Drilling Fluid Issues for Extended Reach Drilling

Many issues related to drilling fluid have to be considered when planning to drill extended reach well (ERD). These issues can pose challenges to the completion of the operations. These challenges include: 

- ECD management 
- Torque and drag 
- Barite sag 

Drilling fluid is one of the most important factors when drilling extended reach well. Oil based muds have been for long time the best choice when planning for such projects, but with the increase of the HSE restrictions and environment protection strategies, the inhibitive water based muds systems are getting more popular. 

The invert emulsion muds have been successful due to their ability to stabilize reactive clays, keep hole stability, resist to contamination and they have also the ability to provide low ECD, excellent hole cleaning and cutting suspension with also stable mud properties. 

When shale inhibition is expected while drilling, water based muds can be good choice. Potassium based, non-dispersed, polymer mud containing glycol or silicates are all good candidates for safer operations. 

Glycol drilling fluids provide many advantaged such as enhanced shale inhibition, improved filter cake, reduced fluids loss and reduced dilution rates. The glycol solubility can be controlled by adjusting the glycol concentration and mud system salinity. 

Silicate drilling fluids have more ability to stabilize shales resulting in gauge holes. Silicate muds are low solids polymer made in seawater with the addition of the soluble silicate. The inhibition mechanism is provided by the precipitation which occurs when the fluid is in contact with the shale surface ions leading to seal totally or partially the pore spaces. The silicate skin helps with the contribution of the hydrostatic pressure to support the borehole wall. 

ECD Management 

The main feature of ERD wells is the high ratio of horizontal departure to the total vertical depth. The mud weight / fracture gradient window decreases with the length increase of the annulus due to the contribution of the annular pressure losses. The increased length of annulus can also limit the pumps rate to achieve optimum solids transport especially in enlarged hole sections. The ECD has to be controlled by using a drilling fluid with minimum plastic viscosity without losing the cleaning and suspension features of the fluids. The quality of fluids products have an impact on the rheology, some developed organo-clays have better ability to suspend solids without affecting enormously the plastic viscosity. 

Hole Cleaning 

Transporting cuttings in high angles sections of ERD wells is critical especially in long 12 1/4'' sections. Using three mud pumps is essential to provide better hole cleaning. Deviated Hole in the range of 40 to 65 degrees is considered as the most challenged part of the hole to clean, this is due to the tendency of cutting to slide down the hole and form cuttings beds. Close monitoring of stand pipe pressure, cuttings volumes and torque and drag helps to detect cutting beds formation. 

Excessive ECD can be induced by increasing flow rates leading to downhole losses. When ECD is an obstacle, controlled drilling can be performed to minimize the concentration of cuttings in the borehole. The controlled drilling can be associated with sweeps and short trips to perform operations safely. 

Using larger drill pipe diameter is a good option to deal with the pump pressure issue which can be a problem for deep deviated wells. Larger drill pipe allows for higher flow rates but the annulus clearness will decreases leading to higher ECD. 

Torque and drag recording and drill string pick up and slack off weight give good indication about the hole cleaning. In some cases, pumping high density or tandem pills with increased drill string rotation while circulating help to get efficient hole cleaning. The amount of cuttings on shakers have to be monitored because it is the straight forward feedback about hole cleaning effectiveness. 

Hole cleaning can be optimized by applying turbulent flow due the limitation of the laminar flow to move cuttings from the low side of the hole, but turbulent regime has some drawbacks such as hole erosion with high level of filtrate invasion as the filter cake fails to form or to be removed. Getting to the limit of the laminar flow at high flow rates associated with sweeps and mechanical practices can be a good option for moving cuttings. 

Hole Cleaning Sweeps 

Pumps rate can be limited leading to insufficient cuttings returns related to the drilling progress. Pumping high-density pills at maximum pump rates associated with drillstring rotation improves the cuttings transportation. The pill should cover from 100 m to 150 m of the annulus for better performance. It has to be taken in consideration, the excess of the pressure when pumping the high density pills. In cases where the rotating motion is limited, the high density pill is less effective, an other option to improve cuttings removal is pumping tendem pill which is consisted of low viscosity pill followed by a weighted viscous pill. The low viscosity pill should be pumped at turbulent rate to disturb the cutting bed and move it to the main annular flow path, the weighted pill with its increased buoyancy can help to lift the disturbed cuttings. The return of pills on surface should be monitored for cleaning analysis and evaluation. 

Circulating Clean 

Long and high deviated holes need more times to clean up, it can reach four times bottom ups for 12 1/4'' sections. It can be long but it is worth it due to many problems which can be faced during tripping. The drillstring should be rotated and reciprocated at moderate parameters during the circulation to avoid any stuck and cavings generation. 

Drill String Contribution In Hole Cleaning  

Drillstring rotation and reciprocation can improve hole cleaning by disturbing cuttings beds allowing for enhanced transportation. Rotating the drillstring helps to move the cutting from the lower side of the borehole to the upper side. After sliding, it is recommended to rotate the drill string before making a connection, this will help to remove the cuttings beds from the low side and transport it up away from the BHA to avoid any mechanical stuck pipe events. Rotation can contribute in the hole cleaning overall performance, however it can cause hole damage. This option should be applied with care. 


While tripping cuttings beds can be dragged up with the drillstring leading to cutting accumulations and may pack-off around the BHA. Torque and drag can be considered as good indications to monitor this phenomena. Pumping high density sweeps or tendem sweeps prior to tripping is a good practice to avoid such a problem. 

Torque Reduction 

Smooth well profile is important for torque and drag point of view. Minimizing dogleg severity and well tortuosity contributes in reducing the torque and drag level. Rotary steerable systems perform directional drilling while continuously rotating resulting in smoother wellbore and improved hole cleaning. Friction factor can be reduced by using some mud lubricants which have the ability to improve the lubricity of water based muds approaching the oil based mud features without hurting the environment. 

Ballooning Effect 

Ballooning effect is characterized by cycles of losses and gains of drilling mud. The ballooning is based on opening and closing induced micro fractures. When the ECD exceeds the fracture pressure, a steady radial fracture propagates leading to mud losses, and when the pumps are switched off, the ECD falls below the fracture pressure leading to fractures closing and pushing the lost mud to flow back into the wellbore. If the ECD exceeds the fracture pressure to a certain level, the fracture propagation will be unstable and causes severe mud losses. Both, water and invert emulsion drilling muds produce this phenomena. Granular lost circulation materials can be used to reduce the severity of ballooning in combination with drilling parameters control. 

Barite Sag 

Barite sag is one of the challenges faced when drilling ERD wells. It can lead to many problems such as: mud weight fluctuations, downhole losses, well control situations and stuck pipe. Barite sag can be accelerated when the mud is static or at low shear rates for example when slow circulation, running casing or even wireline logging. It can be controlled by applying suitable drilling fluid design and good drilling practices. Increasing the viscosity of the drilling mud can help to minimize barite sag formation, also rotating the drill string at high level can also contribute to eliminate this phenomena. 

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