At Elk and Antelope IOC was rather forth coming with pressure data, DST data, Absolute Open Flow data etc. But at Triceratops not so much. They did report one well head pressure for T-2 on DST No. 8 which tested the open hole interval from 4,111 - 4,859 feet. This would be the upper zone and it produced 17.6 MMCFD on a 48/64” choke along with 13.6 - 16.3 Bbl/MMCF of condensate. They reported a “corresponding” well head pressure of 1,382 psig which I believe means the flowing well head pressure.
Let’s look first at the press release of May 14,2013 http://www.interoil.com/iocfiles/documen...inal-1.pdf
Here you will see that we discovered two separated reservoirs separated by an impermeable marl streak which is about 200 feet thick. It is my understanding that this is the first well that drilled through this marl streak, so that means the separation between Triceratops-2 and Bwata-1 is the marl streak. So the upper reservoir at Triceratops-2 is the same reservoir that we saw at Bwata-1. The lower reservoir in a new reservoir that has not been seen before. Dave Holland stated that he believes that this lower reservoir is a local phenomenon and we should not expect it to be field wide. IOC has stated that the gas pressure in both the upper and lower zones are on the same pressure gradient line and that the water aquifer in the lower zone has higher pressure than the aquifer seen at Bwata but they did not tells what those pressures are.
I propose to reveal those pressure to you. If you look at http://www.interoil.com/iocfiles/documen...nal_ml.pdf Page 15 you will see a small graph presented on the right side of the chart. Print this chart off so you will have something to work with. You will note that the chart does not have any numbers on either the horizontal axis or the vertical axis. So let’s put those numbers on the chart for them. On the horizontal axis let’s start on the lower left corner and put 1,800 psi and then mark it across the bottom in 200 psi increments from 1,800 psi to 3,200 psi for the right bottom corner. Now let’s put the depth numbers on. You will note that there is a dotted blue line marked gas water contact. Mark the line right below the blue dotted line (on the right side) -1,500 meters sub sea. Now mark each line above and below that line in 100 meter increments with -2,100 meters at the bottom and the line marked “projected highest gas” as +200 meters above sea level. Now you will see that the “projected highest gas” is 1,700 meters above the gas/water contact, the same as the map on the left.
So what can we learn from this exercise?
1. The reservoir pressure at the Bwata-1 gas/water contact is about 2,200 psi at a depth of -1,470 meters. This is read where the Bwata Aquifer pressure gradient line crosses the gas gradient line
2. The aquifer pressure in the lower zone at Triceratops at the same sub sea depth of -1,470 Meters is about 2,590 psi or about 390 psi higher than in the upper reservoir. If you don’t like that method you can take the difference in depth of the two gas/water contacts and multiply by the fresh water gradient of 0.433 psi/ft.
The Bwata gas/water contact was expected to be at a drill depth of 1,885 meters whereas the gas/water contact in the lower zone was found at 1,611meters drill depth. The difference between 1,885 meters and 1,611 meters is 274 meters or 899 feet. If we multiply the 899feet x 0.433 psi/ft we get 389 psi or the same as indicated on the chart.
3. I do not have the exact surface elevation for Bwata-1 but on the chart at http://www.interoil.com/iocfiles/documen...inal-1.pdf page 23 it looks to be about 100 meters above sea level. If we use 100 meters (328 feet) for the surface elevation then the drill depth of the Bwata gas/water contact would be at 1570 meters or 5,151feet. With a pressure of 2,200 psi at that depth the pressure gradient would be (2,200/5,151) or 0.427 psi/ft.
4. At the same sub sea depth in the Triceratops-2 the aquifer pressure is 2,590 psi but the derrick elevation is higher at 415 meters making the drill depth to the Bwata gas/water contact depth (1,470 meters + 415 meters) or 6,185 feet. This makes the pressure gradient to that depth (2,590/6,185) 0.4189 psi/ft. So even though the aquifer pressure is higher in the lower zone than the aquifer pressure in the upper zone the pressure gradient is still slightly below the normal fresh water gradient of 0.433 psi/ft.
Why is all of this worth discussing? Do you remember all of the discussion about not having any swabbing equipment? If the aquifer pressure is less than a fresh water gradient it will not flow to the surface. At the time we thought we were testing gas zones but they turned out to be water zones that would not flow to the surface.
5. Now back to our chart. As we move up in the gas column the pressure becomes less but due to the depth becoming more shallow the zone becomes more and more over pressured. You can see that at the “highest projected gas” the pressure is down to 2,000 psi but on the above mentioned http://www.interoil.com/iocfiles/documen...inal-1.pdf page 23 it looks like the depth is less than 2,000 feet. That would give us a pressure gradient of (2,000 psi/2,000 ft) 1.0 psi/ft. That means that at that highest part of the reservoir the pressure will be too high to drill safely. So the wells will need to be drilled down dip far enough to drill safely. We do not need to drill 5,500 feet above the gas/water contact, 3,000 feet above the gas/water contact would be fine and safe to drill. Even at that down dip location the zone will be over pressured at the top and sub normally pressured at the gas/water contact.
It is common practice to use 1.0 psi/ft as the pressure at which you might expect to fracture the formation. So it would not be surprising to find some gas seeps in the up dip area to the west. Even so it appears that the overburden is capable of maintaining a reservoir pressure of 2,000 to 2,200 psi since that is what we are seeing.
6. The reservoir pressure at the top of the lower zone at -1,130 meters sub sea is about 2,170 psi.
7. The reservoir pressure at the top of the upper zone is about 2,150 psi.
8. As we move up through the gas column from the gas/water contact to the highest known gas the pressure drops from 2,200 psi to 1,990 psi or drop of 210 psi over an interval of 1,700 meters or 5,578 feet. This give us a gas gradient of 0.037 psi/ft.
I hope you now know more than you ever wanted to know about Triceratops pressures.
Additional comment not directly related.
If we move up dip to the south west of Triceratops-2 to drill the reef, the zone will be over pressured at the top and we should not need to swab if we stay well above the gas/water contact. If they want to identify the gas/water contact by DST we will still need to be able to swab. I hope they have arranged for a heli-portable sway unit. I would like to see them drill straight to TD, log and, before any testing, run and cement casing and then do all of the testing inside casing. Be prepared to swab if necessary in zones at or below the gas/water contact.