Petroleum Department Ass. Lect. Waseem Ali Fourth Stage Drilling Problems Drilling Problems 1.3- REDUCTION OF HYDROSTATIC PRESSURE The reduction of hydrostatic pressure is the obvious and most successful method of freeing a differentially stuck pipe. The lowering of the hydrostatic pressure reduces the side loading forces on the pipe and therefore reduces the force required to free the pipe from the filter cake. There are several methods by which this may be achieved. However prior to implementing this action the following factors should be seriously considered: 1. Are there other pressured zones in the open hole section? 2. Will these exposed zones kick if the hydrostatic pressure is reduced? 3. The confidence level in the accuracy of pore pressure estimates made while drilling and the pressure control equipment. 4. The effects of a reduction in hydrostatic pressure on the mechanical stability of all exposed formations. 5. The volumes of base oil or water required to achieve the required reduction in hydrostatic pressure. (This may well influence the method chosen). All the above factors need to be carefully considered prior to reducing the hydrostatic pressure as the potential for inducing a well control problem or formation instability are considerably increased. The following methods for reducing hydrostatic pressure can be used: • circulation & reducing mud weight • displacing the choke • the ‘U’ tube method 1.3.1- CIRCULATION & REDUCING MUD WEIGHT In this method, the drilling mud is circulated and its weight is gradually reduced. The minimum mud weight required to balance the highest pore pressure in open hole should be determined and the mud weight cut back in small stages. Close attention must be made to all kick indicators whilst circulating down (reducing) the mud weight, frequent flow checks should also be made. Whilst reducing the mud weight, tension should be held on the pipe. Petroleum Department Ass. Lect. Waseem Ali Fourth Stage Drilling Problems Disadvantages of this methods are: • It is slow, and remember the force required to free pipe is time dependent. • The volume increase required may overload the surface pit handling capability. This may be a serious problem when OBM is used. • The active volume will be increasing during the reduction in mud weight, making kick detection difficult. 1.3.2- DISPLACING THE CHOKE This method is applicable to floating rigs where BOPS are placed on the seabed. The hydrostatic pressure can be quickly and effectively reduced by displacing the choke line to base oil or water. The well is shut in using the annular preventer and the displaced choke. The advantage of this method is that if any influx is taken, the well can be immediately killed by closing the choke and opening the annular. This action again exposes the well to the active hydrostatic pressure from rig floor to TD. The disadvantage of this method is that the amount of reduction in hydrostatic pressure is limited to the water depth. This may well result in a limited reduction in shallow water, or in the case of deep water, an excessive reduction in hydrostatic pressure. line opened thereby reducing the overbalance. Note that the annular preventer isolates the wellbore from the hydrostatic head of mud in the riser from rig floor to the annular preventer. 1.3.3- THE ‘U’ TUBE METHOD The U-tube method is used to reduce the hydrostatic pressure of mud to a level equal or slightly higher than the formation pressure of the zone across which the pipe got differentially stuck.Clearly, the objective is to free the differentially stuck pipe safely without losing control of the well by inadvertently inducing underbalanced conditions. A pipe free agent should be spotted across the permeable zone prior to adopting the ‘U’ tube method. The mathematics required for the full method is laborious. Problem (1-2): Calculate the volume of oil required to reduce the hydrostatic pressure in a well by 500 psi, using the following data: Mud weight = 10 ppg Hole depth = 9843 ft Drillpipe = OD/ID = 5"/4.276" Petroleum Department Ass. Lect. Waseem Ali Fourth Stage Drilling Problems Hole size = 12.25" Specific gravity oil = 0.8 (6.7 ppg) Solution: Initial hydrostatic pressure = 0.052 x10x 9843 = 5118 psi Required hydrostatic pressure = 5118 - 500 = 4618 psi Thus, New hydrostatic pressure = pressure due to (mud and oil) in drillpipe 4618 = 0.052x 10xY (mud) +0.052x (6.7) x (9843-Y) (oil) Where: Y = height of mud in drillpipe. Therefore = 6927 ft Hence, Height of oil = 9843 - 6927 = 2916 ft π 1 4 144 Volume of oil = capacity of drillpipe x height = × 4.2762 × × 2916 = 290.79 ft 3 = 51.7 bbl Note that when the required volume of diesel oil is pumped inside the drillpipe, the hydrostatic pressure at the drillpipe shoe becomes 4618 psi, while the hydrostatic pressure in the annulus is still 5118 psi. This difference in the pressure of the two limbs of the well causes a back-pressure on the drillpipe which is the driving force for removing the diesel oil from the drillpipe and reducing the level of mud in the annulus. It is only when the annulus level decreases that the hydrostatic pressure against the formation is reduced and the stuck pipe may be freed. When the formation pressure is unknown, it is customary to reduce the hydrostatic pressure of mud in small increments by the U-tube technique until the pipe is free. A variation of the U-tube method is to pump water into both the annulus and the drillpipe to reduce hydrostatic pressure to a value equal to or just greater than the formation pressure. 1.4.- SPOTTING PIPE RELEASE AGENTS The severity of differentially stuck pipe can be reduced by the spotting of pipe release agents. Pipe release agents are basically a blend of surfactants and emulsifiers mixed with base oil or diesel oil and water to form a stable emulsion. They function by penetrating the filter cake, therefore making it easier to remove and at the same time, reduce the surface tension between the pipe and the filter cake. Due to the time dependency of the severity of Petroleum Department Ass. Lect. Waseem Ali Fourth Stage Drilling Problems differential sticking, the pipe release agent should be spotted as soon as possible after differential sticking is diagnosed. Typically the pill will be prepared whilst initially attempting to mechanically free the pipe; ie by pulling and rotating.