ARTICLES

Download PDF    References

1    2   3    4

Managing the Risks Associated with Unpredicted Fracture Propagation

The risks discussed so far relate to equipment and well bore-related events that would lead to delays and, potentially, significant project expenditure. They are unlikely to have safety or environmental consequences and so are seldom of direct concern to the regulatory authorities. However, an understanding of fracture propagation, predicting future growth patterns, and defining the storage capacity of the fractures are of direct concern to both parties because of the potential implications.

Figure 4: Predicted Fracture Growth

In almost all cases, a CRI programme is preceded by a fracture modelling study using numerical predictive models of various degrees of sophis- tication. These indicate the likely propagation behaviour of the fracture relative to the injected volume (see Figure 4). However, even the mostsophisticated three-dimensional (3-D) model predictions are inaccurate because of the assumptions made in creating the model. At the planning stage the rock properties are seldom well understood. The exact lithological sequence is not known and assumptions have to be made for a wide variety of the input parameters. This is particularly the case in respect of the physical rock properties. In order to overcome these limitations, a probabilistic approach needs to be taken, where multiple model runs are conducted using the range of values for each variable. This initial feasibility study determines the range and most likely values of the following features:

• the storage capacity of the sub-surface fracture complex;
• the lateral and vertical growth of the fractures; and
• anticipated pressure changes during the CRI operations.

1    2   3    4