Wellbore Strengthening in Shales With Nanoparticle-Based Drilling Fluids
Authors: Wilson Adam 
Published: October 2016

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 170589, “Experimental Investigation on Wellbore Strengthening in Shales by Means of Nanoparticle-Based Drilling Fluids,” by Oscar Contreras, SPE, University of Calgary; Geir Hareland, SPE, Oklahoma State University; Maen Husein, University of Calgary; and Runar Nygaard, SPE, and Mortadha Alsaba, Missouri University of Science and Technology, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed.
Wellbore strengthening (WS) is the mechanism of increasing fracture pressure of rock at depth. WS in shale formations is controversial because of the poor understanding of the mechanism and limited field success. This paper presents experimental research in which a significant fracture-pressure increase was achieved in shale and the predominant WS mechanism was identified. The main implication of this work is that WS can occur in shale formations by use of oil-based mud (OBM) with the addition of nanoparticles (NPs) and graphite.
Introduction
This research presents an original approach based on the use of in-house-prepared NPs and graphite as WS agents in OBM. Catoosa shale cores that are very sensitive to water and air were used. The NPs used in this research are believed to have a high interaction with clays. The NPs locate on top of the clays and fill the gaps or holes in the clay platelets. They are subsequently captured within the clay layers by strong adhesion created as a result of the negative nature of the clay edges. A strong bridge resulting from the interaction between NPs and clay is believed to be a WS agent. WS in shale formations was experimentally achieved in this research. The hypothesis, previously proposed for sandstone cores, that WS is related to mud filtration was also tested. The experimental procedures involved hydraulic-fracturing experiments of a high operational complexity because of the very sensitive nature of the shale. Optical microscopy, scanning electron microscopy (SEM), and energy- dispersive X-ray (EDX) spectroscopy analyses were conducted in cores after testing. Tip resistance by the development of an immobile mass was identified as the predominant WS mechanism on the basis of the post-testing observations and by ruling out the occurrence of stress caging.
Fracture-pressure increase was quantified by conducting hydraulic-fracturing tests on 5¾×9-in. Catoosa shale cores. A 9/16-in. wellbore was drilled in the core. Overburden and confining pressures were applied on the cores to simulate a normal-faulting regime. Two injection cycles were applied, allowing 10 minutes for fracture healing after the first cycle. The fracturing pressure was increased by 30% when calcium-based NPs (NP2) were used, whereas iron-based NPs (NP1) resulted in 20% increase. The optimum NP concentrations were identified experimentally.
Experimental Analysis
Virgin and recycled OBM containing in-house-prepared NPs and graphite were used for the hydraulic-fracturing tests in shale cores. NPs were prepared within the OBM (i.e., in-situ) from solid and aqueous precursors. Graphite was added later. Very low rheology impact was caused by the NPs and graphite at the low levels used in this study.