NO348119B1 - A method for controlling hydrostatic pressure in a shut in petroleum well subject to gas influx - Google Patents

Description

A METHOD FOR CONTROLLING HYDROSTATIC PRESSURE IN A SHUT IN PETROLEUM WELL SUBJECT TO GAS INFLUX

The present invention is related to well control in the petroleum industry. More specifically the invention is related to a method for controlling hydrostatic pressure in a shut in petroleum well subject to gas influx wherein a circulation outlet of a drill pipe is arranged between the bottom of the well and a surface of the well. The circulation outlet is the lower end portion of the drill pipe.

Gas influx may typically occur when the hydrostatic well pressure falls below the pore pressure of the well formation. Gas influx in a petroleum well may for example arise when an underpressure is caused by the drill pipe when the drill pipe is withdrawn from a bottom portion of the well, especially when withdrawing the drill pipe too fast.

It should be noted that the present invention is related to controlling hydrostatic pressure in a well subject to a minor seepage or percolation of gas from a formation and into the wellbore, and not a so-called gas kick that may result in a possible loss of primary well control which must be controlled by a secondary control such as a Blowout Preventer (BOP).

The present inventor has been engaged in well control and related activities since the late 1970s and have found that a more sophisticated method for circulating gas caused by gas influx out of a shut in petroleum well in a balanced way is required to achieve better control of the hydrostatic pressure in the well, including a bottom hydrostatic pressure during removal of gas caused by gas influx, than the methods know hithereto.

In the petroleum industry handling of gas caused by gas influx in a petroleum well has been dealt with in various ways. One example is to pump mud into the well in an attempt to displace the gas caused by gas influx further down the well and back into the formation. This method is known as bull heading. A drawback by such a prior art method is that the gas is prone to penetrate through a weak portion of the well wherein the weak portion is not at the same location from which the gas influx has flown into the well. A fractured zone is thereby created which represents a leakage point which may result in loss of expensive mud, while at the same time gas influx may continue.

Another example of a known method is to remove gas influx out of a shut in petroleum well by means of using a volumetric method only. The term volumetric method is known by a person skilled in the art. However, by means of a volumetric method only it is not possible to know when gas migrates or percolates into an annular space between a drill pipe and a wall of the well, i.e. it is not possible to know when the gas influx is at a circulation point at an end of the drill pipe. Bleeding off the same volume with respect to pressure change (choke pressure) at surface will result in a hydrostatic underbalance within the well when the flow capacity within the well changes. By the term flow capacity is meant fluid volume per meter of the well. A flow capacity below a drill pipe is much higher than in an annulus between a drill pipe and a wall of the well. A hydrostatic underbalance within the well may result in continued gas influx into the well. A volumetric method alone is also very time consuming since the gas rises or migrates slowly upwards in a high density drilling mud.

Still another example of removing gas influx from a well is a method known as stripping to bottom of a well and then circulating gas out of the well. However, stripping to bottom requires special equipment while at the same time representing a risk of causing leakage in parts of the equipment involved, such as for example a part of a BOP (Blowout Preventer), especially an elastic seal of the BOP configured for sealing against the drill pipe.

An example of prior art volumetric method is disclosed in Linkedin, Carlos M., “Dynamic Volumetric Method applied to oil wells on deepwater operations”, 2015.07.23, wherein the volumetric method comprises:

a) establishing a first curve representing a kill pressure versus a volume of mud to be bled off at surface to maintain a constant pressure at a bottom of the well, wherein a starting point of the first curve is the shut in pressure of the well prior to start circulation plus a safety margin, and the volume is the mud to be continuously bled off caused by gas expanding as it ascends towards a surface of the well;

b) start circulation of mud while keeping the choke pressure the same as the starting point of the first curve;

c) increasing a mud circulation flow rate to a predetermined flow rate;

d) establishing a starting point of a second curve representing a circulation pressure at an inlet of the drill pipe plus a safety margin and a work margin, and regulating the choke pressure with respect to the volume of the mud to be continuously bled off according to the first curve, whereby the second curve is in parallel with the first curve.

