Civolani, Lorenzo (Via Ca' Di Barbieri 148 Monzuno, I-40036, IT)
SMITH INTERNATIONAL INC. (16740 Hardy Street Houston, TX, 77205-0068, US)
Zausa, Fabrizio (Via Verdi, 5 Calvignasco, I-20080, IT)
Civolani, Lorenzo (Via Ca' Di Barbieri 148 Monzuno, I-40036, IT)
|1.||A method for selecting drilling parameters for drilling a wellbore through earth formations, comprising: determining a compressive strength of samples of said earth formations from measurements of loading displacement on said samples, said loading displacement measurements made by an indenter; and selecting said drilling parameters from said determined values of compressive strength.|
|2.||The method as defined in claim 1 wherein said determining said compressive strength comprises : measuring said loading displacement through a preselected displacement range; determining a substantially linear portion of a relationship between loading and displacement of said sample by said indenter; and correlating a slope of said substantially linear portion to said compressive strength.|
|3.||The method as defined in claim 1 wherein said samples are taken from a wellbore during drilling thereof, and said drilling parameters are adjusted in response to said compressive strength determined during said drilling of said wellbore.|
|4.||The method as defined in claim 1 wherein said drilling parameters comprise mill tooth type on a roller cone drill bit.|
|5.||The method as defined in claim 1 wherein said drilling parameters comprise insert type on a roller cone drill bit.|
|6.||The method as defined in claim 1 wherein said drilling parameters comprise gauge protection type.|
|7.||The method as defined in claim 1 wherein said drilling parameters comprise jet nozzle type and orientation.|
|8.||The method as defined in claim 1 wherein, said drilling parameters comprise at least one of bit profile, cutter density, cutter type and cutter impact resistance on a fixed cutter drill bit.|
|9.||The method as defined in claim 1 wherein said drilling parameters comprise at least one of weight on bit, bit rotation rate and drilling fluid flow rate.|
|10.||The method as defined in claim 1 wherein said samples are disposed in a resin, said resin is cured, and a test specimen is generated therefrom by grinding a substantially parallel flat surfaces on said cured resin.|
Methods known in the art for selecting the type of drilling bit are typically based on analysis of data related to the drilling performance achieved on previously drilled ("offset") wells in the vicinity of the wellbore being drilled, and are based on monitoring and analysis of dull ("worn out") drill bits. Other methods known in the art for bit selection include methods for simulating ("modeling") the formation"drillability"and other drilling performance parameters. The drillability of earth formations is affected by mechanical properties of the formations, in particular the compressive strength. Knowledge of mechanical properties is useful to optimize the drilling of the formations. One well known method to determine compressive strength is based on acoustic ("sonic") well log interpretation combined with lithological analysis of formation data. Even though it is sufficiently reliable to calculate rock strength, the prior art method has two limitations: first, that the strength is derived in this method from elastic theory, relating the rock acoustic responses to rock hardness, therefore the measurement is not a direct determination of strength; and second, that the acoustic log is recorded after the formation is completely drilled, and is therefore not useful for predictive analysis in the particular wellbore being drilled.
Summarv of the Invention The invention is a method for selecting drilling parameters for drilling a wellbore through earth formations. The method includes determining a compressive strength of samples of earth formations which are to be drilled from measurements of loading displacement made on the samples. The loading displacement measurements are made by an indenter. The drilling parameters are selected from the values of the compressive strength thus determined.
In one embodiment of the invention, the compressive strength is determined from drill cuttings made during the drilling of a wellbore. The compressive strength thus determined during drilling is used to select the drilling parameters during drilling of the wellbore to improve drilling performance.
The drilling parameters which can be selected by the method of the invention include, but are not limited to, mill tooth and/or insert bit type when roller cone drill bits are used; whether and what type of gauge protection is to be used on the drill bit; type, size and orientation of jet nozzles to be included on the drill bit; and where fixed cutter bits are used, the blade structure, cutter type and density as well as the cutter impact resistance can be selected.
Other drilling parameters which can be selected using the method of the invention include weight on bit, drill bit rotation rate, and drilling fluid flow rate.
Brief Description of the Drawings Figure 1 shows an indenter apparatus used to measure compressive strength of rock samples.
Figure 2 illustrates the principle of measurement of compressive strength from indenter measurements.
Figure 3 shows an empirically determined correspondence between unconfined compressive strength and the indenter measurements.
Figure 4 shows an example Rock Bit Selector plot using as input the compressive strength values determined from the method of the invention.
