Well Construction: Drilling Fluid Functions

Background
Earlier on, we defined drilling fluid and attempted to classify them based on the nature of the basic fluid used in its formulation eg water based fluid, oil based fluid and pneumatic fluids. We also went further to group them into formation dispersant and non formation dispersant fluids. Then we talked about shale inhibition fluids. Extensive research and field results acquired during drilling operations had proven that penetration rate and its response to weight on bit and rotary speed depends on the hydraulic horse power reaching the formation at the bit. Since the drilling fluid flow rate sets the system pressure losses and these pressure losses set the hydraulic horsepower across the bit, it can be concluded that drilling fluid is very important in the determination of drilling cost. An optimum drilling fluid is a fluid properly formulated so that the flow rate necessary to clean the hole results in the proper hydraulic horsepower required to clean the bit for the weight on bit and rotary speed imposed to give the lowest cost and optimal formation penetration rate, provided that the selected drilling fluid and the combination of variables results in a stable borehole which penetrates the desired target. A properly designed drilling fluid will enable an operator to reach the desired geologic objective at the lowest cost. A fluid should enhance penetration rate, reduce hole problems and minimize formation damage.
Drilling fluids are formulated to perform three major functions:

• Control Subsurface Pressure
• Transport Cuttings and clean the hole
• Support and Stabilize the wellbore

Control of Subsurface Pressure

A drilling fluid controls the subsurface pressure by its hydrostatic pressure. Hydrostatic pressure is the pressure exerted by a fluid column and depends on the mud density and the true vertical depth(TVD). 

P  = (K)(Mw)(D)

P  = Hydrostatic Pressure; K = Conversion Constant; Mw = Mud Density; D = Depth

K = 0.052 when D = feet, Mw = ppg, P = psi

K = 0.0069 when D = feet, Mw = pcf, P = psi

K = 0.098 when D = meters, Mw = g/cm3; P = Atmosphere

Borehole instability is a natural function of the unequal mechanical stresses, physical and chemical interaction of the geologic formations with the drilling fluid. The earth's pressure gradient is 0.465psi/ft. It is equivalent to the height of a column of fluid with a density of 8.94ppg, which is approximately the density of seawater. In most drilling applications, fresh water fluids which includes the solid particles incorporated into the water from drilling subsurface formations is sufficient to balance subsurface pressures. Failure to control downhole pressures may result to the influx of formation fluid into the wellbore.

Cutting Transport and Hole Cleaning

Fluid flowing from bit nozzles exerts a jetting action to clear cuttings from the bottom of the hole and the bit and carries this cuttings to the surface. Several factors influence cuttings transport. If the cuttings generated at the bit is not immediately removed and carried to the surface, they will regrind, stick to the bit ( balling) and retard effective rate of penetration into the uncut rock. The ability of the drilling fluid to effectively remove cuttings from the borehole while drilling also depends on the annular velocity of the drilling fluid. This velocity of the fluid in the annulus depends on the pump output flow rate, fluid density, average cutting particle size, fluid viscosity, side wall friction and the hole and drill pipe geometry. 

Increasing the density of the drilling fluid increases the hydro-static pressure and the carrying capacity through its buoyant effect on the weight of the cuttings. Increasing viscosity also increases the carrying capacity of the drilling fluid. It also improves the gel strength of the drilling fluid therefore its cuttings suspension ability when the fluid is not circulating. Pipe rotation moves cuttings away from the borehole/pipe surface walls to the center of the borehole annulus where the fluid velocity is very high; thereby enhance swift cutting removal from the hole. An examination of drilling fluid annular velocity distribution model across the annulus will reveal that drilling fluids move slowly around borhole walls and drill pipes while the velocity is at its peak in the center of the annular cross section of the bore hole. Hole inclination angle is another factor that determines the effectiveness of drilling fluid in cuttings removal and hole cleaning. Increasing hole angle makes cutting transport more difficult. 

Drilling fluid must have the capacity to suspend weight materials and drilled solids during connections, bit trips, and logging runs, or they will settle to the low side or bottom of the hole. Failure to suspend drilling fluid material might result to fluid density reduction, which in turn can lead to kick and a potential blowout.

At the surface, the drilling fluid must release the cuttings for efficient removal by solid control equipment( Shakers, Desilter, Centrifuge). Failure to adequately clean the hole will result in such hole problems as fill on bottom during trips, hole pack off, bit balling, lost circulation, stuck pipe, kicks and inability to reach the bottom of the hole with logging tools.

Well bore Stability and Support  

Fluid hydrostatic pressure acts as a confining force on the well bore. The confining force acting across a filter cake will assist to physically stabilize the well and prevent the formation from sloughing and caving-in to the hole. 

Borehole stability is also maintained or enhanced by controlling the loss of filtrate to permeable formations and by careful control of the chemical composition of drilling fluid. Most permeable formations have pore space openings too small to allow the passage of whole mud into the formation;however, filterate from the drilling fluid can enter the pore spaces. The rate at which the filtrate enters the formation depends on the pressure differential between the formation and the column of the drilling fluid and then the quality of the filter cake deposited on the borehole wall. 

