| dc.description.abstract | A significant part of the remaining oil and gas resources are present in harsh offshore environments and depleted reservoirs that are challenging to reach with conventional drilling methods. An ever increasing energy demand forces the drilling industry to develop new techniques, to be able drill in these environments and to improve the efficiency of operations to make uncommercial prospects feasible. Drilling in deep-water and depleted reservoirs are limited by a narrow margin between the fracture and the pore pressure gradients. This requires an accurate control of the pressure in the wellbore. Managed pressure drilling (MPD) represents various techniques to control the pressure in the wellbore developed to meet the challenging demand in the industry. These methods introduces a closed pressurized system where the downhole pressure can be controlled by a choke manifold. In addition to narrow drilling window, drilling from a floating rig is challenged by surge and swab pressures in the wellbore due to the heave motion of the drilling rig. These pressure fluctuations are challenging to control during connections when the drillstring is suspended in slips and follows the movement of the rig. Pressure variations, caused by surge and swab, may become so large, dependent on amplitude of heave and length of wellbore, among other factors, that it will be impossible to keep inside the pressure window between the pore pressure and fracture pressure. This calls for alternative methods to be able to attain the desired depth of a well in such conditions. The work on this master`s thesis has been to build a model of a bore hole, sufficiently realistic to investigate the opportunity to utilize a Constant Bottom Hole Pressure (CBHP) MPD method on a floating drilling rig subjected to heave. The bottom hole pressure is kept constant by the use of choke and back-pressure pump, controlled by a control system. The control system utilizes pressure and flow data from the model to calculate the needed back pressure. Most of the work, represented in this thesis is, however, a hydraulic model for the system, including simulations of pressure and flow variations during heave, and corresponding surge and swab pressures. | nb_NO |
