LAG time and LAG time distribution monitoring

Abstract

A method for estimating a cuttings lag time or lag time distribution during a drilling operation includes circulating drilling fluid in a wellbore while drilling. Colored magnetic markers are introduced into the circulating drilling fluid while drilling. A magnetic or electromagnetic trap is used to remove colored magnetic markers from the circulating drilling fluid. The removed colored magnetic markers are detected based upon their color using an optical sensor. Surface arrival times of the detected colored magnetic markers are evaluated to estimate the lag time or the distribution of lag times.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 503,737, entitled “LAG TIME AND LAG TIME DISTRIBUTION MONITORING” filed May 23, 2023, the disclosure of which is hereby incorporated herein by reference.BACKGROUND

[0002] Drilling fluid (mud) is pumped downhole while drilling a subterranean wellbore. The fluid emerges from the drill string at the drill bit and creates an upward flow through the wellbore annulus which carries drill cuttings to the surface. The fluid and cuttings are commonly examined at the surface to evaluate the formation layers though which the wellbore is drilled. The depth at which the cuttings were generated may be determined from a depth log generated while drilling and the cuttings lag time (the time it takes the cuttings to reach the surface).

[0003] A theoretical lag time may be calculated from the well architecture (e.g., including the wellbore and drill string diameters) and the fluid flow rate. However, in practice the theoretical lag time does not provide a consistently accurate measure of the cuttings lag time. As such, drilling operators commonly make occasional lag time measurements while drilling a well. For example, a drilling operator may introduce particulate into the drilling fluid and estimate the lag time based on the arrival time of the particulate at the surface.

[0004] While the above described methods for estimating a cuttings lag time may at times be commercially serviceable, there is room for further improvement. For example, identifying the introduced particulate amidst the drill cuttings in the arriving drilling fluid can be difficult such that the estimated cuttings lag time may be subject to error. Moreover, known methods are time and labor intensive. There is a need in the industry for improved lag time measurement methods.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a more complete understanding of the disclosed subject matter, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0006] FIG. 1 depicts an example drilling rig including a system for estimating a cuttings lag time or lag time distribution while drilling.

[0007] FIG. 2 depicts a flow chart of one example method for estimating a cutting lag time or lag time distribution during a drilling operation.

[0008] FIG. 3 depicts one example colored magnetic marker for use in method shown on FIG. 2.

[0009] FIG. 4 depicts a plot of magnetic marker counts versus time for a synthetic example.

[0010] FIG. 5 depicts a plot of magnetic marker counts versus time for another synthetic example.DETAILED DESCRIPTION

[0011] Embodiments of the present disclosure include systems and methods for estimating a cuttings lag time or lag time distribution during a drilling operation. In one example embodiment, a disclosed method includes circulating drilling fluid in a wellbore while drilling. Colored magnetic markers are introduced into the circulating drilling fluid while drilling. A magnetic or electromagnetic trap is used to remove colored magnetic markers from the circulating drilling fluid. The removed colored magnetic markers are detected based upon their color using an optical sensor. Surface arrival times of the detected colored magnetic markers are evaluated to estimate the lag time or the distribution of lag times.

[0012] A theoretical lag time can often be computed from the flow rate of the drilling fluid and the geometry of the wellbore (e.g., including the depth of the wellbore, the diameters of the wellbore sections, and the diameters of the drill string sections). While a theoretical lag time may be serviceable in some operations, it is prone to error, especially in operations in which there is wellbore caving or washout, pumping difficulties, and / or hole cleaning difficulties. Such difficulties can lead to significant lag time errors and corresponding cuttings depth errors (e.g., up to and exceeding 50 meters of depth error). While methods are known for measuring the lag time, these measurement methods tend to be manually intensive and prone to operator error (e.g., requiring an operator to visually identify particular matter added to the circulating drilling fluid). There is a need in the industry for improved lag time measurement methods.

