| Managed preesure drilling (MPD) is an innovative approach which can be used to solvethe drilling problems induced by narrow safety pressure window in deep carbonate reservoirs.The precise wellbore flow model is not only the premise to realize accurate wellbore pressurecontrol, but also the key for safe and efficicent drilling. Constant Bottomhole Pressure MPD(CBHP MPD) technology displays great advantage in solving above problems, therefore adetailed study is described in this thesis.The entire process of CBHP MPD flow model was established based on the traditionalsingle and multiphase flow model. The model can be used to study flow pattern and pressurechange rules under different drilling conditions more comprehensively, systematically andaccurately. The model include single liquid phase steady flow in drilling and gas-liquidmultiphase transient flow after gas invasion.In the thesis, the research mainly focuses on the following aspects:(1) Based on the energy exchange mechanism of individual control-unit components ofwellbore and formation under different drilling conditions, the physical model and unsteadymodel during circulation, circulation-stop and gas kick were established by using heat transfertheory and fluid mechanics for predicting the temperature distributions of drilling fluids insidethe drill string and in annulus and formation. In the model, Joule-Thomson effect, frictionalheat of fluid flow and the inflow of formation fluid with certain internal energy were consideredand finite difference method was used to solve model. Dynamic wellbore and formationtemperature distribution is revealed in the whole drilling process.(2) Based on dynamic wellbore temperature profile, drilling fluid density change withtemperature and pressure were analyzed, equivalent static drilling fluid density model whichcan accurately describe equivalent static density along the wellbore was built. Hydrostaticcolumn pressure calculation model was corrected. The influence of tool joint, drilling toolrotation and eccentric annulus on friction pressure loss was studied. Considering the factors ofdrilling fluid density, viscosity-temperature characteristic, tool joint, rotation and eccentricannulus of drilling tool, and pressure fluctuation in the process of tripping, single-phase steadyflow model of constant bottomhole pressure drilling was improved, which enhanced wellborepressure prediction accuracy.(3) According to gas kick volume of different well types, gas bubble formation mechanismwas analyzed and gas bubble size calculation model under different gas kick volumes wasobtained. Starting from the force analysis of bubbles movement in different sizes, bubble risingvelocity model was developed, which was used to describe bubble dynamic behavior accurately.And on this basis, fluid flow characteristics and pressure fluctuation rules during the conditionof POOH, make a connection and well shut-in were researched and the relevant flow modelwas presented. With a view to wellbore storage effects during well shut-in, a prediction model of bottom wellbore pressure and wellhead pressure was established. Dynamic migration rulesof gas kick in wellbore was proposed, which laid a foundation for the establishment of transientflow model.(4) Considering the effect of temperature, reservoir flow characteristics, oil/gas phasechange and cuttings, mass, momentum and energy equations were deduced. And transientmultiphase flow model under the conditions of drilling, shut in and well killing process wereestablished to describe coupled multiphase flow quantitatively in wellbore. The dynamic modelwas solved by finite difference method. The dynamic model can truly simulate the wholeprocess from gas kick to well killing. Based on transient multiphase flow theory, dynamiccalculation model of wellhead back pressure was developed. The research results in this thesisnot only lay a theoretical foundation for dynamic well control and pressure prediction, butprovide a strong theoretical support for the precise dynamic control of wellbore pressure. |
PDF Full Text Request