FRAC TANK EXPLOSIONS

Introduction

R.J. Goodman, EHS & Operations Training Manager, XTO Energy

Investigated 4 separate frac tank explosions that occurred on two separate jobs.

8 years of E&P safety experience Degree in Fire Science

References

API RP 2003 Sixth Edition, September 1998. Protection Against Ignitions Arising Out

of Static, Lightning, and Stray Currents

API 545, Working Group, Standard for Lightning Protection for Hydrocarbon Storage Tanks

Learning Objectives

Explore 4 investigations

Understand how static electricity generation relates to frac tank explosions

Demonstrate the usefulness of flowback gas buster systems to prevent frac tank explosions

XTO Energy Explosions

Four Explosions Occurred November 10, 2005 Major County, OK (2

Events) March 31, 2006 Major County, OK April 3, 2006 Major County, OK

All four explosions had the following characteristics in common:

1. An air/foam mixture was pumped down the tubing and returned up the tubing/casing annulus.

XTO Energy Explosions

2. Air/foam assisted flowback operations had been engaged for 12 15 hours

3. Gas, water, oil, sediment and oxygen were piped from the wellhead to temporary non-pressure rated frac tanks

4. Each frac tank was internally lined with an Epoxy type liner

5. Each tank had a screwed together downcomer protruding into the frac tank 2 3

XTO Energy Explosion Site 1Nov. 10, 2005 Major County, OK

Location Configuration

Downcomer entered the front of the tank.

XTO Energy Explosion Site 1Nov. 10, 2005 Major County, OK

Picture of Downcomer

Internal Configuration of Frac Tank: Site 1

XTO Energy Explosion Site 2March 31, 2006 Major County, OK

Downcomer Configuration

Downcomer is chained to the tank

Static Arc Point

XTO Energy Explosion Site 2March 31, 2006 Major County, OK

Oil/Water/Natural Gas contents

Downcomer

2 Part Epoxy Lining on Wall

Static Builds and Arcs from Fluid to Downspout

XTO Energy Explosion Site 2April 3, 2006 Major County, OK

Investigation Conclusions

Static electricity was generated when oil, water, gas and sediment passed through flowline.

The lined frac tanks reduced static discharge through tank walls.

While the tanks were externally bonded and ground, none were internally grounded to the charged liquids.

All explosions occurred while airing back.

Fire Triangle Application

Air/F

oam

Fl

owba

ck

Static

Electricity

Spark

Natural Gas & Oil Vapors

Fire Triangle Application

Air/F

oam

Fl

owba

ckStatic

Electricity

Spark

Natural Gas & Oil Vapors

Oil, Water, Gas & Sediment flowing

Velocity of liquids flowing through a 2 pipe

Insufficient Relaxation time for the charged particles

Insulated Lined Tanks

Air containing 21% O2 is injected down hole

Air containing 21 % O2 is at the tank hatch

Oil and Natural Gas vapors are forced out of the tank and pass through the narrow 1 x 1 tank hatch.

Fire Triangle: AIR An air/foam mixture is pumped down

the tubing and returned up the tubing/casing annulus. The air/foam mixture contains 21% oxygen prior to the mixture being pumped down hole. Therefore, oxygenated gas is returned to the surface after a few hours of stimulation.

The process of pumping an air/foam mixture serves two functions:

1. Lightens formation fluids which allows faster fluid recovery.

2. Pressure forced down the tubing annulus allows the formation fluid to flow more easily up the casing annulus.

AIR

Fire Triangle: AIR

Atmospheric air containing 21% oxygen co-mingle with oxygen enriched flowback gases at the tank hatch. The oxygenated atmosphere

presents a problem because the ignition source (flowback downcomer) penetrates the tank where maximum oxygen saturation occurs.

AIR

Fire Triangle: FUEL

The Fuel Sources can be one of the following: Natural Gas Oils that emit vapor

Fuel vapors collect inside the tank and are forced through a 1 x 1 hatch on the tank.

FUEL

Fire Triangle: Static Generation

Turbulent contact of dissimilar fluids such as water or gas flowing through a liquid hydrocarbon. API RP 2003, page 3

Fluid velocity and turbulence are key components in static generation.

Nonconductive Flammable Liquid -Oil is considered nonconductive and holds a charge better than produced water. Saltier liquids allow electric charges to flow easier through them.

HEAT

Fire Triangle: Spark GapAPI RP 2003 4.1.3.4

Loose floating conductive objects or debris inside the container.

Conductive downcomer which does not reach the bottom of the tank.

Gage rods or side wall probes which are not connected to the bottom.

Gage tapes, sample containers or thermometers which are lowered into the tank vapor space.

Ungrounded couplings or hoses in the tank.

HEAT

Fire Triangle: Static Spark

In order for ignition to take place there must be enough heat generated to ignited the air/fuel mixture.

In the case of the frac tank explosions, heat is generated when a static generated spark jumps the gap created between the downcomer and charged liquid below.

HEAT

Typical Frac Tank Configuration

Review: Why is Static Energy building?

