Successful Application of an End-to-End Advanced Workflow for Reservoir Stimulation: Case Studies from a High-Temperature Gas Formation in Ukraine

An end-to-end workflow was developed for optimization of a carbonate stimulation treatment in a high-temperature environment. The workflow includes advanced core testing, treatment fluid and design considerations, and production simulations. A case history validated the workflow and production results. A well in central Ukraine was selected by an operator as a candidate well for a production stimulation feasibility study. The reservoir and well information were reviewed, and a reservoir model was built in an advanced matrix stimulation simulator. Core flow tests with 3D wormhole geometry visualization were used to calibrate the simulator's fluid-rock interaction parameters. Various skin and permeability profiles were subjected to iterations in the simulator with different fluids and treatment schedules. Optimum fluids were chosen according to the risk analysis, and key design objectives were considered based on the drilling records, mud type, workover history, and petrophysics. Analysis of the drilling history revealed the necessity of using special stimulation fluids for removing damage from the oil-based mud. Analysis of the well and reservoir conditions showed that the use of nonmodified, emulsified, and organic acids was not recommended. Instead, a single-phase retarded stimulation fluid was chosen as the main reactive agent. Use of chelates was declined due to higher cost-to-efficiency ratio in comparison with the single-phase retarded stimulation fluid. The core flow tests along with 3D wormhole geometry visualization allowed optimizing the treatment parameters with respect to wormholing efficiency. The core flow test data were used to calibrate the matrix stimulation simulator with representative fluid-rock interaction curves. Due to interval length and heterogeneity, the use of diverters was recommended to increase wellbore coverage during the treatment. A polymer-free viscoelastic surfactant-based diverter was selected to alter the injection profile. An optimum treatment schedule developed in the matrix simulator included cleanout by coiled tubing equipped with a high-velocity jet, a low-rate coiled tubing matrix treatment, and a high-rate bullhead treatment. The designed treatment was successfully pumped. The post-stimulation production showed a significant increase in productivity index, without issues in cleanup. Similar workflow stimulation treatments were deployed on five subsequent wells, which have also shown very positive production response. This work provides validation that a sound and rigorous engineering approach with advanced modeling and novel chemistry solutions can revive and significantly increase productivity of carbonate reservoirs. It was the first application of such a workflow and described stimulation fluid technology in Ukraine and Europe.