If I were to say that 2008 was the year of geomechanics, specialists would think I had lost my grip. Geomechanics has been studied and used in oil and gas prospection, drilling and production for decades. I started writing this editorial as an educational piece and slowly realized that a lot has been going on in this field recently, and that if 2008 is to be the year of anything in upstream IT, then I reckon it has to be geomechanics. Before I defend that claim, I had better explain what geomechanics is all about.
A popular dinner table or coffee machine dialog starts with the question, ‘When you take petrol (sic) out of the ground, does the surface of the earth sink?’ The correct answer is something like, ‘Err, that’s a good question,’ or ‘Um, it depends...’ A simpler answer is, ‘No.’ As oil or gas is produced from a reservoir, water flows in to replace the oil and generally, the rock itself has strength that holds up the overlying strata.
Consider the Ghawar field in Saudi Arabia, the largest in the world. According to Saudi Aramco’s Shiv Dasgupta, speaking at the Las Vegas SEG meet last month (more on pages 6&7), satellite InSar radar has showed no surface deformation despite the fact that some 58 billion barrels (about 10 cubic kilometres) of oil have been extracted from the field since first oil in 1951.
How does Ghawar hold up? The reservoir is a limestone with a large amount of pore space and great permeability, but the rock matrix, the framework of the reservoir, is strong enough to hold up the overlying strata.
But it is not like this everywhere. Think what would happen if you pumped a seriously large amount of water into a reservoir, much more than required to replace the produced oil? Eventually, the pressure exerted on the overburden will exceed the weight of the rock, causing the overburden to start ‘floating’ on the injected water producing not subsidence, but perhaps a small rise in the ground surface. This is an ‘overpressured’ reservoir, actually quite a common situation in nature.
To understand these issues, imagine an earth made of a strong, porous rock filled with water to the surface. If the rock is strong, it is holding all of its own weight. If you could place a pressure gauge inside the rock at depth, you would measure the ‘lithostatic’ pressure, the weight of a unit area column of the rock from measurement point to the surface. If you moved the gauge into the pore space, you would measure a considerably lower pressure, the hydrostatic pressure, of a unit column of fluid above the measurement point. That’s fine for strong rocks like Ghawar’s limestone or a consolidated sandstone. But what happens when you put a decent section of unconsolidated sediment into the above situation? Above, the pressure regimes are as before, lithostatic in the rock and hydrostatic in the pore space. But below the shale, and depending on the shale’s own strength, suddenly some or perhaps all of the weight of the rock above is transferred to the fluids in the unconsolidated shale.
This is an overpressure situation which can have lots of consequences, both good and bad. If the shale is capable of producing hydrocarbons, a bit of heat will get the cooking process going (given enough time) and the geopressure will expel the hydrocarbons into nearby reservoirs where they will hopefully be trapped (otherwise... think of the pollution!). If you are drilling through these zones, you may have to watch out for the sudden changes in pressure regime, particularly as you penetrate an isolated reservoir within the shale, which may produce a mud ‘kick’ or even a blow out. Inside the reservoir, peculiar pressure regimes may help or hinder production and upset the fluid flow models that reservoir engineers play with.
For earth scientists of a certain age and background, like myself the North Sea’s Ekofisk field was an eye opener. Although the mechanism was more complicated than the process I describe above, the production platform was sinking by a few meters as the oil was produced, and was jacked up in a rather extraordinary feat of engineering. The complex process that caused Ekofisk to sink was more to do with a gas cloud in the overburden than the reservoir itself. Interestingly, this gas cloud was very easy to see on the seismics although it was interpreted by all as a fossil collapse structure over the field. An early example of seismically derived geomechanics in a sense.
Today, a lot of activity takes place in ‘geomechanically sensitive’ environments like offshore West Africa and the Gulf of Mexico. Problems vary from unconsolidated sediments, through mobile salt to high pressure high temperature environments. The drill bit enters all these like a ‘foreign body’ with results that can surprise. Speaking at the SEG in front of an image of a burning rig, Alan Huffman (Fusion Petroleum Technologies) described geopressure as having ‘serious HSE consequences’ as well as being key to prospect viability. A Schlumberger/Hess presentation showed how geomechanics is combined with life of field seismics to mitigate well failure.
Geomechanics has come of age technically but also commercially with the acquisitions—by Halliburton, of geomechanical boutique Knowledge Systems (OITJ May 2008), and Baker Hughes of Geomechanics International (OITJ April 2008). Schlumberger and Weatherford jumped the 2008 gun with their acquisitions of, respectively VIPS (May 2007) and Advanced Geotechnology (April 2007).
Finally, a compelling example of the consequences of getting geomechanics wrong was evidenced at the 2008 AAPG Cape Town conference where a debate on the disastrous 2006 Lusi blowout in Java, Indonesia, and the resulting spectacular mud volcano put the probable cause as the drilling of a gas exploration well.
Finally an apology for this late issue. Yes, it is now 2009 (Happy New Year by the way) and yes, this is the December 2008 issue. Unfortunately, our conference attendance was very intense in the fall and we have a considerable ‘backlog’ of writing. OK I’ll own up, I took some time off over the festive season too! But if you think about it, a week or two of cumulative slippage over 12 years of publishing isn’t too bad...
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