Thermoelectric Nanomaterial Improves Shale Stability
Authors: Carpenter Chris 
Published: October 2014

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164000, ’Thermoelectric Nanomaterial: A Demiurgic Approach Toward Improvement of Shale Stability,’ by Prateek Pandey, SPE, Pratik Kapadnis, SPE, Bilal Ghansar, SPE, and Sagar Mandan, SPE, Maharashtra Institute of Technology (Pune), prepared for the 2013 SPE Middle East Unconventional Gas Conference and Exhibition, Muscat, Oman, 28-30 January. The paper has not been peer reviewed.

Large amounts of clay minerals are present in shale; shale’s resulting instability is a major problem while drilling. This paper introduces a novel idea to improve shale stability by using low-temperature drilling fluid and cost-effective thermoelectric and nickel-iron (Ni-Fe) nanomaterials to alter the mud temperature. The thermoelectric nanomaterial reduces the mud temperature as electricity is applied, while the Ni-Fe nanomaterial helps to increase the mud temperature.

Introduction

Shale instability is a result of several factors. (For a detailed discussion of these factors, as well as three models used to determine stresses and to calculate formation pore volume, please see the complete paper.) Shale instability has considerable negative effects on the cost and speed of drilling operations. Three factors that predominantly affect the shale stability are chemical, thermal, and hydraulic effects. Because of these factors, a change in pore pressure (and thus a change in the stress on the wellbore) is experienced. Improved shale stability can be achieved by minimizing the effect of these factors. Chemical effects can be minimized by upholding the salt concentration in drilling mud, but countering thermal effects can be a formidable task.

This paper discusses a novel process to miniaturize the thermal factor influencing shale stability. The thermoelectric nanomaterial is specially used to cool down the mud temperature. The second nanomaterial used exhibits self-heating properties and thus increases the temperature of mud. The nanomaterials are used in a way that depends upon the respective conditions needed to maintain the optimum temperature difference between the formation and the drilling fluid.

Rock Failure

Wellbore instability comprises compressive failure, hydraulic fracturing, and radial spalling. Compressive failure is observed when the heating of formation takes place (i.e., the mud temperature is greater than the formation temperature). Because of the heating process, expansion of shale takes place, thereby promoting compressive failure. Compressive failure increases the chances of pipe sticking. This compressive failure is determined by the compressive shear failure potential.

Hydraulic fracturing is the failure that occurs when fracture inside the formation takes place along the radial direction. This occurs when the temperature of the mud is lower than that of the formation. Lost circulation is observed because of hydraulic fracturing. Hydraulic-fracturing failure is determined by the hydraulic-fracturing potential. This hydraulic-fracturing-potential value is equal to the tensile stress.