Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N3/40—Investigating hardness or rebound hardness
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/003—Generation of the force
  • G01N2203/0053—Cutting or drilling tools
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/02—Details not specific for a particular testing method
  • G01N2203/022—Environment of the test
  • G01N2203/0244—Tests performed "in situ" or after "in situ" use

Definitions

• the invention relates generally to the field of wellbore drilling More specifically, the invention relates to methods for selecting the type of drill bit used, and the drilling parameters used to drill a wellbore so as to optimize the overall drilling performance
• 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 "d ⁇ llability” and other drilling performance parameters
• the d ⁇ llability 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 thological 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
• 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
• 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
• 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
• 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
• 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 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 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.
• 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.
• 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 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.
• 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.
• 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 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.
• 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.
• 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.
• 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
• ROP rate of penetration
• WOB weight on bit
• RPM drill bit rotary speed
• 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.
• the lithology column is generated and unconfined rock compressive strengths (UCS) are calculated.
• UCS unconfined rock compressive strengths
• 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, d ⁇ llability 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:
• 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
• 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:
• FCBS Fixed Cutter Bit Selector
• 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).
• Hexdraulic 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.
• 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.

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• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
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• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
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• Devices For Checking Fares Or Tickets At Control Points (AREA)
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Cited By (18)

Citations (1)

• 1999
  • 1999-10-04 IT IT1999MI002066A patent/IT1313324B1/it active
• 2000
  • 2000-10-04 WO PCT/EP2000/009710 patent/WO2001025597A1/en active Application Filing
  • 2000-10-04 AU AU11332/01A patent/AU1133201A/en not_active Abandoned

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