[0013] FIG. 1 depicts an example drilling rig 20 including a system for estimating the cuttings lag time or a lag time distribution while drilling. The drilling rig 20 may be positioned over a subterranean formation (not shown). The rig 20 may include, for example, a derrick and a hoisting apparatus (also not shown) for raising and lowering a drill string 30, which, as shown, extends into wellbore 40 and includes, for example, a drill bit 32 and one or more downhole measurement tools 38 (e.g., a logging while drilling tool or a measurement while drilling tool) in a bottom hole assembly (BHA) above the bit 32. Suitable drilling systems, for example, including drilling, steering, logging, and other downhole tools are well known in the art.

[0014] Drilling rig 20 further includes a surface system 50 for controlling the flow of drilling fluid used on the rig (e.g., used in drilling the wellbore 40). In the example rig depicted, drilling fluid 35 is pumped downhole (as depicted at 92), for example, via a conventional mud pump 57. The drilling fluid 35 may be pumped, for example, through a standpipe 58 and mud hose 59 in route to the drill string 30. The drilling fluid 35 typically emerges from the drill string 30 at or near the drill bit 32 and creates an upward flow 94 of mud through the wellbore annulus 42 (the annular space between the drill string and the wellbore wall). The drilling fluid 35 then flows through a return conduit 52 to a mud pit system 56. It will be appreciated that the terms drilling fluid and mud are used synonymously herein.

[0015] The circulating drilling fluid 35 is intended to perform many functions while drilling, one of which is to carrying drill cuttings 45 to the surface (in upward flow 94). The cuttings are commonly removed from the returning mud via a shale shaker 55 (or other similar solids control equipment) in the return conduit (e.g., immediately upstream of the mud pits 56). The drilling fluid 35 is generally reused and recirculated downhole. Formation gases that are released during drilling may also migrate to the surface in the circulating drilling fluid. These gasses are commonly removed from the fluid, for example, via a degasser or gas trap 54 located in or near a header tank 53 that is immediately upstream of the shale shaker 55 in the example depiction. The cuttings 45 and gas are commonly examined at the surface to evaluate the formation layers though which the wellbore is drilled. The depth at which the cuttings and / or gases were generated may be determined from the depth log generated while drilling and the lag time (the time it takes cuttings to reach the surface).

[0016] With continued reference to FIG. 1, example drilling rig 20 may further optionally include a magnetic or electromagnetic trap 80 deployed in the surface system 50. The trap 80 may be located upstream of the shale shaker and, as described in more detail below, may be configured to collect magnetic markers from the returning drilling fluid. An electromagnetic trap may be advantageous in that it may be turned on and off to collect and release the magnetic markers. An optical sensor (or digital camera) 82 may be deployed and configured to identify the collected magnetic markers (e.g., by color or size). Note also that FIG. 1 further depicts a plurality of magnetic markers 150 being injected into the drilling fluid at an injection port 51 located between mud pump 57 and the drill string 30 (e.g., between the standpipe and mud hose or proximate a rig top drive).

[0017] Drilling rig 20 may further include a control room 70 (e.g., a laboratory trailer or other facility including one or more instruments suitable for making various physical and chemical measurements of the drill cuttings and formation gases in the drilling fluid). Moreover, the control room 70 may include a computer system configured, for example, to estimate a distribution of lag times from using input from optical sensor 82. The computer system may include one or more processors (e.g., microprocessors) which may be connected to one or more data storage devices (e.g., hard drives or solid-state memory) and user interfaces as is well known. The computer system may further include, or be configured to execute, processor executable instructions stored in the data storage device. The executable instructions may be configured, for example, to receive and input from the optical sensor and to estimate the lag time distribution.

[0018] FIG. 2 depicts a flow chart of one example method 100 for estimating a cuttings lag time or lag time distribution during a drilling operation. The method includes circulating drilling fluid in a wellbore while drilling at 102. For example, the drilling may include rotary drilling in which the drill string is rotated from the surface using a top drive or the BHA is rotated using a mud motor. Colored magnetic markers (e.g., painted or coated iron pellets) may be introduced into the circulating drilling fluid at 104 while drilling. The markers may be introduced, for example, at a fluid inlet port located between the mud hose and the drill string. In such embodiments, the markers are transported by the drilling fluid down the drill string, through the nozzles in the drill bit, and are carried back to the surface along with the drill cuttings in the wellbore annulus.

