Drag Bit Wear Model
Abstract: A performance model for Polycrystalline Diamond Compact (PDC) bits has been modified to include the wear prediction of individu~l cutters. Wear predictions are based on the geometry of each cutter, the rock type, the forces acting on the cutter, cutter velocity, and cutter temperature. The results of predicted and actual cutter wear for different 8-1/2 in. field worn PDC bit designs are shown. The worn geometry of each cutter was meas~ ured and compared to the model's prediction.Laboratory performance tests we… Show more
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Cited by 9 publications
References 8 publications
Drilling operations became more expensive and complicated day by day due to many reasons affecting directly the daily drilling cost. One of the most effective cost reductions was the fixed cutter bit solution which effectively achieved higher drilling progress and reduces overall well cost. Also, in some cases PDC bit may raise the well cost due to slow down in the penetration rate or stopped drilling to retrieve mechanical parts from the hole due to bit fatigue and followed by extra trips for junking or fishing operations. The present study focuses on most of the factors affecting fixed cutter bit design, drilling parameters that influence the bit cutting structure wear and led to the bit poor progress. All previous PDC bit mathematical models used before for determining the cutters wear value not considered real methodology on rig site to assure cutters wear and gives a proper decision to terminate the PDC bit run. Study investigated four mathematical models had calculated the PDC cutters wear using the influences of the rock strength, rock temperature and mechanical drilling parameters. These models are theoretical and applied within the lab test devices, most of models not achieved significant benefit to use in rig site applications. New model had developed to compute PDC wear value as function of surface torque arises from the friction between the drill string, bit interaction with well bore and rock on bottom. Both resistance created by the string and bit are converted to output data realized by the gauge in front the driller in rig floor and mud logging unit. Analog or digital data reflects the torque obtained from rotating the bottom hole assembly (Drill pipe, string stabilizer and bit) against the wall of hole and formation strength. Mathematical model calculates the theoretical torque for string tool joint, stabilizer and PDC bit cutters allocated in nose up to gauge area. In reality, when BHA rotates off bottom torque created is representing the summation of string tool joint and stabilizer. An additional torque realizes on surface when BHA on bottom and WOB applied then this bit torque value will be compared by model bit torque. This later obtained bit torque percentage that represents the cutters wear value preoperational to cutter height. New mathematical model had created visual analog graphs will help to take decision when stop drilling and terminate the PDC bit run. Field validation test had run in two main concessions in Egypt Western desert and Gulf of Suez with total 6 PDC bit runs using different bit size, type, cutter sizes and different bottom hole assembly (Rotary-steerable). The filed results showed significant correction in cutter wear magnitude between the model calculation and field runs, this error factor less than 10% in hard formation and less than 20% in medium to soft formations. Author set the correction in model and treated in easy way to be used on field by driller and drilling engineers to help them how to determine the PDC bit cutting structure wear value.
Drilling operations became more expensive and complicated day by day due to many reasons affecting directly the daily drilling cost. One of the most effective cost reductions was the fixed cutter bit solution which effectively achieved higher drilling progress and reduces overall well cost. Also, in some cases PDC bit may raise the well cost due to slow down in the penetration rate or stopped drilling to retrieve mechanical parts from the hole due to bit fatigue and followed by extra trips for junking or fishing operations. The present study focuses on most of the factors affecting fixed cutter bit design, drilling parameters that influence the bit cutting structure wear and led to the bit poor progress. All previous PDC bit mathematical models used before for determining the cutters wear value not considered real methodology on rig site to assure cutters wear and gives a proper decision to terminate the PDC bit run. Study investigated four mathematical models had calculated the PDC cutters wear using the influences of the rock strength, rock temperature and mechanical drilling parameters. These models are theoretical and applied within the lab test devices, most of models not achieved significant benefit to use in rig site applications. New model had developed to compute PDC wear value as function of surface torque arises from the friction between the drill string, bit interaction with well bore and rock on bottom. Both resistance created by the string and bit are converted to output data realized by the gauge in front the driller in rig floor and mud logging unit. Analog or digital data reflects the torque obtained from rotating the bottom hole assembly (Drill pipe, string stabilizer and bit) against the wall of hole and formation strength. Mathematical model calculates the theoretical torque for string tool joint, stabilizer and PDC bit cutters allocated in nose up to gauge area. In reality, when BHA rotates off bottom torque created is representing the summation of string tool joint and stabilizer. An additional torque realizes on surface when BHA on bottom and WOB applied then this bit torque value will be compared by model bit torque. This later obtained bit torque percentage that represents the cutters wear value preoperational to cutter height. New mathematical model had created visual analog graphs will help to take decision when stop drilling and terminate the PDC bit run. Field validation test had run in two main concessions in Egypt Western desert and Gulf of Suez with total 6 PDC bit runs using different bit size, type, cutter sizes and different bottom hole assembly (Rotary-steerable). The filed results showed significant correction in cutter wear magnitude between the model calculation and field runs, this error factor less than 10% in hard formation and less than 20% in medium to soft formations. Author set the correction in model and treated in easy way to be used on field by driller and drilling engineers to help them how to determine the PDC bit cutting structure wear value.
