Modelling of the Transient Pressure Build-up in Long Wellbores
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Drilling of long oil wells becomes increasingly more common in the oil and gas industry, as cost rises and complexity of undrilled reservoirs increases. This introduces a challenge that grow in severity with the length of the well, the transient pressure period. The transient phenomenon occurs when the flow system of the well experience a disturbance, requiring a change in the steady state pressure. When the wellbore is long, the systems necessitate a significant amount of time to attain a new steady state value, up to several minutes. During this period, the rig crew has little knowledge of the actual well pressure. The disturbance may be anything affecting the well flow conditions, such as a pack-off, kick or change in mud pump discharge. This work focuses mainly on investigating the pressure transient occurring during a change in the mud pump discharge. The present thesis develops two in-house numerical solvers for the governing equations of the pressure propagation in the wellbore. One is based on the Method of Characteristics (MOC), developed by Wylie and Streeter (1993), created to solve fluid transients in systems. The other method uses the MacCormack method (MCM) to provide a programmable solution to the governing equations. Simulations from the in-house models were compared to the simulations done with OLGATM, a commercial oil and gas simulator, along with field data from four different North Sea wells. The models correctly predicted the transient period in wells of a medium to long length (4000 - 9000 mMD). The duration of the pressure transients exhibited quadratic growth in length. Sensitivity analysis of the pressure transient with respect to wave speed, wellbore length, bit pressure loss, mud pump ramp-up, mud flowrate and friction was performed. Additionally the consequences for wellbore stability, pack-off detection, kick detection, pressure integrity testing and managed pressure drilling were studied. Protracted pressure transients are due to attenuation of pressure signals. The attenuation forces the pressure signals to propagate several times through the well in order to attain SS. Wave speed and friction are determining factors; wave speed dictates the speed of the pressure signal propagation, while friction determines the signal attenuation. The knowledge provided by the present thesis may contribute to lowering the operational and economical risk connected with drilling long wellbores.