1、DRILL BIT SELECTION,One of the more confusing aspects of oilwell drilling to the young and old alike is the numerous types of drill bits. The largest manufacturer of drill bits makes 34 types and 33 sizes (diameters) and many more on request.IADC organization has a system of assigning a designation
2、code to each bit type.,ROLLER CONE,Series Type Feature Feature (2),ROLLER CONE,Example Code: 5 2 2 R5 = code for “insert bits“ 2 = code for “soft to medium formation“ 2 = code for “roller bearing air cooled“ R = code for “reinforced welds“,FIXED CUTTERS,Cutter type and body material, Cross-section p
3、rofile, Hydraulic design, and Cutter size and density,FIXED CUTTERS,First code designations are D - natural diamond/matrix body M -polycrystalline diamond cutters/matrix body S - polycrystalline diamond cutters/steel body T - thermally stable polycrystalline diamond cutters/matrix body O - other Sec
4、ond code designations are long taper/deep cone long taper/medium cone long taper/shallow or no cone (parabolic) medium taper/deep cone medium taper/medium cone medium taper/shallow or no cone(rounded) short taper/deep cone (inverted) short taper/medium cone short taper/shallow or no cone (flat),Thir
5、d code designations arebladed/changeable jets bladed/fixed ports bladed/open throat ribbed/changeable jets ribbed/fixed ports ribbed/open throat open face/changeable jets open face/fixed ports open face/open throat alternative codes radial flow cross flow other,Fourth code designations arecutter siz
6、e; large, medium, and small cutter density; light, medium, heavy,Example Code: M 5 8 2M = code for “polycrystalline diamond cutters/matrix body“5 = code for “medium taper/medium cone“8 = code for “fixed ports/open face“2= code for “medium cutter size/light cutter density“,Drill Bit Characteristics,T
7、eeth length and size Number of teeth (indexing) Cone offset Cone bearing type Heel row teeth configuration Gauge configuration Journal shaft angle (how flat or round the bottom hole pattern),The teeth length and size and cone offset affect the ROPThe next three items on the list have a large affect
8、on the bit life. Item 7 affects hole straightness.,Roller cone drill bit characteristics of importance in regard to selection are:,ROCK FAILURE MODELS,Two rock failure models are recognized: the tension and shear,It has been argued that the shear type action is more efficient than the tension model.
9、,DRILL BIT SELECTION CRITERIA,A popular equation for cost comparisons is the total cost per foot equation. If the bit were operated to its optimal life at its optimum weight and speed, then the cost would be the lowest cost per foot for the bit.,Is the bit life if the bit is terminated by Bearing fa
10、ilure, Total tooth wear, or The reduction of its drilling rate to an uneconomical one. p.s. Formation changes often mask the latter optimal life and care must be exercised.,OPTIMAL LIFE,ExampleFollowing performance data is given:ITEM STEEL TOOTH INSERT TOOTHBit Cost $277.88 $1521.32Trip 6:18 hrs/min
11、 6:32 hrs/minRotation Time 10:01 hrs/min 58:42 hrs/minDown Time None NoneBit Footage 180 ft 880.5 ftRig Cost Rate 375 $/hr SameFraction Tooth Wear 1/2 Not ApplicableFraction Bearing Wear 6/8 8/8Average Drilling Rate 18 ft/hr 12.5 ft/hrBit Size 7-7/8 in. 7-7/8 in.Bit Weight 30,000 lbs 40,000 lbsRotar
12、y Speed 85 rpm 60 rpm,The plot shows that had the steel tooth bit been pulled at its optimal life (6 hours and 34 minutes) it would have had a lower cost per foot: $34.05/ft versus $35.47/ft.,Trip times may be taken from the IADC tour sheet or may be taken from the Geolograph unit or some other devi
