The common drilling fluid systems used in drilling operations were discussed in the previous articles; however, special mud systems used for treating particular difficult wellbore conditions or used in completion and workover operations will be discussed in this article.

Glycol-based Mud Applications and Gas Hydrate in Deep Water Operation

Gas hydration is a crystalline solid phase that consists of a gas molecule surrounded by a cage of water molecules. It forms when water and gas combine under conditions of relatively high pressure and low temperature. Gas hydrate occurs naturally in sediments associated with deep permafrost environments at appropriate pressure and temperature conditions where sufficient gas is present. These conditions are common in deep water operations. At greater water depths, hydrate can occur within a stability zone that extends into the marine sediments to depths of tens to hundreds of meters beneath the seafloor, and the thickness of a hydrate stability zone varies with temperature, pressure, composition of the hydrate-forming gas, underlying geologic conditions, and water depth.

Glycol-based Mud is an engineered mud system designed to replace the conventional O/SBM for deep water and highly reactive shales operations. This system utilizes cloud-point Glycol that operates in conjunction with PHPA and other polymeric material to provides enhanced shale inhibition and hydrate suppression. 

Hdd Mud Recycling Systems

Glycol-based system is the enhanced KCL/PHPA system that shows the flexibility of polymer systems. Its state of solubility changes according to the temp to which it exposes. The salinity type and concentration in combination with PloyGlycol property provide cloud point to match the temp of shale. It is referred to as Gly-Drill-System. The cloud point of fluids is the degree of temp at which PloyGlycol changes from being soluble to insoluble.

PolyGlycol is a type of chemical when dispersed in water exhibits a temperature related behavior that results in the ability of pores plugging when it becomes insoluble to minimize fluids entry, and suppress gas hydrate when  it returns water soluble. The PlolyGlycol emerges from the system as cloud of microscopic droplets (thermal activated micro emulsion), when the proper conditions are present.

Besides, Gly-Drill-System is more inhibitive system used to drill reactive clay and shale formations in deep water drilling when the hydrate suppression is a major problem. The used NaCl and KCL salts, polymers and glycol in the system provide formation inhibition and hydration suppression. The ionic exchange between salt and shale causes water explusion where the water molecules within shale are replaced by the insoluble microscopic droplets of polyglycol that coat out on cuttings and borehole walls.

The mechanism of temperature related behaviors exhibition of Glycol system can be demonstrated as the system exposes to cold temperatures at shallow depths and to high temp at greater depths. When the designed solution exposes to degree of temp of shale that match the cloud point of PloyGlycol, the thermal activated micro emulsion forms of PolyGlycol emerge as insoluble state and plug the pores of shale for  minimizing fluids entry. During circulation up the annulus, temp is lowering down to fall below the cloud point in front of zone containing gas hydrate concern and raiser where low temp and high pressure condition allows hydration to take place; PloyGlycol property returns to the its normal state that is soluble in water to prevent water molecule interaction with shales and suppress gas. Polymers provide inhibition through encapsulation by the adsorption on shale surface and prevent water from migration,  and the high salinity and cation exchange of slat with clay reduces the swelling and hydrate tendency.      

Reservoir Drill-in and Completion Fluids  

Conventional mud systems containing higher solids and non-acid soluble additives cause formation damage; therefore, they are not recommended to be used in drilling production zones and/or in completion operation to prepare a well for production. Formation skin damage is caused by utilizing mud containing high solids, non-acid soluble LCM, and other products such as silicate that cause gelation and porosity and permeability reduction.

Hence, a special mud system call drill-in fluid is used when drilling reservoir sections and in a well constructing and completion. Drill-in fluid has special attributes that make it distinctive from others to be used for such purposes. It develops easily removable filter cake on borehole walls to prevent filtration and formation skin damage. The materials used are acid soluble and exhibit good drilling properties for hole cleaning, lubricity, fluid loss control, bridging, inhibition, and the compatibility with reservoir fluids.

Drill-in fluids are formulated with a particular type of salts that is compatible with formation fluids instead of conventional solids content materials for providing the required density, and bridging agents with proper particle sizes distribution for filter cake development rather utilizing solids content fluids loss control products to bridge specific formation. Drill in fluid is an engineered calcium carbonate/polymer system, which encompasses water as a base fluid, salt for inhibition, alkalinity control materials, viscosifiers, and a good controlling between FLC and bridging agents.

Well-known systems used in reservoir drilling and well completion are Formate-based fluids. They are the alternative to standard brine systems. Formate mud is composed of water soluble salt of alkali materials such potassium/sodium/cesium for forming brines of high densities and shale inhibition, calcium carbonates and different sized NaCl salt as bridging agents, Xanthan as viscosifier , and the polymeric Polyanionic Cellulose PAC and FLC to enhance borehole integrity and suppress shales reaction.

The formate systems have additional unique properties for instance: density and temperature stability in high temperature as they do not generate viscosity or gel strength in stop time and HPHT conidtions, non-corrosion, environment acceptability; resistance to contamination, and solid-free. These attributes make the formate systems an ideal option for drilling reservoirs, associated shales and salt water zones, completion and workover, slim hole drilling, deep and HPHT operations.      

 Silicate Mud System        

Soluble silicate materials are manufactured by fusing of SiO2 with sodium or potassium carbonate at higher temperature reaches 1200 c. This type of drilling fluid is designed to drill very reactive shales, and to improve formation integrity and minimize shale hydration where the conventional OBM, Polymer based mud and other inhibitive mud cannot be used because of their toxicity or high cost.

The system is formulated with high pH +/-12 where almost all polymers can be utilized except PHPA that reacts in low PH solutions. Silicate system uses sodium or potassium silicate to enhance inhibition and mechanical wellbore stability particularly in high fractured and/or unconsolidated formations.

The soluble silicate starts out as monosilicate that polymerizes rapidly to form negatively charged oligomers. Oligomers are polymers that have only a few repeating units; they are small enough to penetrate the pores within shales and can be transported by diffusion or hydraulic flow.

Typically all shales have pore fluids with lower pH close to neutral ~ 7. When the olimogers of silicates solution are diluted down in such neutral pH, they may overcome their natural repulsion and coagulate to form gel networks. Shale pore fluids free polyvalent ions such as calcium and magnesium will instantly react with oligomers to cause water expulsion and form insoluble precipitates for sealing the micro fractures. The gelled precipitation of  silicates provides a physical barrier that prevents further mud filtrate invasion and pressure penetration to isolate pressure between wellbore and shale formation; consequently, reaction of shale is inhibited and gauged hole and wellbore stability is maintained.

Finally, silicates are continuously depleted during operation due to losses, polymerization and precipitation; additives such as KCL and Sodium silicate are added to maintain its concentration.