More Than Log Analysis!

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!