Research On Multiphase Flow Law Coupled With Heat And Mass Transfers Between Wellbore And Formation For Underbalanced Drilling

Underbalanced drilling is often done to reduce formation damage, to avoid lost circulation problems, to increase rates of penetration, etc.The previous research has focused on the wellbore multiphase flow under the condition of mass exchange for underbalanced drilling. When the underbalanced drilling is applied in the deep formation or the formation of high geothermal gradient, the temperature changes caused by mass transfer between wellbore and formation will lead to a significant impact on wellbore fluid, pressure profile, and other parameters. Based on previous studies, the theories and methods about heat transfer, computational fluid dynamics and permeation fluid mechanics are introduced in this study. In the state of underbalanced drilling, we further study on flow regime and its recognition of gas-liquid two-phase flow in the annulus and drill pipe, the calculation of flow parameter, the unsteady formation fluid invasion while drilling, the unsteady heat transfer between wellbore and formation, the gas-liquid two-phase flow under the condition of heat and mass transfer. The heat and mass transfer coupled calculation model of multiphase flow between wellbore and formation is established. The purpose is to provide theoretical basis and technical support for the design and construction of underbalanced drilling in deep formation or high geothermal gradient formation.(1)During underbalanced drilling, the total transient production rate depends on reservoir characteristics as well as the bottomhole pressure and well penetration rates. Part of the complexity arises from the fact that the boundary conditions change with time. Thus, the drilled part of the reservoir is divided into two elements:the newly drilled segment and the old open segments along the wellbore. An approximate mathematical model that describes the single oil or gas flow in the newly drilled segment of the reservoir during underbalanced drilling is presented. The model considers the dynamic relationship among the fluid, porous medium and the flow conditions present in underbalanced operations. Once the model is solved, using a Laplace Transform to determine the local inflow rate of the newly drilled segment. An analytical model that predicts the production rate of the old drilled segments is introduced. The production rate takes into consideration the changing underbalance and penetration rate. The transient well influx law during underbalanced drilling process is revealed by simulation.(2)On the basis of thermodynamic, heat-transfer theory and fluid mechanics, the heat-transfer mechanism between wellbore and formation is studied considering the heat conduction, convective heat transfer, Joule-Thomson effect, friction heat and formation fluid invasion. A new transient heat-transfer mathematic model is introduced during underbalanced drilling. We proceed by first presenting two analytical solutions for tubing and annulus temperatures under various assumptions. The influence law of underbalanced drilling parameters on wellbore temperature is obtained. Compare to heat generated from well friction, frictional pressure loss is not one of the significant parameters affecting downhole temperatures. The liquid injection rate, gas injection rate, and the inlet temperature of the underbalanced drilling fluid at the drillpipe were found to significantly affect the temperatures in the annulus. Therefore, wellbore temperature can be regulated within a rational range by adjusting the liquid injection rate and the gas injection rate.(3)For the operational conditions of underbalanced drilling, such as the high downhole pressure and large pipe diameter, the effect of annulus geometry, inclination, high pressure, large diameter on gas-liquid two-phase flow are considered based on many related experimental and theoretical research at home and abroad. The flow pattern transition boundaries, flow behavior prediction models are modified. The steady-state two-phase flow model of the whole well is considering velocity profile coefficient, swarm effect coefficient, slippage effect and liquid holdup of slug flow. The predicted pressure is larger than Beggs-Brill empirical models and Hasan-Kabir mechanical models. But comparing with measured data, the established model has a high precision, and it can meet the requirement of field applications.(4)On the basis of the fluid mechanics, the seepage mechanics and the thermodynamics, the coupled multiphase-flow model of heat and mass transfer is established considering the effect of the temperature, the formation fluid invasion, the oil-gas phase transition in the different operational conditions including the normal circulation drilling, the influx and shut in. Through the analysis of different conditions, the corresponding initial and boundary conditions are given. Governing equations are discretized using staggered grid finite difference approximation and form a set of consistent equations where wellbore primary variables are phase velocities, phase fractions, pressure, and temperature. The simulations show:the effect of temperatures downhole affect the bottomhole pressure insignificantly, reservoir gas influx is found to significantly affect the temperatures and pressures in the annulus. After the reservoir gas flows into the annulus, bottomhole temperature decreased, gas volume fraction increased, the bottomhole pressure increased first and then decreased. After shut in, the chock pressure increased rapidly as well as bottomhole temperatures and bottomhole pressure, reservoir gas production rate decreased rapidly. Finally, the theoretical model is verified by an example.