Learning to Speak Cajun: Geomodeling for Well Planning

Luke is back at the Cognitive Whiteboard, looking at the importance of learning the language and processes of your colleagues when planning wells.

Click on the image below for a detailed view of this week's whiteboard.

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Hello, welcome back to the Cognitive Whiteboard. My name's Luke, and today I'm going to use my outrageous Australian accent to try to speak Cajun. I want to talk you through a drilling history that I've been involved with, a six-well campaign where something went wrong and we used geomodeling methodologies to make sure that it wouldn't happen again. 

So, let me talk you through the field firstly. It's a salt raft on the West African coast that has a number of different reservoir units. The main productive interval was this yellow one through here where the lower two units particularly were of much better quality and we'd had a lot combining production across those zones. The field had initially been exploited on primary decline and then water injection, so it's quite a complex reservoir management story. 

What we needed to do was make a better job of exploiting the shallowest unit of that main reservoir. We had, however, a reservoir above us that had been under production for some time, and it had some changes that had resulted in the pressure regime associated with that that we hadn't accounted for properly. So, in the pre-drill pressure prediction, we essentially said that the   pressure was going to be relatively consistent across the field. The fracture gradient would therefore be also pretty similar, and our mud weight window required only one casing depth, and we would drill through both the shallow unit and the target reservoir with the same open hole section. 

When we drilled it, however, we encountered a problem, and the problem was this. We had had, above that target, the biggest producer of that shallow reservoir, and so it had been causing a lot of pressure depletion locally in that area. We had a subsurface blowout at that point. We had a kick that we hadn't accounted for, we didn't anticipate it, it hadn't been observed before, a small kick that would've been easily contained by the well design. However, with this depletion above here, the impact of that on the gradient of the shallow reservoir with that additional kick weight resulted in a fracture inside here and massive losses of our mud system into that reservoir unit. We even ended up producing quite a lot of that drilling mud from the reservoir later on.

So, we had a situation where that well design didn't work. The problem was we had five other wells that were designed identically that needed to do the same job, and from the driller's perspective, they were not going to go near it and touch that with a bargepole. We were just post Macondo, so everyone was very sensitive around our drilling parameters. We didn't want to have anything going wrong, even more than normal, and so what they wanted to do was redesign with an additional casing string that would have required us to case between the two units that are quite close together preventing us then from being able to get a horizontal section into the target reservoir. 

That sub-vertical production would have resulted in a roundabout 60 million barrels of lost reserves. So, we really needed to make sure that that had to happen. This was a field in late production life, so it was probably the last campaign that would ever exploit that particular reservoir, and we really wanted to make sure that this is what had to be done.


We spent some time with the drillers and we realized that they do a lot of their benchmarking in 1D. They're doing that in these kinds of pressure elevation plots, so they just dump all of the wells, essentially, onto these kinds of diagrams and look out for these red spots where there's some crossover and say, "Well, there's a one in X number of chance of that happening, therefore, that's not an acceptable risk." 

What we were able to do by building a geomodel that went all the way through to the surface and incorporating all of the basic rock properties of these reservoirs but actually copying the pressure matched current day pressures from all of our simulation history models. We were able to show that the three-dimensional relationships of these pressures today under the current poor pressure regime meant that the other wells were going to be safe with that design. In fact, they're all placed underneath injectors surprisingly, and the effect was actually reversed. 

So, by putting all of this together, inverting our model into drilling mud weight, we were able to generate a example using geomodeling technologies that communicated very, very clearly to the drillers that the wells in the remaining campaign would be safe. So, we executed the plan as we had initially designed the wells, and they all came in safely, and they're now producing very nicely. So, without doing this kind of a work and linking it all together into a single story using those three-dimensional models and the current pore pressure matched simulation models, we would've missed some 60 million barrels of additional reserves. 

I hope you liked this little example. I would love to hear your stories around similar problems during a production scenario. Thank you very much.