Arrangement and method for regulating bottom hole pressures when drilling deepwater offshore wells

Abstract

An arrangement and a method to control and regulate the bottom hole pressure in a well during subsea drilling at deep waters: The method involves adjustment of a liquid/gas interface level in a drilling riser up or down. The arrangement comprises a high pressure drilling riser and a surface bop at the upper end of the drilling riser. The surface bop havs a gas bleeding outlet. The riser also comprises a bop, with a by-pass line. The drilling riser has an outlet at a depth below the water surface, and the outlet is connected to a pumping system with a flow return conduit running back to a drilling vessel/platform.

Description

Where:

• P_bh=Bottom hole pressure
• P_hyd=Hydrostatic pressure
• P_fric=Frictional pressure
• P_wh=Well head pressure
• P_sup=Surge pressure due to lowering the pipe into the well
• P_swp=Swab pressure due to pulling the pipe out of the well

Controlling bottom hole pressure means controlling these five components.

The Equivalent circulation Density (ECD) is the density calculated from the bottom hole pressure P_bh)
ρ_E·g·h=P_bh  (1)
Where:

• • ρ_E=Equivalent Circulation Density (ECD) (kg/m3)
  • g=Gravitational constant (m/s²)
  • h=Total vertical depth (m)

For a Newtonian Fluid, the pressure in the annulus can be calculated as follows assuming no wellhead pressure and no surge or swab effect:

$\begin{array}{cc}{P}_{\mathrm{bh}}={\rho }_m·g·h+\frac{128·\eta ·L_1·Q}{{\pi }^2·{\left(D_0-d_{\mathrm{ds}}\right)}^3·{\left(D_0+d_{\mathrm{ds}}\right)}^2}& \left(2\right)\end{array}$

For a Bingham fluid, the following formula is used:

$\begin{array}{cc}{P}_{\mathrm{bh}}={\rho }_m·g·h+\frac{128·\eta ·L_1·Q}{{\pi }^2·{\left(D_0-d_{\mathrm{ds}}\right)}^3·{\left(D_0+d_{\mathrm{ds}}\right)}^2}+\frac{16·{\tau }_0·L_1}{3·\left(D_0-d_{\mathrm{ds}}\right)}& \left(3\right)\end{array}$
Where:

• ρ_m=Density of drilling fluid being used
• η=Viscosity of drilling fluid
• L₁=Drillstring length
• Q=Flowrate of drilling fluid
• D₀=Diameter of wellbore
• d_ds=Diameter of drillstring
• g=Gravitational constant
• h=Total vertical depth
• τ₀=Yield point of drilling fluid

FIG. 5 is an is an illustration of parameters used to calculate the ECD/dynamic pressure and the height (h) of the drilling fluid in the marine drilling riser using the low riser return and lift pump system (LRRS).

From eq. 4 (Newtonian Fluid ), it is seen that in order to keep the bottom hole pressure P_bh) constant, an increase in flowrate (Q) requires the hydrostatic head (h) to be reduced.

$\begin{array}{cc}{P}_{\mathrm{bh}}={\rho }_m·g·h+\frac{128·\eta ·L_1·Q}{{\pi }^2·{\left(D_0-d_{\mathrm{ds}}\right)}^3·{\left(D_0+d_{\mathrm{ds}}\right)}^2}+P_{\sup }+P_{\mathrm{swp}}& \left(4\right)\end{array}$

The expression for calculating swab and surge pressure is not shown in Eq. 4. However, when moving the drillstring into the hole, an additional pressure increase (P_sup) will take place due to the swab effect. In order to compensate for this effect, the hydrostatic head (h) and/or the flowrate (Q) would have to be reduced.

When moving the drill string out of the hole, a pressure (P_swp) drop will take place due to the surge effect. In order to compensate for this effect, the hydrostatic head (h) and/or the flowrate (Q) would have to be increased.

The swab and surge effects, are as described above, a result of drill string motion. This motion is not caused due to tripping only, but also due to vessel motion when the drill string is not compensated, i.e. make and break of the drill string stands.

