(10-20-2015, 06:59 AM)Thylacine-2 Wrote: Ah So, Tusker ! Thank You! Turning to our resident Petroleum Engineer....Pet, could the bad cement job be related to extreme vugginess?
Cementing Seminar
First let’s remember that the objective here is to get the casing cemented right at the top of the pay zone so none or very little of the pay zone should be exposed to this cement job. They would have drilled in to the formation only far enough to see carbonate cuttings at the surface. Once they determined that they had reached the pay zone drilling would have been stopped. It is unlikely that the high porosity reservoir caused the bad cement job. Since the formation was not open they would have been drilling with full mud returns to the surface and no loss of circulation. As the top of the zone gets deeper and closer to the gas/water contact the zone requires less mud weight to control. The depth is reported to be -1,875 meters sub sea (-6,152 feet) and the derrick floor is about 78 meters above sea level so that makes the true vertical drill depth about 1,950 meters (+/- 6,400 feet). The actual drill depth should be a little more in the deviated hole. The reservoir pressue at this depth is about 3,640 psi and would require a mud weight of about 11 lb/gal to balance. So they were probably using about 12 lb/gal mud for a little safety factor.
What can be done to maximize the possibility of getting a good cement job? We would like for the heavier cement slurry to displace “all” of the mud surrounding the casing. To accomplish this there are several things that can be done.
1. Centralize the pipe in the hole so that we have equal area on all sides of the pipe for the cement slurry to flow through. If the pipe is not centralized the cement will tend to flow through the larger area and leave some of the mud not displaced, especially if the pipe is laying on the low side of the hole as it would tend to do in a deviated hole. We have centralizers for that purpose which are run of the outside of the casing.
2. We would even like to remove the mud filter cake off of the face of the formation (not much of a problem in shale like we have). For the purpose of removing the filter cake we also run “scratchers” on the outside of the pipe. We would like to have nothing between the pipe and the formation but cement which gives us the best chance of having good cement bonding to both the casing and the formation.
3. Before cementing we condition the drilling mud to reduce the viscosity of the mud which makes it easier for the heavy cement slurry to displace from around the pipe.
4. Before we start pumping cement we pump a few barrels of water down the drill pipe ahead of the cement to help clean the formation so that the cement will have a better chance of bonding to both the pipe and formation.
5. While pumping the cement around the casing we move the pipe. This can be reciprocation up and down or rotation. Since the liner will be run on the drill pipe it is most likely that we would reciprocate the pipe.
6. We want to use a little excess cement so that we will completely fill the annulus between the liner and the formation all the way back to the top of the liner inside of the 9 5/8” casing. We then release the drill pipe from the top of the liner and reverse circulate any excess cement out of the hole.
7. The difference in the weight of the mud and cement also helps displace the mud . The mud, in this case, is about 12 lb/gal while the cement slurry will be about 18 lb/gal if I remember correctly.
After giving the cement time to set we will run in the hole with the drill pipe and clean out any cement that might have been left in the hole at the top of the liner. We pressure test the top of the 7” liner in the 9 5/8” casing. If it leaks we have to pump cement into the 9 5/8” x 7” annulus and re-test until it holds pressure.
Then we clean out to bottom and run cement bond logs.
Assuming that we did all of the above and still have a bad cement bonding as shown by the cement bond log, what needs to be done?
Using the cement bond logs we see where the bonding needs to be improved. We perforated the casing at the spot where the bonding needs to be improved and then pump cement through the perforations to improve the bonding. We do not want to leave any channels in the cement behind the casing that might allow gas to escape to the surface. (Remember BP and the Macondo blow out in the Gulf of Mexico? ) Once we are satisfied that the cement bonding is good we drill out the 7” casing shoe and pressure test that to a gradient of 1 psi/ft. of depth. The 12 lb/gal mud would give us about ) 0.62 psi/ft of pressure so we would need a surface pressure of (0.38 psi/ft x 6400 feet) about 2,432 psi at the surface. If the casing shoe fails the pressure test we will again have to squeeze cement around the bottom of the casing, drill out and re-test.
After we get a good test we are ready to drill through the formation.
I am going to watch the rest of the football game. Already missed the first hour.
Have a good day/evening!