1. As oil, water, gas and sediment travel through the flowline they become positively charged.

2. Charged particles are projected into a tank and do not have sufficient relaxation time.

3. Charged particles are further charged when they fall into the liquid level below.

4. The liquid level in the tank contains non-conductive flammable liquids.

HEAT

Review: Why is Static Energy building?

5. As the Charged liquids continue to build, they look for a path to ground.

Lined tanks further insulate the non-conductive flammable liquids and increase static build up.

Caution: Static can build in an unlined tank if build up surpasses discharge to ground.

6. Eventually, the charges build enough to arc from charged liquids inside the tank up to the elevated downcomer.

7. When the air/fuel mixture reaches the appropriate ratio BANG!!!!!

HEAT

What Are The Mitigation Options?

Remove the air? Cant do that because we want faster

stimulation. Air will always be at the tank hatch.

Remove the fuel? Not unless we want to be out of a job!

Remove the ignition source? Now were talking!

Mitigation Option 1Grounding, Bonding & Charge Reduction

Most people think externally grounding the tank is the best answer.

API RP 2003 4.5.3 Grounding Storage tanks on grade-level foundations

are considered inherently grounded for dissipation of electrostatic charges regardless of the type of foundation. The addition of grounding rods and similar grounding systems will not reduce the hazard associated with electrostatic charges in fluid.

Mitigation Option 1Grounding, Bonding & Charge Reduction

Internal grounding or bonding is the better answer. API RP 2003 4.5.9.1.b.2

The tank should have a metal plate with a surface area no less than 30 in. per 100 gallons located at the tank bottom, and bonded to an external ground. The plate provides an electrical path between the liquid contents and ground through which the charge can dissipate.

Mitigation Option 1Grounding, Bonding & Charge Reduction

API RP 2003 4.1.5. To Prevent charge generation:

Avoid Splash and Misting Operations Limit initial fill rates and maximum flow rates Use sufficient relaxation time downstream of

pumps and filters Ground conductive fluids while filling insulated

containers Remove or ground spark promoters in tanks Use sufficient waiting period before sampling

Mitigation Option 2Gas Buster Systems

Use the right equipment for the task at hand. For years companies have used tanks

designed to hold non-pressurized liquids as flowback tanks. This process will continue to work until the forces of nature and the fire triangle align. After the forces align, you will experience your first fire or explosion.

Mitigation Option 2Gas Buster Systems

Gas buster systems eliminate static build up and arc gaps.

This process is accomplished by: Using an unlined open top tank (grounding

principle) Flowback fluid and gas enter the tank through a

2 line. After a few feet of travel, the line size increases to 4 then to 8 then to 16 and finally to 24 in diameter. (charge relaxation)

The increase in line diameter serves 3 functions: Decreases fluid velocity Increases fluid charge relaxation time Increases surface area and allows charges to escape

Progression of Gas Buster Piping

2 Line 4 Line 8 Line 16 Line 24 Line

The line splits at a header system. The header split, along with the increase in pipe diameter, slows the velocity of fluid!

Mitigation Option 2Gas Buster Systems

Slots approximately one foot in length are cut in the bottom of the 24 section of piping. (too rich or too lean to ignite) The slots allow the liquids to fall out the bottom

while allowing the gases to escape up the sides. Large tank hatches located along the sides

of the tank can be opened to allow for faster dispersion of gas vapors. This process eliminates the rich gas volume

between the fluid and the top of the closed tank while reducing static build up potential.

Gas Buster Piping and Vents

Side Vents at back of Frac Tank

Gas Buster System in Wyoming

Gas Buster System in Wyoming

Mitigation Option 3Slotted Downcomer System

Use a slotted downcomer that extends from the end of the riser at the top of the tanks back hatch to approximately 1 from tank bottom. Submerge the downcomer in fluid. Submerging the downcomer will bond the charged liquid to the downcomer and should eliminate spark gap. Cutting 1 vertical slots in the downcomer pipe should allow a lot of the gas to break out of fluid and escape upward out of the hatch (less agitation). This is considered a poor-boy gas buster.

Ensure the downcomer is bonded to the tank hatch lip. Per API RP 12R1, discharging flowback contents

into the fluid should reduce the risk of sand or solids causing metal contact spark.

Mitigation Option 3Slotted Downcomer System

Larger diameter pipe will allow the velocity of fluid and debris to decrease and result in lower static potential.

If flowing back through 100 of 2 3/8 tubing with steel connections increase the output or downspoutdiameter to 4. Doubling the pipe diameter decreases the velocity by approximately 4 times thus lowering the static potential significantly.

API RP 2003 recommends using uncoated frac tanks. This provides protection against ignition arising out of static, lightning and stray currents while allowing for maximum gas/air dilution.

Allow no plastic or rubber connections in the flowback line.

Summary

Multiple frac tanks have exploded throughout the U.S.

Static builds up any time fluids, gases and solids flow through pipe or hoses at a sufficient velocity.

Static dissipation is hampered when it is generated inside lined tanks.

Static may not dissipate through unlined tank walls when nonconductive flammable liquids are involved in the flowback process.

Flowing through an open top working pit with gas buster system is the safest option to prevent closed top frac tank explosions.

Questions?