Detailed Description The invention uses a technique for directly determining the compressive strength of a sample of earth formation known as the"indentation technique". The indentation technique can be described as a quick, low cost test to determine rock mechanical properties from small rock fragments. Being an"index"test it allows production of an index value, directly related to rock properties, by means of a simple statistical analysis and simple rules of thumb. Research directed to establishing the theory and response of the indentation test is described in references numbered 1 through 6 in the Appendix.
The indentation test can be described as the measure of the penetration of an indenter, shown generally at 10 in Figure 1, the indenter having a stylus thereon with well defined geometrical features, shape and dimensions, under precise loading conditions, into a small rock fragment 16 produced by the cutting action of a drill bit (see references 7 and 8 in the Appendix), such fragments being known a"cuttings". The principle can be summarized as follows: a a substantially constant load is applied to the cuttings surface, which is shaped by proper flattening, in order to assure a load applied in a normal direction, this being necessary to correctly interpret the testing results; 'the indenter 10 is forced at a constant loading rate up to the maximum penetration defined; and a loading-displacement curve is then recorded, such as on a computer, and is analyzed to recover the index value and correlated with the mechanical rock behavior.
The principle of measurement using the indenter 10 is illustrated in Figure 2, wherein the rock sample is shown at 16, the indenter stylus at 18, and the correspondence between load applied and displacement of the indenter stylus 18 is shown in the graph at 22.
The variables affecting the response of the indentation test are important to recognize in order to interpret the test results properly. A suitable way to prepare cutting samples and a standard test procedure have been defined to assure consistent results. One aspect of the indentation test, when performed on drill cuttings, is the sample preparation. In this embodiment of the invention, the cuttings are embedded in epoxy resin (as shown at 20 in Figure 2) in order to create a disk-shaped specimen. It should be understood that other means for assuring that the indenter applies normal force to the cuttings can be developed, and that the sample preparation method described here is not meant to limit the invention.
After the resin is cured, one face of the specimen is trimmed and cut parallel to the opposite face to expose a sufficient number of cuttings to enable force. It should be noted that when dealing with highly porous rock, it is possible for the resin to invade the pores, which biases the measurement. To mitigate the infiltration, a viscous resin having a short cure time can be used.
A 1 millimeter cylindrical flat indenter stylus has been used in testing the invention, but the size and shape of the indenter stylus are not limitations on the invention Six to eight indentations are typically performed on each specimen by applying a constant penetration rate equal to about 0.01 millimeters per second, to a maximum penetration depth of about 0.3 mm. The corresponding loading-displacement curves are then measured during both a "loading"and"unloading"phase of the measurement. The measurements are typically stored on a computer (14 in Figure 1) for later processing and interpretation. An"indentation index"is defined herein as the slope of a substantially linear part of the loading displacement curve (as shown in graph 22 in Figure 2).
The above described indentation testing procedure has been evaluated to correlate the indentation index with a mechanical parameter (see references 9 and 10 in the Appendix). The results of the evaluation showed that generally six to eight cutting tests on each specimen should be performed so that the test results are representative of formation strength, but a greater number of tests may be performed. The number of such tests on any individual specimen should not be construed as a limitation on the invention. A direct correlation has been determined to exist between the indentation index and the uniaxial compressive strength ("UCS") as shown on the graph in Figure 3.
The samples can be analyzed after a wellbore is drilled, but in one aspect of the invention, the cuttings can be collected and analyzed during the drilling of a wellbore. During drilling of a wellbore, the cuttings can be collected directly at the"shale-shaker"on the drilling rig, at suitable drilled depth intervals, such as every ten meters, or when a lithological variation is detected in the cuttings or by other detection techniques. The collected cuttings can then cleaned to remove drilling fluid ("mud") therefrom and sorted according to size ("sieved") to a size range of between 2 mm and 5 mm. to avoid contamination by"cavings" (rock fragments which have fallen down from the upper wall of the wellbore), although the sieving size range specified herein is not intended to limit the invention. A sufficient number of cuttings can be gathered, embedded in epoxy, as shown generally at 20 in Figure 2, and prepared as previously explained to obtain a disk specimen for each depth interval. Cuttings in each specimen can then be tested, and a collection of loading displacement curves obtained for each specimen. Three possible values of the mechanical properties have been considered: 'the indentation index value for each tested cutting; 'a mean value of the indentation indices for each specimen, computed as an arithmetical average from the slope measured for each loading displacement curve; and 'the mean of all the loading displacement curves from which is calculated a corresponding slope.
The slope thus determined is then used, in conjunction with a relationship such as shown in Figure 3, to determine an unconfined compressive strength of the formation whose cutting are in each specimen. Every depth interval for which a specimen has been made can then be characterized according to mechanical properties, including compressive strength of the rock cuttings in the associated specimen.
