An Innovative and Independent Method for Formation Strengths and Facies Identification Using Real-Time Downhole Drilling Data, and its Application in Geosteering for Optimal Well Placement
Authors: Prasad Umesh , Jonsbraten Fredrik , Holbrough David , Saint Craig 
Published: March 2023

Understanding of rock strength, and its variability along the length of the well, is essential for building an efficient well trajectory during geosteering operations. Traditionally, drill cuttings, surface gas analysis, measurements while drilling (MWD) data and Logging While Drilling (LWD) measurements have been used to optimize trajectories. Rock mechanical properties, derived from petrophysical well logs are key to drilling, production and recovery potential of the well: However, in a vast majority of geosteered wells, LWD data and the derived rock properties are not available thus conforming to the given well trajectory and successful Geosteering is difficult. In comparison, real-time downhole drilling data is usually available but rarely used. An innovative, reliable and robust method is presented which capitalizes on downhole MWD and LWD data.

This method uses downhole weight-on-bit (WOB), rotational speed (RPM), downhole torque (TOR), and rate-of-penetration (ROP), to characterize the mechanical specific energy (MSE) consumed in the drilling process. The specific bit diameter (D), mud-weight (MW) and depth (TVD) of drilling are also used in the model. If the task is to optimize drilling parameters for a new formation (e.g. drill-off-test), then "minimum" MSE is captured. However, if the task is continuous drilling, geosteering, and creating a stable well for its subsequent stage and cluster-wise hydraulic fracture design, then "instantaneous" MSE is used to infer strength of the rocks and their variation along the length of the well.

An offshore well from the North Sea was initially selected to apply the concept of the above technology on several post well data analyses using downhole drilling data together with average ROP and RPM. Further, the same concept was used in a real-time application with downhole drilling data. The gamma-ray, neutron porosity, density and resistivity were analyzed and compared with the MSE obtained. Drilling efficiency was assumed based on prescribed industry standards for calculating confined compressive strength (CCS), Internal Friction Angle (IFA), and unconfined compressive strength (UCS). The UCS estimated at a scale of 1.0-1.5″ scale versus depth-of-cut (scale of 0.1-0.5″) resolution matched well with log based UCS from density, porosity and acoustic logs. Calculated results are compared with lab-based core test data where available. The details of these calculations and successful application to Geosteering are presented.

These strength estimates are of benefit to directional drilling engineers for safe and economic well placement along optimum well trajectory, better well production and economic recovery from successive multi-stage and stage-and-cluster hydraulic fracturing designs. An ‘Efficiency’ Factor’ used in the process is discussed which originates from strengthening of rocks due to friction, chip-hold-down effect on cuttings, strengthening due to dilatancy, and cuttings-extrusion like behavior prevalent in drilling.