Simon to Somerton: A Review of MWD Drilling Resistance-Paper

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This paper revisits early research by Somerton and Simon on using drilling parameters to assess rock resistance and applies their methods to modern monitoring-while-drilling (MWD) data. Somerton’s 1959 laboratory correlation linked penetration rate and applied thrust to a “drilling strength,” but was based on limited tests and did not reliably predict unconfined compressive strength. Simon showed that Somerton’s equation is a special case of a more general formulation applicable to different drilling methods. The authors also compare these approaches with modified Girard and Teale specific-energy equations.<br /><br />The equations were evaluated using field data from overburden and rock drilling in New Hampshire and from a landslide investigation in Normandy, France. The results show that the methods can reveal changes in subsurface materials and support stratigraphic interpretation. Simon’s equation often provided greater detail, while profile smoothness and sensitivity to drilling parameters varied with the equation and bit type. In rock coring, torque helped identify changes such as fractured zones and clay-filled joints; relying on thrust alone may be misleading because thrust is controlled by the driller.<br /><br />The authors emphasize that these empirical measures describe drilling resistance, not intrinsic rock strength, and must be interpreted with care. Results depend on bit geometry and wear, cuttings removal, drilling-fluid conditions, and consistent units. Coring applications also require using the effective area of the bit’s cutting annulus. Further work is needed to account for bit characteristics and fluid pressure and flow, and to establish relationships with geotechnical properties.

Keywords

Somerton

Simon

MWD

drilling resistance

specific energy

rock coring

torque

thrust

stratigraphic interpretation

bit geometry