A new model for computing surge/swab pressure in horizontal wells and analysis of influencing factors

Abstract

In the drilling of horizontal wells in complex formations such as subsalt fractural formation, factors such as high drilling liquid density, rheological uncontrollability, and narrow safety density window of drilling fluid may lead to downhole pressure fluctuation, which, however slightly gives rise to complex downhole problems. In order to accurately calculate the surge/swab pressure of horizontal wells in such formations, this paper, based on hydromechanics and a width-variable flat-plate flow model, introduce a new model for computing surge/swab pressure in horizontal wells. This model takes the effects of velocity drilling string on the boundary conditions of surge/swab pressure into consideration, as no previous model has. With the tripping of drilling string under consideration, and when the annular flow remains the same, we find that, in the computation of swab pressure, the velocity in the inside and outside velocity zones are both larger than those produced by previous ones, and that in the computation of surge pressure, while the velocity in the inside velocity zone is first smaller and then turns greater, the velocity in the outside velocity zone is always larger. A comparison with previous models also reveals larger surge pressure, larger swab pressure at low rate and smaller swab pressure when annular flow rate reaches a certain level.An analysis of major factors that influence surge/swab pressure in this model shows that pressure drop is at the mercy of a number of factors; the surge pressure drop decreases with the increased eccentricity whereas the swab pressure drop increases with the rising eccentricity at low annular flow rate; the surge pressure drop decreases with the rising yield point whereas the swab pressure drop increases with the rising yield point; the surge pressure drop increases largely with the increase of plastic viscosity whereas the swab pressure drop largely decreases with the dropping plastic viscosity; the surge pressure drop increases with run in hole (RIH)speed; the swab pressure drop increases with pull out of hole (POOH) speed at small annular flow rate whereas the swab pressure drop decreases with the POOH speed when the annular flow rate reaches a certain level.The analysis also indicates that the surge/swab pressure is most sensitive to the plastic viscosity of drilling fluid. Then, it is of great significance to monitor the plastic viscosity during the drilling process when other factors are well controlled.