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1. What was being reported on capping and well integrity testing of BP’s gushing well.
After fitting on the new cap and delaying the well integrity test for a day, BP finally got the approval to completely shut off the oil gusher and monitor the pressure build-up within the cased well on Thursday, 1425hrs local time (1925 GMT) 15 July 2010.
Below is an extract from Alexander Higgins Blog: Alexhiggins-integrity well test
“Retired Adm. Thad Allen, the national incident commander, has said that a pressure reading of 8,000 or 9,000 pounds per square inch (psi) inside the new cap would be ideal, while a pressure reading of 6,000 psi would indicate leakage. But pressures in an intermediate range would create ambiguity and force officials to make a tough decision on whether to keep the well shut in or open it back up and resort once again to containment operations.”
“The Washington Post is reporting that an insider in the BP control room says the pressure inside BP’s blowout preventer has only risen to about 6700 psi which may indicate leaks in the well bore down hole, although he cautioned that it is too early to tell.”
“Furthermore, since BP has started the well integrity test several people have sent in tips or posted comments claiming they have seen leaks in various areas on the sea floor and I have even received a phone call about the issue.
I began watching the live feed from Viking Posiedon ROV 1 soon after they off the well, for well over an hour. I witnessed the ROV cruising the sea floor where I saw large fields of cracks and fissures with oil and gas coming out of them. The ROV travelled over what is clearly a badly fractured seabed. Some the oil geysers looked pretty big as well as what I saw coming out of the cracks in the sea floor, both methane and crude. They have not stopped the oil, but is now surfacing from the sea floor all over the place. The video feed did not show the usual tracking data (heading, depth, etc.) There are leaks everywhere. I saw it between 5:00-6:30 EST. I called a friend and got her online as well and she saw the same thing.”
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BP and Retired Adm. Tim Allen were reported to be satisfied that the new cap is holding up to the pressure building within the well. Pressure readings after 41 hours were 6745 psi and rising more slowly at about 2 psi per hour compared to 2 to 10 psi on late Friday. Although puzzled by the slow rise in pressure, they were nevertheless cautiously happy there were “no leakage on the sea floor and the continued rise (albeit slow) in pressure. According to Kent Wells, a BP PLC vice president, if the pressure could rise above 7,500 psi the well would not be leaking. A low pressure reading or a falling one, could mean the oil is escaping.
2. Reported reasons for the puzzling pressure results.
Washington’s blog on 16 July 2010 compiled four potential explanations for the low pressure readings which you can read in full details at washingtonsblog.
(1) There are substantial leaks in the well;
(2) There is leakage in the sands deep under the seafloor; possibility crossflow at the bottom of the well.
(3) There is some kind of blockage in the well: "If it's rising slowly, that means the pipe's integrity's still there. It's just getting around obstacles"
(4) The reservoir has been depleted more than engineers anticipated.
Even though there is some truth in (1) and ROV videos did pick up instances of up-welling of disturbed sediment and particles clouds, the leakage volume is too small to account for the over 50,000 barrels/day oil gush previously and the nature of the leaks does not explain the simple escape path from the well.
While (2) is the most logical reason, the leakage may not necessary be at the bottom of the well. It is more likely to be at the gas saturated weak sub-formation (GWSF) zone, immediately underlying the non-lithified Quaternary sequence. Leakage at this level will not only explain the puzzling low pressure but also how the blowout occurred in the first place. Capping the well now will make a bad situation even worse leading to more disastrous consequences.
If the well had been gushing unhindered for the last 87 days, there is no reason at all for (3).
Reason (4) is disturbingly wishful since the oil gush did not show the slightest hint of slowing down even after 87 days. If the well is assumed to have depleted itself by half (since 6700 psi shut-in pressure is roughly half of the original 13,000 psi static pressure in the beginning prior to the blowout) the volume of oil would be significantly less before the capping.
3. How GWSF affects drilling and cementing the top hole section
When the GWSF hazards were first discussed more than 15 years ago, drillers agreed that cementing the top-hole section of a well would be extremely difficult even with low pressured GWSF zones. This is because as soon as the well bore penetrates the gas-saturated highly fractured (and faulted) top formation, gases would start escaping into the overlying non-lithified Quaternary sediment (soil). The erosion unconformity (U1) is often overlain by permeable layers of sand which further facilitate the discharge of gas and pore fluid into the well-bore; enlarging the well bore as disintegrated sediment particles get gas-lifted up into the water column. Drillers have no control over the escaping gases swirling outside the well casing; other than speeding up and cementing the top hole as soon as possible.