FIG. 5 shows a flowchart to illustrate the input parameters to the converter indicated above, for control of bottom hole pressure (BBP) using the low return riser and lift pump system (LRRS) described above.

Into the converter 100 a set of parameters are put. The well and pipe dimensions 101, which are evidently known from the start, but may vary depending on the choice of casing diameter and length as the drilling is proceeding, the mud pump speed 102, which, e.g., may be measured by a sensor at each pump, pipe and draw-work movement (direction and speed) 103, which also may be measured by a sensor that, e.g., is placed on the draw-work main winch, and the drilling fluid properties (viscosity, density, yield point, etc.) 104.

The parameters 101, 102, 103, 104 are entered as values into the converter 100.

Additional parameters, such as bottom hole pressure 105, which may be the result of readings from Measurements While Drilling (MWD) systems, actual mud weight (density) 106 in the drilling riser, preferably resulting from calculations based on measurements by the sensors 10a and 10b, as explained above, etc., may also be collected before the needed hydrostatic head (level of interface between drilling fluid and air) (h) to gain the intended bottom hole pressure is calculated.

The needed hydrostatic head (h) is input to a comparator/regulator 108

The fluid level (h′) in the riser is continuously measured and this parameter 107 is compared with the calculated hydrostatic head (h) in the comparator/regulator 108. The difference between these two parameters is used by the comparator/regulator 108 to calculate the needed increase or decrease of pump speed and to generate signals 109 for the pumps to achieve an appropriate flow rate that will result in a hydrostatic head (h).

The above input and calculations may take place continuously or intermittently to ensure an acceptable hydrostatic head at all times.

Referring to FIGS. 6 and 7 some effects of the present invention on the pressure will be explained. In the figures the vertical axis is the depth from sea level, with increasing depth downward in the diagrams. The horizontal axis is the pressure. At the left hand side the pressure is atmospheric pressure and increasing to the right.

In FIG. 7 the line 303 is the hydrostatic pressure gradient of seawater. The line 306 is the estimated pore pressure gradient of the formation. In conventional drilling the mud weight gradient 305 indicates that a casing 310 have to be set in order to stay in between the expected pore pressure and the formation strength—the formation strength at this point being indicated by reference number 309—at the bottom of the last casing 315. If drilling with an arrangement and method according to the present invention, the gradient of the mud can be higher, as indicated by the line 310, which means that one can drill deeper.

If however, the pore pressure, indicated by 312, at some point should exceed the expected pressure, indicated by 311, a kick could occur. With the method of present invention the level can be dropped further, down to 302 and the mud weight further increased. The net result is a pressure decrease at the casing shoe 309 with an increase in pressure near the bottom of the hole, as indicated by 307, making it possible to drill further before having to set a casing.

In this way it is possible to reduce the pressure on weak formations higher up in the hole and compensate for higher pore pressures in the bottom of the hole. Thus it is possible to rotate the pressure gradient line from the drilling mud around a fixed point, for example the seabed or a casing shoe.

Another example of the ability of this system is shown in FIG. 6. In this situation a severely depleted formation 210 is to be drilled. The formation has been depleted from a pressure at 205 at which it was possible to drill using a drilling fluid slightly heavier than seawater (1,03 SG) as drilling fluid, with a pressure gradient shown at 203. The fracture gradient of the depleted formation is now reduced to 211, which is lower than the pressure gradient of seawater from the surface, as indicated by the line 201.

With the present invention drilling can be done without needing reduce the density of the drilling fluid substantially and having to turn the drilling fluid into gas, foam or other lighter than water drilling systems, as shown by the pressure gradient 214.

By introducing an air column in the upper part of the riser the upper level of the drilling fluid can be dropped down to a level 202. Ea the case shown a drilling fluid with the same pressure gradient as seawater 201 can be used, but starting at a substantially lower point, as shown by 202.

