Prediction of Rock Strength using Drilling Data and Sonic Logs

Abstract

Uniaxial test (also called unconfined compression test) is one of most important tests used to measure rock strength. It is critical to obtain the rock strength parameters along the wellbore. Awareness of rock strength could better control drilling problems such as pipe sticking, tight hole, collapse, pack off and sand production. Rock strength also controls the drilling rate of penetration (ROP). The calculations of uniaxial Compressive Strength (UCS) are based on a simplified version of the ROP model proposed for tricone bits. The purpose of this research was to predict the uniaxial compressive strength based on sonic logs as a function of sonic travel and formation porosity. For obtaining continuous log strength along the wellbore in Ahwaz oilfield, quantitative relationships were developed between UCS and sonic travel time as well as UCS and both of sonic travel time and formation porosity. They actually are the outcome of regression analysis that resulting relationships are very practical and most often are specified by the researcher to the region of study. In this work, a larger well segment has been analyzed which there is no information break throughout the segment and it is investigated continuously. Using this approach, 3D model was a better predictor of uniaxial compressive strength than obtained equations related to transient time and results have also confirmed that the use of an recommended equations in this study can reduce drilling costs significantly.