Contamination of drilling fluids with drilled cuttings is an unavoidable consequence of successful drilling operations. If the drilling fluid does not carry cuttings and cavings to the surface, the rig either is not “making hole” or soon will be stuck in the hole it is making. The drill cuttings that are separated from the drilling fluid on the surface by the soldis control equipment and some quantity of unrecoverable or economically unwanted drilling fluid are a major source of drilling waste. Drilled and formation solids that are sized smaller than can be removed by the solids control equipment are often reported as drill solids. Some quantitiy of drill solids will accumulate in the drilling fluid and must be removed by the solids control equipment or reduced in concentration by dilution.
Solids control equipment
Before the introduction of mechanical solids-removal equipment, dilution was used to control solids content in the drilling fluid. The typical dilution procedure calls for dumping a portion of the active drilling-fluid volume to a waste pit and then diluting the solids concentration in the remaining fluid by adding the appropriate base fluid, such as water or synthetic oil.
Using solids-control equipment to minimize dilution has been a standard practice for the drilling industry for more than 60 years. Equipment and methods have changed over that time, but the fundamentals behind the process have not:
Solids concentration
Increasing solids concentration in drilling fluid is a problem for the operator, the drilling contractor, and the fluids provider. It is well established that increasing solids content in a drilling fluid leads to a lower rate of penetration (ROP). Other problems that are related to excessive solids concentration include:
Particle size and surface area
From the perspective of both the drilling-fluids specialist and the solids-control technician, the effects of particle size and surface area are perhaps the most important concepts to understand. The fluids industry describes particle size in microns.
One micron (μm) is one one-thousandth of a meter and is equivalent to one inch divided by 25,400. The visual acuity of an unaided eye is approximately 35 μm, and fingertip sensitivity is approximately 20 μm. Drilled solids vary in size from < 1 μm to 15,000 μm in average particle diameter. Colloidal-sized particles are < 2 μm (average particle diameter) and will not settle out under gravitational forces. Ultrafines range from 2 to 44 μm and are unlikely to settle out of a drilling fluid unless it is centrifuged.
Solids of colloid and ultrafine size have the most adverse effect on fluid rheology. Ultrafines and colloids have pronounced effects on mud properties because both particle types have large surface-to-volume ratios. Like bentonite particles, the exposed surfaces of fine drilled solids contain charges that increase the viscosity and gel strengths of the drilling fluid. Unlike bentonite, drilled solids do not plate out on sides of the wellbore to form a compressible and slick filter cake. The viscosity and fluid loss properties of a drilling fluid are difficult to control with high concentrations of drilled solids that are < 20 μm. The effect of drilled-solids degradation can be demonstrated by the fact that the available surface area increases almost 400 times when a particle degrades from 100 μm to 1 μm in diameter (Fig. 1).
Fluid-technology advances have solved many of the problems that contribute to fines buildup in drilling fluids. Today, fluids are highly inhibitive and prevent cuttings dispersion like that shown in Fig. 1; however, the fluids cannot prevent cuttings recirculation or the inherent mechanical degradation that occurs during recirculation. Drilled solids that circulate through the mud tanks and back downhole are subject to mechanical degradation from:
The solids-control equipment must remove solids as far upstream as possible in the surface drilling-fluid system. When drilled solids have degraded in size to ultrafines or colloids, it becomes increasingly more difficult to remove them by mechanical separation or settling.
Separation by settling
Hydrocyclones, centrifuges, and settling tanks rely on settling velocity to concentrate and seperate solids from the drilling fluid. Settling velocity is described mathematically by Stokes’ law (Eq. 1), which states that the velocity at which a particle will settle in a liquid is proportional to the density difference between the particle and the liquid, the square of the particle diameter, and acceleration. The settling velocity is inversely proportional to the viscosity of the liquid or slurry. This means that particles of greater mass are removed more easily than particles of lesser mass.
....................(1)where:
Because particle diameter is squared, it has a great effect on separation efficiency. However, other factors that affect the settling rate should not be overlooked. For example, if a fluid contains both high-gravity solids (HGS) and low-gravity solids (LGS), a centrifuge that recovers barite particles in the 10-μm range also will recover LGS particles in the 13-μm range because both have similar mass and would settle at the same rate. It also follows from Stokes’ law that the settling velocity is lower in viscous and dense fluids; therefore the cut point and the capacity of centrifugal separators will be adversely affected by increasing viscosity and density.
