Drilling engineering is an essential part of the petroleum industry including the reservoir engineering and production engineering. At the beginning of the Petroleum industry the main type of rotary drilling, introduced by Anthony Lucas, was the Overbalanced Drilling. The Overbalanced Drilling method is a method in which the control of the reservoir pressure is accomplished by the hydrostatic pressure of the drilling fluid and the Equivalent Circulating Density – EMW. As a procedure the conventional drilling hasn’t changed very much over the years, the process is still open to atmosphere. But what has changed are the drilling conditions. Nowadays we are drilling deeper wells, wells with complex trajectories, reservoirs have higher pressures, because of the production of oil and gas from reservoirs we have depleted zones etc.. All of this factors means that our wells are more expensive to drill comparing to those in the past. Because of that we need to find some new solutions in order to make the well economically feasible. Methods like Underbalanced drilling and Performance drilling were introduced. Performance drilling is a drilling method in which the density of a mud used is selected to provide as low as possible overbalance pressure to the bottom of the hole in order to increase the ROP and prolong the bit life. Underbalanced drilling is a method in which BHP that is used is smaller than the reservoir pressure. That means that we are allowing a certain amount of influx into our well from the reservoir. Main objective of the Underbalanced drilling is to minimize the reservoir damage induced by the drilling fluid. Subsurface rocks, through which we are drilling, are defined by the drilling pressure window. Drilling pressure window describes the lower and upper pressure values of hydrostatic pressure for our drilling project. Lower pressure margin describes the formation or reservoir pressure and the upper margin describes the fracture pressure of the rock. Real problem originates when the pressure window becomes narrow. In those cases, the use of above mentioned methods is not safe because the use of these methods can potentially result in kick, fluid losses or wellbore instability which can lead to down stuck pipe situation or even loss of downhole equipment situation. Proper solutions for these subsurface environments is the use of the Managed Pressure Drilling – MPD.
2. MANAGED PRESSURE DRILLING AND ITS VARIATIONS
By IADC definition the Managed Pressure drilling is an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure limits and to manage the annular hydraulic pressure profile accordingly. Main difference between the MPD and the conventional Overbalanced drilling – OBD is the type of the circulation system. OBD circulation system, as mentioned before is a system open to atmosphere. This kind of fluid circulation system can result in a significant wellbore stability issues because of the common changes in BHP. Fluctuations of the bottom hole pressure are the result of the needed arrestment of the circulation in order to make a new connection, to be more precise, the ECD component is lost while there is no circulation. MPD as a method has the ability to keep bottom hole pressure constant, through the application of a certain amount of surface backpressure. MPD has a closed pressurize circulation system similar to the UBD circulation system, but MPD does not invite the influx of formation fluid during drilling like UBD. Instead, the ability to apply varying degrees of backpressure with a closed and pressurized mud returns system is key to enabling technology to contain and safely control any influx that may be incidental to the MPD operation. The difference between the UBD can also be in the type of drilling fluid. In UBD we can use compressible fluids like air or mist and foam where in the MPD drilling we are using incompressible drilling fluids. The usage of incompressible drilling fluids results in almost instant change in BHP. To finally distinct between these two methods we can say that the UBD is a reservoir-issued method and the MPD is drilling-issued method. MPD includes faster corrective action to deal with observed pressure variations. Dynamic changes of the bottom hole pressure by varying the amount of surface pressure enables some of the projects that were declined due to costs, to be reconsidered again and in the end drilled with this method. But the control of the bottom hole pressure can also be accomplished by the change in the mud density, rheology, fluid level, friction regarding the type of the fluid that is used, hole geometry and combinations of these factors. MPD technology uses tools and techniques which may mitigate the risks and costs associated with drilling wells using the overbalanced drilling. The system is designed to complement the existing well control equipment and an important feature is that there is no change in existing rig well control equipment. Since the same equipment is used as in conventional drilling in combination with added equipment for managed pressure drilling performing, the operations can employ either of the systems utilizing easy procedures switching procedures from one method to the other. Additional equipment is added to enhance the corntol of the first barrier. The main components of the MPD systems are:
1. Rotating Controlling System – RCD, and
2. MPD choke valve.
3. Precise mass flow meter
4. Surface gas separator (degasser) if needed downstream of the choke manifold.
Systems with this equipment can vary from manual to fully automated systems.