Boring or penetrating the earth – Automatic control – Of fluid pressure below ground
Reexamination Certificate
2002-03-07
2004-06-29
Bagnell, David (Department: 3672)
Boring or penetrating the earth
Automatic control
Of fluid pressure below ground
C175S038000, C175S048000
Reexamination Certificate
active
06755261
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to drilling subterranean well bores of the type commonly used for recovery of oil or gas. More particularly, this invention relates to an improved method and system for automatically reducing the well fluid circulation rate while circulating a kick out of a well in response to one or more of a plurality of monitored well or surface equipment conditions relating to the fluid circulation system.
BACKGROUND OF THE INVENTION
Drilling subterranean wells typically includes circulating a drilling fluid (“mud”) through a fluid circulation system. The circulation system typically includes a drilling rig and mud treating equipment located substantially at or near the surface of the well. The drilling fluid may be pumped by a mud pump through the interior passage of a drill string, through a drill bit and back to the surface through the annulus between the well bore and the drill pipe.
A primary function of drilling mud is to maintain hydrostatic fluid pressure control of fluids in the formations penetrated by the well bore. Weighting agents may be added to a mud to achieve the desired mud density. Overbalanced drilling techniques typically practice maintaining a hydrostatic fluid pressure in the well bore and on the formation equal to or slightly overbalanced with respect to formation fluid pressure (“pore pressure”), both when circulating and when not circulating the mud. In underbalanced drilling techniques, hydrostatic pressure in the well bore is maintained at least slightly lower than formation pore pressure by the mud, supplemented with surface well control equipment. If the well bore encounters a zone having a substantially higher pore pressure than the hydrostatic fluid pressure in the mud, an influx of formation fluid may be introduced into the well bore. Such occurrence is known as taking a “kick.”
When a kick is taken, the invading formation liquid and/or gas may “cut” the density of the drilling fluid in the well bore annulus, such that as more formation fluid enters the well bore, hydrostatic control of the well bore may be lost. Such occurrence may be noted at the drilling rig in the form of a change in pressure in the well bore annulus, changes in mud density, and/or a gain in drilling fluid volume in the mud system tanks (“pit volume”). When a kick is detected or suspected, mud circulation is conventionally halted and the well bore closed in/shut in to measure the pressure buildup in the well bore annulus, pit gain and shut in drill pipe pressure. Appropriate well-killing calculations may also be performed while the well is closed in. Thereafter, a known well killing procedure may be followed to circulate the kick out of the well bore, circulate an appropriately weighed mud (“kill mud”) into the well bore, and ensure that well control has been safely regained. When a circulated kick enters long, narrow, and/or restrictive choke lines, such as may be encountered with a deepwater floating rig, the operator may anticipate this condition and briefly shut in the well.
Typically, the intent of the operator while circulating a kick out of a well is to hold pump rate constant at a normal or high rate, and only change the pump rate if an excessive or undesirable condition arises. It is common practice during the course of drilling the well bore to frequently measure and record the slower mud pump rates and corresponding pump circulation pressures required to circulate the mud. These slow mud pump rates, e.g., about one-half to one-third of the normal circulation rate while circulating a kick out of a well, may be used to more slowly and carefully circulate the kick out of the well bore. The cumulative number of pump strokes from a mud pump or a plurality of mud pumps required to circulate the hole may thus be measured or assumed, and is generally known to the well operator.
One of the most common techniques for killing the well and circulating an appropriate kill fluid is the “constant bottom hole pressure” method, whereby bottom hole pressure may be maintained substantially at or above formation pore pressure. Two variations of this method exist: the Driller's method and the Wait and Weight method. The Driller's method may be utilized when kill weight fluid is not yet available for circulation. In the Driller's method, the original mud weight may be used to circulate the contaminating fluids from the well bore. Thereafter, kill weight mud (“KWM”) may be circulated into the drill pipe and the well bore. Although two circulations may be required to effectuate the Driller's method, this method may be quicker than the subsequently discussed variation.
In the Wait and Weight or “Engineers” method, KWM is prepared and then circulated down the drill string and into the well bore to remove the contaminating fluids from the well bore and to kill the well, in one circulation. This method may be preferable to maintain the lowest casing pressure during circulating the kick from the well bore and may thereby minimize the risk of damaging the casing or fracturing the formation and creating an underground blowout.
A substantially constant bottom hole pressure may be maintained in both methods. In both methods, pressure within the casing and/or drill pipe may be controlled by adjusting a choke conducting mud from the well to a mud reservoir. To further control pressure, the mud pump rate may be maintained at one of the previously measured or assumed low or reduced circulation rates. In the Driller's method, a constant drill pipe pressure may be maintained during the first circulation, which may include the shut in drill pipe pressure (“SIDPP”) plus the slow rate pump pressure, plus a nominal safety factor, e.g., fifty psig. During the second circulation, the casing pressure may be held constant while the KWM is circulated to the bit, and then the drill pipe pressure held constant while the KWM is circulated from the bit to the surface. In the Wait and Weight method, a substantially constant bottom hole pressure may be maintained during the one circulation of KWM. KWM may be circulated down the drill string while maintaining drill pipe pressure at a calculated, predetermined pressure while the mud pump is maintained at a constant rate. The drill pipe pressure may gradually decrease as KWM is circulated to the bit. After KWM reaches the bit, the drill pipe pressure may be held constant until the KWM reaches the surface. A combination method is known which may combine portions of each of the above two methods. After the well is shut in and the pressures recorded, pumping of original weight mud may begin while the original weight mud is being weighted up to KWM, as the kick is being pumped out of the well bore.
Each of the above methods may be time consuming and may require extensive planning, calculations, monitoring, human intervention and/or coordinated regulation of components, rates and pressures during execution of the respective method. In addition, each method preferably uses a substantially constant pump rate (reduced circulation rate) in order to maintain control of the process during execution of the respective method, hopefully while maintaining a substantially constant bottom hole pressure. The Wait and Weight method also may require constructing a graphical or tabular pumping schedule of pump pressure versus volume pumped. In the event it becomes necessary to change pumping rates and/or interrupt pumping while executing the procedure to circulate the kick out of the well, it frequently may be necessary to record new shut in and circulating pressures, and recalculate a new pumping and/or pressure schedule. While circulating a kick out of the well, it is common for the drilling operator to monitor a plurality of signals relating to the fluid circulation system and, in response to one or more of those signals exceeding on an acceptable limit, the operator shuts the well back in and restarts the procedure of circulating the kick out of the well.
Following completion of the kill procedure, new pressure readings
Bagnell David
Browning & Bushman P.C.
Smith Matthew J
VARCO I/P, Inc.
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