A new drilling model has been developed for the interpretation of downhole drilling conditions using surface and downhole drilling data. This model is based on the analysis of the interaction of the drill bit (PDC or impact) and the formation under in situ pressure conditions. It combines downhole and surface drilling data with MWD formation measurements to provide detailed drilling analyses for real-time applications.
An expression for Drilling Response developed from the model provides information on lithologic changes and allows interpretation of downhole drilling conditions. Drilling Response is a function of lithology, pore pressure, mud pressure and bit properties and is computed from the following drilling parameters: rate of penetration, bit revolutions, and downhole weight and torque on bit. Techniques and expressions allowing computation of formation porosity and pore pressure on a foot by foot basis have also been developed based on the Drilling Response.
Another key feature of the model is the identification of inefficient drilling conditions. Inefficient drilling conditions are expressed in the form of Drilling Alerts for real-time applications, allowing the operator to take the necessary action to rectify the problem at the wellsite. Drilling Alerts also allow the operator to continuously monitor the effects of any remedial actions.
Bit condition is evaluated using Drilling Response and torque analyser. This allows the operator to assess bearing and tooth wear conditions for impact bits and cutter wear for PDC bits.
Field Examples from sedimentary basins illustrate the usefulness of the model and its applications at the wellsite and in post-drilling analysis.
The advent of Measurement-While-Drilling (MWD) has made it possible to obtain measurements of drilling parameters very near the bit, and these measurements can provide significant information about the formation being drilled and the condition of the bit during drilling. Among the measurements that can now be made by an MWD sub placed near the bit are weight on bit (WOB), torque on bit (TOB), formation gamma ray, and directional measurements. Rotational speed (RPM) and rate of penetration (ROP) are calculated from surface measurements.
Due to friction along the drill string, weight applied to the formation at the bit is generally lower than weight measured at the surface. For the same reason, surface measured torque may not bear any resemblance to actual torque at the bit. Because the torque measurements are made near the bit, they more accurately represent the response of the bit to formation interaction. ROP is also a reflection of bit/rock interaction; consequently, for a fixed WOB, RPM, and mud density, variations in these two parameters should provide a direct indication of formation changes.