In Total’s 1988 Sisi-2 blowout (see figure 1), the gas discharge quickly developed into an uncontrollable massive gas blowout at 800m below sea floor. Figure 2 shows the geological setting for the 1991 Barton-BT5 blowout which occurred years after 4 highly problematic trajectories had been drilled. The early warning signs had been ignored and misinterpreted, culminating into a major disaster which almost shut down the Barton-A platform. With ROV videos showing scary pictures of blown out craters, fissures and spewing gas columns on the seafloor, Sabah Shell Berhad had no choice but to shut down the platform before the feared collapse culminate into an even bigger disaster. Barton-A would not be standing today almost 20 years later, had the voice of geological reasons on the delayed mechanism of GWSF hazards, not prevailed at the very last minute.
BP’s Macondo blowout, like PTTEP’s Montara, Total’s SiSi-2, Shell’s Barton-BT5 and Bajt-F blowouts are all disasters waiting to happen. It was only a matter of timing and depth. The GWSF hazards zones can be as large as a few km in lateral extent and hundreds of metres deep; depending on the geological structure. Since the chance of sealing the top-hole section in a GWSF zone is virtually zero, the best alternative (precaution wise) is to move the well location off the apex of the GWSF hazards zone. Figure 3 is a map showing previous problematic wells and the extent of the damaged seafloor caused by their near-disaster massive gas discharge at a shelf-edge zone.
4. How BP’s Macondo Well blew.
Figure 4a illustrates a normal sealed (cemented) top-hole section of a well. Figures 4b to 4d illustrate the difficulty in sealing the top-hole section passing through a GWSF hazards section. The enlarged and irregular well-bore also acts like a vertical conduit connecting the open fractures (fissures) in an already fractured formation. Loss Circulation is common as the heavier high pressured mud invades the fractured formation and in the process forces the formation fluid and gas deeper (laterally) into the formation and upwards into the overlying permeable sandy layers. But this is only a temporary equilibrium. As more gases and formation fluid are squeezed out and displaced, more drilling mud will invade the fractured formation. The GWSF zone in the vicinity of the well bore essentially becomes a hydraulically connected extended gas-charged pressurized (EGCP) zone; the extent of which depends on the drilling practices and geological nature of the GWSF hazards zone. Drilling mud will be forced back into the well whenever the pressure in the well drop below the charged-up pressure in the EGCP zone and vice versa. This is the nightmare scenario drilling through the GSWF hazards zones.
Even if a well manages to “drill thru and safely cement” the top-hole section, the poorly cemented top-hole section in a hydraulically connected EGCP zone will continue to act like a dynamic “spring-loaded” charging system; just waiting to blow on the slightest mistake. Despite the nightmares, Transocean’s drilling crew managed to control the well until it reached their targeted depth and oil reservoir. Displacing the drilling mud with seawater (too early) was a mistake that triggered the blowout. As the pressure inside the well continued to drop, the mixture of mud, clay slurry and eventually gas were forced back into the well. With continuous gas kicking-in at the top-hole level and no mud column to counter the gas bubbles’ rapid ascent through the riser, it was like “sucking” the oil out of the reservoir through a “straw”. Naturally, the well’s bottom seal gave way to the high pressure oil gushing out of the reservoir.
5. Making a bad situation worse
Capping the well from the top is a bad idea. The “Top Kill” attempt was doomed to fail because there is simply no way to overcome the multiple flow-path from a badly damaged top hole section. Capping the well at the top would only make a bad situation worse. Once the pressure in the well builds up, the oil would be forced back into the highly fractured GWSF zone again; recreating an even bigger EGCP zone.
There is a possibility that some of the methane gas could have been vaporized insitu from “frozen” methane hydrates, thus providing endless supply of methane gas. As the heavier hydrocarbons from the reservoir warm up the hydrates, more methane gas is vaporized and squeezed through the fissures. They eventually filter up towards the sea floor. Basically, the pressure increase in the well will slowly taper off but never reaching the maximum as the EGCP zone gets bigger and bigger.
Methane and lighter hydrocarbons will filter through the sandy sequence, fissures and fractures to reach the seafloor. The heavier oil will remain trapped within the EGCP zone. Given time, the fragile sedimentary (soil) structure will fail, resulting in uncontrollable gas and oil seeps. In reality the weak sedimentary (soil) structure will likely fail first before the steel casing and lining. Keeping the cap on and the pressure high will only lead to uncontrollable consequences.
BP well engineers should be aware that the EGCP zone acts like a secondary gas reservoir connected by the well in question. Calculating the correct mud weight might be tricky given the many unanswered questions and uncertainties. It might be prudent to carry out a detailed high resolution 3x3 km geophysical investigation over the ill-fated well. The evidence to date confirms the damage had extended far beyond the normal vicinity of the well. Concentrating on the well alone would be missing the forest for the trees. A correlation of the pre-incident and post-incident survey data would be priceless in understanding the causes of the blowout and oil spill disaster. The mistakes learnt and knowledge earned will go a long way to ensure our environment may never suffer from another mega mishap of this kind again.
