One of the success keys in drilling projects is the Drilling fluids which are circulated down hole through the drill string and drilling bit then up in the annular space towards the surface. The drilling fluids program which includes design, formulation, and reactions with the formation and also surface treatment are crucial for operations.
Drilling fluids have many functions which can involve:
- Controlling the formation pressure to do not allow formation fluids to come into the well which can lead to well control situations
- Hole cleaning and that means suspending and removing drill cuttings from the wellbore
- Provide mechanical and chemical borehole stability
- Allow for reliable formation rocks information
- Drilling bit and string lubrication
- Minimize the formation damage
- Limit the level of the corrosion
Many functions can be listed which are related to the drilling fluids. The important ones are detailed below.
1. Hydrostatic Pressure
While performing drilling operations, part of the rock is removed and has to be replaced by the drilling fluids for two reasons:
- Supporting and stabilizing the drilled rock
- Overcoming the formation fluids pressure do not allow them to enter into the wellbore and this issue is related to safety.
The hydrostatic pressure is generated by the column of the drilling fluids. This pressure has to be calculated correctly and accurately because exceeding this planned limit can lead to damaging the formation or even worse formation fracture can be faced leading to drilling fluids losses and well control situations. Gathering data about the formation stresses, strength and pore pressures is very important to planning drilling fluids muds and casing setting depths.
The pressure generated by the drilling fluids in static situation can be estimated as following
Hydrostatic Pressure HP = 0.052 x MW x TVD
Where
MW is the mud weight in ppg
TVD is the total vertical depth in feet
When circulating fluids downhole additional impact on the formation is generated which can be added to the hydrostatic pressure level, this impact is represented by the equivalent circulated density and can be calculated as follows:
ECD = MW + [APL / (0.052 x TVD)]
Which
ECD is the equivalent circulating density in ‘’ ppg’’
APL is the estimated annular pressure losses ‘’Psi’’
2.Cuttings Transport and Hole cleaning
Cuttings are generated while drilling and have to be removed from the wellbore to allow for better drilling progress. Cuttings are transported to the surface by the drilling fluids then they are separated by the solid control equipment. The transporting of the cuttings is related to many factors including the annular velocity which should be greater than the slip velocity. The drilling fluids should help to remove the cuttings away from the area where they are generated below the drilling bit, and should also have the ability to suspend the cuttings when the mud pumps are shut off for any reasons for instance making connections. Removing the cuttings can be affected by the mud pumps rate, mud properties and hydraulics. The drilling fluids properties can help improving the lifting capabilities and the suspension of the cuttings but the volumetric flow is important to sweep area below the bit and transport the cuttings to the surface. Poor hole cleaning and cuttings transport can overload the annular space which can affect the mud density and lead to excessive hydrostatic pressure and this can happen especially when drilling surface formations where the rate of penetration are higher. One of the solutions when drilling surface section is controlling the rate of penetration in order to avoid facing severe losses and may be total losses. For this reason the flow rate is critical for overall drilling performance.
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Fig 1 - Stuck Pipe Due to Pack Off |
3.Filtration Control
Fluid loss of the drilling fluids is related to the level of the overbalance between the hydrostatic pressure generated by the column of the drilling fluids and the pore pressures, formation permeability, filter cake permeability and the temperature. Uncontrolled fluid loss can raise many issues while drilling such as reduced penetration rate, poor hole cleaning, formation damage and differential sticking. Mud filtration can become more critical when drilling through depleted formations, zones with high level of permeability and naturally fractured layers and this can be more severe when hydrostatic pressure is higher. Also, it can be mentioned here that the level of invasion into producing zones can cause formation damage and affect the production level from this affected zone or layer. A good filter cake can help to maintain fluid invasion under control, unlike the thick filter cake leads to excessive torque and drag and in some cases to stuck pipe by differential sticking mechanism.
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Fig 2 - Stuck Pipe by Differential Mechanism |
4.Mechanical Stability
The mechanical instability or rock failure results from the imbalance between the different stresses. When drilling a new hole, the stresses have to be rebalanced to avoid wellbore collapsing. There are many factors which can affect the shale stability and they are listed as follows:
- Proper Drilling fluids density to keep the borehole walls stable and avoid cavings
- Erosion which can be caused by uncontrolled hydraulics
- Contact with drill string equipment like drill pipe which can with high rotating level destabilize the formation
- High fluid losses can contribute in weakening the formation
Rock instability can result many downhole problems which increase the non-productive time NPT and lead in worse cases to loss the hole. Tight spots can be faced when running in or out of the hole and result in excessive wear on drilling bit and stabilizers. Formation cavings can come into the drilling fluids system and when the mud can not transport these cavings to surface they can cause stuck pipe which is considered may be the most costly problem. Dealing with stuck pipe takes rig time and consequently extra project budget. In some cases, it becomes impossible to recover the fish and the project has to be modified which can include very expensive operation such as setting a cement plug, side tracking the well to come the original point.
In this mechanism, it can be listed three types of subsurface stresses. The overburden pressure which is the pressure exerted by the formations layers weight above the concerned zone. The pore pressure is the pressure of the formation fluids inside the pore spaces and this pressure helps in supporting the overburden pressure. The matrix stresses which are considered as a part of the overburden pressure and it is supported by the formation physical structure, the matrix stresses can be divided into three components one vertical and two horizontal.
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