There are two ways of considering high pressure when planning wells. High pressure can be considered the ability to drill through the ‘window’ between the pore pressure and the fracture gradient. This narrow drilling window can occur at any depth, but is often located below one to two km. When the pore fluid pressure is well above normal it creates a narrow drilling window between the pore pressure in the formation and the fracture pressure. To drill through this window requires careful monitoring and control of the mud system, and ensuring that the mud weight cannot exceed the fracture at the last casing point.
An alternative way, sometimes used for describing high pressure, is to hardwire a number. In Europe, the HPHT [high pressure, high temperature] environment requiring additional well planning consideration would be wherever the subsurface pressure was higher than 10,000 psi and high temperature above about 150°C.
High pressure is there for a reason; something has caused it to be there. So it is important to understand what the sedimentation rate is and how fast sediment has been accumulating, because that influences the pressure in the sense that as rocks get buried they get compacted. This in turn means that they have to get rid of some of the fluid and the porosity goes down.
What we do at GeoPressure Technology, pre-drill, is to start off by considering, ‘what’s the context, and what sort of pressures might we anticipate as a result of those mechanisms?’ What we then do is examine what sort of data we have that can corroborate any of our ideas about what the pressures might be?
Imagine you’re exploring in an area that has never, ever been drilled. What data might be available for a pre-drill pressure analysis? The only data available is remote data, which is either gravity data or seismic data, and seismic data tends to be much more important.
If you move from a true frontier area to an area where quite a lot of wells have been drilled, you increase the ability to predict with confidence, as the knowledge and confidence is built by knowing what the rocks are and being able to test the pressures in any of those wells that have been drilled.
In terms of saying how confident we are, it just depends on what amount of information we’ve got from either surrounding wells, or, if you don’t have any wells, what’s the quality of the seismic data we’re using?
Non-reservoir rocks comprise the majority of the section that we drill, and that’s also true while drilling. While drilling through the non-reservoir section we try and get a sense of what the pressure might be if we drill into the reservoir. Reaching a reservoir with a mud pressure too low will be dangerous, causing a ‘kick’ - so reading the signs in the non-reservoir rocks is critical to good well drilling practice.
A lot of people don’t appreciate that when drilling in non-reservoir rocks you’re just looking for subtle indications of the relationship between the mud pressure and whether it can hold back the formation pressure. If you suddenly arrive in the reservoir, but you haven’t spotted those things as you’ve been drilling, then you’re suddenly going to have something called a ‘kick’, which you’ve got to handle and handle fast.
The first indication of this is that you’re receiving more mud back than you’re pumping in. In which case you must be receiving extra fluid from somewhere else, and that’s the interpretation that’s made - coming most likely from somewhere near the bit.
There are a number of operational things to consider here; one is that you want to increase the mud weight to hold back the pressure and extra heavy mud has to be pumped down. But how heavy? Sometimes heavy mud can be pumped in straight away and sometimes it is necessary to shut in and monitor the well. And by shutting the well in and monitoring it, you’re allowing an interpretation to be made of exactly what the pressure is where the influx is happening, and what mud weight is going to be required to ‘kill’ the well.
However, there can be some dangers when shutting in a well; you’re building up pressure all the way up through your borehole, and of course, what you’re concerned about is that the pressure is not so high that it would create a fracture so that fluid would then leak out of the borehole into the rocks.
In this instance, time is of the essence. Imagine for a moment that you’ve got an influx of fluid coming into the borehole - you’re now replacing high density mud being lost to surface with low density material coming in from the formation, so you’re lightening the total mud weight designed to hold back the formation pressure. So, all the time you’ve got an influx and you’re not handling it or not recognising it, you’re losing the capacity to be able to handle it. As such a very quick response is really important. And, from my experience over many years of working with companies, the problem areas are mostly involved where high pressure wasn’t expected, and so they weren’t monitoring closely and weren’t thinking, ‘if we have an influx, this is what we’ve got to do’.
And you don’t have a lot of time sometimes to react to an influx. You know you’re in a high-pressure area, and you can anticipate it, even if you’re drilling exactly to the book, if you drill and you respond very quickly. Nearly every single incident I’ve ever been involved with or know about has been handled professionally and without incident. However, I think the recent experience in the Gulf of Mexico indicates just how important it is to monitor pressures until the well has been successfully abandoned. The pressure and the challenge of that high pressure remain there until the well has been satisfactorily secured.
In my opinion the oil industry does a phenomenally good job in preparing for and drilling, in the main, in extremely hostile conditions. If you look back to the 1960s, we were looking at drilling in the North Sea for the first time and at that time that environment was considered the frontier. This then was 600ft of water, or 200 metres, was the shelf edge. Now we’re drilling in 5000 to 10,000ft and most of those wells, in fact almost all of those wells, are drilled safely and responsibly and without incident.
This is all to satisfy a global demand for crude oil. So, until we’ve weaned the world’s population away from oil we are going to have to supply an energy mix that includes crude oil from these high-pressure regions. We have to go and find oil where it exists, and I think the oil industry is prepared to accept what is needed in terms of regulation and new technology, and works very hard to get that technology right.
Ultimately I don’t think we should allow the press and popular opinion to think that the industry is failing because there have been one or two incidents. The industry, in the main, is doing a phenomenally good job. The rare incidents highlight the risks and remind us all of the high standards of safety and respect for the environment needed, especially in the future when more of our global oil will be sourced from hostile and other high pressure settings.IKON - GeoPressure Technology
Richard Swarbrick is the director of GeoPressure at Ikon Science. The GeoPressure Technology business is a training and consultancy for oilfield pressure problems, offering 'best practice advice' for the knowledge and interpretation of pressure data.
For further information please visit: www.ikonscience.com/geopressure
*Reproduced from PESA News Resources, No 107, August/September 2010,www.pnronline.com.au/article.php/178/849