Reversible Invert Emulsion Drilling Fluids: A Quantum Leap in Technology
- A.D. Patel (M-I LLC)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 274 - 279
- 1999. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 2.2.3 Fluid Loss Control, 2.7.1 Completion Fluids, 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 2.5.2 Fracturing Materials (Fluids, Proppant), 2 Well Completion, 4.3.1 Hydrates, 5.4.10 Microbial Methods, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.1 Drilling Operation Management, 1.14 Casing and Cementing, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.8 Formation Damage, 1.6 Drilling Operations
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A novel, invert emulsion fluid useful in the drilling, completion, and workover of subterranean wells has been invented. The new invert emulsion drilling fluid can readily and reversibly be converted from an invert, i.e., water-in-oil emulsion, to a regular, i.e., oil-in-water emulsion, and back to the original invert emulsion using an acid-base chemical switch. The surfactants used in the reversible invert emulsion fluids are strong invert emulsifiers in the presence of lime. However, the same surfactants become regular emulsifiers in the presence of water-soluble acid. The reversible emulsion property of these surfactants allows the drilling fluid to be changed from an invert emulsion to a regular emulsion at different stages of the drilling/completion operation. During the drilling stage, the drilling fluid can be an invert emulsion achieving all the performance benefits associated with oil-based muds (OBMs). During the completion stage and afterwards, the emulsion can be reversed by adding water-soluble acid, thus providing excellent filtercake cleanups, improved production in an openhole completion, better cementing, and efficient cuttings cleanup and waste minimization. The reversible system is simple and easy to run without any exotic changes in current operations or other additives than those commonly used in OBMs. The reversible drilling fluid is versatile and relatively inexpensive and versatile, especially as compared to new, exotic fluids on the market today. The reversible feature allows the mud to achieve performance, production, minimal environmental impact, and cost control. The paper discusses the overall impact of reversible invert emulsion technology in meeting the environmental and functional requirements of drilling operations. The laboratory data show that this system can provide drilling performance similar to OBMs at various oil/water (O/W) ratios, mud weights, and in the presence of commonly encountered contaminants. This paper compares the new reversible fluids with previous state-of-the-art invert emulsion drilling fluids. These new stable, reversible invert emulsion drilling fluids can readily be reversed to regular emulsions using inorganic acids such as hydrochloric acid or organic acids such as citric acid. The paper discusses the application of reversible emulsion technology on efficient removal of filter cake and improved production in openhole horizontal well completions. The environmental impacts of this system on washing and disposal of drilled cuttings are discussed.
Conventional OBMs and SBMs are typically designed for high emulsion stability and strong oil-wetting characteristics. Oil-wetting characteristics of invert emulsion drilling fluids are beneficial to the drilling operation. These invert emulsions are very stable and strongly oil-wet the solids, the drillstring, and the formation which are in contact with the continuous oil phase. This eliminates drilling problems associated with WBMs reacting with water-sensitive shales. Invert emulsion drilling fluids provide formation of a thinner filter cake, excellent lubrication, enhanced rate of penetration, and superior hole stability.
This preferentially oil-wetting characteristic is as detrimental to the completion and workover of a well as it is beneficial to drilling operations. Disadvantages associated with state-of-the-art invert emulsion drilling fluids are: filtercake cleanup, bonding between the cement and the formation, oil retained on the cuttings, and disposal of cuttings and used fluid. Conventional oil-based muds leave residual mud in the column and oil-wet formation and casing which results in a substantial reduction in the cement bond strength that can be achieved between water-wet formation and casing.
Oil retained on the cuttings has been a major issue in the increasingly stringent limitations for cuttings discharge offshore.1 Conventional OBMs and SBMs produce oil-wet drill cuttings which, when discharged into the sea, form localized cuttings piles in some receiving environments. The formation of cuttings piles is highly dependent on oceanographic conditions such as water depth and currents. Various methods have been proposed as alternatives to seabed disposal that includes detergent wash, solvent extraction, high-temperature distillation, transportation to shore, and downhole injection of cuttings. However, these methods have limitations, such as cost, low efficiency, limited feasibility, and safety hazards. Trends show that it is more difficult to wash synthetic-based mud from drilled cuttings than conventional OBMs. This may be attributed to stronger emulsions and synthetic-wetting characteristics of conventional SBMs.
In addition, the invert emulsion fluids may cause formation damage due to mud invasion into the formation. The muds and filter cake can change wetting characteristics of the formation and form emulsion blocks. Formation damage affects the injectivity or productivity in pay zones and ultimately effects the economics of the well. The challenge for industry is, therefore, to significantly reduce the environmental impact of its operations while retaining the operational performance advantages of invert emulsion drilling fluids.
A potentially attractive alternative to these muds is a modification which would exhibit the best performance for drilling as with OBMs while retaining all the advantages of WBMs related to environmental issues, cleanup during completion, and production of the well. A novel reversible invert emulsion fluid technology has been developed that can be used in invert emulsion drilling fluid applications and can provide the benefits associated with WBM. The formulated drilling fluid system is similar to conventional oil-based muds in terms of both composition and performance. Except for primary emulsifiers, the products used are the same as used in conventional oil-based muds and are used at the same concentrations. The drilling fluid system is simple, easy to put together and maintain, and can be engineered by personnel familiar with other, conventional invert emulsion drilling fluid systems.
Emulsions, Emulsifiers, and Wettability
An emulsion is formed between two liquids by lowering the interfacial surface tension of one liquid with an emulsifier, or surfactant, to enable that liquid to form a stable dispersion of fine droplets in the other liquid. The lower the interfacial tension, the smaller the droplets of the dispersed liquid and the more stable the emulsion.
In most regular emulsions, the oil phase is dispersed as fine droplets in the continuous water or aqueous phase. This is commonly known as an oil-in-water (O/W) emulsion. A water-based mud containing oil as the internal phase is an example of this type of emulsion.
In an invert emulsion, the aqueous phase is the dispersed phase and the oil phase is the continuous phase. This is known as a water-in-oil (W/O) emulsion. The present state-of-the-art OBMs are invert emulsion drilling fluids.
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