[0019] With continued reference to FIG. 2, the magnetic markers may be removed from the circulating drilling fluid after they return to the surface using a magnetic or electromagnetic trap at 106. The removed markers may be detected and identified at 108, for example, using an optical sensor or a digital camera. Magnetic marker detection events (e.g., arrival times and / or magnetic marker counts) may then evaluated at 110 to estimate the lag time and / or lag time distribution of the drill cuttings.

[0020] FIG. 3 depicts one example colored magnetic marker for use in method 100. In the depicted example, marker 150 includes a magnetic iron (or steel) pellet (such as a ball bearing) 152 encapsulated in a colored coating (such as a paint or a colored polymer coating) 154. FIG. 3 depicts various stages of an example encapsulation process. The coating thickness and the pellet size may be adjusted to adjust the size (e.g., diameter) and density of the magnetic markers 150. Substantially any suitable color may be utilized. Advantageous colors are those that are highly visible in the drilling fluid, for example including red, yellow, green, blue, orange, and white. Moreover, to promote similar transport properties, the marker 150 may have a similar size to normal drill cuttings, for example, having a diameter in a range from 0.1 mm to 10 mm (e.g., from 0.2 mm to 2 mm). Moreover, the marker may have a similar density to the formation rock, for example, ranging from 1 g / cm3 to 5 g / cm3 (e.g., from 1.5 g / cm3 to 3 g / cm3).

[0021] With reference again to FIGS. 1 and 2, the trap 80 may advantageously include an electromagnetic wand. The electromagnet may be energized as the wand is introduced into and moved in the returning drilling fluid. The electromagnet may then be deenergized after the wand is removed from the drilling fluid to release the collected magnetic material. For example, the collected magnetic material may be released onto a pad or into a tray in sensory range of an optical sensor or digital camera. After the magnetic material has been released, the optical sensor (or digital camera) may automatically detect the colored magnetic markers in the released magnetic material. The released magnetic material (including any present markers) may further be washed or rinsed prior to the optical detection or digital image acquisition.

[0022] In one example embodiment, an optical detector may be configured to automatically detect the arrival of the colored magnetic markers based on color (e.g., red). Detection of one or more colored markers may then be taken as an arrival time (or a sequence of arrival times) of the colored makers. The arrival time(s) may then be used to estimate (compute) total transit times (e.g., the arrival time minus the injection time), lag times (e.g., the total transit time minus a theoretical time for the markers to reach the drill bit), and a lag time distribution.

[0023] In another example embodiment, a digital camera may be configured to take one or more digital images of the released magnetic material. The digital image(s) may then be evaluated using artificial intelligence (e.g., a trained neural network) to identify and quantify the magnetic markers in the image. For example, selected objects in the digital image maybe identified using a segmenting algorithm. Color features (e.g., red, green, and blue intensities, etc.) may then be extracted from the identified objects. The extracted color features may be further evaluated to identify and count any colored magnetic markers in the released magnetic material. The number of magnetic markers in each image may be tracked with time, thereby enabling a distribution of arrival times to be determined and a corresponding distribution of lag times to be estimated. The distribution may include, for example, a range of lag times or a weighted or normalized range of lag times (e.g., weighted by the number of magnetic markers identified at each time interval or in each image).

