Boring or penetrating the earth – Automatic control – Of advance or applied tool weight
Reexamination Certificate
1999-09-17
2001-09-25
Schoeppel, Roger (Department: 3671)
Boring or penetrating the earth
Automatic control
Of advance or applied tool weight
C175S057000, C175S094000
Reexamination Certificate
active
06293356
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to earth boring and drilling, and more particularly to a method of and system for optimizing the rate of penetration in drilling operations.
DESCRIPTION OF THE PRIOR ART
It is very expensive to drill bore holes in the earth such as those made in connection with oil and gas wells. Oil and gas bearing formations are typically located thousands of feet below the surface of the earth. Accordingly, thousands of feet of rock must be drilled through in order to reach the producing formations.
The cost of drilling a well is primarily time dependent. Accordingly, the faster the desired penetration depth is achieved, the lower the cost in completing the well.
While many operations are required to drill and complete a well, perhaps the most important is the actual drilling of the bore hole. In order to achieve the optimum time of completion of a well, it is necessary to drill at the optimum rate of penetration. Rate of penetration depends on many factors, but a primary factor is weight on bit. As disclosed, for example in Millheim, et al., U.S. Pat. No. 4,535,972, rate of penetration increases with increasing weight on bit until a certain weight on bit is reached and then decreases with further weight on bit. Thus, there is generally a particular weight on bit that will achieve a maximum rate of penetration.
Drill bit manufacturers provide information with their bits on the recommended optimum weight on bit. However, the rate of penetration depends on many factors in addition to weight on bit. For example, the rate of penetration depends upon characteristics of the formation being drilled, the speed of rotation of the drill bit, and the rate of flow of the drilling fluid. Because of the complex nature of drilling, a weight on bit that is optimum for one set of conditions may not be optimum for another set of conditions.
One method for determining an optimum rate of penetration for a particular set of conditions is known as the “drill off test”, disclosed, for example, in Bourdon, U.S. Pat. No. 4,886,129. In a drill off test, an amount of weight greater than the expected optimum weight on bit is applied to the bit. As the drill string is lowered into the borehole, the entire weight of the drill string is supported by the hook. The drill string is somewhat elastic and it stretches under its own weight. When the bit contacts the bottom of the borehole, weight is transferred from the hook to the bit and the amount of drill string stretch is reduced. While holding the drill string against vertical motion at the surface, the drill bit is rotated at the desired rotation rate and with the fluid pumps at the desired pressure. As the bit is rotated, the bit penetrates the formation. Since the drill string is held against vertical motion at the surface, weight is transfer from the bit to the hook as the bit penetrates the formation. By the application of Hooke's law, as disclosed in Lubinsky U.S. Pat. No. 2,688,871, the instantaneous rate of penetration may be calculated from the instantaneous rate of change of weight on bit. By plotting bit rate of penetration against weight on bit during the drill off test, the optimum weight on bit can be determined. After the drill off test, the driller attempts to maintain the weight on bit at that optimum value.
A problem with using a drill off test to determine an optimum weight on bit is that the drill off test produces a static weight on bit value that is valid only for the particular set of conditions experienced during the test. Drilling conditions are complex and dynamic. Over the course of time, conditions change. As conditions change, the weight on bit determined in the drill off test may no longer be optimum.
It is therefore an object of the present invention to provide a method and system for determining dynamically and in real time an optimum weight on bit to achieve an optimum rate of penetration for a particular set of conditions.
SUMMARY OF THE INVENTION
The present invention provides a method of and system for optimizing bit rate of penetration while drilling. The method of the present invention substantially continuously determines an optimum weight on bit necessary to achieve an optimum bit rate of penetration for the current drilling environment and maintains weight on bit at the optimum weight on bit. As the drilling environment changes while drilling, the method updates the determination of optimum weight on bit.
The method of the present invention determines the optimum weight on bit to achieve the optimum bit rate of penetration by building a mathematical model of bit rate of penetration as a function of weight on bit. As long as actual bit rates of penetration fit the mathematical model, the mathematical model validly represents the conditions. Whenever the actual bit rates of penetration do not fit the model, conditions have changed. When the method detects a change in conditions, the method fetches an updated mathematical model and computes an updated optimum weight on bit based upon the updated mathematical model.
In one of its aspects, the method of the present invention maintains the weight on bit at the optimum by displaying a currently determined weight on bit and the optimum weight on bit to a human driller. The human driller maintains optimum weight on bit by matching the displayed currently determined weight on bit to the displayed optimum weight on bit. In another of its aspects, the method of the present invention maintains optimum weight on bit by inputting the currently determined weight on bit and the optimum weight on bit to an automatic drilling machine.
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King Charles H.
Pinckard Mitchell D.
Noble Drilling Services Inc.
Pillsbury Madison & Sutro LLP
Schoeppel Roger
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