Publication US 2019/0120003 A1 discloses a method of drilling a subterranean wellbore using a drill string includes injecting a drilling fluid into the subterranean well bore via the drill string and removing the drilling fluid from an annular space around the drill string (the annulus) via an annulus return line, oscillating a pressure of the drilling fluid in the annulus, determining a wellbore storage volume and a wellbore storage coefficient for each drilling fluid pressure oscillation, and using the wellbore storage volume and wellbore storage coefficient to determine a proportion by volume of gas and a proportion by volume of liquid in the annulus during that drilling fluid pressure oscillation. The wellbore storage volume is a change in a measured flow rate over a time period. The wellbore storage coefficient is the wellbore storage volume divided by a pressure change over the time period.

Publication US8678085 B1 discloses a method and system to aid and/or train well control personnel by measuring the actual hydraulic delay and pressure attenuation of operator choke changes during well control operations or simulations. This provides the choke operator with an anticipated drill pipe pressure as soon as the choke is adjusted, accounting for hydraulic delay, pressure attenuation and prior choke adjustments that are currently travelling through the wellbore as well as reflections of the transient pressure waves against the pumps and choke. The technique that is described utilizes only three inputs, and works without knowledge of or inputting data such as well depth, pipe and hole geometry, mud properties, temperature, water depth, land, offshore platform or floating drilling rigs.

Publication US8517111B2 discloses methods and systems for drilling subsea wells bores with dual-gradient mud systems include drilling the subsea well bore while employing a subsea pumping system, a subsea choke manifold and one or more mud return risers to implement the dual gradient mud system. When a well bore influx is detected, the well bore is shut in, and components determine if pressure control may be used to circulate the influx out of the well bore, the size of the influx, and how much the mud system weight will need to be reduced to match the dual gradient hydrostatic head before the influx reaches the subsea pump take point. The subsea pumping system, subsea choke manifold, and mud risers are isolated while the influx is circulated up one or more fluid passages in the drilling riser package using the surface pump, through the wellhead, and out the surface choke manifold.

Sule, I. et al., Journal of Petroleum Science and Engineering, Volume 174, March 2019, pages 1223-1235, discloses a nonlinear model predictive control of gas kick in a managed pressure drilling system.

A person skilled in the art will know that it is normally not possible to predict a location in a petroleum well where any gas influx takes place.

By known methods, it is also not possible to predict when gas arrives at the circulation outlet of the drill pipe. Thus, it is not possible to have full control of the hydrostatic pressure within the well and especially the hydrostatic pressure at the bottom of the well. Thereby, problems and unwanted situations may arise. Such problems may be related to for example loss of drilling fluid into a formation, inflow from the formation, and excessive pressure at a surface of the well which may be harmful to for example a choke forming part of a well circulation system.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claim. The dependent claims define advantageous embodiments of the invention.

According to the invention, there is provided a method for controlling hydrostatic pressure in a shut in petroleum well comprising gas caused by gas influx, wherein a circulation outlet of a drill pipe is arranged between a bottom of the well and a surface of the well. The method comprises:

a) establishing a first curve representing a choke pressure versus a volume of mud to be bled off at surface to maintain a constant pressure at a bottom of the well, wherein a starting point of the first curve is the shut in pressure of the well prior to start circulation, and the volume is the mud to be continuously bled off caused by gas expanding as it ascends towards a surface of the well;

b) starting circulation of mud while keeping the choke pressure the same as the starting point of the first curve;

c) increasing a mud circulation flow rate to a predetermined flow rate;

d) establishing a starting point of a second curve representing a circulation pressure at an inlet of the drill pipe, and regulating the choke pressure with respect to the volume of the mud to be continuously bled off according to the first curve, whereby, the second curve is in parallel with the first curve; and

e) finding when the gas starts percolating into an annulus between the drill pipe and a wall of the well by closely observing when it is necessary to further increase the choke pressure independently of the first curve to make sure that the circulation pressure follows the second curve, and in due time increasing the choke pressure accordingly.

By the term choke pressure is meant a pressure monitored at the choke. By the term circulation pressure is meant a pressure monitored at the inlet of the drill pipe.

When the gas ascends towards a surface of the well, there will be an increase in pressure at surface due to expansion of the gas and the movement of gas upwards in the well bore. Such an increase in pressure will, if not bled off, influence the pressure in the well, including a bottom hole pressure of the well. The effect of step a) above is therefore to balance the bottom hole pressure with the reservoir pressure while gas influx ascends or migrates slowly towards the surface of the well. The bottom hole pressure is thus balanced with the reservoir pressure by removing or bleeding off the increase in pressure caused by gas expanding while ascending in the well.

