Abstract
Compressed bentonite in the form of pellets or plugs is used for the abandonment of production wells for the oil and gas industry. The design of the abandonment systems is based on the hydro-mechanical behaviour of the compressed bentonite defined by mechanical parameters that are used from published data rather than quantified for the used material by laboratory investigations. This paper presents an experimental study on characterising the swelling and shear strength behaviour of raw and polymer (polyvinylpyrrolidone, PVP) treated bentonite. Dislodgement tests consist of three hydrated bentonite plugs inserted in steel casings with the failure mechanism characterised. The bentonite used comes from a local mine (in Queensland, Australia) and is comparable to other bentonites usually used for the abandonment of wells or for other problems where mineral sealing is required (e.g. basal clay barriers of landfills). The experiments have shown that polymer treated bentonite shows significantly larger shear strengths than raw bentonite with simultaneously less swelling. More compressed samples also showed higher shear strengths and less swelling. The dislodgement tests have characterised for the first time the cascaded failure mechanism of a series of plugs forming an abandonment system. This investigation is the first step towards the development of an improved design for abandonment systems for wells using bentonite plugs.
Highlights
Compacted bentonite can be used as material to produce plugs or pellets for the abandonment of wells [1] [2] [3] [4]
Dislodgement tests with several hydrated plugs inserted in steel casings have been conducted for characterising the failure process of a short section of an abandonment system
With regards to the shear strength measured in the direct shear tests, PVP treated samples have revealed a more pronounced frictional behavior compared to raw bentonite
Summary
Compacted bentonite can be used as material to produce plugs or pellets for the abandonment of wells [1] [2] [3] [4]. Bentonite products in different forms have been used for abandonment operations on various well types. Shallow water wells and seismic shot holes can be plugged using coarse grained bentonite [6]. ZoniteTM is a compressed bentonite product for well abandonment operations, exhibiting a large nodule geometry. Being irrespective of the product used, the bentonite is expected to become nearly saturated after placing in a borehole. It is important to understand the saturation process in detail to provide input parameters for predictive modelling of the plug transient geomechanical properties and well abandonment performance
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