From Tony Mason this morning: "this particular reservoir has been shut-in for over 20 years and may have “recharged” – hence we are producing more oil than originally anticipated along with some gas."
An article on re-charging:
http://trilogymedia.com.au/Thomas_Gold/recharging/
Recharging of oil and gas fields.
There have been numerous reports in recent times, of oil and gas fields not running out at the expected time, but instead showing a higher content of hydrocarbons after they had already produced more than the initially estimated amount. This has been seen in the Middle East, in the deep gas wells of Oklahoma, on the Gulf of Mexico coast, and in other places. It is this apparent refilling during production that has been responsible for the series of gross underestimate of reserves that have been published time and again, the most memorable being the one in the early seventies that firmly predicted the end of oil and gas globally by 1987, a prediction which produced an energy crisis and with that a huge shift in the wealth of nations. Refilling is an item of the greatest economic significance, and also a key to understanding what the sources of all this petroleum had been. It is also of practical engineering importance, since we may be able to exercise some control over the refilling process.
The debate about the origin of all the petroleum on Earth lies in the center of the subject. If we really knew that it is only biological materials, which, in their decay, could produce hydrocarbons, then the quanities that could ever be produced would be limited by the biological content of the sediments. But then the clear and strong association of petroleum with the inert gas helium would have no explanation; the finding of hydrocarbon gases, liquids and solids on most other planetary bodies in our solar system which have surface conditions quite unsuitable for surface life, could not be understood; the presence of hydrocarbons which we now find in abundance in basement rocks would also remained unexplained.
If we accept the fact, now known full well, that hydrocarbons are a common constituent of the cosmos and the planetary condensations that formed in it, then we have a totally different viewpoint. Hydrocarbons are stable down to great depths and the high temperatures there, contrary to many statements that have been made that the temperature reached at depths between 30,000 and 40,000 ft would dissociate most of the hydrocarbons. But these calculations are seriously in error, because they ignored the strong stabilizing effect of pressure at depth, that had been calculated by Soviet (Ukrainian and Russian) thermodynamicists.
The existence of diamonds, crystals of pure carbon that form at pressures which are not reached on earth at depths of less than 140 kilometers, proves that unoxidized carbon exists at such depths, and also carbon-bearing liquids must flow there that can deposit carbon at high purity. High pressure fluid inclusions in diamonds prove that liquid or gaseous hydrocarbons were present at their formation. Present day meteorites give us examples of the solids responsible for the building up of the Earth; among those only one class, the carbonaceous chondrites, contain much carbon, mostly in unoxidized form. That this material is present in the Earth's interior in large abundance is shown by the distribution of noble gases and their isotopes that have emerged into our atmosphere and show distributions that are strikingly similar to those in carbonaceous chondrites, but dissimilar to those of any other class of meteorites. The presence of this type of material would account for a continuous supply of hydrocarbons to the atmosphere, as the outer layers of the mantle heat up over time and make fluids form from the solid hydrocarbons that were included in the forming Earth (as also in most of the other planets and their satellites, in the asteroids, comets and interplanetary dust grains). Such fluids are less dense than the rocks, and buoyancy forces will propel them upwards.
Rocks and lower density fluids can co-exist at any level in a solid planetary body, provided that the pressure of the pore fluids is sufficiently high to make the differential pressure between rocks and fluids less than the crushing strength of the rocks. For a static case (with no upward flow of the fluid), this would result in pressure domains, within which the fluid pressure shows a pressure gradient with depth given just by the density of the fluid (the "head"), and where the bottom of each domain is at the level at which the fluid pressure is insufficient to maintain pore spaces against the higher pressure of the rock. (See Figure 1.) It is assumed here (for the static case) that this makes a complete barrier. As for the top of any domain, this cannot be at a level higher than that at which the fluid pressure equals the rock pressure, since fluid pressures in excess of this value cannot be maintained in rocks that on a large scale and in long time-intervals, have no tensile strength and therefore cannot resist the intrusion of the fluids and the generation of new pores.
