**2. Newly discovered value in surface drilling data and mechanical specific energy (MSE)**

Ouenes et al. [2] introduced the use of surface drilling data to simultaneously estimate the rock geomechanical properties, pore pressure, stresses, porosity and natural fractures needed to guide the steering of horizontal wells within the most frackable rock in real time, and additionally provide a completion design for optimal hydraulic fracturing when drilling is finished. The Mechanical Specific Energy (MSE) [3] computed from commonly available surface drilling data such as torque (T), rate of penetration (ROP), weight-on-bit (WOB) and bit diameter (D) has been widely used to improve drilling efficiency. The Mechanical Specific Energy is defined as

$$\text{MSE} = 4\left(\frac{\text{WOB}}{\pi \text{D}^2}\right) + \left(\frac{480 \text{N} \times \text{T}}{\text{ROP} \times \text{D}^2}\right) \tag{1}$$

All the components used in the MSE equation are commonly measured at the surface during drilling operations. Most horizontal shale wells currently being drilled use a drilling motor which requires the use of a different term for the Rotation Speed N\*. When using a motor, the MSE requires the use of the formula given below in Eq. (2) where N is the rotational speed of the drill pipe, Kn is the mud motor speed to flow ratio and Q is the total mud flow rate.

$$N^\* = N + K\_n \times Q \tag{2}$$

Once the MSE is available, it can be used for multiple purposes including deriving geomechanical properties, pore pressure, stresses, porosity, and natural fracture indicators. To fully take advantage of the derived MSE, it should be further combined with other drilling information. Once MSE is available, the Unconfined

Compressive Strength (UCS) can be derived from it in multiple ways. However, the UCS derived from the MSE needs to be corrected as the bit performance is also significantly influenced by the differential pressure.
