Macondo Well Deepwater Horizon Blowout Lessons for Improving Offshore Drilling Safety (2012) / Chapter Skim
Currently Skimming:
2 Well Design and Construction
2 Well Design and Construction
Pages 19-44
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 19... ...
The original well plan was to drill to a total depth of 19,650 feet, but this was modified during drilling and the actual total depth was 18,360 feet, as discussed below. Before the well was drilled, design teams estimated pore pressures and strengths of geo 1 Temporary abandonment refers to a set of normal procedures used by rig personnel to secure a well after drilling has been completed, so that the rig, along with its blowout preventer and marine riser, can be moved from the well site.
|
From page 20... ...
: 1. Measurements showed that pore pressures were increasing at a faster rate than anticipated, combined with a period of lost circulation of drilling mud at 12,350 feet, indicating that the well could not be continued without setting protective casing.
|
From page 21... ...
FIGURE 2-1 Original wellbore architecture planned for Macondo well. Source: BP 2010, p.
|
From page 22... ...
. The lower mud weight should not have been needed at this depth on the basis of the original plan and was an indication that pore pressure and fracture pressure in part of this interval were considerably less than had been anticipated.
|
From page 23... ...
Depth and diameter values on the y-axis correspond to the final well bore architecture shown in Figure 2-2. The equivalent mud weight (EMW)
|
From page 24... ...
A reservoir containing salt water that had a pore pressure equivalent to 14.1 ppg was also exposed in the wellbore. As discussed below, the difference between the mud weight needed to prevent flow of salt water and the mud weight above which reservoir fracture could occur was only 0.2 ppg.
|
From page 25... ...
FINDINGS Beginning of Hydrocarbon Flow That Led to the Blowout As part of the temporary abandonment process, a negative pressure test was used to indicate whether a cement barrier and other flow barriers had isolated formation fluids from the wellbore. To conduct the test, rig personnel purposely reduced the hydrostatic pressure inside the well.
|
From page 26... ...
The drill pipe pressure should not have built up between tests, but this could have been attributed to the heavy mud leaking past the annular BOP. The mud volume that flowed out beyond what was necessary to account for mud compressibility should have made this test a failure, despite the drill pipe pressure having bled to zero.
|
From page 27... ...
The largest pore pressure required that the mud weight be at least this high to prevent salt water flow from the reservoir. The fracture mud weight was just above 14.2 ppg, as evidenced by lost circulation at 18,260 feet (see BP 2010, 17–18, and Figure 2-3)
|
From page 28... ...
Cement slurries are often heavy, with densities around 16.4 ppg. Use of a high-density slurry is not a problem so long as the density of the slurry, along with the density of the mud, does not create a pressure in the well that exceeds the fracture pressure of exposed reservoirs.
|
From page 29... ...
To make a foamed cement slurry that has a density of 14.5 ppg at the bottom of the well, the foam quality7 had to be 17.4 percent at bottom hole conditions of 245F and 13,321 psi (see Appendix D for the calculations) .8 At the surface, where the conditions in the mixer were about 600 psi and 60F, the foam quality had to be 66 percent, producing a foamed slurry of about 6 ppg, to allow for the substantial compression and heating that were to occur as the foam was pumped to the bottom of the well.
|
From page 30... ...
The tail cement might have fallen all the way through the foam, unless it was stopped by viscous forces, or it might have mixed with the foam and changed the quality of the foamed cement. In either event, the tail cement slurry remaining inside the shoe track at the end of pumping would be dispersed in the foam and would not retain its intended composition; that is, there would be no concentration of dense tail cement inside the casing as originally planned (see Figure 2-5, right side)
|
From page 31... ...
has shown that foamed cement begins to establish compressive strength at about the same time as the base cement (Class H in this case) , but the strength of the foam continuously lags that of the base cement as curing time increases.
|
From page 32... ...
Table 2-1 shows that as little as 5 percent mud contamination reduces the foam compressive strength to less than the 999-psi pressure differential created during the negative test. On the basis of the general relationship illustrated by the Chevron data and the assumption of the foam algorithm, if mud contamination ex 9 A positive pressure test was used to assess the integrity of the production casing and other mechanical barriers by intentionally increasing the pressure within the production casing.
|
From page 33... ...
Finding 2.5: Foamed cement that may have been inadvertently left in the shoe track would likely not have developed the compressive strength of the un-foamed cement, nor would it have had the strength to resist crushing when the differential pressure across the cement was increased during the negative test. Float Collar with Flapper Valves and Differential Fill Tube The float collar had two flapper-style backflow valves that were held open by a differential fill tube, as shown in Figure 2-8.
|
From page 34... ...
The practice of allowing mud to fill the casing by flowing up through the differential fill tube is a time-saving step that would not be needed if the casing were filled with mud from the top as it is run, a much slower process but one that reduces the possibility of debris entering the casing. Several factors pointed to the probable failure of the flapper valves in the float collar.
|
From page 35... ...
There are alternative possibilities for the point of entry into the casing. As discussed above, the most likely possibility appears to be the combination of weak cement inside the casing and leaking flapper valves in the float collar.
|
From page 36... ...
that no debris is inside the casing that might plug the float collar or shoe and (b) that the open hole section is stable, has no hydrocarbon flow entering the borehole, and is free of debris before cementing.
|
From page 37... ...
A float collar with two float valves in it was used in the Macondo well. A reamer shoe rather than a float shoe was used so that the differential fill tube could be installed in the float collar.
|
From page 38... ...
Thus, the possibility of mud-filled channels or poor cement bonding existed. Cement Bond Log Whether to run a cement bond log was up to the discretion of the operator because MMS rules did not require a bond log if no lost circulation occurred during cementing operations.
|
From page 39... ...
A redesign of the completion could also have provided sufficient depth below the producing formations so that the cement bond log could examine the presence and quality of the cement throughout the productive interval. A sufficient margin of safety should be used for the ECD while fluids are circulating so that even with unforeseen pressure surges or rate and fluid property fluctuations, the possibility of fracturing is reduced.
|
From page 40... ...
The rig personnel explained the pressure increase by using an erroneous theory termed the "bladder effect." Observation 2.2: Had an attempt been made to bleed off the drill pipe pressure at the end of the negative test, the communication with the reservoir would likely have been discovered. Instability of the Foamed Cement Foam is inherently unstable, and the extent to which it is stable is sensitive to its chemical makeup and the environment to which it is exposed.
|
From page 41... ...
Observation 2.4: The pumping sequence of cement slurries and other fluids used for cementing the Macondo well subjected the volume of the lead cement slurry to contamination by the spacer or mud that was placed ahead of it. If it was heavily contaminated, the slurry would not have established a cement cap with the compressive strength of uncontaminated cement.
|
From page 42... ...
Use of the Long String Production Casing The use of the long string of production casing has already been cited as a reason the casing could not be reciprocated or rotated during cementing operations. One alternative to using the long string is to run a production liner on the drill pipe.
|
From page 43... ...
There is no standard for this margin of safety. As a guide until a reasonable standard is established, industry should design the ECD so that the difference between the ECD and the fracture mud weight is a minimum of 0.5 ppg.
|
From page 44... ...
All tests should have established procedures and predefined criteria for acceptable performance and should be subject to independent, near-real-time review by a compe tent authority. This includes the timing of the start and the magnitude of the pressure tests compared with the amount of time needed for strength development of the cement, the results of the pressure tests, verification that the flapper valves have closed and the other mechanical seals are holding, and evaluation of cement bond logs.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.