03-14-2012, 09:25 PM
I have found another morsel of information to add to our knowledge base for the Triceratops Field. Perhaps it only matters to me but it has a bearing on some of my previous writing, so I must discuss it in order to improve on or correct some of my past assumptions. There is some good news and bad news here. I have previously discussed over pressured gas zones due to the height or thickness of the gas column. Because I had no pressure information on Bwata-1 or Triceratops-1 I assumed the aquifer was normally pressured with a pressure gradient of 0.433 psi/ft.. That was apparently a bad assumption. I now have information that leads me to believe the aquifer of the Triceratops Field is sub-normally pressured with a pressure gradient of 0.350 psi/ft. The gas/water contact is thought to be near -1,500 meters (4922 feet) sub sea. This gas/water contact will then be at a drill depth of 6,283 feet drill depth at Triceratops-2. The effect of this change in pressure gradient means that the pressure at the gas/water contact at Triceratops-2 will be 2,200 psi instead of the previously assumed 2,720 psi or the gas reservoir pressure is 520 psi lower than I had previously estimated. The gas column is about 2,200 feet thick and by using a gas gradient of 0.04 psi/ft we can calculate the pressure at the top of the reservoir at Triceratops to be [2,200 psi - (2,200 ft x 0.040 psi/ft)] 2112 psi. If we assume the 9 5/8”casing shoe is at a drill depth of 1,250 meters (4100 ft) this would give us a pressure gradient of only (2,112/4,100) 0.51 psi/ft. This would only require a mud weight of 9.8 lb/gal. This is a little over pressured but not nearly as much as I had previously estimated. With this lower mud weight requirement and some loss of circulation material in the mud it is possible that they could drill the pay zone without losing circulation. Or since they will be rigged up to do pressure managed drilling they could drill into the top of the reservoir with an under balanced mud system. This could be done using fresh water with about 350 psi of back pressure on the rotating head at the surface.
The bad news is the reservoir will contain about 19% less gas that it would have if it had been normally pressured.
The good news is that everyone who matters already had this information. That would be IOC, Knowledge Reservoir and all of the prospective bidders (people who have had access to the data room). The Knowledge Reservoir estimate of Gas Initially In Place (GIIP) http://www.interoil.com/presentation/201..._Final.pdf chart 25 should have used the correct pressure data so no correction should be required for their number. When I did my calculation of GIIP I simply increased the pore volume to account for the Phase 3 seismic data and then increased Knowledge Reservoir’s number proportionally for the additional pore volume, so my number also does not have to be corrected. Wonderful, my number was self correcting. Until we get the new data for porosity and net/gross pay percentage I will stick with my estimate of 11.335 TCF for GIIP and 7.345 TCF for the sales gas resource number.
Other good news is that I have been very worried that it would be impossible to drill the highest part of the structure to the West http://www.interoil.com/presentation/201..._Final.pdf chart 18 due to that part of the field being 4,000 to 4,900 feet above the gas/water contact and being relatively shallow for the pressure involved. It now appears that the pressure in the very highest part of the reservoir will only be about 2,000 -2,040 psi. I believe these wells can be drilled without any problem.
Here is a bonus for you. I am going to guess that the shut-in bottom hole pressure of DST No. 1 (reservoir pressure) will be 2,117 psi at a depth of about 4,200 ft.
With the lower reservoir pressure the flow rate on DST may be less than I previously guessed i.e. 10-15 MMCFD but, if we have excellent permeability and a very large bottom hole choke (or no choke) it is still possible to achieve these rates. For example with a flowing surface pressure of 1,000 psi on a ¾ inch surface choke the flow rate would be 14+ MMCFD.