Boring or penetrating the earth – With signaling – indicating – testing or measuring – Measuring or indicating drilling fluid rate of flow
Reexamination Certificate
2002-02-19
2004-01-13
Dang, Hoang (Department: 3672)
Boring or penetrating the earth
With signaling, indicating, testing or measuring
Measuring or indicating drilling fluid rate of flow
C073S152220, C073S152270, C166S250070, C175S050000
Reexamination Certificate
active
06675914
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD OF THE INVENTION
The invention relates to a method and apparatus for the removal of undesirable materials on the wall of an earth formation so as to allow the measurement of formation characteristics such as pressure. More particularly, the invention relates to a device that creates a wave discharge by pulsing a volume of fluid so as to produce a resonant oscillation in the fluid. The wave discharge is directed in the form of a concentrated beam against at least partially non-permeable membranes formed on the earth wall of a borehole in order to remove these materials from the wall of the borehole. Still more particularly, the described device creates oscillations that produce the wave discharge by using a Helmholtz resonance frequency in pulsing a fluid volume. The wave discharge will disintegrate mudcake formed on the earth formation borehole wall to allow the unobstructed measurement of formation pressure within the formation.
BACKGROUND OF THE INVENTION
The efficient recovery of subterranean hydrocarbons such as oil and gas is assisted by obtaining reliable data about the physical conditions in a formation of interest. For example, a target formation typically includes hydrocarbon fluids that are under high pressure. Accurately measuring the formation pressure where such pressurized materials reside promotes safe and cost-effective operations in nearly all phases of hydrocarbon recovery. However, techniques for measuring formation pressure must overcome a number of technical challenges. One obstacle to pressure measurement is the mudcake that drilling mud tends to deposit on the wall of the wellbore.
A wellbore is typically filled with a drilling fluid such as water or a water-based or oil-based drilling fluid. The density of the drilling fluid is usually increased by adding certain types of solids that are suspended in solution. Drilling fluids containing solids are often referred to as drilling muds. The drilling fluids cool and lubricate the drill bit and carry the cuttings uphole to the surface. The solids in drilling fluids also increase the hydrostatic pressure of the wellbore fluids. By selecting drilling fluids weighted to a particular density, the column of drilling fluids creates a pressure downhole, which is greater than the pressure of the fluids in the formation. When the drilling fluid pressure is greater than the formation fluid pressure, the well is said to be in an over balanced condition. Conversely, if the formation pressure is greater than the fluid column, then the well is said to be in an under balanced condition. Control of formation fluids flowing into the well under high pressure minimizes the risk of a well blowout.
While an over balanced condition prevents well blowouts, it also has disadvantages, such as increased drilling costs due to slower penetration into the formation. Drilling fluid pressure in excess of formation pressure slows the penetration of the drill bit into the formation. In certain well environments it is preferred to maintain a neutral or slightly under balanced condition so as to achieve drilling speeds faster than those achieved while drilling in an over balanced condition. Drilling Practices Manual, Preston Moore, P. 18-22 Pennwell Publishing, 1974. Consequently, it is desirable to maintain a neutral balance or a slightly under balanced condition to maximize drilling penetration into the formation.
Drilling fluids create a mudcake as they flow into a formation by depositing solids on the inner wall of the wellbore. The mudcake on the wall of the wellbore tends to act like a filter and tends to isolate the high-pressure fluids of the wellbore from the relatively lower pressures of the formation. The mudcake helps prevent excessive loss of drilling fluid into the formation. The static pressure in the wellbore and the surrounding formation is typically referred to as hydrostatic pressure. Pressure in the formation beyond the mudcake gradually tapers off with increasing radial distance outward from the wellbore.
The measurement of formation pressures during drilling operations assists in locating strata most likely to produce hydrocarbons efficiently. Typically after the borehole is drilled, the well is logged by lowering a package of sensors downhole that gather data about the formation. Pressure data is useful in judging when a formation contains hydrocarbons and when such a formation may economically produce hydrocarbons. Often a wellbore may pass through more than one hydrocarbon-bearing formation, and formation pressure data assists the drilling engineer in determining whether to halt or continue drilling.
Further, the ability to monitor formation pressure during drilling is important to the desired practice of continuously adjusting the drilling mud density. This facilitates drilling through the maximum amount of formation in the shortest amount of time.
To maintain the proper condition during drilling, whether neutral, over balanced or under balanced, it is necessary to measure the pressure of the formation fluids at the vicinity of the drill bit. However, the dynamic environment near the drill bit makes measurement of the formation fluids particularly difficult during logging while drilling (LWD) operations. In addition, the mudcake that forms on the wall of the borehole presents a further difficulty in determining formation fluid pressure at the bit during drilling. This mudcake forms a relatively non-permeable barrier between the instrument on the one side and the formation fluids on the other. The mudcake barrier hinders accurate measurement of the pressure of the formation fluids.
Prior art sensors are generally not capable of measuring formation fluid pressure during drilling. Consequently, rig personnel must closely monitor the drilling fluids flowing from the borehole for signs of increased formation fluid pressure. This often entails temporarily halting the drilling operation to allow pressure measurement of the formation. Once the drilling fluids show evidence of formation fluids flowing up the borehole, drilling is stopped and corrective measures are taken. However, this approach has particular drawbacks; and, it would be desirable to determine formation fluid pressure at the bit during drilling.
One such prior art instrument is a reservoir description tool (RDT) such as that disclosed in U.S. Pat. No. 5,644,076 (the '076 patent) entitled “Wireline Formation Tester Supercharge Correction Method”, incorporated herein by reference in its entirety. The RDT of the '076 patent includes a pressure sensing element mounted within a chamber of a housing having a piston to create a vacuum within the housing chamber. Hydraulic pads force the housing against the borehole wall; and, as the piston retracts to create a pressure reduction, a drawdown pressure removes the mudcake lining from the borehole wall. Fluids in the formation then enter the housing chamber allowing the pressure-sensing element to take a pressure reading. This tool allows only stationary measurements because drawdown pressure requires a tight seal between the housing and the borehole wall. This is undesirable because, aside from being time consuming, stationary measurements provide only discrete data points, not a continuous log. The drawback to discrete data points is that the fluid pressure between the discrete data points may vary dramatically and unpredictably.
Another borehole tool for removing the mudcake to measure the pressure of the formation fluids is disclosed in U.S. Pat. No. 5,969,241 (the '241 borehole tool) incorporated herein by reference. The '241 borehole tool measures pressure from within the borehole. A portion of the borehole wall is isolated from the surrounding borehole fluids by placing the chamber of the '241 borehole tool against the borehole wall. The chamber comprises a recess in an exterior surface of the '241 borehole tool. This patent describes an acousti
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