Geophysics and the Drilling Location. Where is it, and how big is it?

When Sandy Lucas approached me about writing this piece my first thought was that ‘we all know what we do, why explain house building to house builders?’ However, as I thought about it, it occurred to me that given my present location maybe the thing to do was to contrast how things are done in Canada with a typical play versus an offshore location (say the NW shelf of Australia).

So moving forward in time, where are we? The data has been shot (hopefully 3D) and has been delivered to the processing shop to begin the transformation into useable data for interpretation and while this isn’t really what we are here to talk about, it nonetheless plays a crucial role in the process going forward. Here I will assume that everyone is fairly clear on a standard processing flow that may or may not include PSTM for a Canadian 3D. An average size of around 50 sq. kms. gives you a delivery date of somewhere around a month to a completed product loaded on the work station and ready to go. At this point we come to our first big difference: the offshore project will have an average size of about 500 sq. kms. and will most certainly use PSTM, with the majority of projects these days using PSDM due to a tiny little velocity problem that has us looking at structures that don’t close in time but will in depth. The end result is a processing time that is now more like 4 – 6 months to that same completed product.

Now the real work that we are here to talk about begins. Your data is loaded, you open up the project and at that instant your manager walks into the office and says ‘so are you done yet, how many locations do we have?’ This is a truism world wide much to the dismay of the interpreter! However, forging onward our vaunted geophysicist works through the data, eventually producing a set of maps outlining the potential on the property being evaluated. Here again we run into some differences going forward as the drilling location is picked. Our Canadian based “wizard” will sit down with the geologist and patiently explain why the picks are where they are often repeating ‘no that most likely isn’t a channel’! Once this detailed evaluation process is complete, coordinates are picked and the location is sent off to drilling. Depending on the amount of well control in the area the geophysicist may or may not be asked to provide SS elevations for the major tops and the zone of interest. This can usually be accomplished with a simple velocity function or, depending on the software available, a depth conversion may be run for the area surrounding the well location. In the case of the offshore geophysicist there may not be any well control on the 3D at all, in which case a velocity function from the closest well will be brought in and applied to the drilling curve supplied from the drilling engineer. If the 3D has already been depth processed then depths for each horizon are read off directly and entered into the well plan. Otherwise the data is transferred into a software package like Petrel and the data is depth converted based on a geologic model determined by the geologist and geophysicist. In this case the geophysicist is always the lead in determining depth to tops supplied to the drilling department. The interesting part is that due to the sparse control uncertainties for tops can be as much as +/- 40m, an uncertainty that would cause Canadian well site geologists to ask if the office guys really knew what they were doing!

So at this point where are we? The well is now drilled and miracles of miracles no one has walked into your office and said ‘but you said that it would be there’! In fact there are several people walking around the office talking about what a great discovery they just made! This is all good news as it means that your hard won idea has in fact worked. At this point the logs are examined, pay calculated and reserves determined, again in the Canadian environment for an average location there will be a determination of go forward economics based on the cost to tie in the well. In Canada this is generally a relatively small number due to the tremendous amount of infrastructure already in the ground. “Go forward” numbers as determined by the reservoir engineer in conjunction with the geologist and geophysicist are a mainstay of our business. Despite what the logs show, the money for the well is now sunk so the question is whether or not the completion and tie-in can be paid out with an acceptable rate of return. This allows the company to boost production and cash flow from a well that is not a 100% success. Our overseas team however faces a completely different “go forward” set of problems. In almost every case the well will be P&A’d as a matter of course unless it is a development case. In our situation however infrastructure is most likely tens of kilometers away and the team must now rework the data based on the new information. As complete a reservoir model as possible will be built to determine the potential size of the discovery, which may take several days or weeks depending on the complexity of the problem. If the data has not been depth migrated it may be at this point to attempt to improve the image of the pool. While it seems like a long slow process the issue is that in today’s climate a 300bcf discovery may not be economic due to location so it is crucial that as accurate a model as possible is created.

So in summary what have we seen? We have two geophysicists that have taken a data set from start to finish, resulting in the picking of a drilling location. Both are involved with the prognosis determination provided to the drilling department and both are involved with the post drill evaluation of the prospect. The main difference is the time involved in each area. While the offshore geophysicist seems to have the luxury of months to ponder what to do next he faces the issue of huge development costs that need to be taken into account, a sometimes daunting task. The Canadian geophysicist, on the other hand, has made her determination usually within a day’s time and moved on. Simple right? Well, not necessarily so, as the data must now be incorporated into the overall plan since that manager is headed back down the hallway to find out ‘where we go next’! In either case the race to find the next one has begun again.