[0024] In example embodiments, the segmenting algorithm may include a neural network such as a pixel by pixel convolution neural network (CNN) or other artificial intelligence and / or machine learning algorithms. In example embodiments, the algorithm may employ a Mask Region-Based Convolutional Neural Network (Mask R-CNN). The Mask R-CNN may be configured to identify magnetic material including the magnetic markers in the digital images and thereby generate a segmented image. For example, the Mask R-CNN may produce, for example, bounding boxes and mask images. The bounding boxes may be defined as a set of x-y coordinates in an image that indicates an image region that contains an object of interest. The bounding box may include a confidence score that ranges from 0 to 1 (e.g., with greater values indicating higher confidence regarding) for each object of interest. The mask image may indicate (e.g., highlight or otherwise bound) regions of interest that have a confidence score that exceeds a threshold.

[0025] The identified elements in the segmented image may then be further evaluated to extract color features. Various color related features may include, for example, average (such as mean, median, or mode) red, green, and blue intensities or distributions of or standard deviations of red, green, and blue intensities and / or an average luminance of each particle as well as a histogram, a variance, a skewness, and / or a kurtosis of the red, green, and blue intensities. The extracted color features may be further evaluated to identify and then count the colored magnetic markers in the segmented image. For example, the colored magnetic markers may be identified, for example, according to a location of the element in a multi-dimensional space of extracted color. The identified elements may then be classified (e.g., as colored magnetic markers of a particular color or other non-marker material) according values of those features, for example, that cause like particles to cluster in the aforementioned multi-dimensional feature space.

[0026] FIG. 4 depicts a plot 180 of magnetic marker counts versus time for a synthetic example in which the marker counts may be determined using a trained neural network as described above. In the depicted plot the individual marker counts are shown as dark circles. A best fit is shown at 182. It will be appreciated that a number (or a distribution) of arrival times (and therefore lag times) may be indicated by the plot. For example, a first arrival time (the earliest arrival) is shown at 184. A last (or latest) arrival time is shown at 186. A peak arrival time (the time at which the greatest number of magnetic markers are identified or counted) is shown at 188. It will be further appreciated that each of these arrival times may be evaluated to determine a distinct lag time from which a distribution of lag times may be obtained.

[0027] In another example embodiment, a plurality of (at least first and second) colored magnetic marker sizes and / or densities may be employed to evaluate the influence of cuttings size and / or density on the lag time or lag time distribution. For example only, a set of magnetic markers may include markers having a plurality of (e.g., three) distinct sizes (such as 0.3 mm, 1 mm, and 3 mm) or a plurality of distinct densities (such as 1.5 g / cm3, 2.5 g / cm3, and 3.5 g / cm3). Each marker size or density may be distinguishable by having a corresponding marker color (such as 0.3 mm red markers, 1 mm green markers, and 3 mm blue markers or 1.5 g / cm3 red markers, 2.5 g / cm3 green markers, and 3.5 g / cm3 blue markers). The markers may be introduced to the drilling fluid simultaneously and then uniquely identified at the surface based on color (e.g., as described above). Arrival times and / or marker counts with time may be evaluated for each marker size (and color) to estimate the lag time and / or distribution of lag times for cuttings of various sizes.

[0028] FIG. 5 depicts a plot 190 of magnetic marker counts versus time for another synthetic example in which three distinct marker sizes were used and the marker counts were determined using a trained neural network as described above. In the depicted plot the individual marker counts are shown as diamonds (small markers), squares (medium sized makers), and triangles (large markers). Corresponding best fits are shown at 192, 194, and 196. It will be appreciated that a number (or a distribution) of arrival times (and therefore lag times) may be estimated for each of the marker sizes as described above with respect to FIG. 4. In the depicted example, the distribution of arrival times (and therefore lag times) increases with increasing marker size indicating potential hole cleaning issues, especially for the larger marker (and therefore cuttings) size.

[0029] With reference again to FIG. 2, in another example embodiment, multiple sets (at least first and second sets) of colored magnetic markers may be utilized, with each set being characterized by a distinct color. The individual sets of colored magnetic markers may be introduced into the drilling fluid sequentially at 104 at some predetermined time interval (e.g., at five or ten-minute intervals). In one example embodiment, six distinct sets of magnetic markers may be utilized. The individual sets may advantageously have distinct and highly distinguishable colors, for example, red, yellow, green, blue, orange, and white. Of course, other colors can be used. By sequential introduction it is meant that a first set (e.g., the red set) is introduced at time zero. After a first time interval (e.g., a five minute delay), the second set (e.g., the yellow set) is introduced. And so on. The entire sequence may then be repeated any number of times while drilling progresses.

