Aquifer water samples are a key component of carbon capture and storage (CCS) site characterization and are needed to satisfy data requirements in many jurisdictions (The EU, 2009). Often, these aquifers must be drilled with a water-based mud (WBM) system due to the potential for interaction with potable water or the marine environment. As part of the Northern Endurance Partnership (NEP) CCS project, high-quality aquifer samples were required from the Bunter Sandstone formation to assess the far-field water chemistry. Sampling formation water in a WBM system is often difficult as the fluids are miscible and determining contamination can be challenging. To assist with this an environmentally acceptable WBM system was developed with a tracer chemical to monitor the sample pump out. This system was successfully deployed in a shallow aquifer sampling job using a wireline formation tester, resulting in fluid samples with <3% contamination. Results from the water sampling were then validated using data acquired from other sources (core plugs from whole core, wireline logs and formation pressures).
The Northern Endurance Partnership (NEP) project is planning to store CO2 in the Bunter Sandstone aquifer in the Southern North Sea (SNS) (NEP team, 2021). Phase 1 of the project involves the development of the Endurance store. Near to the proposed Endurance carbon capture and storage (CCS) site the Bunter Sandstone is significantly uplifted by a salt diapir and is raised up close to the seabed. As part of site characterization a shallow aquifer sample (at approximately 200m BML) was required to characterize the fluid properties in the wider Bunter Sandstone aquifer.
It was determined that the best way to acquire this data was to obtain a jack up rig and drill a short borehole into the aquifer to allow collection of fluid samples on wireline. A closed circulation system was desired to control the drilling fluid, minimize invasion and obtain a high quality fluid sample.