Proper understanding of the strength of rocks, and its variability along the length of the well, is essential for efficient and economic drilling operation. Traditionally, the industry has used log-based strength estimates calibrated to strength measured on core samples. However, coring and core testing is costly and time consuming and downhole logs may also be left out of the program to manage costs. In comparison, drilling data is almost always available as the well is drilled. An innovative and robust method is presented which capitalizes on availability of drilling tools, which measure key drilling data downhole. As the measurements are acquired downhole, uncertainties associated with surface-to-downhole conversions are reduced. Reliable results are available over the length of the wellbore, irrespective of complexity in well trajectory. The work also reviews the development of tools measuring downhole-drilling data.
This method uses downhole weight-on-bit, rotational speed, downhole torque, and rate-of-penetration to characterize the downhole mechanical specific energy (MSEDownhole) consumed in the process. The bit diameter, mud-weight, and depth of drilling are also accounted for.
If the task is to optimize drilling parameters for a new formation (e.g. drill-off-test), then the parameters with the "minimum" MSEDownhole are captured. However, if the task is for stage and cluster-wise hydraulic fracture design, then "instantaneous" MSEDownhole is used to infer confined compressive strength (CCS). The CCS together with internal friction angle (IFA) provides unconfined compressive strength (UCS) using Mohr-failure envelope inversion.
The MSEDownhole is compared to Drilling Strength over the same interval. Drilling Strength is defined as Weight on Bit / (Bit Diameter * Penetration per Revolution), and has been used to estimate rock strength. The comparison between MSEDownhole and Drilling Strength highlights the differences in the estimated strength from the two methods.
Current work shows the results from 14 drilling simulator tests, in shale and limestone, under typical ‘drill-off-test.’ The minimum-MSE obtained was transformed to CCS using user-defined ‘efficiency factor.’ The CCS was translated to UCS using basic Mohr-failure envelope and compared with core test data. Utilization of lab tests for calibration greatly improves the trust in this conversion.
The concept of ‘instantaneous MSE’ was applied in a Gulf-of-Mexico well where drilling parameters obtained from downhole sensor were maintained in a close range. Formation evaluation logs were used to compare UCS obtained.
The CCS and UCS estimates benefit drilling engineers, geoscientists, and completion engineers. The less known ‘Efficiency Factor’ is also discussed and reviewed.