Local magnetic field changes during gas injection and extraction in an underground gas storage

Abstract

Repeated measurements of the total magnetic field in an array consisting of 40 stations were performed across the Hutubi underground gas storage (UGS) to investigate the possible effects of piezomagnetism. We conducted five field measurements using proton precession magnetometers with a sensitivity of 0.15 nT spanning an entire cycle of gas extraction (from November 2016 to March 2017) and gas injection (from April 2017 to October 2017). The changes in the local magnetic field (LMF) in different stages were obtained after data processing, which included diurnal variation reduction, secular variation reduction and main magnetic field elimination. Negative variations with maximum amplitudes of −1.9 nT (−2.6 nT) and positive variations reaching 0.8 nT (1.2 nT) in the LMF over a four-month (six-month) period were observed separately with a constantly increasing pressure during gas injection. Positive variations reaching 1.5 nT near around the NW edge and negative variations reaching −1.5 nT near around the SE edge of the UGS were observed over a five-month period (from November 2016 to March 2017) during gas extraction with a constantly decreasing pressure. The LMF changes reveal a negative correlation with both increasing and decreasing pressures in the UGS. This result provides powerful field evidence for the existence of piezomagnetic effects in a middle-scale controlled source experiment.