Extended Reach Drilling Challenges - Part I-

Equivanlent Circulating Density - ECD - management

The ERD are characterized by their long horizontal displacement. The annular pressure loss (APL) increases with the annulus length increase, and this is not always associated with formation strength increase. The ECD is related to many factors: hole enlargement, drill pipe internal and external diameters, mud properties, nozzle area, temperature, and annular pressure loss. Many of these parameters are affected by downhole environment, others are chosen while drilling operations planning. 

In narrow formation pressure gradient/fracture gradient window, the pump rate can be limited to control the annular pressure loss and reduce the ECD but this can affect the quality of solids transport and causes fluctuation of the ECD which can generate lost circulation. The ECD and hole cleaning are interconnected, the poor hole cleaning can increase the ECD. Also the amount of cutting presents in the drilling fluid can affect the density of drilling fluid. A considerable increase in mud density can make ECD exceeds the normal level because the ECD is a combination of static density of the drilling fluid and the annular pressure loss. The pump rate has to be balanced to achieve sufficient cutting transport and minimizing cutting beds formation without generating excessive ECD.

ECD is not a fixed value, there is always a difference between the calculated value and the actual value, and this is due to the variation of mud properties and drilling parameters while drilling. The pressure while drilling measurement process (PWD) can help to monitor the ECD by providing the real time value of annular pressure which can help to determine the real time value of the ECD and this can be used to optimize drilling performances and avoid formation fracturing.

Drilling fluids for ERD are designed to generate a flatter rheological performance to reduce fluid rheology impact on the ECD. ECD limit drilling is also a technique used to improve performance. There is always a difference between the predicted ECD and the real time ECD which is determined using the PWD. The drilling operations are curried till the level when the real time ECD exceeds the predicted ECD by 1 ppg, because the real time ECD is more reliable and it is related instantaneously to the level of hole cleaning. 

Building an appropriate invert emulsion fluid around a base fluid with low kinematic viscosity can improve enormously ECD control. Optimizing rheology can be performed by using a high quality weighting agent.

ERD Hole Cleaning Challenges

Hole cleaning procedures for ERD include pump rate increase/annular velocity, pipe rotation, the control of the rate of penetration. The pipe in ERD is not in the center especially in the high deviated and horizontal section of the well. This eccentricity divides the velocity into two regimes: high velocity into the upper part of the hole and low velocity into the lower part of the well below the drill pipes. The cuttings are transported better with high velocity but the gravity tends to make cuttings fall into the low velocity area. Moving and rotating the pipe is the only way to transport the cuttings to the upper part to improve hole cleaning. Creating turbulent flow around the drill pipe can reduce the formation of cuttings beds and improve the process of cleaning.

Procedures which include higher mud viscosity, higher flow rate and rotating the drill string is not always the solution because these steps can generate an increase in annular pressure loss (APL), hence an excessive ECD, especially in the case of narrow equivalent mud weight/fracture gradient window.

Hole cleaning sweeps are not always efficient for ERD. The sweeps will follow the high velocity area and bypass the cuttings beds formed in the lower part of the hole if they are not supported by the pipe rotation. Also, it has been proved that short trips are a waste of time. The cuttings beds are removed from the parts touched by the short trips but when drilling is restarted the beds will be formed again. 

Fig 01- Bladed Drill Pipe for Hole Cleaning Enhancement 

Hole cleaning and drilling transport become difficult and harder in drilling an ERW, and these processes are harder with inclination varies between 40° and 65°. Cuttings in such inclinations tend to slide down the hole and form beds. When the inclination exceeds 65° till the level where the hole becomes horizontal, the cuttings form dunes bellow the drill string and BHA. This situation can cause many problems, for example: stuck pipe. Avoiding the inclination of the well in the upper part or larger diameter sections can provide better hole stability and this can contribute more in hole cleaning. Hole enlargement can be a real risk. The velocity decreases in regions where the hole size increases and this can affect the ability of hole cleaning.

For extended reach drilling, the most appropriate drilling fluids are those of potassium based, polymer mud with silicates or glycol. These types of drilling fluids are used when shale inhibition is required. Mixed metal silicates can be used if the inhibition is not needed. Drilling fluids with average plastic viscosity have to be used carefully, because they can help to minimize the ECD and avoid reaching ECD limits but this can have a significant impact on the hole cleaning process. Drilling fluids which have low kinematic viscosity have proven better hole cleaning and lower ECD especially in the case of large diameters.

In some cases, reducing the rate of penetration becomes the only way to keep an appropriate hole cleaning, but this can have an impact on the cost of the well.

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