Management and Engineering of Fire Safety and Loss Prevention

Onshore and offshore

BHR Group Ltd

• Publisher: Routledge
• Year: 2006
• ISBN:
  9781135378929
• Edition: 1
• Topic: Architecture
• Subtopic: Architecture General

The papers presented deal with the general methods and techniques, from a range of disciplines, as they can be applied to specific engineering and fire safety situations. The circumstances described include a variety of large scale plant applications in the petrochemical industry. As such this book is a valuable reference for fire engineers, petroleum engineers and legislators working in today's multi-disciplinary design engineering team. These proceedings address five major areas of importance on and offshore: risk assessment, operations and operational safety, research, risk reduction and design safety, detection and control, and protective systems.

SESSION E:OPERATIONS AND OPERATIONAL SAFETY

CONTINGENCY PLANNING TECHNIQUES TO REDUCE RESPONSE TIME FOR TACKLING OFFSHORE WELL BLOWOUTS

by
KEN FRASER

NORTH SEA WELL CONTROL ENGINEERING LTD

Summary

This paper discusses Contingency Planning Methods which can be applied to predict the nature and correct response for potential Offshore Well Blowouts. The paper lists the procedures to be followed to collate relevant data and how they can be applied to reduce response times.

Index

1.Causes of Blowouts on Offshore Rigs.

2.Effective response prediction and management.

3.Application of collected data to reduce emergency response time.

1.1Causes of Blowouts on Offshore Rigs

1.0Introduction

Formation pressures are controlled during normal drilling and workover operations by means of the hydrostatic head exerted by the column of fluid in the well. This hydrostatic head is manipulated by adjusting the drilling or workover fluid density.

Such pressure control is termed Primary Control.

If Primary Control is lost in the well then Blowout Preventors, Surface Valves and /or Tubing Valves are used to provide a means of closing in the well to trap pressures and prevent well flow whilst a denser fluid is circulated into the well bore to regain Primary Control.

The use of this equipment is termed Secondary control..

During Production Operations, Primary Control is removed to allow the well to flow. Consequently, only Secondary Control can be used during Production. This is usually in the form of Surface and Downhole Safety Valves and a Xmas Tree.

Only if both Primary and Secondary well control is lost can a Blowout occur. The fact that they are lost does not mean that a Blowout will occur — only that the conditions then exist which would allow Blowouts to happen. Whether or not there will be a Blowout will depend on the exposed formations, their fluids and potential productivity.

For the purposes of this paper the rig operations during which a Blowout could occur are treated in 3 ways:

1.During drilling

2.During workover

3.During production operations.

1.1Blowouts during Drilling Operations

1.1.0Introduction

Any porous and permeable formation will try to produce its contained fluid into the Wellbore once Primary Control has been lost. Providing Primary Control is maintained

ie P Hydrostatic Head of Mud > P Formation

the well will not flow. When the well formations begin to flow, the well is termed to be kicking or ‘taking a kick'

All kicks occur therefore when Primary Control is lost.

The ‘kick’ is the precursor to the Blowout during drilling operations.

Causes of Kicks during Drilling

Since drilling is a dynamic process, the potential causes of a kick at any time are directly associated with the activity in progress at that time.

The Drilling Process can be generally considered in 5 modes.

1.Tripping in the hole

2.Drilling ahead

3.Making connections

4.Circulating

5.Tripping out of the hole

1.1.1Causes of Kicks while Tripping in the Hole

If the trip out of the hole has been completed without incident and the well is static during BHA handling then it is likely that Primary Control is effective. When we run in the hole we can affect this Primary Control in two ways:

a)Pressure surging creating losses

b)Stringing out of swabbed in gas

1.1.1a)Pressure Surging

That is by running in the hole the surge pressure caused by lowering the string is sufficient to break a weak formation causing a loss of hydrostatic head in the annulus. This loss of hydrostatic head can allow previously controlled formations to flow into the wellbore.

NB. It is loss in hydrostatic head which causes the kick and not the pressure surging itself. Providing that the hole is kept full then Primary Control will be maintained even with induced losses down hole.

1.1.1b)Stringing Out of Swabbed in Gas

When tripping out of the hole it is possible to swab in small amounts of gas as the BHA passes reservoir rocks without a clear indication at surface. Normally this phenomena exhibits itself in the form of TRIP GAS.

Trip Gas is a common occurrence on wells and indicates how easy it is to swab wells in. What is worth commenting on is how trip gas is usually recorded at ‘bottoms up’ circulation. This indicates that the swabbed in gas has stayed in the same spot in the wellbore during the Round Trip or if it has migrated, it has not migrated very far.

The migration speed of gas in mud is often quoted as being 1000'/hour however in the Author's view this figure is meaningless since the actual migration speed depends on influx make up, solubility, formation porosity, mud type, lithology and bore hole geometry.

All that can be said with any certainly is that the influx will not sink, it will either stay put or ascend.

If the influx ascends then it expands as the hydrostatic head exerted on it reduces. As it expands it should become detectable, however this depends on the sensitivity of the Rig Instrumentation and the vigilance of the crew.

Since most muds are engineered with a TRIP MARGIN built into them a small amount of swabbed in trip gas expansion will not immediately allow the well to kick, however it will obviously reduce the overbalance.

If there is a gas influx sitting in the Open Hole occupying a height of say y ft in 12 1/4 Open Hole and then a 8” BHA is run into it the height of the influx will become ± 2y ft.

This obviously reduces the hydrostatic head on the bottom of the hole and could be enough to allow a new influx to enter the well bore.

1.1.2Causes of Kicks Whilst Drilling Ahead

Kicks occur whilst drilling ahead for three reasons:

a)Penetrating a higher pressured zone.

b)Penetrating a loss zone and losing Primary Control.

c)Losing Primary Control due to gas cutting.

1.1.2a)Penetrating a Higher Pressured Zone

If a reservoir type rock is penetrated whilst drilling ahead which has a higher pressure than can be primarily controlled by the drilling mud then the well can kick.

1.1.2b)Penetrating a Loss Zone

If a new formation is drilled which has insufficient strength to support the hydrostatic head of a full column of mud to surface then the fluid level in the well will fall until it reaches an equilibrium point. At this point the hydrostatic head exerted by the mud will balance the formation strength of the newly drilled weaker formation.

If there are any reservoir type rocks higher up the hole which have a formation pressure higher than the new hydrostatic head exerted by the shorter mud column then the well can kick.

1.1.2c)Losing Primary Control due to Gas Cutting

As rock which contains gas in its pore spaces is drilled, this gas is circulated towards the surface together with the drilled cuttings. As it rises the hydrostatic head which is compressing it is reducing and As the gas expands it occupies a greater percentage of the annular volume hence reducing the hydrostatic head of mud below it.

Clearly any amount of released gas from cuttings will to some degree reduce the hydrostatic head of the annular mud column, however small...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• Acknowledgements
• SESSION A:RISK ASSESSMENT
• SESSION E:OPERATIONS AND OPERATIONAL SAFETY
• SESSION B:RESEARCH, RISK REDUCTION AND DESIGN SAFETY
• SESSION C:DETECTION AND CONTROL
• SESSION D:PROTECTTVE SYSTEMS