After the strength of the cuttings in each of the specimen is determined by the indentation testing as just described, a drill bit optimization analysis is then performed to determine the most likely optimum drill bit cutting structure and other bit design features, as well other drilling parameters such as hydraulic requirements, gauge protection, the axial force (weight) applied to the bit, and the bit rotation rate. The optimization can be performed for both roller cone and fixed cutter drill bits. Several different types of drill bit optimization systems are known in the art. For example, one such system is sold under the trade name DBOS by Smith International, Inc., Houston, Texas. Other drill bit optimization systems known in the art include: RSATM service sold by Reed Hycalog, Houston, Texas; GEOMECHANICSTM service sold by Dresser Industries. Inc.
(now owned by Halliburton company); and ROCK service sold by Baker Hughes incorporated, Houston, Texas. It is to be clearly understood that the particular type of drill bit optimization program is not meant to limit the invention. The foregoing optimization programs are provided herein only as examples of programs that are useful with the method of this invention.
An important input parameter to the drilling bit optimization program or analysis is the compressive strength of the formations through which the wellbore is drilled. In the prior art, as previously explained, acoustic well logs, combined with other well logs, have been used to predict formation drillability (see references 12,13,14 and 15 in the Appendix) by estimating compressive strength. The use of indentation testing, as provided in this invention, to predict drillability can have several advantages when used in a drill bit optimization program or analysis: 'indentation testing does not correlate a dynamic property with a mechanical one, but provides a direct measure of properties related to the mechanical features of the rock; indentation testing allows a continuous monitoring of formation strength along the wellbore section being drilled; indentation testing is a"while drilling"measurement performed directly at the rig site and allows the possibility to adjust the forecasted strength with the values measured during drilling.
A brief description of the parameters calculated, for example, by the DBOSTM drilling optimization service sold by Smith International, Inc. will be presented analyzing therefor the main characteristics, and particularly those aspects related to formation strength. It should be clearly understood that the method of this invention can be used with any type of drilling optimization program or analysis which uses compressive strength, as an input parameter.
The first step of the typical drilling optimization program is formation analysis, through reconstruction of the lithologies within a given stratigraphic section. Formation analysis relies on: 'offset well log information, such as from gamma Ray, acoustic velocity, bulk density, and neutron porosity; "mud log"analysis to provide formation descriptions of accessory minerals not determined from well log response, and to verify the well log determined lithology; 'rate of penetration ("ROP") data, weight on bit ("WOB") data and drill bit rotary speed ("RPM") for better post-drilling analysis of drill bit performance.
Next step of the process is bit performance analysis. Bit record information, directional surveys, and/or real time ROP and drilling parameters from mud log data are incorporated in the bit performance analysis. Figure 4 shows such an analysis in a"well log"plot format.
Variables affecting drill bit performance within a formation having a particular drillability can be evaluated techniques such as by bar graph distribution or zoom-in on a single bit run. Once data have been input to the program, the lithology column is generated and unconfined rock compressive strengths (UCS) are calculated. Offset wellbore data can be divided into sections or intervals, based on"geomechanical units" (often incorporating several geological units) or "lithologic units". Several parameters are evaluated statistically for each drillability interval: Lithology Normalized ("LN") Porosity is evaluated and is used later to more accurately determine bit hydraulic design and nozzle requirements. Gas zones are identified and rock strengths are then corrected; Unconfined rock compressive strengths are then statistically calculated and, combined with the dominant rock type, later applied to rock bit selection and cutter density and bit profile recommendations for fixed cutter bit applications. Further sand content (quartz-bearing formations) combined with rock strength contributes to the model's determinations of formation abrasiveness; Fractional volume of shale variations from interval to interval, together with acoustic velocity (or transit time) are used to determine optimal types of hydraulic nozzling, where porosity logs are unavailable.
The Rock Bit Selector ("RBS") output of the DBOS program compiles the results of the bit type and feature selection in a"well log"plot format (as shown in Figure 4). The following are presented on the RBS plot: source data from any offset wells including gamma ray, acoustic travel time or velocity, any well log-determined lithologies, the calculated unconfined rock strength), log normalized porosity, drillability intervals, and true vertical depth; 'a mill tooth bits output column which indicates bit types based on combinations of dominant rock type and formation strength, ranging from International Association of Drilling Contractors (IADC) standard bit code 11, to 21-type bits. Note that intervals having similar drillability become apparent when observing the bit type suggested in the output column. tungsten carbide insert (TCI) bits column, ranging from IADC Code 41 to 83-type bits.
These selections are made in parallel with the mill tooth bit selection.
'insert type bits column with the various insert types depending on specific application.'DD' bits are designed with diamond enhanced TCI inserts (of various shapes) deployed across the entire bit face and are suitable when quartz-bearing rocks dominate the overall lithology.