[0030] Upon their return to the surface and removal from the drilling fluid, the markers may be detected and identified by set according to their color, for example, using an optical sensor or a digital camera as described above. Under normal drilling conditions, the same sequence (e.g., red, yellow, green, etc.) may be observed. In this way the lag time or lag time distribution may be monitored with time as drilling progresses. Coding of the color sequence may enable appropriate quality control of the lag time and synchronization with other drilling measurements and may advantageously enable lag time changes to be quickly identified and the cause of the change to be identified.

[0031] It will be understood that the present disclosure includes numerous embodiments. These embodiments include, but are not limited to, the following embodiments.

[0032] In a first embodiment, a method for estimating a cuttings lag time or lag time distribution during a drilling operation includes circulating drilling fluid in a wellbore while drilling, the circulating drilling fluid traveling downhole through a drill string and uphole through a wellbore annulus, the circulating drilling fluid carrying drill cuttings generated by the drilling to a surface location; introducing colored magnetic markers into the circulating drilling fluid while drilling; removing ones of the colored magnetic markers from the circulating drilling fluid at the surface location using a magnetic or electromagnetic trap; detecting the removed colored magnetic markers using an optical sensor based upon a color of the colored magnetic markers; and evaluating arrival times at the surface location of the detected colored magnetic markers to estimate the lag time or the distribution of lag times.

[0033] A second embodiment may include the first embodiment, wherein the colored magnetic markers comprise magnetic iron pellets encapsulated in a colored coating.

[0034] A third embodiment may include any one of the first through second embodiments, wherein the colored magnetic markers have a diameter in a range from 0.2 mm to 2 mm.

[0035] A fourth embodiment may include any one of the first through third embodiments, wherein the colored magnetic markers have a density in a range from 1.5 g / cm3 to 3 g / cm3.

[0036] A fifth embodiment may include any one of the first through fourth embodiments, wherein the removing comprises inserting an electromagnetic wand into the circulating drilling fluid at the surface location; energizing the electromagnetic wand to capture magnetic material in the circulating drilling fluid; removing the electromagnetic wand and the captured magnetic material from the circulating drilling fluid; deenergizing the electromagnetic wand to release the captured magnetic material in sensory range of the optical sensor.

[0037] A sixth embodiment may include the fifth embodiment, wherein the optical sensor is a digital camera and the detecting comprises using the digital camera to take a digital image of the captured magnetic material; and evaluating the digital image to identify the removed colored magnetic markers in the captured magnetic material.

[0038] A seventh embodiment may include the sixth embodiment, wherein the evaluating the digital image comprises identifying selected objects in the captured magnetic material using a segmenting algorithm; extracting color features from the identified selected objects; and evaluating the extracted color features to identify and count the removed colored magnetic markers in the digital image.

[0039] An eighth embodiment may include the seventh embodiment, further comprising repeating the detecting the removed colored magnetic markers and the evaluating the arrival times at the surface location to obtain at least a first arrival time and a last arrival time and to thereby estimate the distribution of lag times.

[0040] A ninth embodiment may include any one of the first through eighth embodiments, wherein the introducing the colored magnetic markers comprises simultaneously introducing at least first and second sets of colored magnetic markers having (i) corresponding first and second different sizes or first and second different densities and (ii) first and second different colors; and the evaluating the arrival times comprises estimating at least first and second lag times or at least first and second distributions of lag times corresponding to the at least first and second sets of colored magnetic markers.

