Drilling Fluids – 5.1 – Mud Types

Drilling Fluids can be classified into two main systems according to the liquid phase in which mud mixture based on either aqueous or non-aqueous; water-based system or oil-based system. However, it also can be categorized according to the engineered designing of meeting a certain objectives; conventional mud is used in normal operations, while special mud is typically involved special engineered design for specific operations in challenging wellbore conditions.

Because of swelling and viscosity creation property of Bentonite in water, it is used in water-based mud systems for making up spud mud and sweep and casing go pills. Spud Mud refers to mud used for drilling first upper layers that are unconsolidated. Mainly low cost bentonite instead of expensive polymer used to have viscous mud that is able to carry big size cuttings. Bentonite has higher solids content that can build thick filter cake for supporting borehole wall from collapse or washout. Moreover, Hi-VIS pills primarily made up from bentonite that normally used for sweeping to clean a wellbore and to overcome tight spots problems or used as a casing go pill that facilities running casing.

Formations with fracture gradient below water pressure gradient might be encountered underground. In other word, such weak cannot be drilled with water only. Hence, dry air, aerated mud, foam, or emulsion mud are recommended. Drilling with air has many advantages; cuttings brought up to surface by air are uncontaminated and fast drill without loses or fracturing unconsolidated formations. However, air can be used in drilling formation containing water, therefore, aerated or foamed mud is used instead. Aerated/foamed mud?not only overcome dry air drilling issue of water formations, but also full return, downhole pressures balance, and proper well cleaning with no cuttings fall back can be achieved. On the contrary, quality of cuttings samples are not good and span life of bit is reduced.

For achieving excellent drilling efficiency and overcoming many wellbore stability and integrity such as loss circulation and stuck pipe, special WBM called emulsion mud is used. Emulsion means water is the base liquid phase while the mixed surfactant is only dispersed in water to lower density. Techniques use chemicals emulsifiers to disperse oil as the external phase instead diesel as an example. This sort of mud was developed to meet difficult drilling targets such as water sensitive shale formation and expected weak/thief zones.

Owing to the fact of problems confrontation of high solids dispersed WBM for instance:?native clay formation interaction, thick filter cake and tight hole, higher rheological properties, formation damage, and low drilling performance; nowadays water-based polymer mud system is greatly preferred. Polymer mud system usually refers to mud system in which the polymer is the main treating agent. It is also referred to polymer-based mud as moderate density and low solids non-dispersed mud.?Polymer can be natural, modified, or synthetic and its applications in drilling as viscosifier, de/flocculants, filtration controller, and shale stabilizer. Ploymer mud system has many advantages over bentonite mud. Polymer mud has low solid content, medium density, low viscosity, low fluid loss, better shear thinning behavior, appropriate flow pattern, and wall building ability. This mud system should have a strong ability of inhibiting hydration dispersion of cuttings and shale near the borehole well, thereby getting the function of anti-sloughing, leak-proof, anti-shrinkage and preventing fall-block. In addition, it increase penetration rates, improve core recovery, minimize formation damage, retain a near-gauge hole, can suspend and exclude cuttings effectively to ensure quickly and efficiently drilling. On the other hand, polymers are attacked by bacteria, have temperature limitations, more expensive, and special care is required.