Abstract
Chemical reactions with reservoir fluids and geology related in-situ stress changes may cause damages to cement sealing material in plugged and abandoned oil, gas and CO2 wells. To avoid leakages, a legitimate monitoring technique is needed that could allow for early warning in case such damages occur. In this paper, we test the utility of oil and gas well cement with a conductive filler in sensing stress changes. To this end, we have measured the resistance response of Portland G—oil and gas well cement with carbon nanofibers (CNF) to axial load during uniaxial compressive strength test. Simultaneously, the microseismicity data were collected. The resistance of the nanocomposite was measured using two-point method in the direction of loading. The resistance changes were correlated with acoustic emission events. A total of four different material response regions were distinguished and the resistivity and acoustic emission changes in these regions were described. Our results suggest that the two complementary methods, i.e., acoustic emission and resistance measurements, can be used for sensing stress state in materials including well cement/CNF composites. The results suggest that the well cement/CNF composites can be a good candidate material to be used as a transducer sensing changes in stress state in, e.g., well plugs up to material failure.
Highlights
IntroductionOur results suggest that the two complementary methods, i.e., acoustic emission and resistance measurements, can be used for sensing stress state in materials including well cement/carbon nanofibers (CNF) composites
It is clear from the tomography imaging that the cement/carbon nanofibers (CNF) material is rather homogeneous at the millimeter scale
The tomography image taken from the same batch of cement/CNF material but imaged at smaller field of view (5.5 mm) and without metal objects (Figure 3b) indicates that there are two types of inhomogeneities present in the material
Summary
Our results suggest that the two complementary methods, i.e., acoustic emission and resistance measurements, can be used for sensing stress state in materials including well cement/CNF composites. The results suggest that the well cement/CNF composites can be a good candidate material to be used as a transducer sensing changes in stress state in, e.g., well plugs up to material failure. This implies that the responsible authorities have to use only partial data while judging on whether or not installed well plugs are fulfilling their role and they have no control over when eventually the plugs fail to be a proper barrier due to chemical or mechanical degradation. The chemical degradation can result from interaction of the plug with reservoir fluids [2,3], while the mechanical failure can result from e.g., subsidence, shear displacement along discontinuities (e.g., rock interfaces, fractures) or change of in-situ stresses. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
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