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P E T R O P H Y S I C S
Just what exactly is Petrophysics? This brief is for those of you seeking elucidation.
Where Does Oil & Gas Come From?
Contrary to popular myth, oil (and gas) are not found in gigantic caves
underground. Hydrocarbons are found inside rocks - not all rocks, just
those with gaps between the rock grains. These gaps are called porosity
and can best be visualised by thinking about how a sponge holds water
within itself.
Unfortunately, porosity alone is insufficient for a rock to contain
hydrocarbons. Since oil and gas are both lighter than water, they will
float on top of it. Since most of the underground rock with porosity
contains only water, when hydrocarbons are generated from deeply buried
organic material they try to move upward to float on top. When the
rising hydrocarbons reach a barrier - a rock with no path through it,
the hydrocarbons cannot rise anymore, so they accumulate in the rock
beneath this "seal". A "pool" of hydrocarbons will build up below the
seal and within the rock porosity until part of the column can find a
path upward. No more hydrocarbons will accumulate thereafter below that
particular seal.
How Do We Find These Traps?
The traps are generally formed with a domed or "anti-clinal" shape i.e.
the hydrocarbons accumulate below an "upside-down bowl" shape. Oil
exploration personnel look for these structures in the rocks below
ground by looking for patterns within sound waves reflected from such
structures. This technique is called "seismic acquisition &
interpretation".
Once it is thought a trap is present in the sub-surface, the only way
to be sure it's got hydrocarbons in it is to drill a hole from the
surface to the underside of the "trap".
Petrophysics or How Do We Know What's In There?
This point is where Petrophysics comes in. It's a Petrophysicist's job
to interpret all the data acquired up to and including the drilling
phase to determine just what is in the trap.
To assist with this process, a number of measuring tools are lowered
into the drilled hole on the end of an electric cable or "wireline".
These tools take a range of measurements including the natural
radioactivity of the rock, it's density, the speed of sound through the
rock and it's electrical resistivity. The Petrophysicist must interpret
all the available information to determine the porosity and whether
hydrocarbons are present and if they are, how much is present and will
it flow out of the rock?
To understand how difficult this problem is, visualise drilling a
tunnel 20 cm (8 inches) across that is 3 kilometres (approximately 2
miles) long into the base of a mountain. The problem we need to solve
is "what is the porosity of the rock, what fluids does it contain and
how much of each - all at the end of the tunnel? Whatever we have
measured at the end of the tunnel is then assumed to be representative
of most of the mountain!
Detective Work
Have you ever tried looking at a jigsaw puzzle with half of the pieces
randomly removed? Is it difficult to work out what the picture is
supposed to show? Sometimes it's impossible, other times it is
straightforward.
Petrophysics is similar. Basically the problems are normally
under-determined i.e. there are more unknowns than knowns, so some
assumptions must be made if the required reservoir properties are to be
calculated. This "interpretation" is where the detective work comes in.
A good Petrophysicist will gather ALL the available data and consider
what it means BEFORE he decides what assumptions to impose on his well
evaluation.
Computing Guru
To fit all these pieces of data together and test all the different
scenarios in a realistic time frame, excellent computer skills are
required. Good Petrophysicists tend to be amongst the most computer
literate of oil company personnel.
At the Centre
Once the Petrophysicist has estimated the required reservoir properties
he passes these onto the geologist and reservoir engineer for use in
their modelling. The geophysicist and petrophysicist will also work
together to calibrate the seismic measurements. In fact, data quality
controlled and interpreted by the Petrophysicist is used by all other
sub-surface disciplines. If the Petrophysics are poor, then all other
interpretation work in a subsurface development suffers. The
Petrophysicist provides the foundation upon which sub-surface models
are built!
Tested Everyday
Unlike many jobs, Petrophysics gives you the opportunity to learn from
your mistakes. When you recommend an interval for flow testing, you
find out quickly whether or not you are right! If you are wrong, the
mistake tends to be an expensive one, so you try hard not to make any!
This almost daily testing keeps a good Petrophysicist on their toes,
ensuring a fun and challenging job!
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