Wells – Processes – Assembling well part
Reexamination Certificate
1999-04-12
2001-11-13
Tsay, Frank (Department: 3672)
Wells
Processes
Assembling well part
C166S081100, C175S218000
Reexamination Certificate
active
06315051
ABSTRACT:
The present invention relates to a method for drilling wells, particularly drilling for hydrocarbons.
In drilling wells for hydrocarbons, particularly petroleum, the drill string is rotated to drive the drill bit and mud is circulated to cool, lubricate and remove the rock cuttings formed by the drilling.
As the drill penetrates into the earth, more tubular drill stems are added to the drill string. This involves stopping the drilling whilst the tubulars are added. The process is reversed when the drill string is removed, e.g. to replace the drilling bit. This interruption of drilling conventionally means that the circulation of the mud stops and has to be re-started on recommencement of the drilling which, as well as being time consuming, can also lead to deleterious effects on the walls of the well being drilled and can lead to problems in keeping the well ‘open’.
Additionally the mud weight is conventionally chosen to provide a static head relating to the ambient pressure at the top of the drill string when it is open while tubulars are being added or removed. This weighting of the mud can be very expensive.
We have now invented a method and equipment for drilling wells in which the tubular members forming part of the drill string can be added or removed during continuous circulation of mud in a closed system such that relating the mud weight to the static head below the drilling head is no longer necessary.
According to the invention there is provided a method for drilling wells in which a drill bit is rotated at the end of a drill string comprising tubular members joined together and mud is circulated through the tubular drill string, in which method tubular members are added to or removed from the drill string whilst the circulation of mud continues.
The method enables there to be continuous rotation of the drill string while tubulars are added or removed and for there to be continuous vertical motion of the drill string by addition or removal of tubulars.
The method provides for the supplying of mud, at the appropriate pressure in the immediate vicinity of the tubular connection that is about to be broken such that the flow of mud so provided overlaps with flow of mud from the top drive, as the tubular separates from the drill string. The separated tubular is then totally separated from the drill string by the closure of a blind ram or other preventer or other closing device such as a gate valve. The separated tubular can then be flushed out e.g. with air or water (if under water) depressured, withdrawn, disconnected from the top drive and removed. The action of the said blind ram is to divide the pressure chamber into two parts such that the separated tubular may be removed from the upper depressurised part without loss of mud to the environment the drill string continues to be circulated with mud at the required pressure from the lower part of the chamber.
Preferably there are means which seal off the circulating mud and other fluids to prevent environmental contamination whilst they are still circulating.
In a preferred embodiment of the invention a tubular can be added using a clamping means which comprises a ‘coupler’ and the top end of the drill string is enclosed in and gripped by the lower section of the coupler, in which coupler there is a blind preventer which separates the upper and lower sections of the coupler, the tubular is then added to the upper section of the coupler and is sealed by an annular preventer and the blind preventer is then opened and the lower end of the tubular and upper end of the drill string joined together.
In use, the lower section of the coupler below the blind preventer will already enclose the upper end of the drill string before the tubular is lowered and when the tubular is lowered into the coupler the upper section of the coupler above the blind preventer will enclose the lower end of the tubular.
The tubular can be added to the drill string by attaching the lower section of the coupler to the top of the rotating drill string with the blind preventer in the closed position preventing escape of mud or drilling fluid. The tubular is lowered from substantially vertically above into the upper section of the coupler and the rotating tubular is then sealed in by a seal so that all the drilling fluid is contained, the blind preventer is then opened and the tubular and the drill string brought into contact and joined together with the grips bringing the tubular and drill string to the correct torque.
The lower end of the tubular and the upper end of the drill string are separated by the blind preventer such that the tubular can be sealed in by an upper annular preventer so that when the blind preventer is opened there is substantially no escape of mud or drilling fluid and the tubular stand and drill string can then be brought together and made up to the required torque.
To remove another tubular from the drill string the tubular spool or saver sub under the top drive penetrates the upper part of the pressure chamber, is flushed out with mud and pressured up; the blind ram opens allowing the top drive to provide circulating mud and the spool to connect to and to torque up the into the drill string. The pressure vessel can then be depressured, flushed with air (or water if under water) and the drill string raised until the next join is within the pressure chamber, the ‘slips and grips’ ram closed, the pressure chamber flushed with mud and pressured up and the cycle repeated thus avoiding pollution of the environment, either above or below the water.
Preferably the coupler includes slips which support the drill string while the top drive is raised up to accept and connect another driver.
The method can be used in drilling in which a drill string is rotated from a top drive rotating means and drilling fluid is circulated down the drill string in the conventional way.
The making and breaking of joints can be carried out using conventional rotating grips which can be outside the coupler but preferably are within the coupler.
As the mud, drilling fluids or other circulating fluids can be kept segregated from the environment there is the capacity to reduce pollution and this is particularly advantageous subsea where it reduces the risk of contamination of the sea-water particularly with oil based muds which will not be able to enter the marine environment. Additionally water may be excluded from the mud where well bores could be damaged by water.
The pressure isolation means that the mud weighting is not based on the ‘static head’ as in conventional drilling, but is based on the pressure profile required over the exposed formation of the borehole, and is determined by the mud inlet and return pressures, the characteristics of the exposed formation and the properties of the returning mud, and so expensive weighting additives which can be required to be added to the mud in conventional drilling to provide adequate weight of mud need not be used except for emergency kill stocks.
This makes it much easier to ‘hold the hole open’ and allows for the choice of lighter drilling muds which can result in considerable savings in costs over conventional drilling methods.
The method of the invention enables a steady and controllable pressure to be maintained on the exposed formation wall down the borehole at all times from first drilling until cementing the casing and this can be achieved in overbalanced, balanced or underbalanced drilling. This enables the ROP to be safely maximised and formation damaged to be minimised. The method of the invention is particularly valuable for use in underbalanced drilling where its true benefits can be achieved by controlling the downhole pressure to any desired value between losing circulation and well bore collapse which can maximise the rate of penetration. The downhole pressure can be easily and immediately altered without changing the mud weight while tubulars are added and removed and is therefore much safer to use when ‘kicks’ occur.
The method of the invention can be remotely controlled e.g. by computer assiste
Bartlett & Sherer
Coupler Developments Limited
Sherer Ronald B.
Tsay Frank
LandOfFree
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