Conical Inserts are preferred over standard chisel inserts where shale fractional volumes are low and where rock strengths indicate that at least an IADC Code 44-type cutting structure should be used. gauge protection column, wherein an abrasion function is determined by a combination of sand (quartz) content and rock strength. If sand content exceeds a predetermined"normal" condition, the program will generate an indication of how extreme the abrasive wear characteristics the particular formation is likely to have. The DBOS program then calculates an indication of the need for enhanced gauge protection based on the abrasion indicator and/or bit dull gauge wear conditions from any offset well data. In this column three increasing levels of abrasiveness are defined: 1. Soft formation-Standard (no enhancement needed), soft'OD' (diamond enhanced heel row applied to aggressive soft formation bits), D/E Chisel (diamond inclined gauge chisel).
2. Medium formation-'D'feature (diamond Semi Round Top's-SRT-in the gauge row position),'DOD' (diamond SRT's in both gauge and heel row positions).
3. Hard Formation-'OD'feature (diamond SRT's again in the heel row position-for hard formation bits).
Hydraulics/Nozzling. The hydraulics column indicates jet nozzle requirements to maintain proper bit cleaning, assuming adequate mud flow rates and hydraulic horsepower. Based on lithology normalized (LN) porosity and/or acoustic velocity and shale content analysis, indicators are presented for the following jet types: 1. Standard (no enhancement).
2. Crossflow (asymmetric flow patterns).
3. Center jetting.
4. Extended Nozzles A Fixed Cutter Bit Selector ("FCBS"), similar to the Rock Bit Selector previously described, uses the formation characterization, rock strength and offset dull bit condition to assess the aspects of PDC/natural diamond bit design appropriate for the application. Again a well log plot form is generated, showing: Source Data from the offset well as previously described.
Blade/Body Design to evaluate the"architectural"aspects of the bit head.
Density : either or both cutter density and blade count as a quantitative function of rock strength. Density ranges from light (3-4 blades) to heavy (12 or more blades).
Bit profile, evaluates both PDC and Natural Diamond bits. A statistical histogram, for a certain depth interval, indicates the optimum survivable profile.
Hydraulic Design or blade"architecture", refers to the height of the blade above the body or relative openness of the bit face. Hole cleaning is determined considering shale porosity derived from the log normal porosity curve. If no porosity data is available then shale volume and/or acoustic velocity analysis can be used.
Cutter size determinations, based on a combination of dominant rock type and acoustic velocity. Optimal PDC cutter sizes can be evaluated statistically across the interval of interest, ranging from 19 mm (3/4") for softer formations to natural diamond stone sizes (5-6 stones per carat) to impregnated bits for harder formations. Statistical distributions that overlap cutter sizes would justify multiple cutter bit designs.
Abrasion, as introduced above, based on combinations of sand content and rock strength.
The DBOS program determines a sand content with respect to the rock strength at each foot as a "normal"abrasive condition. If the actual sand content exceeds this condition, the program calculates how excessive or abrasive the formation is likely to be given such sand content. The data are presented in a well log plot format and, indicating increasing levels of abrasivity, recommend for premium abrasive resistant cutters where appropriate.
Impact resistance, measuring the rate of change in rock strength for two consecutive depth levels. In a manner similar to the calculation for abrasivity, the program determines how much the impact excess is likely to be generated by the particular formation. The data are presented in well log plot format just as for the abrasion plot. This output indicates where premium impact resistant cutters are indicated and/or where bit vibration reducing systems are advisable for the intended application.
Gauge Protection, as a continuous function of formation abrasivity and/or offset bit gauge wear condition. The data, presented in well log plot format, indicate heavy gauge protection requirements and suitable gauge pad technology where appropriate.
A methodology to improve bit selection using direct measurements of compressive strength made on drill cuttings as input data in a drill bit optimization system has been developed. Some possible advantages can be pointed out considering a direct measure of formation compressive strength instead of an estimated one: 1. The testing conditions are extremely fast and simple but sufficiently reliable to correctly determine rock strength.
2. Portable equipment exists to run this test, allowing the possibility of applying the whole methodology as a real time tool to adjust the predicted drilling plan.
3. Continuous information can be recovered along the whole wellbore.
4. It is an inexpensive methodology.
5. The possibility of directly measuring rock compressive strength at the drilling rig-site can allow the selection of more aggressive cutting structures compared to those generally used in earth formations, thereby improving drilling performance.
Those skilled in the art will appreciate that the foregoing description is only one embodiment of the invention, and that other embodiments of the invention can be readily devised which do not depart from the spirit of the invention. Accordingly, the invention should be limited in scope only by the attached claims.
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ISMR/SPE Symp., pp. 85-92,1989.
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