13、ce. A chart versus depth must be constructed for each rig for ultimate analyses.,TRIP TIME,Example,A bit is run at a depth of 8,000 feet and pulled at a depth of 8,500 feet. How many hours is the trip time for this bit?,Trip T=(5.2+5.7)/2=5.45 hr,From the chart a round trips at 8,000 feet and 8,500
14、feet require 5.2 hours and 5.7 hours, respectively.,OPTIMAL WOB & RPM,The purpose of selecting bit weight and rotary speeds are to1. produce the lowest drilling cost per foot 2. control the direction of the drill hole 3. find overpressured zones 4. satisfy equipment limitations (solids removal, BHA,
15、 etc.) 5. develop a satisfactory cuttings description log,CONTOUR METHOD,The method consists of constructing the previous figure in a two dimensional presentation.,Example,Six insert bits have been run and their performance and total cost per foot data are listed in the table.,BIT # ROT. TIME FOOTAG
16、E TCFHr:Min $/FT1 58:42 734 35.282 39:05 360 51.513 101:21 855 48.994 62:07 614 44.245 47:16 517 41.276 56:33 658 38.14,The optimal WOB and RPM are 40,000 lb and 50 rpm ($35.00/ft),ANALYTICAL METHOD,The same procedure could be used for determining optimal WOB, and RPM for the steel tooth, roller bea
17、ring bit; however, an Galle and Woods analytical method is available for these bits.,DEFINITION OF TERMSBS = Bit diameter; inch W = Actual weight on bit; 1000 pounds W = Equivalent weight on a 7-7/8 inch bit; 1000 pounds TT = Trip time; hrs R = Cost rate of the drilling operation; $/hrs T = Time; hr
18、s G = Bit Cost + Trip Cost; $ TC = Trip cost; R * TT; $ D = Fraction of tooth height worn away; 8ths B = Fraction of bearing life consumed; 8ths DT = Down time; hrs RT = Rotation time; hrs F =Bit footage; ft,Galle and Woods published the following relationships for the rate of tooth wear.,TOOTH WEAR
19、,A= Correlation coefficient for tooth wear and is called the formation abrasiveness factor,Integration of the tooth wear equation gives bit rotation time,Tooth wear Equation,Rotary Speed, N=80,BEARING WEAR,S= correlation coefficient called the drilling fluid factor,Others published the following rel
20、ationship for the rate of bearing wear.,Rotary Speed, N=80,RATE OF PENETRATION,C= correlation coefficient called the formation drillability factor,The values of and are determined from drill-off test and range between.6 1.0.4 1.0p= 1.0 if flat tooth wear (non-sharpening) p= 0.5 if self sharpening,Bi
21、t footage (F) is equal to Rate of Penetration x Rotation Time,Equation of ROP,So far, RT is determined either by setting D=1 (complete bit tooth wear) or B=1 (complete bearing wear) Whichever comes first.,D RT F,D RT F,D RT F,D RT F,D RT F,D RT F,D RT F,Rotary Speed, N=80,The total cost per foot equ
22、ation is,D RT F TCF,D RT F TCF,D RT F TCF,D RT F TCF,D RT F TCF,D RT F TCF,Rotary Speed, N=80,The parameters A, C & S are usually computed with data gathered from other similar drill bits which were run in similar formations, at similar depths, with similar bottom hole assemblies, with similar hydra
23、ulics, with similar muds, and similar rigs.,No direct method for determining the optimal weight and speed is available for the insert bit with journal bearings; however, analyses of several insert bit performances at various weights and speeds near the manufacturers recommended values usually deline
24、ates the optimal values.,MATHEMATICAL CONSTRAINTS,The restrictions are that the life of the bit is ended when:,TCF,D,Dopt,Example Calculate an optimal bit weight and rotary speed for the steel tooth bit in the first problem if from a previous bit the following data were noted.,W = 40,000 lb N = 80 rpm p = 1.0 = 1.0 = 0.7 RT = 10 hr F = 180 ft,and all other data remain as in the first problem.,DULL BIT GRADING,BIT TOOTH WEAR,BEARING WEAR,GAUGE,