[0041] A tenth embodiment may include any one of the first through ninth embodiments, wherein: the introducing the colored magnetic markers comprises sequentially introducing at least first and second sets of colored magnetic markers having corresponding first and second different colors at a predetermined time interval; the removing ones of the colored magnetic markers comprises removing ones of the first set of colored markers at a first time and ones of the second set of colored markers at a second time; the detecting comprises detecting the ones of the first set based upon the first color and detecting the ones of the second set based upon the second color; and the evaluating comprises evaluating the arrival times of the first set to estimate a first lag time or a first distribution of lag times and evaluating the arrival times of the second set to estimate a second lag time or a second distribution of lag times.

[0042] In an eleventh embodiment, a system for estimating a cuttings lag time or lag time distribution during a drilling operation includes a plurality of colored magnetic markers configured for introduction into circulating drilling fluid on a drilling rig; a magnetic or electromagnetic trap configured to remove ones of the colored magnetic markers from the circulating drilling fluid at a surface location on the drilling rig; an optical sensor or digital camera; and a processor configured to receive measurements from the optical sensor or the digital camera, evaluate the measurements to identify the removed colored magnetic markers based upon a color thereof, and evaluate evaluating arrival times at the surface location of the detected colored magnetic markers to estimate the lag time or the distribution of lag times.

[0043] A twelfth embodiment may include the eleventh embodiment, wherein the plurality of colored magnetic markers comprise magnetic iron pellets encapsulated in a colored coating; and the colored magnetic markers have a diameter in a range from 0.2 mm to 2 mm or a density in a range from 1.5 g / cm3 to 3 g / cm3.

[0044] A thirteenth embodiment may include any one of the eleventh through twelfth embodiments, wherein the magnetic or electromagnetic trap comprises an electromagnetic wand configured for insertion into the circulating drilling fluid at the surface location to capture magnetic material in the circulating drilling fluid.

[0045] A fourteenth embodiment may include the thirteenth embodiment, wherein optical sensor or digital camera comprises a digital camera configured to take a digital image of the captured magnetic material, wherein the processor is further configured to identify selected objects in the captured magnetic material using a segmenting algorithm, extract color features from the identified selected objects; and evaluate the extracted color features to identify and count the removed colored magnetic markers in the digital image.

[0046] A fifteenth embodiment may include any one of the eleventh through fourteenth embodiments, wherein the plurality of colored magnetic markers include at least first and second sets of colored magnetic markers having corresponding first and second different sizes or first and second different densities and first and second different colors; and the processor is configured to estimate at least first and second lag times or at least first and second distributions of lag times corresponding to the at least first and second sets of colored magnetic markers.

[0047] In a sixteenth embodiment, a method for estimating a cuttings lag time or lag time distribution during a drilling operation includes circulating drilling fluid in a wellbore while drilling, the circulating drilling fluid traveling downhole through a drill string and uphole through a wellbore annulus, the circulating drilling fluid carrying drill cuttings generated by the drilling to a surface location; introducing first and second sets of colored magnetic markers into the circulating drilling fluid while drilling, the first and second sets of colored magnetic marker having corresponding first and second different colors; removing ones of the first set of colored magnetic markers and ones of the second set of colored magnetic markers from the circulating drilling fluid at the surface location using a magnetic or electromagnetic trap; detecting the removed ones of the first set of colored magnetic markers and the removed ones of the second set of colored magnetic markers using an optical sensor based upon the first and second different colors; and evaluating arrival times of the first set of colored magnetic markers to estimate a first lag time or a first distribution of lag times and arrival times of the second set of colored magnetic markers to estimate a second lag time or a second distribution of lag times.

[0048] A seventeenth embodiment may include the sixteenth embodiment, wherein the first and second sets of colored magnetic markers have first and second different diameters or first and second different densities; and the first and second sets of colored magnetic markers are introduced into the circulating drilling fluid at the same time.

[0049] An eighteenth embodiment may include any one of the sixteenth through seventeenth embodiments, wherein the first and second sets of colored magnetic markers have identical diameters and densities; and the first and second sets of colored magnetic markers are introduced into the circulating drilling fluid at different times.