Large filtrate inversion into the formation will not just destabilize the near well bore formation through shale hydration and eventual sloughing to chemical reactions between the driiling fluid filtrate and formation materials. Drilling Fluid filterate inversion into the formation can also cause reservoir formation damage or loss of well if drilling through a payzone section of the well.Drilling fluids which produce low quality filter cake may also cause tight hole conditions including stuck pipe, difficulty in running casing and poor cement jobs.

Other Minor Functions of drilling fluids includes: 

• Support of the weight of tubulars
• cool and lubricate the drill bit and drill string
• Transmit hydraulic power to bit
• Provide medium for wireline logging
• Assist in subsurface geological data acquisition and formation evaluation 

In addition to the above essential functions of a typical drilling fluid, they are other benefits that can be gained by proper selection and control of drilling fluid, such as to:

• Minimize Producing Formation Damage
• Reduce Corrosion
• Minimize lost circulation
• Reduce Pressure Losses
• Improve Penetration Rates
• Reduce Environmental Impact and improve safety 

Factors to consider while selecting drilling fluid for a given hole section includes the following:

• Cost and availability of materials
• Application and performance
• Available equipment and technology
• Nature and pore pressure regime of formations to be drilled
• Geological and exploration concerns
• Effect on reservoir productivity
• Environmental impact and safety

 Cost and availability of materials: The short and long term cost effect of using the drilling fluid needs to considered prior to selecting a particular drilling fluid system. Short term cost as per the cost that must be incurred to acquire and use a selected drilling fluid in comparison with other alternative drilling fluids that can be used to drill the hole. But the long term cost of a drilling fluid system may include its impact on total well cost and the fluid's effect on well productivity throughout the useful life of the well. Certain fluid systems can be easily sourced in a particular environment. For instance, sea water is readily available while drilling offshore or when attempting to dynamically kill a flowing shallow gas well.

Application and Performance:  Historical performance data of a similar type of fluid used in an offset well should be reviewed and pilot testing performed to ensure the suitability of any selected drilling fluid for a particular drilling operation. This will ensure that only fluid that will bring about the greatest hole stability and minimal total well cost is selected. So drilling fluid systems should be selected to provide the best performance.

Available Equipment and Technologies:  Equipment and technology selection is very important in well planning. Logistics and manpower planning is a major consideration in mud program development especially when operating in remote areas. Product efficiency, shelf life, packing, transportation cost, warehousing, inventory and volumes should be considered. The knowledge base and field experience of the mud crew should be considered and adequate plans made for training in the use of the selected fluid and associated equipment system. 

Nature and Pore Pressure Regime of Formation to be drilled: In fact this can be said to be the most important criteria when it comes to drilling fluid selection. Because of the need to control the flow of formation fluid into the well bore, the pore pressure of the formation must be over balanced by the hydrostatic pressure exerted by the drilling fluid on the bottom of the hole and side walls of the borehole/formation. As explained earlier, the hydrostatic pressure of a drilling fluid column depends on its density and the true vertical depth of the hole. Drilling fluids also do chemically react with the rock formations within the earth and this can lead to adverse hole problems. the compatibility of the drilling fluid with the formation to be drilled needs to be considered prior to spudding the well. The bottom hole temperature of the hole section also affects the choice of drilling fluid needed for a hole section. 

Production Concerns: Petroleum production personnel are personally concerned with minimizing formation damage. Drilling fluid and other activities which affect insitu formation characteristics needs to be considered while selecting a particular type of fluid and additives.

Geological and Exploration Concerns: The effect of drilling fluids on cuttings analysis and interpretation is very important to the petroleum explorationist. Apart from data transmitted by logging while drilling/measurement while drilling systems, cuttings analysis and interpretation provides the geologist the insight needed to understand the nature of the subsurface formation being drilled. Cuttings are usually analysed to locate oil and gas finger prints. It is also analysed to ensure that the hole is not being overloaded as such hole cleaning efficiency can be monitored by monitoring the solid content of the drilling fluid.

Environmental Impact and Safety: As more environmental laws and regulations are enacted, certain drilling fluids can no longer be used in some areas. Environmental and safety consideration equally determines the handling system required for each fluid and the method of disposing the fluid after use. In most areas like the Arabian Gulf, they is zero discharge policy of drilling fluids to the sea or onshore environment. In this region, earth pits are not allowed and if allowed, the hole must be well lined with a waterproof and leak proof material. The effect of drilling fluid and its additives on the personnel is equally a concern. It had been reported widely that diesel based drilling fluid is usually carcinogenic and irritating to the eyes and skin. These had lead some governments to disallow the use of diesel as a base fluid for mud systems.

Conclusion

Drilling mud program is critical to well planning and as such must be given all necessary attention to ensure a successful drilling operation program development and well construction. 

Note: You are free to ask questions regarding drilling mud program and report development. You can also enquire about mud chemicals/additives/storage/handling/circulation/hole cleaning/well control equipment. 

Just send me an email; kalu.ekwueme@gmail.com We are still admitting people into our discussion forum. You can join by sending me a whatsapp message on: 2348168085342.