Blowout simulation leads to recommendations for high-risk contingencies
A serious blowout in a gas exploration well in the heavily populated Sichuan Basin of China raged for 13 days until operators finally were able to extinguish the inferno on the third attempt. The scenario was unveiled in a presentation, “Disaster Aftermath: High Risk Region Needs a Better Contingency Plan,” by Lei Zhou, principal consultant for the SPT Group, which conducted a series of simulations of the 2007 disaster. “We followed the pump schedule into the simulation,” Mr Zhou said in the presentation at the 2009 IADC Well Control Conference of the Americas and Exhibition on 26 August in Denver. “We learned that abnormal pressure zones can be encountered in an exploration well due to formation uncertainty.” In this case, there was a sudden increase in ROP at 14,058 ft. The well was located in the second-largest gas field in China, characterized by high production rate and high permeability in highly connected reservoirs.
Preliminary control trials included first circulating 116 barrels of lost circulation contour material (LCM) with 1.6 specific gravity (sg) weight, then pumping 1.8 sg of mud at a pump rate of 5.7 bbl/min. That resulted in a high gas rate out of the well and mud gas separator shaking. Mud weight was gradually increased in subsequent trials.
The gas well fire in China lasted 13 days and took three attempts to extinguish.
In the first attempt to extinguish the fire, operators circulated 1,571 barrels of 2.05 kill mud at 12.6 bbl/min in an attempt to shut in the well with reverse circulation and then cement it. But the procedure proved unsuccessful after the casing pressure increased to 6,092 psi within four minutes while the well was shut in. The well reignited through four open discharge lines, with flames shooting 82 ft to 148 ft.
In the second attempt, operators circulated 2,088 barrels of water followed by 2,390 barrels of 2.2-sg kill mud. “The fire went out during the kill process, but the casing pressure increased to 5,366 psi and kept rising, causing a surface pipe joint leak,” Mr Zhou said. After opening five discharge lines, the fire reignited. In the third try, the operators devised a plan to “establish friction by circulating water, kill the well with heavy mud and then cement the well,” Mr Zhou said.
The operation involved circulating 799 barrels of water followed by 4,352 barrels of 2.2-sg mud. “BHP (bottomhole pressure) continued to build while free gas was replaced by kill mud,” he explained. After five hours of the kill operation, all gas was out of the annulus. The well has been capped, and another one is now operating nearby.
SPT’s simulation of the kill verified the procedures and determined that casing strength ranging from 7,687 psi (10 ¾ in.) and 12,618 psi (7 5/8 in.) can create additional challenges for well control.
The blowout resulted in a series of recommendations for avoiding future disasters in high-risk regions, including the establishment of effective emergency rescue procedures both by the operator and the government; key well control engineering technology and equipment standards; a high-risk region safety and risk management contingency plan. A well control rescue, training and research base also was recommended.