If we consider the case of a slow upward migration of fluids (liquids or gases), then this picture changes to one in which each domain
Idealized stacked pore pressure domains that make up a stepwise approximation to the rock pressure.
Pc is the critical pressure at which the pore fluid pressure cannot support the rock against crushing.
will be stacked on another one below, all the way down to the level of origin of the fluid. The fluid pressure would thus make a stepwise approximation to the pressure in the rocks. Now none of the barriers can be absolute, since they would be torn open by the fluids that arise from deeper and higher pressured domains. But the barriers would be torn open in each case only to the point at which the flow to the overlying domain causes it to suffer a pressure drop resembling that of the static case. This rule will apply whatever the nature of the rock. The heights of the domains will be determined by the rock and fluid densities and the crushing strength of the rocks; this height has been found to be between 10,000 ft and 15,000 ft in many sedimentary rocks, and in excess of 20,000 ft in granitic basement rocks. The upward seepage of methane is very widespread all over the Earth, as is shown by the great extent of methane hydrates on the ocean floors and in permafrost regions on land, where mostly no shallow source of methane can be invoked.
Vertically stacked domains of hydrocarbons have been found in all cases where drilling was sufficient to display them. The consistent tendency to find hydrocarbons below any producing region has been given the name of "Koudyavtsev's Rule", after the important Russian petroleum investigator who discovered this effect and collected a very large number of examples of it from all parts of the world. This rule would be the consequence of a deep origin of hydrocarbons and a steady process of outgassing.
With this picture in mind we would readily understand that refilling of hydrocarbon fields is possible and even probable. But if merely the steady upward flow from deep sources had been responsible, the refilling time scales would be much too slow to be of commercial interest, or to match the speed that appears to have been observed. A limit to the global average of that flow speed can be derived from the approximately known supply of carbon to the atmosphere over time. On that basis a large gas field may be recharging in times reckoned in tens of thousands of years, still very short compared with many millions of years, as had been the widespread belief. But observed refill times of just a few tens of years cannot be explained by this. However another effect will set in when a field is under production and the pressure in its domain is thereby diminished. The pressure difference between the producing domain and the one below it will then be increased, resulting in a higher rate of flow through the low permeability layer that divides these domains, or it may even result in a physical rupture of that layer.
There is an analogous case known in Kuwait. The extraction of goundwater at the shallow levels results in the disintegration of the barrier to the oil levels just below, and the water in the wells is suddenly replaced by oil. The delicate pressure balance that had established itself, just up to the level that the strength of the rock could bear, had been upset. Similarly in stacked domains of hydrocarbons, the lower domains will be opened quickly, once the upper ones had been depleted and the fluid pressure thereby reduced sufficiently. This process can be fast, just as it is in Kuwait, where we had the advantage that a different liquid (water) filled the upper domain, so that one could identify the rupture to the oil filled domain below.
This type of refilling process thus allows exploitation of the domain below that from which production had been obtained before. In turn, when this lower domain had suffered a sufficient pressure loss, the process may continue to the next lower domain. How much more than the original content of a hydrocarbon field can be produced in any one case will depend on numerous details of the formation, but present indications are that it is often at least double. The present global gas and oil glut appears to be due to this effect, and we have not yet seen the end of it, or any indication that it will end soon. Gas fields will be subject to faster refilling than oil fields, and moreover the volumes of gas in lower domains will in general be greater due to the higher pressures there and the higher compressibility of gas. Gas will thus become more plentiful than oil for this reason alone, but gas seems to be generally more plentiful and more widespread than oil. The environmental advantages of changing from coal or oil to gas, by far the cleanest of all combustible fuels, are very large, and the changeover is at present still handicapped by the mistaken belief that the supplies of gas will run out soon.
Thomas Gold
September 1999
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