Modeling of wellbore effects in Managed Aquifer Recharge monitoring

Arnaud Levannier
Schlumberger Water Services
 ALevannier@slb.com, Delftechpark 20, PO box 553, 2600AN, Delft, The Netherlands

An adequate monitoring system is critical in any Managed Aquifer Recharge (MAR) field implementation and can target different objectives including the following-up of:

1. Operation performance, whereby  the inputs/outputs of the system are controlled in order to optimize the injection/recovery scheme (often targeting the highest achievable flow rates under current design);
2. Storage integrity in case of Aquifer Storage and Recovery (ASR), whereby the pressure response and freshwater plume distribution are monitored to detect any issue of underground leakage (e.g. through the caprock), of unplanned displacement of the stored water due to hydrogeologic heterogeneity, or of storage contamination;
3. Environmental impact, to insure the hydrodynamic and geochemical effects of the MAR system continuously meet the local regulations.

Monitoring the freshwater plume location/distribution often includes repeated measurements or vertical profiles of the electrical conductivity (EC) in the available observation or pumping wells. The representativity of such measurements in long-screened wells can be affected in some cases by wellbore effects like mixing and cross-flow inside the wells. In particular, cross-flows can be observed when vertical heterogeneity exists in the geological formation and vertical hydraulic gradients are observed. Such effects can not only significantly bias the monitoring results but as well can increase the mixing effect and deteriorate the ASR recovery efficiency.

A reservoir engineering simulator was used to model both the aquifer and well behavior in the framework of typical ASR projects in confined and unconfined aquifers settings. The conditions where these cross-flow effects can be observed are first assessed as well as their potential impacts in terms of ASR monitoring and operations. Solution measures are then also proposed relying on optimized monitoring design based on advanced aquifer characterization.