Improper well control procedures lead to uncontrolled flow of hydrocarbons to the surface which is referred as blowout. KG basin is a proven petroliferous pericratonic basin formed on the continental passive margin located on the East Coast of India. Major blowouts in KG basin have led to number of risks related to loss of human lives, environmental pollution and loss of material assets. The geological complexity of the field and the presence of over pressure zones mainly in East Godavari sub-basin have led to major disasters in the past years. This is a study to know the facts about what went wrong and why went wrong in a blowout occurred in KG basin in East Coast of India.
A raging blowout occurred in an exploratory well in the East Godavari District of Andhra Pradesh in January 1995. The well was spudded in September 1994. The drilling progressed with an 8.5 inch bit which had stuck at the depth of approximately 2727 m MD which was confirmed by conducting a stretch test. An attempt has been made to retrieve the drill rods by unscrewing which successful. Unfortunately they were unable to retrieve the drill bit and drill collars and were left in the bore. It was reported no kick off pressure recorded until this stage.
As alleged, in the evening uncontrolled flow of gas to surface at high pressure led to a blowout. The spewing gas caught fire immediately. The remaining pipes inside the bore were thrown out by the enormous pressure of the gas. The pressure of the gas was estimated around 281 kg/cm2. The sound resembled like a rocket engine and the level was so high that none can hear anything up to 700 m distance from the site. In the night the intensity of the light caused by flame was so high that it is visible up to a distance of more than 2 km from the well site. The gas flames were about 200 feet high with temperature to more than 50 degrees centigrade. About 1 million cu.m/day of gas spewed out with a deafening sound.
The main reasons for the causes of blowout in KG Basin based on the struck up of drill string are:
In plastic formations such as salt dome, if the pressure caused by the drilling mud is lower than the formation pressure, the formation deforms causing the hole to collapse as salt is viscous-elastic in nature. Reaction of clay minerals with the drilling mud causes swelling and sloughing that cause well bore problems and result is pipe stuck up.
The other type of stuck up which cause due to differential pressure. This occurs in open hole when the pipe comes in contact with a permeable formation having a pore pressure much less than the pressure caused by the drilling fluid. In this case the string is held in place due to the differential pressure. This situation can be recognized by increased over pull on connections due to increase in frictional forces in the well bore.
As KG basin contains huge gas resources, drilling through these gas reservoirs is of high risk. The main reason for blowout in drilling gas reservoirs too fast is due to fact that the gas contained in the formation being drilled becomes mixed with the drilling mud as the bit penetrates the gas bearing strata thus causes the drilling mud to become gas cut and lightened to such an extent that it will not overcome the formation pressure.
The recommended safety procedures are:
As excessive pressures have been encountered in the East Godavari sub-basin it is advisable to have a detailed study of the geological setting of the basin in terms of presence and degree of over pressures and different regimes in different parts of the reservoir leads to compartmentalization of the reservoir.
Various signs of kick such as increase in the ROP caused due to reduced differential pressure between mud pressure and the pore pressure, loss of circulation, pressure decrease in well bore due to gas cut mud, increase in return flow rate and increase in pit level must be monitored constantly and the information should be passed on to the crew working at the well site. Training must be imparted to the crew to avoid occurrence or at least to limit or lessen the ferocity.
The rig and equipment’s used for drilling should be well adapted to the performance of the task required. The equipment should be routinely tested for any defects and should be rectified before using at the site.
Wells must be designed carefully with casing seats selected scientifically and the location should be checked for any weak formations. The rough depth estimate of high pressure zones should be known from exploration of geologists before drilling a well. The proximity of high pressure formations should be known before hand through ROP to avoid the possibility of formation breakdown.
Implementation of new exploration and drilling technologies such as high resolution 3D seismic, cyber rigs should be used for efficient development of oil and gas fields.
References
https://www.indiatoday.in/magazine/special-report/story/19950215-ongc-struggles-to-control-blaze-erupting-from-its-gas-well-in-andhra-pradesh-806879-1995-02-15