Abstract

Bentonite has been identified as a cost-efficient and durable solution for alternative plugging options in shallow wells due to its ductile, self-healing, and chemically stable properties. In this work, we evaluate the application of bentonite as a sealant for deep geo-energy wells. The shear strengths of the bentonite-casing and bentonite-rock interfaces were investigated at various downhole conditions, i.e. fluid salinity, fluid saturation, pressure, temperature, and casing sizes. A sliding shear test was performed to measure the cohesion and friction angle at the interfaces of different bentonite systems. Furthermore, dislodgement tests were conducted using a custom-built test cell with realistic downhole geometry, both small- and large scale, to measure the minimum axial force to displace the plug in a casing. Results show that bentonite plug lengths of less than 50 m in casing sizes from 7-5/8" to 13-3/8" would be sufficient to meet the necessary criteria for alternative sealants from regulators. Interfacial shear capacity of the bentonite are negatively influenced by the fluid salinity. Achieving high initial shear capacity is more challenging in larger casing sizes, and curing the plug at elevated pressure and temperature can improve the initial strength. After failure occurs, rehydrated plugs are observed to self-heal and regain or surpass their initial interfacial strength.

Introduction

The abandonment operations of wells in the Netherlands and in mature hydrocarbon basins worldwide are to substantially increase in the following years as many oil and gas fields are approaching the end of their economic life. The decommissioning cost for the Netherlands alone is estimated to be 7 Billion EUR, of which 46% is estimated to be well plug and abandonment (P&A) operations (Nexstep, 2020). In addition to decommissioning operations in mature hydrocarbon basins, hundreds of new wells for other subsurface geo-energy applications are planned to be drilled in the near future – e.g to achieve the Dutch geothermal energy production goals of 50PJ in 2030 (EBN, 2018). Finding economic and durable solutions for sealing and well plugging would largely contribute to cost savings of abandonment activities.

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