Design Of Hydraulic Parameters In Unconventional Well Kill Methods

Unconventional well kill refers to a variety of well kill methods except the driller’smethod, the engineer’s method and other conventional methods of constant bottom holepressure well control. Because of the special well control conditions, the flow mechanismand patterns of borehole fluid are complicated. At present, the hydraulic parameter designof unconventional well kill is limited to the field experience calculation model due to itscomplicated process. Therefore, it is significant to study special well kill methods underdifferent working conditions and design the related hydraulic parameters.According to the comparison experiment of gas rising velocity in Newtonian andnon-Newtonian fluid in static vertical flow, the factors which affect the bubble flow slipvelocity, including the bubble diameter, the density and the viscosity of the liquid phase areanalyzed. The results indicate that larger bubble diameter and smaller viscosity of theliquid phase lead to higher bubble slip velocity. Yet the density of the liquid does not muchaffect the bubble slip velocity. For non-Newtonian fluid, higher viscosity of the liquidphase and larger bubble diameter lead to higher drag coefficient. Meanwhile the density ofthe liquid phase does not much affect the drag coefficient. Based on the experiment dataanalysis, a new model is put forward to calculate the gas slip velocity in staticnon-Newtonian fluid according to the relationship between drag coefficient andinfluencing factors including gas bubble diameter, liquid viscosity and liquid density.An experiment of bubble slip velocity affected by wellhead back pressure is carriedout in vertical static annular two-phase fluid. The results indicate that under the pressure of1MPa to3MPa, a turning point exists in the curve of gas slip velocity vs. gas fraction. Andthis turning point corresponds to the critical gas fraction of bubble flow instability. Underthe pressure of4MPa to6MPa, the turning point of the gas slip velocity is not observed.The critical gas fraction will increase with the increase of liquid flow under the samewellhead back pressure. A new calculation model of the gas slip velocity under high pressure is regressed based on the boundary of the critical gas fraction when the bubbleflow instability occurs.In the condition of drilling well without mud, the whole well kill process is dividedinto three stages considering the characteristic of bottom hole pressure and wellheadpressure. Then the multi-phase flow controlling equations are established. For each stage,the corresponding secondary equation, boundary conditions and the discrete solutionmethod are given. The simulated computation demonstrates that the higher the wellheadpressure is allowed, the lower kill mud density is needed to kill the well successfully. For acertain wellhead pressure, the kill rate needed to kill the well successfully decreases withthe increase of kill mud density. The relationship of kill rate and kill mud density can beused as the criterion of kill.While applying the bullheading kill method, the fluid flow resistance should beovercomed to push the wellbore fluid back to the formation. The flow resistance information is related to the formation conductivity, the liquid saturation and the forehead ofthe multi-phase flow. The calculation results indicate that the bottom hole gas-liquidtwo-phase flow and the pressure difference between bottom hole and forehead of themulti-phase flow increase with the increase of the liquid saturation and the volume of theliquid pressed in, during the process of bullheading. A constant rate of the kill fluid is notsuitable when the gas kick is large, for the formation at bottom hole will have a risk ofbeing fractured when trying to press back all the invaded gas kick. For well kill of gaswells, the earlier well kill starts and the larger kill rate is adopted, the smaller maximumcasing pressure of the kill process will occur.In case of the gas well blowout in off-bottom condition, considering the gas criticalvelocity, the zero net liquid holdup and the falling back of the kill fluid under the bit, amethod of well kill for blowout in off bottom condition is put forward. The method ofcalculating the pressure profile along the wellbore under the drilling bit is obtained, whichperfects the conventional dynamic well kill. The simulated calculation results indicate thatthe kill rate of conventional dynamic killing method is excessive compared to that of theimproved kill method at the same kill mud density. Furthermore, the bottom hole pressureof conventional dynamic kill method is also smaller than that of the improved method at the same kill rate when the gas velocity is lower than the gas critical velocity.