In all drilling formations it is important that we try to improve productivity, whilst ensuring that we control both risk and cost. To be able to do so we must attempt to understand the formation being drilled as this is the main factor that will decide on the type of drill bit, mud mixing/pumping/recycling plant and drilling fluid to be used. A drilling fluid cannot only significantly improve the efficiency of the entire operation, but also lower cost and direct impact on the environment.
Their are multiple principals for using a drilling fluid. These include:
- Removes material (cuttings) from the borehole: The drilling fluid carries the drilled material to the ground surface either by viscosity or velocity.
- Cools and lubricates the bit: The drill bit becomes hot due to friction generated during the drilling process. As the drilling fluid passes through the bit and exits the jets/nozzles, the excess heat is removed and carried up the borehole.
- Cleans the drill bit: When the drilling fluid exits the bit jets, the fluid’s velocity removes material from the bit teeth and the cuttings from the bit formation interface. This prevents the cuttings from being re-cut or re-ground.
- Controls fluid loss: As the fluid moves from the borehole into the formation, clay particles are deposited on the borehole wall. The clay particles form a barrier limiting the amount of drilling fluid penetrating the formation. This barrier, called a filter cake, is important for the stability of the borehole. Additionally, well development time is reduced if fluid loss to the formation is limited.
- Stabilizes the Borehole: The drilling fluid’s weight in the borehole must overcome the formation pressure to preventthe borehole from collapsing. Also, the fluid prevents formation swelling by “coating” the formation with an impermeable barrier.
- Lubricates the drill pipe: The drilling fluid reduces friction between the drill pipe and the rising cuttings, and also between the drill pipe and the formation.
- Suspends cuttings: When the mud pump stops, the drilling fluid velocity stops. The fluid must have enough gel strength to keep the drilled material (cuttings) in suspension until the mud pump actrivates.
Pure Water: Being the simplist form this may work in the hardest rock formations using foam mist, but does not work as efficiently in alluvial materials/sedimentary formations. Water will also dissipate is porous formations like sands and gravels, which may then collapse. Shales will tend to hydrate, sloughing back in to the borehole and in clays it will usually create a high wash out casing the clay to dissolve.
Foam: Normally created by mixing a chemically stabilised detergent with water. This additive is most commonly used in air drilling, with the mixture being injected in to the air stream, as a fine mist or in larger quantities. When drilling consolidated rock with compressed air, large clouds of dust are usually blwon out of the borehole. This foam injection assists in dampening the dust cloud, helping protect the health of both the operaqtors and the equipment. In less cohesive/stable formations it will help keep the hole walls clean, preventing small water-bearing fissures being blocked by damp drill collars of cuttings that form behind the bit.
Drilling Mud: Produced by mixing a hydrateable clay with water. The most commonly used being bentonite. Once mixed the mud has a higher density than water meaning that that the fluid cannot escape from the formation ensuring a positive hydrostatic head is maintained. This positive head means that the formation cannot collapse as a tight thin wall cake is produced supporting the boreholes structure. Other additional benefits include good gel strength to suspend cuttings even when the pump is turned off and the prevention of wash-out.
Liquid Polymer: A synthetic, polyacrylamide based drilling fluid. A small quantity will mix efficiently with salt and fresh water, developing a high viscocity drilling fluid, encorporating good lubrication and clay encapsulation. These are effectively designed to have the same effect on the physical properties of water as mud, ensuring improved handling properties on the surface and stabilisng the borehole wall.
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PDS TK60 Liquid - Click Here