The first curve may for example be presented in a so-called “Kill Sheet” known to a person skilled in the art.

In step d) above, the starting point of the second curve representing the circulation pressure, will be above the first curve. This is due to the additional circulation pressure required to overcome the flow resistance caused by i.a. friction through the drill pipe.

A flow area of the petroleum well below the circulation outlet of the drill pipe is larger than a flow area in the annulus between the drill pipe and a wall of the well. Thus, a flow capacity of the well is larger below the circulation outlet of the drill pipe than in said annulus. As mentioned above, the term flow capacity is the fluid volume per meter of the well.

After having established the second curve representing a circulation pressure, a hydrostatic bottom hole pressure is substantially maintained by means of controlling the choke pressure to make sure that both the first curve (choke pressure versus volume to be bled off) and the second curve (circulation pressure versus volume to be bled off) are followed.

When the gas influx starts percolating into the annulus, an operator that carefully controls that the circulation pressure follows second curve, will observe that it is necessary to increase the choke pressure, i.e. further reduce a flow through the choke at a surface of the well to make sure that the second curve is followed. Thus, in step e) the operator balances a hydrostatic column of fluid in the annulus between the drill pipe and the wall of the well, with fluid within the drill pipe. A so-called U-flow is thereby prevented. By preventing U-flow, drilling mud flowing out of the circulation outlet of the drill pipe starts replacing gas below said circulation outlet, thereby knowing that the well is substantially in hydrostatic balance.

From the above it should be understood that according to the present invention, a method is provided for, in a safe and controlled way, circulating gas caused by gas influx out of the well by finding a crossing point wherein the hydrostatic pressure in the well is controlled by changing from the volumetric- method in step a), via a volumetric-circulation method in steps b)-d), to a circulation method in or immediately after step e). By the term immediately after is meant within some seconds, for example less than a minute after observing that it is necessary to increase the choke pressure.

In a preferred embodiment, the volume of mud to be bled off at surface in step a) is measured by means of increase in fluid volume in a circulation tank forming part of a well circulation system. Alternatively, or additionally, the volume of mud to be bled off at surface in step a) may be measured by means of a volumeter forming part of a well circulation system. In such an alternative, a volumeter may be arranged downstream of the well circulation system, i.e. downstream of an outlet of the well.

As drilling mud replaces gas below the circulation outlet, the hydrostatic pressure in the well will increase due to a difference in density between the mud and the gas. To prevent an excessive pressure in the well caused by mud replacing the gas in the well, the method may after step e) comprise substantially preventing increase in hydrostatic well pressure by reducing the circulation pressure.

The regulation of the circulation pressure may comprise stepwise reducing the circulation pressure by temporarily shutting in the well between each step. Such a stepwise reduction of the circulation pressure may be achieved by repeatedly pausing the circulation after a certain time or a number of so-called pump strokes, and measuring a new, reduced shut in pressure, and thereafter resuming the circulation with a circulation pressure being the same as the reduced shut in pressure. This stepwise process may be repeated until the measured shut in pressure of the drill pipe and the choke are the same. When the measured shut in pressure of the drill pipe and the choke are the same, all of the gas caused by gas influx has been circulated out of the well in a safe way with fully balanced wellbore pressure.

When all gas influx has been circulated out of the wellbore, a volume of mud being circulated into the well will be the same as the volume of mud being circulated out of the well.

Thus, a volume within any circulation tank that may form part of the well circulation system, will be constant, and/or a volume of mud flowing through the drill pipe and into the well will be the same as a volume of mud measured by any volumeter at an outlet of the well circulation system. A volume of mud flowing into the well may be measured by means of pump strokes, or the volume may be measured by a volumeter configured for measuring a flow through the drill pipe.

In the following, examples of preferred embodiments illustrated in the accompanying drawings are described, wherein:

Fig.1 illustrates a shut in petroleum well prior to circulation, wherein a gas influx is between a bottom of the well and a circulation outlet of a drill pipe;

Fig.2 illustrates a first curve representing a choke pressure versus a volume of mud to be bled off at surface to maintain a constant hydrostatic pressure at a bottom of the well;

Fig.3 illustrates the shut in well in fig.1 where circulation has started and the gas caused by gas influx has risen to the outlet of the drill pipe;

Fig.4 illustrates the first curve in fig.1 and an additional second curve representing a circulation pressure, wherein the first curve comprises a breakpoint illustrating when the choke pressure has deviated from the first curve to secure that the circulation pressure strictly follows the second curve when the gas influx has risen to the outlet of the drill pipe as shown in fig.3;

Fig.5 illustrates the shut in well as illustrated in figs 1 and 3, but wherein substantially all of the gas caused by gas influx has risen into an annulus between the drill pipe and a wall of the well; and

Fig.6 illustrates a combination of fig.4 and a subsequent stepwise reduction of a pressure bled off during temporary shut in of the well.