A mathematical diagnostic drilling model was derived from the balance of forces acting at the PDC bit cutter. The model combined the torque and the drilling rate equations, cutter's geometry and rock properties. It was verified using the laboratory drilling data from several research reports as 11 as the field drilling data collected by the authors. Based on the drilling model, a new method was developed for the in-situ measurements of the PDC bit condition and for the lithology change detection. In this technique, a diagnostic plot is made by correlating two dimensionless groups containing measured values of torque, weight on bit, rotary speed and penetration rate. Several laboratory and field data (presented in the study) confirmed linearity of such a plot. The diagnostic plot is a unique indicator of the bit-rock interaction and it is independent from the bit operational variables. Moreover, the instantaneous wear of a PDC bit can be computed from coordinates of the straight line points. This method is feasible for a graphical use supplemented with a computer program. The technique was further verified by comparing the predicted and the measured PDC bit wear from the MWD records in the Gulf Coast area. Also provided, were the examples of a correlation between rapid formation changes and discontinuities in the diagnostic plots. The new method contributes to the PDC bit drilling theory. Its importance lies in the MWD software development for the purpose of the in-situ rock detection and the PDC bit evaluation and control. Introduction Polycrystalline Diamond Compact (PDC) bits are a high-tech revival of the earliest bit types, drag bits. By using the state of the art materials, drag bits have been competitively reintroduced into the oil well milling technology. Absence of moving parts and the high wear resistance of synthetic diamonds make a drag bit a long lasting bottomhole tool. The success of the PDC's in the petroleum drilling industry is well documented in the literature. PDC bits are extremely sensitive to formation properties and operating conditions. Recent studies on PDC drilling performance in the harsh environment such as geothermal and hard rock drilling showed an average two-fold increase of penetration rate and bit life as compared to conventional bits, which resulted in cost reduction 10 to 15%. However, in the case where the PDC bit life was reduced 50%, the cost savings were cut in half. Therefore there is a very good reason to improve bit life in any potential application of PDC bits, in order to take advantage of their high penetration rates. Early detection of formation changes and appropriate adjustment of the operational variables are very important measures to save the bit. Such detection can be made possible with the instantaneous drilling data acquisition (MWD) system. Development of the drilling data acquisition systems has been in progress for many years in the petroleum industry. However, without having an appropriate data processing tool (a drilling model), some of the information will become invaluable. To date, a few predictive models have been proposed. Ziaja developed a mathematical model of a single PDC cutter penetration assuming a circular cut and an absence of cutters interaction. The model was designed for a core bit. The single proportionality constant was used to up-scale from the single cutter load and penetration to the weight on bit and drilling rate. Data from one field run of a core bit was used to verify the model. Glowka used experimental data from laboratory drilling in hard rocks and developed a power-function correlation between cutter penetration and stress at the wearflat area. His analytical work also included derivation of the single cutter wear equation as a function of penetration per rotation and footage. P. 137^
SPE Member Introduction "Bit whirl", the backwards whirling motion of polycrystalline diamond compact (PDC) drill bits, has been identified as a significant contributory factor in the premature failure of PDC bits in the field. Two SPE papers have discussed bit PDC bits in the field. Two SPE papers have discussed bit whirl and the testing of various potential solutions, and one described the field testing of PDC bits modified to produce a "low friction gauge". This paper describes the design and subsequent testing in the laboratory and then in the field of a PDC bit designed from scratch to resist backwards whirling motion by incorporating various design features including the "low-friction gauge" concept described by Warren et al (described below as an "Antiwhirl bit"). It details the importance of the magnitude of the lateral-component of the resultant cutting force on the bit and the angular extent of the bearing pads that react with this force, and describes and explains the directional behaviour of Antiwhirl bits in the field. ANTIWHIRL BIT DESIGN General The 8 1/12" (215.9mm) diameter Antiwhirl PDC bit that is the subject of this paper is shown in figures 1 and 2. The cutting forces arising from every PDC cutter on the bit are calculated using a mathematical model, which has itself been validated against laboratory tests. They are then summed vertorially to give the expected weight and torque requirements of the bit, together with the magnitude and direction of the lateral component of the resultant cutting force ("out of balance force"). The actual cutter positions on each bit, measured using a coordinate measuring machine, are input to this model to ensure that the performance of the bit is not compromised by any unfavorable build-up of manufacturing tolerance. In each of the laboratory and field tests described below, this measurement confirmed that the bit used had no cutters significantly out of position. The PDC cutters are arranged so that the out off balance force is directed towards two large unaggressive (or "low friction") bring pads. The absence of any aggressive cutting elements in the region of these pads should ensure that the bit does not start to whirl provided that the resultant force is directed towards them. This idea has been used in gun drilling for many years but was only recently introduced to rock drilling by Warren et al. P. 243
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