Screen selection
Obviously, shale-shaker screens are important for controlling the concentration of LGSs. What often is overlooked is the impact that proper screen selection can have on the other functions of a mud system:
Waste volumes
The combined waste volume of cuttings that are created while drilling and the excess or spent drilling fluid might be the best measure of performance and cost savings offered by a fluids system. The volume of spent mud determines what the mud-maintenance and disposal costs are and affects the long-term liabilities that are associated with waste disposal. Even under ideal situations, the volume of wet cuttings generated can easily exceed hole volume by a factor of two or more (three is a good rule of thumb). Minimizing the volume of spent mud and cuttings is the key to effective waste management. The increase in volume of the wet cuttings stems only partly from the added volume of cavings, washouts, or drilling a nongauge hole.
Cuttings are not discharged from mechanical separators as dry particulate matter. Much of the volume increase comes from the effect of surface-to-volume ratio. As drilled cuttings are ground down by the bit or dispersed from fluid interaction, they become thoroughly wetted with the drilling fluid. This fluid is known as ROC and is difficult to remove mechanically. Furthermore, a certain amount of carrier fluid usually discharges from mechanical separators with the cuttings. Unless measures are taken to dry the cuttings, the volume of drilled cuttings that is discharged will be more than double that of the theoretical gauge hole. Hydrocyclones in particular discharge very wet cuttings. The volume of spent mud that is created will depend largely on mechanical solids-removal efficiency.
Total fluids management
The importance of viewing fluids, solids control, and waste management as a process that must be designed to meet specific drilling conditions cannot be overemphasized. This process design is the key to helping improve the economics and minimizing theenvironmental impact of drilling activities. Many operators prefer a total fluids management approach that integrates fluids, solids control, and waste management to deliver a cost-effective wellbore in a safe and successful manner.
During the project planning stage, the following questions should be asked to help ensure successful field operations:
Nomenclature
| d | = particle diameter, L, μm |
| g | = acceleration or G-force (constant 980 cm/s2), L/t2 |
| η | = viscosity of the liquid, m/Lt, cp |
| Vs | = settling velocity because of G-force, L/t, cm/s2 |
| ρl | = density of the liquid = SG |
| ρp | = density of the particle = SG |
References
Bouse, E.E., Carrasquero, J.E., Corpoven, S.A.: Drilling Mud Solids Control and Waste Management, 23660-MS,http://dx.doi.org/10.2118/23660-MS
Dearing, H. L. (1990, January 1). Material Balance Concepts Aid in Solids Control and Mud System Evaluation. Society of Petroleum Engineers. http://dx.doi.org/10.2118/19957-MS
Rengifo, R., Browning, W. K., Bernal, G., Carruyo, F., Figueroa, V., & Medina, J. (2005, January 1). Evaluation and Optimization of Solids-Control Equipment Systems Reduce Waste Volumes, Improve Safety, and Lower Costs. Society of Petroleum Engineers. http://dx.doi.org/10.2118/93935-MS
Dahl, B., Saasen, A., & Omland, T. H. (2006, January 1). Successfull Drilling of Oil and Gas Wells by Optimisation of Drilling Fluid Solids Control- A Practical and Theoretical Evaluation. Society of Petroleum Engineers. http://dx.doi.org/10.2118/103934-MS
Aase, B., Omland, T. H., Jensen, E. K., Vestbakke, A. T. L., Knudsen, B. S., Haldorsen, F., … Peikli, V. (2013, March 21). Criticality Testing of Drilling-Fluid Solids-Control Equipment. Society of Petroleum Engineers. http://dx.doi.org/10.2118/159894-PA
Getliff, J.M., Silverstone, M.P., Dowell; Shearman, A.K., Lenn, M., Hayes, T., IBS Viridian Ltd: Waste Management and Disposal of Cuttings and Drilling Fluid Waste Resulting from the Drilling and Completion of Wells to Produce Orinoco Very Heavy Oil in Eastern Venezuela, 46600-MS, http://dx.doi.org/10.2118/46600-MS