Any positional indications refer to the positions shown in the figures.

In the figures, same or corresponding elements are indicated by same reference numerals. For clarity reasons, some elements may in some of the figures be without reference numerals.

A person skilled in the art will understand that the figures are just principle drawings. The relative proportions of individual elements may also be distorted.

In figures 1, 3 and 5, reference numeral 1 illustrates a petroleum well comprising gas G caused by gas influx from a surrounding formation. A drill pipe 3 has been withdrawn from a bottom portion of the well 1 so that an outlet 5 of the drill pipe 3 is arranged between a bottom of the well 1 and a surface of the well 1. A BOP B is indicated in a top portion of the well 1 and seals around the drill pipe 3 by means of elastic seals, such as rubber. The well 1 may be hundreds or even thousands of meters long. The location of the outlet 5 of the drill pipe 3 is between the bottom and the surface of the well 1. However, a location of the outlet 5 close to the bottom or close to the surface of the well 1 should be avoided. As will be understood from the disclosure herein, the method according to the invention has substantially no practical effect if the outlet is arranged close to the bottom. If the outlet 5 is arranged close to the surface of the well, the method according to the invention may be challenging to control. By the term close is meant a few hundred meters, for example but not limited to 600-700 meters. In one embodiment the outlet 5 of the drill pipe 3 is closer to the surface of the well 1 than the bottom of the well.

Figures 1, 3, and 5 illustrate a substantially constant diameter vertical well 1 housing a drill pipe 3 having a body with constant diameter.

The drill pipe 3 forms part of a circulation system S. The system S comprises a circulation pump P, an appurtenant manometer M1 arranged at an inlet of the drill pipe, and a choke arranged downstream of an outlet of the well 1, wherein the choke is provided with a manometer M2 for measuring a choke pressure. The system S further comprises a circulation tank T being in fluid communication with the drill pipe 3 via a circulation line L, and a pump P arranged downstream of the tank T for circulating drilling mud between the tank T and the drill pipe 3.

The manometer M1 is configured for measuring a shut in drill pipe pressure known as SIDPP by a person skilled in the art, and for measuring the circulation pressure when the mud is pumped into the well, i.e. during circulation.

Turning now to fig.1 illustrating the shut in well 1 in a situation without circulation of drilling mud through the drill pipe 3. The gas G caused by gas influx is at a distance below the outlet 5 of the drill pipe 3. For illustrative reasons, the gas is shown relative concentrated. However, a person skilled in the art will appreciate that the gas will be more distributed within a portion of the well 1 than appearing in the illustration.

As the gas ascends or rises slowly through the drilling mud within the well 1 towards the surface of the well, it will expand. The expansion of gas will displace drilling mud out of the well 1 and into the circulation tank T. The volume of drilling mud in the circulation tank T will increase as illustrated by ΔV in fig.1.

Based on the situation illustrated in fig.1, a first curve Pchoke related to pressure versus an increase in volume ΔVT is established, as illustrated in fig.2.

A starting point PSchoke of the first curve Pchoke is the shut in pressure, the SIDPP, of the well 1 prior to start circulation, and the volume is the mud to be continuously bled off caused by gas expanding as it ascends or migrates towards a surface of the well 1. The curve may for example be presented in a kill sheet known to a person skilled in the art.

An inclination of the curve Pchoke is determined by ΔP/ΔV (change in pressure versus change (increase) in volume in the tank T.

ΔP= ρmud x Gr, wherein

ρmud is the density of the drilling mud,

Gr is the factor of gravity; and

ΔV is the change in volume in the tank.

After having prepared the first curve Pchoke the mud pump is activated, and circulation of mud is started and increased to a predetermined flow rate. The choke pressure is kept the same as the starting point PSchoke of the first curve Pchoke. The circulation of drilling mud is illustrated in the in figures 3 and 5 by means of a downward arrow in the drill pipe 3.

