Historically Reaming While Drilling (RWD) operations have been restricted to softer formations. The objective in this study is to create a thermoporoelastic model of Mechanical Specific Energy (MSE) for RWD in order to shed important new light on the decision process how to use RWD for a specific formation as well as a recommendation for reamer-pilot size ratio.
The analytical approach developed in this paper calculates thermoporoelastic coupled time-dependent stress, pore pressure and temperature variations for an inclined borehole that is drilled through permeable or impermeable formation and which is subjected to far field three dimensional in-situ stresses. Apparent rock strength of the rock in Depth of Cut (DOC) zone beneath the reamer can be determined by using Mohr-Coulomb theory. By using Apparent Rock Strength (ARS) the analytical Mechanical Specific Energy (MSE) can be estimated afterwards. This parameter can be used as a key decision to determine whether a specific formation is a good candidate for RWD or not.
A proper combination of pilot and reamer is absolutely critical to optimize bottom hole assembly (BHA) performance and durability. For many years, a trial-and-error technique has been employed by the drilling industry to determine advantages or drawbacks of reamer in the BHA and the optimum reamer-to-pilot-size ratio and bit characteristic for drilling, which requires a significant investment of time and money.
The comparison of reamer's MSE, to destroy rock around the wellbore, with bit's MSE, to destroy rock at the bottom of the hole, shows how efficiently reamer can drill rock around the wellbore in certain formation, and in some other formations the drawbacks are outweigh of its advantages. This in turn makes it possible to determine proper formation characteristics to employ reamer in the BHA and also the maximum reamer/bit size ratio for certain rock characteristics in order to support efficient drilling operations.
Considering that so fare in the market there is no evidence of a specific model to predict rock strength bellow the reamers, this research and study show its degree of novelty since it propose a model to fill this gap, which can in the future and after tuning be used as a reference application in the petroleum industry for decision making and project cost analysis. Validation in different environments is still to be concluded as a next step in this research.