Already successfully applied to exploration wells, this method centres on the accurate determination of minimum horizontal stress. This is a vital parameter when:

• attempting to minimise loss of drilling fluids into a formation via  fractures induced by the drilling process
• planning the paths of complex production wells (especially horizontal or high-angle sections)
• ensuring that the rate of hydrocarbon production does not damage weak reservoirs.

All these aspects contribute to considerable operational savings and increased profit.

The method

Drilling continues until a predetermined depth has been reached and the borehole is protected by metal casing. Before drilling resumes, however, a well integrity (extended leak-off) test is conducted on the unprotected section of the borehole just below the casing.

The first step is to increase the pressure of the drilling fluid until the borehole wall starts to fracture. Drilling fluid then continues to be pumped down until the fracture extends into the undisturbed formation beyond the wall, when pumping stops to allow the fracture to start closing.

From a graph of pressure versus time, drillers can roughly calculate the minimum value of the horizontal stress in the rocks required to close the fracture – a safety threshold. 

Statoil’s innovation is to add a third step systematically to the procedure. In this, the pressure is released by allowing the drilling fluids to flow back up the well to the surface – thereby making sure that the fracture has indeed closed.

A far more accurate measurement of the minimum horizontal stress can be obtained from this part of the graph.

That in turn allows the next well interval to be drilled safely – the procedure being progressively repeated at each casing shoe as the borehole deepens.