Abstract
Bentonite plugs manufactured with and without a polymer binder were studied using Small Angle Neutron Scattering (SANS) and Ultra Small Angle Neutron Scattering (USANS) techniques. We have subjected the samples to hydration by evaporating water and then applied CD4 methane at a Zero Average Con-trast (ZAC) pressure (about 48.3 MPa/7000psi) to measure the accessible and inaccessible pores and channels. The data show that introduction of moisture has an immediate effect on the smallest pores, with swelling preventing access by the methane; however the larger network appears to be little affected by the addition of the moisture. The addition of a binder to the plug creates a greater number of open pores in the micron and above size regime, indicating that the polymer is vital in creating larger channels, which aids the movement of mois-ture throughout the plug, at the expense of sealing the smallest ones. This sug-gests that the polymer binder helps in the self-healing process in the clays and makes the structure more stable. The process of plug manufacturing and drying produces a chaotic system of aggregates in the sample, which forms packed clusters 130 nm in size. Addition of moisture destroys this structure. Addition of water to the dried plug material showed that at 18 weight% a dual layer of water has formed in the montmorillonite layer of the clay, separating the montmorillonite layers by 17 A. Increase to 33 weight% of water has added an extra layer with basal spacing of 19 A. The clay hydration with vapor and directly deposited liquid appear to delaminate some of the clay affecting the fractality index at the micron sizes. Mechanical compression had a significant effect on the polymerised sample, encouraging larger water-conducting channels to form.
Highlights
Over the last century, a large number of bore wells have been drilled worldwide so that prospecting, utilization and geological assessment of available deposits can be made
Each Small Angle Neutron Scattering (SANS) measurement used a beam of thermal neutrons, with a wavelength of 1.498 Å and beam diameter of 15 mm aimed at the cell and collimated down to 10 mm at the sample
The raw data collected by the instrument detector has to be processed before it can be used to extract the information of interest
Summary
A large number of bore wells have been drilled worldwide so that prospecting, utilization and geological assessment of available deposits can be made. The result is increased bore drilling activity worldwide. Due to changing local geological conditions (such as those due to fluid and gas movement, seismic activities and varying pH levels), these plugs can fail, resulting in pollutants reaching underground aquifers and eventually venting into the atmosphere. Bentonite is a mineral mixture containing primarily the clay mineral montmorillonite that has been used for shallow water well abandonment in a variety of forms [3] and has been slowly gaining acceptance for plugging oil and gas wells [4]. Plugging wells with bentonite is supposed to be cheaper than using concrete plugs [4] Plugs made of this material are claimed to be more reliable than those made of concrete. While concrete can fail to seal or later crack due to ground movement or tectonic activity, hydrated bentonite is deformable and self-sealing in such conditions
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