When the circulation of drilling mud is at a predetermined flow rate, a starting point PSc of a second curve Pc is established as illustrated in fig.4. The starting point PSc of the second curve Pc represents a circulation pressure at an inlet of the drill pipe 3. The starting point PSc of the circulation pressure is the sum of the starting point PSchoke of the first curve Pchoke (the shut in pressure) and an additional circulation pressure required to overcome the flow resistance through the drill pipe 3. A differential pressure between the starting point PSchoke of the first curve Pchoke and the starting point PSc of the second curve Pc, i.e. a distance between the two parallel lines Pc and Pchoke, depends i.a. on a length of the drill pipe 3.

During circulation, the choke pressure measured by a manometer M2 is regulated with respect to the volume of the mud to be continuously bled off according to the first curve (Pchoke), whereby the second curve Pc is in parallel with the first curve Pchoke.

In fig.3, the gas starts percolating into an annulus A between the drill pipe 3 and a wall W of the well 1. The annulus A has a reduced flow capacity with respect to the flow capacity of the well 1 below the outlet 5 of the drill pipe 3. Due to the reduced flow capacity in the annulus A, an operator controlling the choke pressure based on the increased volume ΔVT will observe that the choke pressure Pchoke has to deviate from first curve by further increasing the choke pressure Pchoke in order to keep the circulation pressure along the second curve Pc. Therefore, the choke pressure can no longer be based on the first curve Pchoke. This is illustrated by means of a breaking point Pdev in fig.4.

Thus, the breaking point Pdev of the first curve Pchoke in fig.4 represents a change in controlling the hydrostatic pressure within the well 1 by means of the volumetric method via the volumetric-circulation method, to controlling the hydrostatic pressure within the well 1 by means of the circulation method.

When the gas starts flowing into the annulus A as illustrated in fig.3, a hydrostatic pressure below the outlet 5 of the drill pipe will increase as the gas G having a density ρgas is replaced by the mud ρmud having a higher density than the gas G. The increase in pressure Pi is given by the formula.

Pi = (ρmud- ρgas) x (ΔVT/C) x Gr, wherein

ΔVT is the increase in volume within the tank T,

C is the volume capacity in m<3>/m of the well 1 below the outlet 5 of the drill pipe 3; and

Gr is the factor of gravity.

Fig.5 illustrates a situation wherein substantially all the gas G flows in the annulus A. In this situation, substantially all gas G below the outlet 5 of the drill pipe 3 has been replaced by drilling mud having a higher density than the gas G that has percolated into the annulus A. Thus, if the circulation pressure Pc is maintained, a hydrostatic pressure Pu at a bottom of the well 1 will, since there is no gas, increase to a pressure defined by:

Pu = ρmud x ( V0/C) x Gr.

As the gas G rises within the annulus A, the fluid volume within the tank T will rise. However, when all gas G has been circulated out of the well W (not shown), the fluid volume within the tank T will be reduced to a lower level. This is illustrated as ΔVT(+/-) in fig.5, and by means of the upward arrow in a top layer V0 and the downward arrow through ΔVT(+/-) and V0, respectively of the fluid in the tank T.

In fig.4, the increase in pressure due to increase in choke pressure, may cause a minor, temporary increase in the hydrostatic pressure within the well 1. Such a minor increase in hydrostatic pressure within the well 1 makes sure that a hydrostatic underbalance situation within the well is avoided, and thereby prevents a new situation of influx of gas.

However, when all of, or substantially all of the gas has percolated into the annulus A, as shown in fig.5, the increase in hydrostatic pressure in the well may be undesired for parts of the well 1.

To at least reduce such an undesired increase in pressure caused by the replacement of the gas by the heavier mud, the circulation pressure is reduced with respect to the predetermined flow rate upon which the second curve Pc is based, i.e. with respect to the predetermined flow rate stated in item c) of the first aspect of the invention stated above.

Turning now to Fig.6 illustrating a principle of an embodiment of the method according to the invention.

Fig.6 illustrates the curves similar to those shown in fig.4, and an additional curve ΔPR versus time, wherein ΔPR illustrates the choke pressure Pchoke to be bled off to avoid increase in hydrostatic pressure in the well below the circulation point caused by gas still rising in the annulus A. The choke pressure Pchoke is measured by the manometer M2.

