Chalk influx and solids production mitigation in the North Sea

Chalk fields in the North Sea such as Ekofisk and Valhall are some of the most important oil sources in Norway with much of the reserves yet to be extracted. Production in these fields is challenging and is associated with massive chalk influx events entailing up to cubic meters of chalk flowing like toothpaste, filling the tubing, and killing well flow. This behavior, which resembles soil liquefaction, results from the unique nature of chalk reservoirs. Chalk consists of remnants of planktonic algae called Coccolithophorids, has high porosity (35-50%), low permeability (1-2 mD) and strong water sensitivity, with even small amounts of water reducing its strength and stiffness by as much as half. This research and development target a growth in profitability by increasing productivity, improving hydrocarbon recovery, and lowering capital and operational expenditure by optimizing chalk control completions, mitigating well collapse problems, and reducing infill drilling in mature reservoirs. The project addresses both fundamental understanding of the destabilizing mechanism in chalk fluidization and methods for its prediction and mitigation. Physical experiments are conducted on SINTEF's True Triaxial Test System to increase the understanding of chalk influx mechanisms. This unique facility reproduces the geometry, stress, and flow conditions around a producing well. Prediction methodology is studied through analytical and numerical modeling. The combined knowledge is used to develop the ChalkPredictor software for use by our industrial partners to monitor and control their producing wells. Mitigation systems are studied to answer the question of how to better design these systems to optimize their performance. The project has had its kickoff meeting in April 2021. A PhD student has been recruited and will start in Jan 2022. The experiments that are currently running are designed to reproduce the in-situ field conditions of a reservoir that has been depleted and subsequently repressurized using pressure support. This is a realistic case from North Sea chalk reservoirs. Depletion may collapse the weak reservoir chalk and thus after repressurization the stability of producing wells may be compromised. The current study compares the results from virgin versus depleted-repressurized reservoirs in terms of critical drawdown for chalk influx events.

Chalk fields in the North Sea such as Ekofisk and Valhall are some of the most important oil sources in Norway with much of the reserves yet to be extracted. Hydrocarbon production in these fields is challenging and is associated with massive chalk influx events; this may entail several cubic meters of chalk flowing like toothpaste, filling the tubing and killing well flow. Chalk production has received limited research attention and the mechanisms behind chalk influx fluidization are not yet well understood. Currently, the weak, high porosity parts of the North Sea chalk fields are operated on the limit of their stability and operators are continuously pushing the boundaries to improve the production and recovery rate and reduce the operating costs by increasing the applied drawdown, avoiding or limiting chalk support and applying chalk management to manage chalk influxes and well failures. High depletion and water breakthrough in these mature fields exacerbates the problem. Mitigation of chalk influx is expensive and often results in impaired well productivity or well loss. An understanding of the main driving mechanisms for chalk influx coupled with an optimization of mitigation systems and a user-friendly Chalk Influx Predictor methodology for field operations can lead to considerable commercial benefits from increased productivity and recovery rate and reduced capital costs. The activities are based on an experimental part which tests chalk influx at conditions as close as possible to the field in terms of anisotropic stresses, saturation fluids (including water breakthrough), and reservoir depletion. Upscaling from laboratory to the field is studied. Mitigation systems are tested and optimized including proppants/frac-packs and screens liners. Theoretical and numerical models use the experimental findings to advance the understanding and develop a methodology for chalk influx control and management of the production parameters.