Boring or penetrating the earth – Automatic control – Of fluid pressure below ground
Reexamination Certificate
2001-01-26
2002-11-26
Bagnell, David (Department: 3672)
Boring or penetrating the earth
Automatic control
Of fluid pressure below ground
C175S038000, C175S048000, C175S066000
Reexamination Certificate
active
06484816
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to drilling subterranean well bores of the type commonly used for oil or gas wells. More particularly, this invention relates to an improved method and system for regaining hydrostatic fluid pressure control of a well bore after the well bore receives an influx of fluid from the formation. The methods and system of this invention may facilitate more timely circulation of the fluid influx out of the wellbore while circulating a more dense fluid into the well bore to regain hydrostatic control of the well bore.
BACKGROUND OF THE INVENTION
Drilling subterranean wells typically includes circulating a drilling fluid (“mud”) through a drilling fluid circulation system (“system”). The circulation system may include a drilling rig and mud treating equipment located substantially at the surface. 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 of the well bore 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. Traditional overbalanced drilling techniques typically practice maintaining a hydrostatic fluid pressure 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 wellbore encounters a zone having a higher pore pressure than the hydrostatic fluid pressure in the mud, an influx of formation fluid may be introduced into the wellbore. Such occurrence is known as taking a “kick.”
In the well bore drilling industry, it is common practice to frequently during the course of drilling the wellbore, measure and record slow mud pump rates and corresponding pump circulation pressures required to circulate the mud at the reduced rate with the mud pumps. Such measurements may be made at such rates as may be used in circulating a kick out of the well bore, e.g., one-half to one-third of the normal circulation rate. Additional determinations may also be performed, including the cumulative number of pump strokes required to circulate the hole.
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 wellbore, hydrostatic control of the wellbore may be lost. Such occurrence may be noted at the drilling rig in the form of a change in pressure in the wellbore annulus, changes in mud density, and/or a gain in drilling fluid volume in the mud system tanks (“pit volume”).
Typically when a kick is detected or suspected, mud circulation is 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.
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 first variation may be known commonly as the “Driller's 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, the driller's method variation may be quicker than the subsequently discussed variation.
The second variation of the Constant Drill Pipe Pressure method may be commonly known as the “wait and weight” method, or the “Engineer's” method. In the “wait and weight” 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 in that this method may 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 either method, pressure on the casing and/or drill pipe may be controlled by adjusting a choke conducting mud from the casing to a mud reservoir. In addition, to further control pressure the mud pump rate may be maintained at one of the previously measured rates and corresponding pressures. 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, pre-determined pressure schedule 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 aforementioned 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 typically uses a substantially constant pump rate in order to maintain control of the process during execution of the respective method. The wait and weight method also may require constructing a graphical or tabular pumping schedule of pump pressure versus volume pumped, to follow during the procedure. Further, in the event it becomes necessary to change pumping rates and/or interrupt pumping during execution of the kill procedure, it frequently may be necessary to record new shut-in pressures, new circulating pressures, and recalculate a new pumping and/or pressure schedule. A key component of each method may be adhering to a substantially constant pump rate during the procedure and maintaining a substantially constant bottom hole pressure.
Typically, the intent of the operator is to hold pump rate constant, and only change the pump rate after circulation has started if some excessive or undesirable condition arises. For example, when a circulated kick enters long, narrow, and/or restrictive choke lines, such as may be encountered with a deepwater floating rig. In anticipation of this, the operator may collect slow circulation data at up to three discrete rates.
Following completion of the kill pro
Bagnell David
Browning & Bushman P.C.
Halford Brian
Martin-Decker Totco, Inc.
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