With time, the bacterial degradation of organic components of drilling fluid starts. It is usually detected through increase in carbonate alkalinity. Bactericide is a generic name for chemicals that are added to avoid bacterial degradation of the organic components of drilling fluid.

Blocking materials are solids in the mud system that block the pore throat openings of permeable rock. Blocking material stops the flow of mud solids into the formation. However, the filtration loss continues through the reduced gaps. Drilled solids, barite, and Calcium Carbonate are common blocking materials in the mud system.

Types of Losses

Causes, Effects, and Control of Seepage Loss

Deflocculation is the process of breaking up the flocculent state. Defolocculants are thinning agents, which reduce the viscosity to disperse the particles flocking together in the mud system. It should be noted that deflocculated mud is not the same as dispersed mud. Traditionally, tannins, lignosulfonates, and lignites are used as deflocculants but many high-performance WBMs use short-chained polymers or surfactants instead.

A deflocculated mud usually shows much improved filter-cake qualities, lower yield point, and gel strengths. Filter-cake quality is improved because the platelets become detached from each other and can lie flat to form a thin, low-permeability filter cake. Lowering yield point and gel strength beyond a point may not be desirable and should be adjusted through appropriate chemical treatment. If yield points and gels are lowered too far, the suspension and cutting capacity of the mud could reduce.

Drilling and completion fluids are prepared by adding various liquid and solid chemicals in a liquid phase. Dispersants are aimed at breaking up solids and liquids as fine particles or droplets, thus improving the separation of particles to disperse well in the system and avoid any settling. Without dispersant, the particles may lump together and settle out of the liquid phase, changing rheological properties of the fluid system. Dispersants are also used in case of oil spills. They break up oil into very small particles, which get easily diluted or dispersed in water.

Drilling Fluid also known as mud is a sticky mixture of various solid materials with liquid. While drilling, the drilling fluid is continuously circulated in a close loop from the rig's mud system through the drill string, annulus, and back to the mud system. It performs various important functions and a properly designed mud system greatly contributes to the success of the drilling process.

Functions of Drilling Fluid

Equivalent Circulation Density (ECD) is the dynamic density of circulating mud in a borehole and is used to calculate the pressure exerted on the borehole wall when the mud is being circulated. Any fluid column exerts a pressure equivalent to its hydrostatic head on the walls of its container. Similarly, the drilling fluid also exerts pressure on the borehole, which is equivalent to the hydrostatic pressure of the mud column and is a function of the mud density. However, the pressure effect changes in dynamic conditions and is reflected as Equivalent Circulation Density (ECD).

More about Equivalent Circulation Density (ECD)

Significance of ECD and factors affecting it

Flocculation is the process where clays, polymers, or other charged particles become attached and form a fragile structure. After the clay is depressed in the mud system, the platelets arrange themselves according to their electrical surface charges. Positively charged edge to negatively charged face and floc together.

Flocculation is not desirable, since it results in a high yield point and high gel strength. Common causes of flocculation are high ionic strength, a high concentration of shale in the mud and high temperature, or contamination with an influx from the formations. Deflocculants are used for neutralizing the charges on the clay surfaces.

High Gravity Solids (HGS) are dense materials that are used as weighting material to increase the specific gravity of drilling fluid. Barite and Hematite are commonly used HGS in drilling fluid. Specific gravity to Barite is 4.2 whereas, for Hematite, it is 5.505.

Commonly known as HPWBM, the High Performance Water Based Mud systems are reformulated polymer water based systems designed to replicate the performance benefits of Oil Based Mud systems. HPWBM are designed to be able to achieve OBM benefits like shale stability, clay inhibition, lubricity, high ROP, while minimizing bit balling without the downside of adverse environment impact. In many wells a regular Water Based Mud (WBM) is used in shallow sections but due to water sensitive formations and high friction, Oil Based Mud systems are required in deeper horizons. In operations where cutting collection and environmentally safe waste disposal costs are high, HPWBM can prove to be more cost effective solution.

Also known as Hydrostatic Head, Hydrostatic Pressure is defined as vertical pressure exerted by the weight of fluid. While planning a well, specific gravity of the drilling fluid (mud) is selected to ensure that the hydrostatic pressure exerted by the column of mud is more than pore pressure at any point in the well. This acts as a primary barrier for well control by ensuring that the formation fluid is held back from flowing into the well at any point of the time.

Invert Emulsion Mud system has water-in-oil emulsion. It has oil as continuous phase and brine as internal phase. Typical oil/brine ratios range from 95/5 to 50/50 in invert emulsion systems. Invert emulsions are usually tightly emulsified, low fluid loss oil muds. Relaxed Invert Emulsion mud is the one with relaxed fluid loss control, which have proven to improve drilling rates.

Laminar flow occurs when fluid flows in parallel layers with layers sliding past one another smoothly without any lateral mixing. There are no cross-currents perpendicular to the direction of flow. Laminar flow has a parabolic pattern with velocity of the layers the centre of a conduit being the maximum and gradually reducing towards the walls. A low Reynolds number below 2300 usually indicates laminar flow where as the flows with Raynolds number from 2300 to 4000 would indicated in transition between laminar and turbulent flow.

