For example, Figure 5 shows the results of multiple fracture growth simulations for a field injection project, predicting that the most likely (P50) fracture radius will be 570ft, with a possible range of 504ft (P10) to 635ft (P90).
These simulations provide guidance on the necessary well spacings from the injection point, the number of injection wells required for a particular field development, the pressure regime for the tubing design and the specification for the surface equipment.
Figure 5: Fracture Growth Predictions
Once the injection well has been drilled and logged, the measured rock properties and exact lithological sequence can be input into the model, improving its accuracy. The first injection sequence, with its information regarding the cycle of injection pressure for particular pumping rates, slurry densities and viscosities, then provides validation of the model.
This cycle of monitoring, model update and validation, as depicted in Figure 6, needs then to be repeated at intervals during the slurry injection project, such that the model can be continually refined and the projected ranges of capacity, fracture growth and pressure development, narrowed. This provides the assurance that the fracture growth is developing as predicted. Deviations from the modelled pressure trends during the injection phases then provide early warning signals of unexpected fracture development. (7) What-if statistical analysis can then be used to evaluate the likely causes of the pressure deviations and steps taken to mitigate the consequences of the unintended fracture growth.
Figure 6: Fracture Modelling
Assurance
Summary
To fully manage the regulatory and project risks associated with the sub-surface disposal of oil- contaminated exploration and production (E&P) waste streams and to provide overall assurance of the process, four elements are required, as follows.
- The CRI process needs to be de-coupled from the drilling operation such that the latter is not hindered by equipment failures or limited capacity.
- The slurry properties and injection sequence need to be programmed using a solid transport model that simulates changes to the slurry under down- hole conditions. Continuous monitoring of the surface properties will then minimise the risk of settling during periods when the slurry is static.
- The fracture model needs to be periodically updated as new information is obtained, such that the range of uncertainty is reduced.
- The injection pressure needs to be continually monitored. Deviations from the predicted trend can then be analysed to explain the fracture behaviour and steps be taken to manage the consequences.
Acknowledgements
The author is indebted to A Wilkinson, Dr Q Guo and Dr S Marinello of M-I SWACO, for their contributions in developing this new approach to CRI Assurance, and to M-I SWACO for permission to publish this paper
Category:
Environment
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