Measuring and testing – Borehole or drilling – During drilling
Patent
1994-11-21
1996-09-03
Williams, Hezron E.
Measuring and testing
Borehole or drilling
During drilling
731515, G01B 518, G06F 1715, E21B 4500
Patent
active
055512860
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of determining the rate of penetration (ROP) of a drill bit from measurements made at the surface while drilling.
In the rotary drilling of wells such as hydrocarbon wells, a drill bit is located at the end of a drill string formed from a number of hollow drill pipes attached end to end which is rotated so as to cause the bit to drill into the formation under the applied weight of the drill string. The drill string is suspended from a hook and as the bit penetrates the formation, the hook is lowered so as to allow the drill string to descend further into the well. The ROP has been found to be a useful parameter for measuring the drilling operation and provides information about the formation being drilled and the state of the bit being used. Traditionally, ROP has been measured by monitoring the rate at which the drill string is lowered into the well at the surface. However, as the drill string, which is formed of steel pipes, is relatively long the elasticity or compliance of the string can mean that the actual ROP of the bit is considerably different to the rate at which the string is lowered into the hole. The errors which can be caused by this effect become progressively larger as the well becomes deeper and the string longer, especially if the well is deviated when increased friction between the string and the borehole wall can be encountered.
Certain techniques have been proposed to overcome these potential problems. In U.S. Pat. No. 2,688,871 and U.S. Pat. No. 3,777,560 the drill string is considered as a spring and:the elasticity of the string is calculated theoretically from the length of the drill string and the Young's modulus of the pipe used to form the string. This information is then used to calculate ROP from the load applied at the hook suspending the drill string and the rate at which the string is lowered into the well. These methods suffer from the problem that no account is taken of the friction encountered by the drill string as a result of contact with the wall of the well. FR 2038700 proposes a method to overcome this problem in which the modulus of elasticity is measured in situ. This is achieved by determining the variations in tension to which the drill string is subjected as the bit goes down the well until it touches the bottom. Since it is difficult to determine exactly when the bit touches the bottom from surface measurements, strain gauges are provided near the bit and a telemetry system is required to relay the information to the surface. This method still does not provide measurements when drilling is taking place and so is inaccurate as well as difficult to implement.
By contrast, in FR 2,165,851 (AU 44,424/72) there is employed a mathematical model describing the drill bit cutting rate--the model necessitates a knowledge of the drill depth, the drill rotational speed, and the weight on bit, and its use involves the application of a Kalman-Bucy filter--to derive an ROP value. This method suffers from the obvious problems of having to know what is really going on at the bit, and the model utilised applies only to roller cone bits. The later GB 2,129,141 A tries to deal with the problem in a related way, applying Kalman filtering to a model that treats the drillstring as an elastic cable, and provides a downhole bit-acceleration measurement device (together with a "motionless tool" sensor necessary for correcting certain errors in depth measurement). Though quite useful, this method, like that of the aforementioned FR 2,165,811, suffers from its requirement for knowledge of downhole conditions.
A simpler method is proposed in U.S. Pat. No. 4,843,875 (incorporated herein by reference) in which ROP is measured from surface measurements while drilling is taking place. This method uses the following model: .LAMBDA. is the drill string compliance and h is the axial force at the surface (.DELTA. is the difference operator taken over some time interval .tau.). Using the assumptions that over any time interval .tau.' (typically 5 minutes) d
REFERENCES:
patent: 2688871 (1954-09-01), Lubinski
patent: 3777560 (1973-12-01), Guignard
patent: 4843875 (1989-07-01), Kerbart
patent: 4928521 (1990-05-01), Jardine
patent: 5398546 (1995-03-01), Jeffryes
Kanak Wayne I.
Lee Peter Y.
Schlumberger Technology Corporation
Wiggins J. David
Williams Hezron E.
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