The maximum available weight on bit (WOB) for a tangential horizontal well is planned with the drill bit and BHA in the horizontal section, which is the important case. Let the horizontal plan which has been solved in a previous example be as shown in the sketch. Let the drillpipe be composed of 8,000 feet of
5"(0D) by 4.276"(ID) drillpipe with an average weight of 21.6 ppf and the BHA be 90 feet with average diameters of 7" by 2.5"( 114 lb/ft). Mud weight is 11.0 ppg.
The gravity weight of the drillpipe or any other section of the drill string is its buoyed weight times its true vertical length.
For the above drill string, the gravity weight of the drillpipe is
Gr. Wt. = (1 - * 21.6 * (5000 + 1,204.04) = 111,484 lb
The gravity weight of the drillpipe below and including the tangent section and BHA is
DRAG (AXIAL FRICTION)
Axial friction forces may be precipitated by three factors; the buoyed weight of the drill string lying on the low side of the drill hole; the bending of the drill string in a dogleg; and the packing of the drill string in a cuttings bed. The "hanging up" of a component of the drill string on a ledge or other unique obstruction would not be continuously present while drilling and is therefore not thought of as friction. In the following equation the V sign is used if the dogleg is in the shape of an arch and the sign is used if the dogleg is sagging.
The equation for frictional drag is gravity dogleg packing
■W , /DLS * LOS\ , Ff = L|_|I * Bf * Ws * DL ± ji * 2 * T * sin(-%-+ BPJi
Ff = axial friction force; lb
Bf = buoyancy factor; lb/lb
Ws = air weight per foot of a section of the drill string; lb/ft
DL = departure length of a section of the drill string; ft
T = axial tension in the section; lb
DLS = dogleg severity in which the section lies; deg/lOOft
LOS = length of the section in the dogleg; ft
BP = axial packing force of a cuttings bed; lb
\l = coefficient of friction between the drill string and hole; lb/lb i = index representing each section of the drill string
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