much smaller bandwidth, typically 5-10kHz vs. 50-100kHz saturation bandwidth, the measurement results obtained using the above-described method are less sensitive to lateral motions of the tool, and in particular are not affected by lateral displacements during the wait time period which do not exceed 1/4" -1/2". In accordance with another preferred embodiment of the present invention, the tool used to make the measurements can be fitted with accelerometers, such as those manufactured by Analog Devices, to monitor peak acceleration values along all three axis during the measurement interval. Measurements, during which the peak accelerations indicate that the tool may have been displaced by more than allowable by the extent of the saturation region, are discarded before stacking to further improve the accuracy of the proposed method. In accordance with another preferred embodiment, the tool is further fitted with hardened steel stand-offs, which, in an in-gauge borehole, allow lateral tool displacements only within the range given by the saturation width. Naturally, the tool may further be provided with accelerometers, as described above, for further accuracy. While the invention has been described with reference to a preferred embodiment, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and form of the invention without departing from its spirit and scope which is defined in the following claims .

Claims: CLAIMS : 1. A method for determination of petrophysical properties of a geologic formation using an NMR logging tool, comprising the steps of : (a) generating at least one radio frequency pulse covering a relatively wide range of frequencies to saturate nuclear magnetization in a cylindrical volume around the tool ; (b) transmitting a readout pulse sequence at a frequency near the center of the range of covered frequencies, the readout pulse sequence following a predetermined wait time after the saturation pulse; (c) receiving at least one NMR echo corresponding to the readout pulse sequence; (d) repeating steps (a) , (b) and (c) one or more times for a different wait time to produce a plurality of data points on a Tl relaxation curve; and (e) processing the produced Tl relaxation curve to derive petrophysical properties of the formation. 2. The method of claim 1 wherein the produced Tl relaxation curve is processed to derive the amounts of bound water, free water and hydrocarbons in the formation. 3. The method of claim 1 wherein the produced Tl relaxation curve is processed to derive the formation pore size distributions and formation permeability. 4. The method of claim 1 wherein the range of frequencies to saturate the nuclear magnetization is between about 50kHz and 100kHz. 5. The method of claim 1 wherein the range of frequencies is covered using a rapid succession of short radio frequency pulses. 6. The method of claim 1 wherein the range of frequencies is covered using a single pulse in a frequency sweep . 7. The method of claim 1 further comprising, prior to the step of receiving NMR echo, the step of applying at least one refocusing pulse to the readout pulse. 8. The method of claim 1 wherein the step of repeating comprises the steps of transmitting readout pulses following wait times the values of which can are equally distributed on a logarithmic scale. 9. The method of claim 1 wherein the step of repeating comprises the steps of transmitting readout pulses following wait times having values of 3ms, 10ms, 30ms, and 100ms. 10. The method of claim 1 further comprising the step of drilling a borehole in said geologic formation concurrently with steps (a) though (e) . 11. A method for determination of petrophysical properties of a geologic formation during the drilling thereof, comprising the steps of: (a) drilling a borehole in a formation; (b) saturating nuclear magnetization in a cylindrical volume around the longitudinal axis of the borehole; (c) providing at least two readout pulses, each readout pulse at a frequency approximately corresponding to the center of the cylindrical volume; (d) receiving NMR echo signals corresponding to said at least two readout pulses; (e) producing a plurality of data points on a Tl relaxation curve from said received NMR echo signals; and (f) processing the produced Tl relaxation curve to derive petrophysical properties of the formation. 12. The method of claim 11 wherein the step of saturating nuclear magnetization in a cylindrical volume comprises the step of generating at least one radio frequency pulse covering a relatively wide range of frequencies. 13. A device for determination of petrophysical properties of a geologic formation using an NMR logging tool comprising: a drilling means; a first means for generating at least one radio frequency pulse covering a relatively wide range of frequencies to saturate nuclear magnetization in a cylindrical volume around the tool; a second means for generating relatively narrow bandwidth readout pulses; a timing means for providing a predetermined wait time between pulses generated by said first and said second means; a means for receiving NMR echo signals corresponding to generated readout pulses; means for processing the received NMR echo signals to produce a Tl relaxation curve to derive petrophysical properties of the formation. 14. The device of claim 13 further comprising accelerometer means for monitoring peak acceleration values along three orthogonal axis of the tool; wherein said means for processing the received NMR echo signals comprises means for discarding echo signals obtained during measurement intervals when peak acceleration values exceed certain threshold. 15. The device of claim 13 further comprising hardened stand-offs, to limit lateral tool displacements within a predetermined range .

Inventors: PRAMMER MANFRED, PRAMMER, MANFRED
Applicants: NUMAR CORP, NUMAR CORPORATION
Application year: 1997
Main classification: E21B47
IPC codes: G01R33, G01V3