A pore pressured of 0,7 SG can be neutralized by low liquid level with seawater of 1,03 SG as shown by 202. This ability gives rise to great advantages when drilling in depleted fields, since reducing the original formation pressure of 1,10 SG at 205 to 0,7 SG at 210 by production, can also give rise to reduced formation fracture pressure, shown at 211, that can not be drilled with seawater from surface, as shown by 201. With the present invention the bottom-hole pressure exerted by the fluid in the well bore can be regulated to substantially below the hydrostatic pressure for water. With the prior art of drilling arrangements this will require special drilling fluid systems with gases, air or foam. With the present invention this can be achieved with a simple seawater drilling fluid system.

It should be apparent that many changes may be made in the various parts of the invention without departing from the spirit and scope of the invention and the detailed embodiments are not to be considered limiting but have been shown by illustration only. Other variations will no doubt occur to those skilled in the art upon the study of the detailed description and drawings contained herein. Accordingly, it is to be understood that the present invention is not limited to the specific embodiments described herein, but should be deemed to extend to the subject matter defined by the appended claims, including all fair equivalents thereof.

Claims

1. A drilling system for compensating for changes in equivalent mud circulation density (ECD) or dynamic pressure in an annulus bore in a well resulting from drilling activities during subsea drilling at great water-depths, comprising:

a high pressure drilling riser extending from a seafloor wellhead to near the surface;

a near surface bop at the upper end of the drilling riser, the near surface bop having an upper high pressure line;

a subsea outlet in communication with the interior of the riser at a point above the seafloor wellhead;

a flow return conduit running back to the surface;

a pumping system suspended above the seafloor and connecting said subsea outlet to said flow return conduit;

a valve adapted to isolate the riser from the pumping system;

means for converting changes in pressure in said riser to an equivalent change in height of drilling fluid in the riser;

means for adjusting the pump rate of said pumping system according to the difference of the height of drilling fluid in said riser and said equivalent change in height of drilling fluid; thereby adjusting the height of drilling fluid in the drilling riser so as to neutralize the changes in pressure in said annulus bore created by said drilling activities by varying the actual amount of drilling fluid in the riser; and

a subsea shut-off device at the sea floor, the shut off device having at least one by-pass line providing communication between the well below the shut-off device and the interior of the riser, the by-pass line containing at least one shut-off valve.

2. A system according to claim 1, further comprising a gas bleeding outlet connected to a choke line in communication with a high pressure choke and stand pipe manifold on a drilling vessel.

3. A system according to claim 1, wherein the pumping system with flow return line is adapted to be launched and run with the riser.

4. A system according to claim 1, further comprising a filling line coupled to the riser substantially below sea level and above said subsea outlet, the filling line being adapted for filling the riser with a gas or liquid.

5. A system according to claim 4, wherein the gas is an inert gas for displacement of the air above the drilling fluid.

6. A system according to claim 1, further comprising a valve in the flow return conduit, and a particle collection box in the flow return line, the valve being adapted for opening and closing the communication between the particle collection box and the flow return conduit.

7. A system according to claim 6, wherein the particle collection box is hanging underneath the pumping system and the particle collection box has a re-circulation and jetting means for breaking down particle size to prevent particle build up.

8. A method for compensating for equivalent mud circulation density (ECD) or dynamic pressure increase or decrease in an annulus bore in a well during subsea drilling at great water-depths resulting from drilling activities, comprising the steps:

maintaining the pressure in the top of a drilling riser extending from a seafloor wellhead to the surface at equal to or lower than atmospheric pressure, said riser configured with a seabed bop and bypass;

monitoring bottom hole pressure in the well for a change in pressure created by drilling activities;

converting a the change in pressure in the well created by drilling activities to an equivalent change in height of drilling fluid in the riser;

adjusting the pump rate of a drilling fluid return pump suspended above the seafloor and connected at a point above the seafloor wellhead to the drilling riser so as to adjust the height of drilling fluid in the drilling riser by the equivalent change in height of drilling fluid so as to neutralize the change in pressure created by the drilling activities by varying the actual amount of drilling fluid in the riser.

