Water-Based Glycol Drilling Muds: Shale Inhibition Mechanisms
- M.S. Aston (BP Exploration) | G.P. Elliott (BP Chemicals)
- Document ID
- Society of Petroleum Engineers
- European Petroleum Conference, 25-27 October, London, United Kingdom
- Publication Date
- Document Type
- Conference Paper
- 1994. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 4.3.1 Hydrates, 4.1.5 Processing Equipment, 5.2.2 Fluid Modeling, Equations of State
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Water based glycol (polyol) muds are becoming increasingly popular, and are now replacing oil based muds in many drilling operations. Apart from exceptional shale inhibition, other benefits include environmental friendliness, case of handling, robustness and good lubricity.
This paper explores the mechanisms of shale stabilisation by glycol muds. Experimental data show shale dispersion, swelling and hardness behaviours for a model reactive shale (London Clay). The data suggest the high level of inhibition obtained is mainly due to a shale hardening effect - the shale actually becomes harder than in its original state. Measurements with a range of salt types show that salt (eg KCl) plays a major role in the inhibiting mechanism.
Adsorption measurements indicate that the glycol displaces water from the shale, resulting in a change in pore fluid composition. In effect, the glycol acts as a penetrating glue which, it is believed through hydrogen bonding effects, strengthens the shale. This mechanism contrasts with other water based mud additives (e.g. PHPA or polyamines) which form a protective or encapsulating layer on the surface of the shale.
The recent track record of glycol-containing water based muds is impressive. Good performance is consistently being achieved in problematic shale sequences, particularly in the softer and highly reactive shales such as the gumbo types found in the North Sea. There would appear to be few, if any, technical drawbacks with these systems, and, with concentrations of the order of 1-10% being sufficient, they are found to be very cost-effective. The mud often remains in good condition throughout the drilled interval. This means that, in some cases, the fluid can be recycled to give further cost and environmental savings - normally recycling of the fluid is only done with oil based muds.
Glycol additives have been researched by BP over the past few years. A brief history of our development work, plus an account of some of the earlier field experiences in the North Sea, has been given by Reidetal. A more detailed and recent case history of success in Papua New Guinea has been given by Twynam et al. Such has been the success with glycols, there is now a great deal of interest in how these additives function. Their mechanism appears not to be straightforward, but by knowing more about how they work, it is hoped that further improvements in mud technology can be made.
The term glycol is used quite loosely in the context of drilling fluid additives. Glycols can in fact vary from the simple dihydric alcohols (eg, ethylene glycol) to complex polymeric systems, such as random polymers containing polyethylene or polypropylene oxides. Bland has given an account of glycol additives, discussing some of the basic types and chemistry.
The molecular weights of glycols can be very high, giving rise to pastes or solid grades.
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