Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-01-14
2003-12-09
Barlow, John (Department: 2853)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C702S009000, C175S303000
Reexamination Certificate
active
06662110
ABSTRACT:
BACKGROUND OF INVENTION
Field of the Invention
The present invention generally concerns apparatus and systems for drilling wells, such as for production of petroleum products and more specifically concerns methods and systems for ensuring efficient well drilling and protection of well drilling systems during drilling operations. More particularly, the present invention concerns a closed loop control system for drilling rig controls, which is responsive to downhole measurement by drilling tools. The measured downhole data is transmitted by measurement while drilling (MWD) telemetry to a digitally controlled switching control regulator (SCR) module via an interfacing computer and utilized to refine the drilling controls by automated correction of driller inputs to the drilling controls of the well drilling system.
For production of petroleum products, such as crude oil, natural gas and mixtures thereof from subsurface reservoirs boreholes are drilled in the earth from the surface to one or more subsurface petroleum bearing zones, typically by rotating a drill bit against the formation. The drill bit may be rotated against the formation by a rotary table or top drive of a drilling rig via multiple interconnected lengths or stands of drill stem to which the drill bit is connected. Alternatively, the drill bit may be driven by a downhole motor, typically referred to as a “mud motor” which is connected to the drill stem or to coiled tubing and which has a rotary drive shaft to which the drill bit is connected. Regardless of the character of the drilling system, the drill stem or coiled tubing defines a flow passage through which drilling fluid, typically referred to as “drilling mud,” is pumped. The drilling fluid is typically a weighted slurry which, even in absence of pump pressure, develops sufficient bottom hole pressure to overcome formation pressure and prevent well blowout in the event a pressurized subsurface pocket is encountered by the drill bit.
A well drilling device, which is typically referred to as a “drilling rig,” for drilling with interconnected lengths of drill stem, is provided with a controllable drill stem handling apparatus including a crown block and a traveling block each having multiple sheaves about which wire cable is laced. The traveling block is typically provided with a hook which typically has supporting engagement with the bail of a swivel apparatus which permits rotation of the drill stem or a rotary table driven kelly to which the drill stem is connected and provides a fluid inlet through which drilling fluid is pumped into the drill stem by one or more mud pumps. The wire cable is fed from a storage spool of a drilling rig drawworks to the sheaves of the crown block and traveling block and provides for supporting, controllably lowering or raising the traveling block and thus the drill stem to thus control engagement of the drill bit against the formation as the drill bit is rotated during drilling. Alternatively, where rotation of the drill stem is accomplished by a top drive system, the top drive mechanism and the swivel assembly are supported, lowered and raised by the hook of the traveling block.
Personnel accomplishing actuating control of the drilling rig is typically an experienced person known as the “driller”. During most phases of rig operation the driller is stationed at a control console which is equipped with a display or multiple displays identifying the various important parameters of the well drilling operation. The wire cable storage spool of the drawworks typically incorporates a brake which is controlled by the driller or by a software program commanded by the driller, permitting controlled payout of wire cable from the spool and thus permitting controlled weight actuated descent of the traveling block and drill stem for controlled penetration of the drill bit into the formation.
As the true objective of rig controls is to achieve a particular set of drilling parameters downhole and at the bit, if the actual measurements of the downhole drilling parameters are not available, one has to compute their values from the surface measurements only. A typical case is to compute the Downhole Weight On Bit (DWOB) from the total weight suspended to the Derrick (Hook load), by subtracting the weight of the pipes, which are suspended in Tension (Wt). This calculated weight on bit is commonly called Surface Weight On Bit (SWOB). Hookload and SWOB are basically related by the following equation:
SWOB=Hookload−(Wt) (1)
The difference is equal to the sum of all the pipes or drill collars, which are below the neutral point of tension/compression (usually the drill collars).
Immediately, some complications become apparent, which can be alleviated by downhole measurements:
Effect of Inclination:The pipes, which are not in tension only, contribute to DWOB through the component of their weight, which is aligned with the borehole, not by their absolute weight. Hence a first complication of the equation:
SWOB=Hookload(Wt)×Cosines(Inclination) (2)
Effect of Flotation in Drilling Mud:
The drill string is immersed in the drilling mud, which has a significant density (pMud), resulting in a flotation force proportional to the weight of fluid displaced by the immerged part of the drill string. Hence a second complication of the equation, with Vstring of the immerged part of the drill string
SWOB=(Hookload(Wt)×Cosines(Inclination))pMud×
V
string (3)
This being a first approximation, given to illustrate the actual complexity of the problem, as the floatation force is vertical, and the drill string may be inclined on a significant part of its length, requiring the knowledge of the well profile (Inclination versus depth) for exact calculation.
The Third Effect is Friction of the Drill String against the Borehole (F):
The friction force is opposed to the direction of the displacement. As the driller can move the drill string up and down when the bit is off-bottom, it is possible to have a surface measurement of the friction forces:
Fric
=½(Hookload going up Hookload going down) (4)
This reduces the actual weight on bit, and can be accounted for in the calculation of SWOB:
SWOB=(Hookload(Wt)×Cosines(Inclination))pMud×
V
string Fric (5)
As drilling of a well progresses, the friction forces can change for several reasons:
inclination changes, coefficient of friction changing as new formations are cut or as the borehole degrades, packing of debris around the drill string, friction of stabilizers increasing when the hole size decreases as the drill bit wears down or when the borehole collapses. The only way to actualize Fric, if no downhole measurements are available, is to stop drilling and repeat the up and down motion to obtain a new value of the difference. Since this activity results in interruption of the drilling process, it is not done frequently. Whereas, the Hook load is constantly adjusted manually by the driller when drilling a 90 ft stand, generally, the up and down motions only occur when connecting a new 90 ft stand. The estimation of Friction is therefore established at each connection, however, thereafter assumed constant when drilling the next 90 ft section.
One can readily identify a number of scenarios where a driller's manual control input based on experience will fail to accomplish the desired result:
Scenario
1
: Stabilizer Hanging Up
If one stabilizer of the drill string is hanging up, the weight of the drill string is not transmitted to the drill bit, and lowering the block (traveling block hook supporting the drill string) to achieve a constant rate of penetration (ROP) will not have the desired effect. In reality, it can cause damage to the drill string by buckling and other consequences of overload.
Scenario
2
: Sudden Reduction in Formation Strength Due to Pressure Imbalance Between Mud and Formation, or Properties of Rock Geomechanics
If the driller maintains the same SWOB command setting, the ROP wil
Bargach Saad
Hache Jean-Michel
Andrews & Kuruth
Curington Tim
Jeffery Brigitte C.
Schlumberger Technology Corporation
Taylor Victor J.
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