9. A method according to claim 8, wherein gas escaping from an underground formation is separated from liquid during offshore drilling, comprising the steps:

permitting gas and drilling fluid in a drilling riser extending from a seafloor wellhead to the surface to form a gas/liquid interface within the drilling riser;

providing a liquid outlet below the gas/liquid interface level and substantially above the seafloor wellhead, said outlet being connected to a pumping system suspended above the seafloor and hence to a return conduit,

providing a gas outlet above the gas/liquid interface level,

closing a near surface bop at the upper end of the drilling riser, and

pumping liquid out of the drilling riser through the liquid outlet, whereby the drilling riser is acting as a gas separator.

10. A method according to claim 9, wherein a flow return line between the liquid outlet and the pumping system is adapted to prevent free gas from entering the return conduit by having a U-shaped V-shaped loop acting as a gas-lock.

11. A method according to claim 10, where the height of the gas-lock can be adjusted by varying the subsea level of the pumping system.

12. A method according to claim 9, wherein the level of the gas/liquid interface between the drilling fluid and the gas in the drilling riser is maintained below sea level so that the pressure in the bottom of the well is lower than the hydrostatic pressure exerted by seawater from sea level.

13. A method according to claim 12, wherein the drilling riser comprises sensors for monitoring the height of the gas/liquid interface level in the riser, the sensors being coupled to a regulating means controlling the pump rate of the pumping system and thereby controlling the height of the gas/liquid interface level.

14. A method according to claim 8, said change in pressure occurring in said riser by drilling activities comprising a change in pressure created by the drill string being moved up or down in the well.

15. A method according to claim 8, said change in pressure in said drilling riser being created by circulation of drilling fluid through the bit.

16. A method for controlling equivalent mud circulation density (ECD) in a well during subsea drilling operations, comprising:

using a high pressure drilling riser extending from a seafloor wellhead and subsea bop to the surface, within which there is drilling fluid present, there being no outside kill or choke lines extending from the surface to the subsea bop, said subsea bop configured with a bypass;

maintaining the pressure in the top of the drilling riser at equal to or lower than atmospheric pressure;

monitoring the height of drilling fluid in the riser;

monitoring bottom hole pressure in the well for a change in pressure;

calculating an equivalent change in height of drilling fluid to the change in pressure;

using a drilling fluid pump suspended above the seafloor and connected to the riser substantially above the seafloor wellhead and below the height of drilling fluid, adjusting the height of drilling fluid in the riser by the equivalent change in height of drilling fluid, thereby adjusting the drilling fluid level in the drilling riser so as to reverse the change in the bottom hole pressure.

17. A drilling system for controlling equivalent mud circulation density (ECD) in a well resulting from drilling activities during subsea drilling operations, comprising:

a high pressure drilling riser extending from a seafloor wellhead to the surface and having a surface bop at the upper end of the drilling riser, the surface bop having an upper high pressure line, there being no kill or choke lines extending from the surface to the seafloor wellhead;

a subsea outlet in communication with the interior of the riser at a point above the seafloor wellhead;

a flow return conduit running back to the surface;

a pumping system suspended above the seafloor and connecting the subsea outlet to the flow return conduit;

a valve adapted to isolate the riser from the pumping system;

means for monitoring the height of drilling fluid in the drilling riser;

means for sensing a change in pressure in the drilling riser;

means for converting the change in pressure in the drilling riser to an equivalent change in height of drilling fluid in the riser;

means for adjusting the pump rate of the pumping system according to the difference of the height of drilling fluid in the drilling riser and the equivalent change in height; thereby adjusting the height of drilling fluid in the drilling riser so as to neutralize the change in pressure by varying the actual amount of drilling fluid in the riser; and

a subsea shut-off device at the sea floor; the shut off device having at least one by-pass line providing communication between the well below the shut-off device and the interior of the riser, the by-pass line containing at least one shut-off valve or pressure regulating valve.