The dotted area illustrated by “(Fig.1)” shown between the first curve Pchoke and the parallel second curve Pc, illustrates the situation in fig.1, i.e. when the gas G is below the circulation outlet 5 of the drill pipe 3. The situation illustrated in fig.3 is indicated by the vertical line indicated by “(Fig.3)”.

The additional curve (ΔPR versus time) shown in fig.6 and indicated by “(Fig.5)” illustrates an example of a stepwise reduction of the circulation pressure wherein the circulation pressure is repeatedly paused or stopped after a certain time or a number of pump strokes. During each pause (seven shown in fig.6), an increase in choke pressure Pchoke is bled off and a new, lower shut in pressure is measured by means of the manometer M1 of the drill pipe 3. Between each pause, the circulation is resumed with a pressure of the last measured shut in pressure. This process is repeated until there is no increase in choke pressure Pchoke during a pause, i.e. the shut in pressure of the drill pipe and the choke are the same during a pause. When there is no increase in choke pressure Pchoke during a pause, this indicates that all of the gas G caused by gas influx has been circulated out of the well in a safe way with substantially fully balanced wellbore pressure. The drill pipe circulation pressure Pc is now the same as the choke pressure Pchoke.

When all gas G has been circulated out of the well 1, a volume in the tank T is constant, and ΔVT(+/-) and V0 has been reduced to zero.

A final shut in pressure PEc indicated in fig.6 is:

PEc = PSchoke – (ρmud- ρgas) x (V0/C) x Gr.

Thus, the pressure PEc is the hydrostatic pressure in the well 1 when all gas G within the well 1 has been replaced by mud.

From the disclosure herein, it should be clear that the method according to the invention allows an operator to know when the gas G starts percolating into the annulus A between the drill pipe 3 and the wall W of the well 1. By knowing this, an operator will be able to circulate gas out of the well in a safe and controlled way with respect to hydrostatic pressure in the well and pressure loads against the surface equipment of the well. By controlling the hydrostatic pressure within the well, a hydrostatic pressure at a bottom of the well will be substantially constant. A constant pressure within the well is an important measure to at least reduce the risk of losing expensive drilling mud into the formation, and also the risk of further gas influx in the well 1. As compared with removing gas caused by seeping of gas into a well by means of the volumetric method only wherein the gas is removed without any circulation, the method according to the invention is far more effective with respect to time.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims (5)

C l a i m s

1. Method for controlling hydrostatic pressure in a shut in petroleum well (1) comprising gas (G) caused by gas influx, wherein a circulation outlet (5) of a drill pipe (3) is arranged between a bottom of the well (1) and a surface of the well, c h a r a c t e r i s e d i n that the method comprises:

a) establishing a first curve (Pchoke) representing a choke pressure versus a volume (V0) of mud to be bled off at surface to maintain a constant pressure at a bottom of the well, wherein a starting point (PSchoke) of the first curve (Pchoke) is the shut in pressure of the well prior to starting circulation, and the volume is the mud to be continuously bled off caused by gas expanding as it ascends towards a surface of the well;

b) starting circulation of mud while keeping the choke pressure the same as the starting point (PSchoke) of the first curve (Pchoke);

c) increasing a mud circulation flow rate to a predetermined flow rate;

d) establishing a starting point (PSc) of a second curve (Pc) representing a circulation pressure at an inlet of the drill pipe (3), and regulating the choke pressure with respect to the volume of the mud to be continuously bled off according to the first curve (Pchoke), whereby the second curve (Pc) is in parallel with the first curve (Pchoke); and

e) finding when the gas starts percolating into an annulus (A) between the drill pipe (3) and a wall (W) of the well (1) by closely observing when it is necessary to further increase the choke pressure independently of the first curve (Pchoke) to make sure that the circulation pressure follows the second curve (Pc), and in due time increasing the choke pressure (Pchoke) accordingly.

2. The method according to claim 1, wherein the volume (V0) of mud to be bled off at surface in step a) is measured by means of increase in fluid volume in a circulation tank forming part of a well circulation system.

3. The method according to claim 1, wherein the volume (V0) of mud to be bled off at surface in step a) is measured by means of a volumeter forming part of a well circulation system (S).

4. The method according to any one of the preceding claims, wherein the method after step e) further comprising reducing the circulation pressure to prevent increase in hydrostatic well pressure.

5. The method according to claim 4, wherein the regulation of the circulation pressure (Pc) comprises stepwise reducing the circulation pressure (Pc) by temporarily shutting in the well (1) between each step.