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- WELL CONTROL
Trình bày: ĐỖ QUANG KHÁNH
Bộ môn Khoan – Khai thác Dầu khí
Khoa Kỹ thuật Địa chất và Dầu khí
Đại học Bách Khoa TP. HCM
Email: dqkhanh@hcmut.edu.vn
- CONTENT
1. INTRODUCTION
2. WELL CONTROL PRINCIPLES
3. WARNING SIGNS OF KICKS
4. SECONDARY CONTROL
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- CONTENT
5. WELL KILLING PROCEDURES
6. BOP EQUIPMENT
7. BOP STACK ARRANGEMENTS
8. EXERCISES
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- 1. INTRODUCTION
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- INTRODUCTION
Ensure that fluid (oil, gas or water) does not flow in an
uncontrolled way from the formations being drilled, into
the borehole and eventually to surface.
This flow will occur if the pressure in the pore space of the
formations being drilled (pf) >= the hydrostatic pressure
exerted by the column of mud in the wellbore (pbh).
It is essential that pf, due to the column of fluid, exceeds
the formation pressure at all times during drilling.
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- INTRODUCTION
If, for some reason, pf >= pbh an influx of fluid into the borehole (known
as a kick) will occur.
If no action is taken to stop the influx of fluid once it begins, then all of
the drilling mud will be pushed out of the borehole and the formation
fluids will be flowing in an uncontrolled manner at surface. This
would be known as a Blowout.
This flow of the formation fluid to surface is prevented by the
secondary control system.
Secondary control is achieved by closing off the well at surface with
valves, known as Blowout Preventers – BOPs.
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- INTRODUCTION
The control of the formation pressure,
either by ensuring that the borehole pressure is greater
than the formation pressure (known as Primary Control)
or by closing off the BOP valves at surface (known as
Secondary Control)
is generally referred to as keeping the pressures in the well
under control or simply well control.
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- INTRODUCTION
When pressure control over the well is lost, swift action
must be taken to avert the severe consequences of a
blow-out. These consequences may include:
Loss of human life
Loss of rig and equipment
Loss of reservoir fluids
Damage to the environment
Huge cost of bringing the well under control again.
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- INTRODUCTION
For these reasons it is important to understand the
principles of well control and the procedures and
equipment used to prevent blowouts.
Every operating company will have a policy to deal with
pressure control problems.
This policy will include
• training for rig crews,
• regular testing of BOP equipment,
• BOP test drills and standard procedures
to deal with a kick and a blow-out.
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- INTRODUCTION
One of the basic skills in well control is to recognise when
a kick has occurred.
Since the kick occurs at the bottom of the borehole its
occurrence can only be inferred from signs at the
surface.
The rig crew must be alert at all times to recognise the
signs of a kick and take immediate action to bring the
well back under control.
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- INTRODUCTION
The severity of a kick (amount of fluid which enters the
wellbore) depends on several factors including
• the type of formation;
• pressure;
• and the nature of the influx.
The higher the permeability and porosity of the formation, the
greater the potential for a severe kick (e.g. sand is considered to
be more dangerous than a shale).
The greater the negative pressure differential (pf to pw) the
easier it is for formation fluids to enter the wellbore, especially if
this is coupled with high permeability and porosity.
Gas will flow into the wellbore much faster than oil or water.
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- 2. WELL CONTROL PRINCIPLES
Two basic ways in which fluids can be prevented from
flowing, from the formation, into the borehole:
Primary Control (PC)
Secondary Control (SC)
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- WELL CONTROL PRINCIPLES
Primary Control: is maintained by ensuring that the pressure due
to the column of mud in the borehole is greater than the pressure in
the formations being drilled i.e. maintaining a positive differential
pressure or overbalance on the formation pressures.
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- WELL CONTROL PRINCIPLES
Secondary Control: is required when PC has failed and
formation fluids are flowing into the wellbore.
The aim: is to stop the flow of fluids into the wellbore and eventually
allow the influx to be circulated to surface and safely discharged, while
preventing further influx downhole.
Fig. Secondary Control -Influx Controlled by Closing BOP's
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- WELL CONTROL PRINCIPLES
First step: close the annulus space off at surface, with the
BOP valves, to prevent further influx of formation fluids
Next step: circulate heavy mud down the drillstring and up
the annulus, to displace the influx and replace the original
mud (which allowed the influx in the first place).
• The second step will require flow the annulus but this is done in
a controlled way so that no further influx occurs at the bottom of
the borehole.
• The heavier mud should prevent a further influx of formation
fluid when drilling ahead.
The well will now be back under primary control.
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- WELL CONTROL PRINCIPLES
PC of the well may be lost (i.e. pbh < pf) in two ways.
The first is if the formation pressure in a zone which is penetrated is
higher than that predicted by the reservoir engineers or geologist. In
this case the drilling engineer would have programmed a mud weight
that was too low and therefore pbh would be less than pf.
Fig. PC - Pressure due to mud column exceeds Pore Pressure
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