The seepage loss or vugular loss of drilling fluid occurs due to high permeability formation, cavernous zones, or coarse beds of gravel. These losses seize once the solid particles in the mud system block the pore throats. In shallow unconsolidated sands or gravel beds, increasing the viscosity of mud also helps to stop or restrict the losses. If the loss rate is not controlled through these means, coarse, fibrous material is used, which is called 'Lost Circulation Materials' or LCM. Lost circulation treatment is most effective when LCM is mixed in batches of approximately 100 bbls. These LCM pills with a high concentration of coarse material are pumped into the well and placed opposite to the thief zones to stop or reduce the rate of drilling fluid loss.

Types of Losses

The solid particles in the mud system that have a density lower than the weighting materials like barite or hematite are called 'Low Gravity Solids' (LGS). Drilled solids and bentonite clay are considered LGS. Solids in the mud system are measured as lbm/bbl or Vol %. Solids percentage is continuously measured on the rig site to maintain their concentration within design limits. The density of LGS is assumed to be 2.60 gm/cc

Mud is another name of Drilling Fluid, which is a sticky mixture of various solid materials with liquid. Drilling fluid is continuously circulated while drilling in a close loop from rig's mud system through drill string, annulus and back to mud system. It performs various important functions and a properly designed mud system greatly contributes to the success of drilling process.

Rheology is the study of the deformation or flow of material under stress or forces. The properties which affect the flow of material are called rheological properties. Mud Rheology refers to the properties of drilling fluid, which help determine how the fluid will flow under different conditions.

Mud rheological properties like viscosity, gel strength, and yield point along with other fluid properties are continuously measured during drilling. Required chemical treatment is carried out to maintain mud properties at the prescribed level to ensure the proper flow of drilling fluid, mud cake build-up, and primary control of the well under downhole conditions.

The fluids which exhibit constant viscosity under constant temperature and pressure conditions are called Newtonian Fluids. Viscosity is the resistance to flow that any fluid exhibits and in the case of Newtonian fluids, it varies only with changes in pressure or temperature. For Newtonian fluids, viscosity is a ratio of shear stress and shear rate. Light oil, water, salt water, sugar solution, glycerin, air, and other gases are examples of Newtonian fluids. Drilling fluid is not a Newtonian fluid.

OBM is the acronym of Oil Based Mud. Oil-based mud is a drilling fluid composed of oil as the continuous phase and water as the dispersed phase. Additional chemicals are added as needed for achieving desired rheological properties. The oil base can be diesel, kerosene, fuel oil, mineral oil or synthetic oil. Barite is used as weighting agent to increase fluid density and organophilic bentonite is used for providing viscosity. There are two types of Oil Based Mud systems, 'Invert Emulsion Mud' and 'Oil Mud'.

OBM is used to address drilling problems such as, clays in the formation that would react or swell when exposed to water-based drilling fluid. Oil Base Mud also provides higher lubricity and reduces torque & drag downhole. Oil muds have the ability to drill formations where bottom hole temperatures exceed water-based mud tolerances. OBM has been used at temperatures approaching 550°F. Environmentally sensitive location however have strict regulations for disposal of drilling fluid and drilled cuttings. In many offshore locations, drilled cuttings need to be collected and shipped to onshore locations to safely treat and dispose as per environmental regulations.

Oil Mud also known as 'All-oil Mud' has external phase as oil and no additional water or brine are added. Oil mud has 95 - 100% oil and 0 - 5% unintentional water, which is picked up from formation during operations.

Oxygen dissolved in water can cause corrosion in the drill string. Oxygen scavenger reacts and neutralizes the dissolved oxygen to reduces corrosive effects of the fluid on metal components of downhole equipment. When fluid returns to surface, it again comes in contact with the atmosphere and begins to pick up oxygen, thus reducing effective concentration of scavenger. Hence oxygen scavenger level is measured and continuously injected at the pump suction. Common oxygen scavengers have sulfite (SO3–2) or bisulfite (HSO3–) ions, which combine with dissolved oxygen in the fluid system to form sulfate (SO4–2). Nickel or cobalt based catalysts are used for this reaction.

The size of solid particles in the mud system is measured in microns. A micron(µ) is 1/1000 of a millimeter or 1/25400 of an inch. API classification of particle sizes is as below:

Particle Size, microns(μ) Particle Classification Sieve Size

Greater than 2000 Coarse 10

2000 to 250 Intermediate 60

250 to 74 Medium 200

74 to 44 Fine 325

44 to 2 Ultra-Fine 400

Less than 2 Colloidal –

SBM is an acronym of 'Synthetic Based Mud'. It's an invert emulsion mud with synthetic fluid as base fluid rather than oil. SBMs have lower toxicity than 'Oil Based Mud' and are also called Low Toxicity Oil Based Mud (LTOBM). SBMs commonly use linear alpha olefins (LAO), straight internal olefins (IO), poly alpha olefins (PAO), vegetable oils, esters and ethers as base fluid. SBMs are more expensive than OBM but are more environmentally acceptable in offshore operations. Since synthetic oil is more biodegradable, environmental regulations in many areas could approve disposing off drilled cuttings in water without the need of any pre-treatment or collection and shipment to onshore location for safe disposal.

Turbulent flow is a flow regime characterized where the flow is irregular and chaotic. Turbulent flow has a random pattern with cross flows. It has rapid variation of pressure and flow velocity in space and time. Flows at Reynolds numbers larger than 4000 are typically turbulent in nature. Flows in the range of Reynolds numbers 2300 to 4000 are in transition zone between laminar and turbulent flow.

As the name suggests, Water Base Mud is a drilling fluid with water as a base fluid.