Drilling fluids are consisted of two major parts: fluid phase and solid phase. During drilling operations, solid cuttings are generated continuously, these solids are considered as inert particles and do not hydrate or react with other compounds within the mud. Drilled cuttings have to be removed from the drilling fluids for better and more efficient drilling performance.
Solid Control can be defined as the process which can help to control the quantity and the quality of the suspended solids in the drilling fluids. This process has to be performed without affecting the percentage of solids which are parts of the drilling fluids systems such as barite and bentonite. It is important to estimate the amount of the generated drilling cuttings during operations that have to be lifted to the surface. The drilling cuttings should be maintained at 5% per volume of drilling fluids for better performance. The generated cuttings are estimated in barrel per hour or tons per hour using the following formula:
Vs= π(1-Φ)d²ROP/1029
where
Vs generated drilling cuttings volume in bbl/hr
Φ formation porosity
d drill bit size ''in''
ROP rate of penetration ft/hr
For a simple example, drilling at 80 ft/hr through 20% porosity formation with 16 in drilling bit can lead to almost 6.5 tons/hr of drilled cuttings. This can show the huge amount of solids generated while drilling operations. Failing to control the optimum cuttings percentage during operations can lead to many problems and increase non-productive time and drilling operations overall cost.
Impact on the filter cake quality
High percentage of drilling cuttings in the mud system can contribute in generating thicker filter cake. Differential stuck pipe can be faced when the drill collars are forced toward the formation due to the differential force generated between the formation and wellbore. This risk can be higher when making a connection or forgetting rotating the drill string during repairing any surface equipment such as top drive or draw works. The thick filter cake contributes in increasing the contact surface between the bottom hole assembly BHA components and the formation leading to higher level of overpull when proceeding to unstuck the drill string.
Swab and Surge can occur when running or pulling too fast compared to the acceptable range of tripping speed. Thick filter cake contribute in increasing the swab or surge pressures leading to well control situation is the case of swab or downhole mud losses in the case of surge.
Thick filter cake cannot be removed effectively during cementing operations and consequently the cement cannot fill the space between the casing and the formation. Several procedures and non-routine jobs can be needed such as squeeze cementing job leading to higher operations costs.
Impact on Plastic Viscosity
The plastic viscosity can be defined as the mechanical friction between particles in the drilling fluids. The plastic viscosity of the drilling fluids can be determined by four factors: the viscosity of the liquid phase, the size, the shape and the amount of solid particles. Related to the last description, the increase of the plastic viscosity can indicate the increase of the solids volume ratio inside the drilling fluids system. This increase can be a straightforward indication of solid control and mud dilution effectiveness.
High plastic viscosity can weaken the impact of the controlled yield point which is an important parameter for hole cleaning and can also reduce the overall performance of solid control equipment.
Impact on Carrying Capacity
Many drilling problems such as stuck pipe can be faced during operations due to poor hole cleaning. Failing to transport the cuttings to the surface can aggravate the situation by giving the cuttings enough time to disintegrate and consequently increase the low gravity content. Hole cleaning is represented by what it is called the carrying capacity index CCI which is empirically estimated as follows:
CCI= (MW x AV x K/400000)
With MW: mud weight, AV: Annular velocity, K is the characteristic viscosity.
The characteristic viscosity K is related to the plastic viscosity PV and yield point YP. any increase in PV will significantly decrease the K and resulting in decreasing the CCI factor leading to bad hole cleaning. This can be illustrated on the next chart
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| Fig 1: Impact of Plastic viscosity PV at Characteristic viscosity K |
Impact on Drilling Fluid Costs
High percentage of drilled cuttings in the drilling fluid system requires using the solid control equipment continuously. This process can affect drilling fluids properties such as the density which is controlled by the amount of weighting agents such as barite. Compensating the quantity of the weighting agents when solid control process affect the overall drilling fluids costs.
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