[0050] A nineteenth embodiment may include any one of the sixteenth through eighteenth embodiments, wherein the removing comprises inserting an electromagnetic wand into the circulating drilling fluid at the surface location; energizing the electromagnetic wand to capture magnetic material in the circulating drilling fluid; removing the electromagnetic wand and the captured magnetic material from the circulating drilling fluid; and deenergizing the electromagnetic wand to release the captured magnetic material in sensory range of the optical sensor.

[0051] A twentieth embodiment may include the nineteenth embodiment, wherein the optical sensor is a digital camera, and the detecting comprises using the digital camera to take a digital image of the captured magnetic material; identifying selected objects in the captured magnetic material using a segmenting algorithm; extracting color features from the identified selected objects; and evaluating the extracted color features to identify the ones of the first set of colored magnetic markers and the ones of the second set of colored magnetic markers.

[0052] Although lag time and lag time distribution monitoring has been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

1. A method for estimating a cuttings lag time or lag time distribution during a drilling operation, the method comprising:circulating drilling fluid in a wellbore while drilling, the circulating drilling fluid traveling downhole through a drill string and uphole through a wellbore annulus, the circulating drilling fluid carrying drill cuttings generated by the drilling to a surface location;introducing colored magnetic markers into the circulating drilling fluid while drilling;removing ones of the colored magnetic markers from the circulating drilling fluid at the surface location using a magnetic or electromagnetic trap;detecting the removed colored magnetic markers using an optical sensor based upon a color of the colored magnetic markers; andevaluating arrival times at the surface location of the detected colored magnetic markers to estimate the lag time or the distribution of lag times.

2. The method of claim 1, wherein the colored magnetic markers comprise magnetic iron pellets encapsulated in a colored coating.

3. The method of claim 1, wherein the colored magnetic markers have a diameter in a range from 0.2 mm to 2 mm.

4. The method of claim 1, wherein the colored magnetic markers have a density in a range from 1.5 g / cm3 to 3 g / cm3.

5. The method of claim 1, wherein the removing comprises:inserting an electromagnetic wand into the circulating drilling fluid at the surface location;energizing the electromagnetic wand to capture magnetic material in the circulating drilling fluid;removing the electromagnetic wand and the captured magnetic material from the circulating drilling fluid; anddeenergizing the electromagnetic wand to release the captured magnetic material in sensory range of the optical sensor.

6. The method of claim 5, wherein the optical sensor is a digital camera and the detecting comprises:using the digital camera to take a digital image of the captured magnetic material; andevaluating the digital image to identify the removed colored magnetic markers in the captured magnetic material.

7. The method of claim 6, wherein the evaluating the digital image comprises:identifying selected objects in the captured magnetic material using a segmenting algorithm;extracting color features from the identified selected objects; andevaluating the extracted color features to identify and count the removed colored magnetic markers in the digital image.

8. The method of claim 7, further comprising repeating the detecting the removed colored magnetic markers and the evaluating the arrival times at the surface location to obtain at least a first arrival time and a last arrival time and to thereby estimate the distribution of lag times.

9. The method of claim 1, wherein:the introducing the colored magnetic markers comprises simultaneously introducing at least first and second sets of colored magnetic markers having (i) corresponding first and second different sizes or first and second different densities and (ii) first and second different colors; andthe evaluating the arrival times comprises estimating at least first and second lag times or at least first and second distributions of lag times corresponding to the at least first and second sets of colored magnetic markers.

10. The method of claim 1, wherein:the introducing the colored magnetic markers comprises sequentially introducing at least first and second sets of colored magnetic markers having corresponding first and second different colors at a predetermined time interval;the removing ones of the colored magnetic markers comprises removing ones of the first set of colored markers at a first time and ones of the second set of colored markers at a second time;the detecting comprises detecting the ones of the first set based upon the first color and detecting the ones of the second set based upon the second color; andthe evaluating comprises evaluating the arrival times of the first set to estimate a first lag time or a first distribution of lag times and evaluating the arrival times of the second set to estimate a second lag time or a second distribution of lag times.

