Boring or penetrating the earth – Automatic control – Of fluid pressure below ground
Reexamination Certificate
2002-04-17
2003-03-04
Dang, Hoang (Department: 3672)
Boring or penetrating the earth
Automatic control
Of fluid pressure below ground
C175S048000, C175S057000
Reexamination Certificate
active
06527062
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 maintaining bottom hole hydrostatic pressure while making a drill pipe connection. The methods and system of this invention facilitate improving hydrostatic control of a well bore while drilling with a reduced equivalent circulating density (“ECD”).
BACKGROUND OF THE INVENTION
Drilling subterranean wells typically requires circulating a drilling fluid (“mud”) through a drilling fluid circulation system (“system”). The circulation system may include a drilling rig located substantially at the surface. The drilling fluid may be pumped by a mud pump through the interior 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. When the circulated drilling fluid arrives back at the surface, cuttings and other solid contaminants are commonly separated from the circulated drilling fluid such that substantially “uncontaminated” drilling fluid may be recirculated.
A primary function of drilling fluid is to provide hydrostatic well control. Traditional overbalanced drilling techniques practice maintaining a hydrostatic pressure on the formation equal to or slightly overbalanced with respect to formation pore pressure. In underbalanced drilling techniques, hydrostatic pressure is maintained at least slightly lower than formation pore pressure by the drilling fluid supplemented with surface well control equipment providing the well control.
As well depth increases, a change in density of the drilling fluid translates into a more pronounced corresponding change in hydrostatic pressure at the bottom of the well bore. Certain formations penetrated by the well bore at deeper depths may not tolerate significant changes in hydrostatic pressure. Hydrostatic pressure changes may result in either a formation fluid influx into the wellbore (a “kick”) or in the drilling fluid invading or being lost into the formation (“lost circulation”). As a result, density control may become more critical as well depth increases.
Drilling fluid is circulated through the fluid system by applying a circulating pressure to the fluid at the surface to pump the fluid through the system. As drilling fluid is circulated through the system, the fluid encounters a series of friction related pressure drops, the sum of which may be roughly equal to the pump pressure required to circulate the fluid (“circulating pressure”). The circulating friction is primarily due to the dynamic interaction between the fluid and the particular conduits through which the fluid is circulating. The mud pump and bottom hole circulating pressure typically remains substantially constant for a particular set of operating parameters.
While circulating drilling fluid, such as when drilling, the bottom hole hydrostatic pressure exerted on the formation is increased above a non-circulating (“static”) hydrostatic pressure by the amount of friction pressure in the well bore annulus. The resulting bottom hole pressure applied to the formation while circulating drilling fluid may be calculated in terms of an equivalent fluid density, commonly called an equivalent circulating density (“ECD”).
When a drill pipe connection is required, circulation is typically terminated for a few minutes while the connection is being performed. When circulation is terminated, the bottom hole hydrostatic pressure on the formation is reduced by approximately the amount of pressure equal to the friction losses in the well bore annulus between the bit and the surface. To maintain well control while circulation is terminated, the drilling fluid density is typically sufficiently high to maintain hydrostatic control under the static conditions.
Another primary function of drilling fluid is to carry cuttings and solid materials, such as weighting agents, to the surface. To prevent cuttings and solid material entrained within the drilling fluid from falling down hole and sticking the drill pipe when circulation is terminated, one or more agents may be added to the drilling fluid to provide a “gel” strength to the fluid and/or increase fluid viscosity. The gel strength of a drilling fluid is a measure of the ability of the fluid to either suspend cuttings in the fluid or the degree to which the fluid may retard the rate at which the cuttings fall back. When movement of a drilling fluid having some degree of gel strength is stopped, the fluid may require the application of an initial pressure (stress) in excess of a minimum threshold pressure to initiate movement (shear) of the fluid. Such fluid may be referred to as a “non-Neutonian” or “Bingham plastic” fluid. The minimum stress required to initiate movement of a Bingham plastic fluid may be referred to as the Bingham yield pressure. Bingham plastic fluids may also require a higher circulation pressure and may generate higher friction pressure drops, than neutonian fluids, thereby resulting in an increased ECD for the plastic fluids.
When the drill pipe connection is completed, the mud pumps are typically re-engaged to regain circulation. To initiate or “break” circulation throughout the system, a startup circulation pressure may be applied to the fluid by the mud pumps and may be transmitted through the circulation system including the bottom hole formations. In certain well bore conditions, the magnitude of the circulation startup pressure (“startup ECD”) required to reach the Bingham yield pressure may exceed the circulating ECD pressure attributable in part to friction pressure as the fluid begins to circulate. Thereby, initiation of circulation of a non-neutonian fluid may have to be conducted slowly to avoid the startup ECD exceeding the ECD. Care may be required during startup and during circulation to avoid the ECD exceeding either or both the pore pressure in the formation and the fracture pressure of the formation matrix, which may result in drilling fluid circulation being partially or completely lost to the formation. Loss of circulation may result in loss of well control, loss of expensive drilling fluids, stuck drill pipe, or other related adverse consequences. Thereby, the startup ECD and the circulating ECD are both disadvantages of prior art.
As circulation is established and drill pipe rotation is commenced, the circulating pressure may reduce to the ECD pressure. The changes in circulation pressure and the corresponding changing hydrostatic pressure exerted upon the formation results in reduced control of hydrostatic pressure exerted upon the formation. In overbalanced drilling, the applied hydrostatic pressure also may be substantially higher than the minimum hydrostatic pressure that may otherwise be required to maintain well control. Those skilled in the industry may appreciate that increased drilling fluid density and hydrostatic pressure may result in reductions in rate of penetration (“ROP”) by the drill bit, further resulting in increase time and well costs. The hydrostatic pressure fluctuations, the complex determinations of actual circulating bottom hole pressure, the increased fluid density, and the resultant decreased ROP are also disadvantages of the prior art.
The disadvantages of prior art are overcome by the present invention. An improved method and system for more accurately controlling well bore hydrostatic pressure and reducing the startup ECD and the ECD are described herein.
SUMMARY OF THE INVENTION
This invention provides methods and systems for drilling a well bore through a subterranean formation whereby the hydrostatic pressure exerted upon the formation by the drilling fluid (“mud”) may be maintained substantially the same regardless of whether the drilling fluid is or is not being circulated. The bottom hole pressure exerted on a formation during periods of drilling fluid circulation may be the equivalent circulating density (“ECD”). The ECD may be at least partially dependent upon circulatio
Elkins Hubert L.
Merit Mark A.
Browning & Bushman P.C.
Dang Hoang
Vareo Shaffer, Inc.
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