Development of experimental correlations between indentation parameters and unconfined compressive strength (UCS) values in shale samples

  • Darwin Mateus Ecopetrol S.A. – Instituto Colombiano del Petróleo, A.A. 4185 Bucaramanga, Santander, Colombia
  • Ricardo Andrés García Universidad Industrial de Santander, UIS, Bucaramanga, Santander, Colombia
  • Néstor Fernando Saavedra Ecopetrol S.A. – Instituto Colombiano del Petróleo, A.A. 4185 Bucaramanga, Santander, Colombia
  • Zuly Calderón Carrillo Universidad Industrial de Santander (UIS), Facultad de Ingenierías Físico-Químicas, Bucaramanga, Colombia
Keywords: well stability, geomechanics, compressive strength, shales, formación paja, Colombia


Unconfined Compressive Strength (UCS) is one of the rock mechanical properties that is important take into account during drilling operations in order to avoid wellbore instability. During drilling operations, UCS variability influences wellbore stability more than other factors such as azimuth, slope, exposure time, and mud weight (Jaramillo, 2004). In last years, the indentation technique has been demonstrated to be an appropriate method for determining rock strength in real time during oil well drilling. This technique implements correlation that allow UCS evaluation from indentation parameters such as Indentation Module (IM) and Critical Transition Force (CTF), that can be measured on small rock fragments obtained during drilling. Shale formations in well drilling have demonstrated to be a hindrance since they represent the most important problem in reservoir stability (Abass, H., A. et al. 2006). Therefore, the main objective of this article is to find experimental correlations that allow the modeling of rock strength by applying the indentation technique to reservoir plug. The importance of this technique is the possibility to get rock strength properties in real time during drilling operations, although, those reservoir sections which do not have neither direct UCS records nor indirect measurements.  Eight Unconfined Compression Tests (UCS) on rock cylinders (plugs) extracted from the Paja formation upwelling were conducted in order to develop the corresponding experimental correlations. Two hundred indentation tests were also simultaneously conducted on shale fragments extracted from each plug surroundings in order to simulate the cavings obtained from reservoir drilled. Results of both tests were correlated using the Minimum Square technique, seeking the best correlation that shall represent result behavior, thus obtaining two 2nd-degree polynomial correlations. Correlation coefficients of 0,6513 were determined for the (IM) - (UCS) correlation and 0,8111 for the (CTF) - (UCS) correlation. This demonstrates that the highest correlation between indentation parameters and (UCS) is obtained with the Critical Transition Force (CTF).


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How to Cite
Mateus, D., García, R. A., Saavedra, N. F., & Carrillo, Z. C. (2008). Development of experimental correlations between indentation parameters and unconfined compressive strength (UCS) values in shale samples. CT&F - Ciencia, Tecnología Y Futuro, 3(4), 61–81.


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