11. A system for estimating a cuttings lag time or lag time distribution during a drilling operation, the system comprising:a plurality of colored magnetic markers configured for introduction into circulating drilling fluid on a drilling rig;a magnetic or electromagnetic trap configured to remove ones of the colored magnetic markers from the circulating drilling fluid at a surface location on the drilling rig;an optical sensor or digital camera; anda processor configured to receive measurements from the optical sensor or the digital camera, evaluate the measurements to identify the removed colored magnetic markers based upon a color thereof, and evaluate evaluating arrival times at the surface location of the detected colored magnetic markers to estimate the lag time or the distribution of lag times.

12. The system of claim 11, wherein:the plurality of colored magnetic markers comprise magnetic iron pellets encapsulated in a colored coating; andthe colored magnetic markers have a diameter in a range from 0.2 mm to 2 mm or a density in a range from 1.5 g / cm3 to 3 g / cm3.

13. The system of claim 11, wherein the magnetic or electromagnetic trap comprises an electromagnetic wand configured for insertion into the circulating drilling fluid at the surface location to capture magnetic material in the circulating drilling fluid.

14. The system of claim 13, wherein optical sensor or digital camera comprises a digital camera configured to take a digital image of the captured magnetic material, wherein the processor is further configured to:identify selected objects in the captured magnetic material using a segmenting algorithm;extract color features from the identified selected objects; andevaluate the extracted color features to identify and count the removed colored magnetic markers in the digital image.

15. The system of claim 11, wherein:the plurality of colored magnetic markers includes at least first and second sets of colored magnetic markers having corresponding first and second different sizes or first and second different densities and first and second different colors; andthe processor is configured to estimate at least first and second lag times or at least first and second distributions of lag times corresponding to the at least first and second sets of colored magnetic markers.

16. A method for estimating a cuttings lag time or lag time distribution during a drilling operation, the method comprising:circulating drilling fluid in a wellbore while drilling, the circulating drilling fluid traveling downhole through a drill string and uphole through a wellbore annulus, the circulating drilling fluid carrying drill cuttings generated by the drilling to a surface location;introducing first and second sets of colored magnetic markers into the circulating drilling fluid while drilling, the first and second sets of colored magnetic marker having corresponding first and second different colors;removing ones of the first set of colored magnetic markers and ones of the second set of colored magnetic markers from the circulating drilling fluid at the surface location using a magnetic or electromagnetic trap;detecting the removed ones of the first set of colored magnetic markers and the removed ones of the second set of colored magnetic markers using an optical sensor based upon the first and second different colors; andevaluating arrival times of the first set of colored magnetic markers to estimate a first lag time or a first distribution of lag times and arrival times of the second set of colored magnetic markers to estimate a second lag time or a second distribution of lag times.

17. The method of claim 16, wherein:the first and second sets of colored magnetic markers have first and second different diameters or first and second different densities; andthe first and second sets of colored magnetic markers are introduced into the circulating drilling fluid at the same time.

18. The method of claim 16, wherein:the first and second sets of colored magnetic markers have identical diameters and densities; andthe first and second sets of colored magnetic markers are introduced into the circulating drilling fluid at different times.

19. The method of claim 16, wherein the removing comprises:inserting an electromagnetic wand into the circulating drilling fluid at the surface location;energizing the electromagnetic wand to capture magnetic material in the circulating drilling fluid;removing the electromagnetic wand and the captured magnetic material from the circulating drilling fluid; anddeenergizing the electromagnetic wand to release the captured magnetic material in sensory range of the optical sensor.

20. The method of claim 19, wherein the optical sensor is a digital camera, and the detecting comprises:using the digital camera to take a digital image of the captured magnetic material;identifying selected objects in the captured magnetic material using a segmenting algorithm;extracting color features from the identified selected objects; andevaluating the extracted color features to identify the ones of the first set of colored magnetic markers and the ones of the second set of colored magnetic markers.

Images