Define Effective Injection Zones and Requirements of High Injectivity of Disposed Water in Heterogeneous Carbonates.
Authors: Ahmad Jamal , Almessabi Wael Sakhr , Alhammadi Sarra Husain 
Published: May 2024

The study examines how subtle geological characteristics like karsts and faults/fractures play a vital role in ensuring effective injectivity in produced water disposal (PWD) wells. It discusses the requirements of produced water disposal and the utilization of integrated geophysical tools to identify heterogeneity and geological features, such as karsts that are proven for good injectivity in produced water disposal wells. Furthermore, it compares the injectivity and performance of two wells targeting the same formation, revealing notable distinctions attributed to rock characteristics.

As water production increases in brown fields, there is a need for better water management as well as disposal. The current water production level in the field ranges from moderate to high, and it is projected to nearly double in the next ten years. This increase necessitates substantial focus on water management, water disposal wells planning and most importantly identifying suitable subsurface layer(s) for long term injection without harming the environment. Underground disposal of the produced formation water has many benefits such as minimum or no damage to the environment (Wenrong et al., 2021). This study uses a multi-disciplinary approach, integrating borehole and advanced seismic data, to identify subtle features that improve injectivity in PWD wells. The use of advanced techniques, such as diffraction seismic (Ahmad et al., 2022) and preliminary results from Distributed Acoustic Sensing (DAS) Vertical Seismic Profile (VSP) (Mason et al., 2023), have significantly improved the understanding of the subsurface by characterizing geological features which are not defined thoroughly using conventional seismic. The integration of supplementary datasets such as dynamic mud losses, logs, Production Logging Tool (PLT) data, injectivity tests, injection performance history and microfrac data have proven to be valuable in delineating effective zones within the target formation.

Karst collapse features have been identified as the most effective targets for good injectivity in PWD wells (Ahmad et al., 2022). The reservoir rocks in the study area exhibit high heterogeneity due to randomly distributed karsts and variable rock quality (permeability) layers, which could easily be distinguished by logs, core, and dynamic datasets. To target good zones, it is important to outline the spatial distribution of geological features and consider this while planning wells. The use of advanced geophysical data has resulted in improved subsurface imaging which could not be achieved with conventional seismic data. Understanding the movement of injected PWD after injection is also imperative. Due to the presence of karsts and limited logs/core in the target layer, modeling proved to be challenging. Therefore, two wells have been chosen from the areas characterized by seismic, their comparative performance suggests that the injectivity of the well (Well-1) near the karst collapse zone is almost twice that of the one drilled in a non-karst zone (Well-2). Additionally, analysis of microfrac data from a similar rock unit without karst collapse features indicated specific pressure requirements for maintaining and propagating fractures that may lead to improved injection results.

As oil production increases, so does the quantity of produced water. Managing and disposing of this water requires additional wells, suitable subsurface target zones and concerns related to water quality and reservoir damage for long term injection. The integrated method presented in this paper improves subsurface understanding and optimum well planning that leads to good injectivity.