Abstract

Chemical treatment is a potential method to decrease shale strength and thereby induce borehole closure and thus the formation of a natural barrier around a hydrocarbon well after completion. A laboratory test was designed in order to investigate whether HCl exposure is a possible way to form such a barrier. The sample used for the test was a hollow cylinder Pierre I shale, with the borehole direction perpendicular to the bedding. The shale contains about 4 wt% of dolomite, CaMg(CO3)2, and 1 wt% chlorite, as a part of the cementation. This is expected to be dissolved by the acid. Borehole measurements during the test showed significant deformations, however the intended borehole closure was not obtained. Post-test CT scans revealed disintegrated material in a limited area near the borehole wall. External radial deformation was relatively small, which also indicates that significant change of properties has happened only in a small area near the borehole. Trends of density decrease towards the borehole may indicate a deeper impact of the acid exposure, without significant mechanical consequences.

Introduction

Formation of a sustainable natural barrier in oil and gas wells is a promising technology for the plug and abandonment operations (P&A) of hydrocarbon wells. In-situ stress is the driving force for the formation of such barriers, while the shear strength and shear stiffness of the shale restrict or prevent the process. Creep reduces shear strength and shear stiffness, and thus contributes to the formation of natural barriers. There are three main creep modes – transient, steady state, and accelerating [1]. In most cases, steady state creep is dominating, and in that case, complete closure of the annulus may take decades or hundreds of years [2]. To obtain a sustainable barrier within reasonable time, enhancement of creep or otherwise reduction in shear strength is beneficial. Chemical treatment is a potential method to achieve this.

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