A Blowout Analysis
Written By: Narendra Vishnumolakala
Along with the increasing demand for hydrocarbons and other sources of energy, the methods for satisfying our energy needs have become more complex. In the case of oil and gas, exploration and production has gone to deep waters and wells are drilled several miles under the ground. Nonetheless, we must obtain the hydrocarbons in a safe manner and without negative environmental impacts. The application of technical, operational and organizational solutions to reduce risk of uncontrolled release or intake of formation fluids through the life cycle of a well is known as well integrity. The consequences from not achieving well integrity can be catastrophic, both for fluids entering the well in an uncontrolled manner or fluids being released to the formation unintended. Uncontrolled fluid flow in the wellbore is one of the most critical safety concerns for the oil and gas industry. The most unwanted types of uncontrolled fluid flow include gas-kicks and blowout. In a variety of operational phases, if the wellbore pressure provided by drilling mud is less than the formation pressure, the formation fluid will flow into the well from the reservoir. This is known as a kick, and is described in many references. The rate of fluid influx is a function of reservoir parameters and the pressure difference between the formation and the wellbore. After detecting a kick, proper well control procedures must be performed immediately to eliminate the fluid influx and prevent further formation fluid from flowing into the well. The wellbore must be isolated from the surface by activating the blowout preventers. Then the remaining fluid influx is circulated out to the surface. Once the influx has been displaced, heavier drilling mud is pumped into the well in order to regain a control of the well. However, as can be seen from the incident history of the oil and gas industry, such procedures do not always succeed. If the kick is out of control, it may lead to a blowout. The crude oil spill and natural gas leakage due to the blowouts have damaged the environment in Alaska, the Gulf of Mexico, and many other places worldwide. Whether the damage is long-term or short-lived is subject to debate. For example, the Gulf War oil spill in 1991 is known as one of the largest oil spills in history. When Iraqi forces invaded Kuwait, they set over six hundred oil wells on fire in January 1991 to achieve their strategic goal—prevent potential landings by US Marines. The fire lasted for about ten months. There were 11 fatalities associated with the Kuwait fires by that time. The oil spill left over 200 oil lakes throughout the desert, and some of them were more than six feet deep. The clean-up work is continuing until today. Different parties conducted research to estimate the volume of oil spilled during the Gulf War. Initially, media estimated the spilled amount could reach about 11 million US barrels at the beginning of the Gulf War oil spill. After all the wells were shut in, the US Committee on Merchant Marine and Fisheries reported to Congress that the volume of the oil spill ranged between four and six million barrels based on the satellite images. In 1993, some independent researchers reported that the maximum amount of spilled oil should be around two to four million barrels according to the historical and incident data.
The obvious drawbacks of the above estimates include a lack of basic physics to understand the behaviour of blowouts; therefore, their estimates are not accurate. Unfortunately, very few papers have addressed the production loss and environmental impact during blowouts which are directly related to the blowout rates. We still cannot perform a comprehensive risk assessment and consequence analysis of blowout events due to the lack of understanding of its mechanisms. Particularly, for the aftermath calculation, the estimation of oil spill volume has to rely on the incident data (barrels of oil recovered, flame or plume height). A research team at the Mary Kay O’Connor Process Safety Centre at Texas A&M University is carrying out study on the advancement of understanding the blowout mechanism. To capture all the physical phenomena during a blowout event, blowouts need to be simulated from the beginning of the event to the time they are brought under control. An analytical or semi-analytical model of blowouts is established taking into account the mass balance in the reservoir, and energy balance, momentum balance, and mass balance in the wellbore. The interaction between the reservoir and the wellbore is also described in this model. The model tries to avoid using any commercial packages as they only provide black box models and therefore limit the flexibility and completeness of the model. It is expected that the model could estimate the blowout rates as a function of time and corresponding volume of spill based on the parameters of the reservoir and the wellbore, and other operational conditions.
The cumulative production loss during a blowout event is directly related to the blowout rates. It depends on the conditions of the wellbore and the reservoir. The interaction between the reservoir and the wellbore also needs to be taken into account. Analytical equations are used to address the relationship between bottom-hole pressure and reservoir pressure by considering both transient and pseudo steady state periods in the reservoir. The reservoir pressure determines if the blowout rate can be sustained, while the blowout rate affects the rate of depletion of the reservoir. As a result, the reservoir model and the wellbore model are studied separately. In the reservoir model, it is assumed that the reservoir is isothermal throughout the operation. The heat transfer model is therefore not considered in the reservoir. Material balance is used to solve the average reservoir pressure at any given time. Due to the complex nature, the wellbore model is treated as a semi-steady state model. It means that all the parameters and variables are assumed to be constant in one single time step. There are several important components that we are interested in in the wellbore model. The pressure loss must be addressed to account the phase behaviour of multiphase flow. The fluid temperature in the wellbore is obtained by heat transfer model. The significant thermodynamics effects are taken into consideration, such as the Joule-Thomson effect and the contribution of kinetic energy to the fluid temperature. The potential existence of sonic velocity during the blowout incidents, which is particularly likely for onshore gas or oil/gas wells, is also studied.
The research on the physics of blowout, such as the study described earlier, will significantly improve our understanding of the mechanism of blowout and will help us conduct a comprehensive analysis of the consequences of blowout events, including the environmental impact.