The Wait & Weight method is one of the well killing techniques used extensively all over the world. To explain briefly, it is a constant bottom hole pressure (BHP) method of circulating the kick out while pumping a heavier kill mud into the well to replace the original mud which was in the well when the kick occurred. This requires that a certain circulating pressure versus pump strokes schedule must be maintained while pumping the kill mud, to keep the formation pressure equally balanced to prevent additional influx into the well. This pressure schedule helps to pre-determine the amount of circulating pressure that must be maintained at the drill pipe pressure gauge at any time when the kill mud is traveling from surface to bit. The standard methodology used for calculating the pressure schedule in vertical wells, assumes a linear relationship between the circulating pressure and the volume of kill mud pumped. If the same method is used in a deviated or horizontal wells, serious problems may occur. This includes over pressuring the formation which may lead to loss circulation, more complications in well control and or loss of well and its production.
The paper is developed to provide an understanding to the rig personnel on the behavior of circulating pressure when the kill mud is being pumped. The circulating pressure schedule depends on two things namely, increase in friction pressure and hydro-static pressure due to pumping of the heavier kill mud from surface to bit. The standard method for calculating the pressure schedule follows two assumptions :
The length of kill mud column increases by the same amount for each incremental increase in pump strokes and
The true vertical height of kill mud column increases by the same amount for each incremental increase in pump strokes.
The first assumption is valid for vertical as well as deviated or horizontal wells, as long as the inside diameters of the different components of the drill string are similar. The inside diameter of the bottom hole assembly components i.e., drill collars and hevi-weight drill pipes are usually smaller than that of drill pipes. But this change can safely be neglected due to the relatively short length of bottom hole assemblies when compared to the drill pipe. However, this assumption shall not be valid for tapered strings, where two different size drill pipes are used.
The second assumption is valid for vertical well with the same considerations as above for inside diameter of all components in drill string. This assumption is valid for horizontal wells only up to the kick off point (KOP). Thereafter, from kick off point to the horizontal entry point i.e., at 90 deviation, the true vertical depth (TVD) depends on the angle build up rate and the length of the build up section (BUS). From horizontal entry point to the end of the well, the true vertical depth for all practical purposes, remains constant. Hence, the standard method cannot be applied for calculating the pressure step down schedule in deviated and horizontal wells.
A true method is developed in this paper to use on deviated and horizontal wells. A comparison of vertical, deviated and horizontal wells as well as a vertical well with tapered drill string is shown to explain the degree of variance between the standard and the true method.
The true method eliminates the two assumptions used by standard method. The difference between the inside diameters of bottom hole assembly components and drill pipe is neglected. The effects of influx expansion in the annulus is ignored.
Let us consider a horizontal well, with a constant build up rate from kick off point to horizontal point.