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formation. The rubble prevents fracture healing, making the use of
proppants unnecessary.
 Damage removal refers to other forms of removing
formation damage, such as flushing out of drill fluids [6].
Flexible coiled tubing can be wound on a large diameter
drum and can be inserted and removed much quicker than tubing
installed from rigid pipe segments. Well workover equipment
including coiled tubing is often mounted on well workover rigs.

Unconventional sources of oil and gas
The descriptions above are valid for conventional oil and
gas sources. As demand increases, prices soar and new
conventional resources become harder to find, production of oil
and gas from unconventional sources become more attractive.
These unconventional sources include very heavy crudes, oil
sands, oil shale, gas and synthetic crude from coal, coal bed
methane and biofuels. Estimates for conventional proven
producible oil and gas reserves vary somewhat. The current
increase in consumption is just under 2 % per year, or 15% - 20%
in a decade for different products, even with energy saving efforts.
If this trend continues the time to go figures quoted above will be
reduced by one third.
Estimates on undiscovered conventional and
unconventional sources vary widely as the oil price; economical
production cost and discovery are uncertain factors. With
continued high oil prices, figures around 1-2 trillion barrels
conventional (more gas than oil) and 3 trillion barrels
unconventional are often quoted, for a total remaining producible
hydrocarbon reserve of about 5 trillion barrels oil equivalent.
Within a decade, it is expected that up to a third of oil fuel
production may come from unconventional sources.

Extra Heavy Crude
Very Heavy crude are hydrocarbons with an API grade of
about 15 or below. The most extreme heavy crude currently
extracted are Venezuelan 8 API crude e.g. in eastern Venezuela
(Orinoco basin). If the reservoir temperature is high enough, the

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