Abstract

Microannuli that develop along the cement-casing interface have been identified as common leakage pathways in wellbores. We have developed an experimental system that allows laboratory testing of wellbore specimens which are comprised of a cement sheath cast on a steel casing. Specimens were produced with a range of flaws including microannuli between the steel casing and the cement. The system allows independent application of confining pressures to 35 MPa and casing pressures to 20 MPa while gas flow is measured through the specimens along the wellbore axis. We present the gas flow results in terms of the hydraulic aperture of microannuli as a function of confining pressure and internal pressure for two different types of microannuli. Hydraulic apertures decrease non-linearly with increasing stress across the microannuli in a manner similar to fractures in rocks and other materials. The hydraulic apertures are more sensitive to changes in confining pressure than casing pressure, consistent with the estimated contact stress that develops across the cement-casing interface.

1. INTRODUCTION

The integrity of wellbore systems - consisting of casing, cement sheath and adjacent rock formation – is of critical importance to many sub-surface operations including CO2 sequestration, oil and gas exploration and production, and geothermal energy development. Wellbore systems can have a significant vertical permeability due to the presence of flaws (factures, voids) that allow for fluid migration along the axis of the wellbore [e.g., 1]. The cement-casing interface, herein referred to as the microannulus, has been identified as a common leakage pathway [2, 3, 4]. The flaws are created by a range of factors during well cementing and subsequent well operations, including incomplete drilling mud removal prior to cementing, cement shrinkage, and changes in pressures and temperatures within the casing [5].

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