The Assumption of Zero Leakage from Permanently Abandoned Wells on the Norwegian Continental Shelf

Quality data is essential for calculations of expected leakage in wells post Permanent Plug and Abandonment (PP&A). Such data may come from surveys studying hydrocarbon leakage to the marine environment. However, recent literature suggests that current regulatory practices for environmental surveys are suboptimal, giving reason to question the assumption that wells have experienced zero leakage from the deep reservoir post PP&A on the Norwegian Continental Shelf (NCS). We investigate whether such an assumption is credible. The credibility of the assumption of zero leakages is investigated through a review of literature addressing the integrity of wells post PP&A on the NCS, with particular emphasis on a 2021 report from the Norwegian Environmental Agency (NEA). Based on the review, the strength of knowledge supporting the assumption that no wells on the NCS have experienced leakages from their deep reservoirs is discussed. The implications of the uncertainty associated with the assumption of zero leakage on the NCS, the rationale for collecting more relevant data, and how these data may be obtained is also discussed in brief. The NEA report details the current regulatory practice for environmental surveys on the NCS. This regulatory practice, as it is described in the NEA report, give limited support to a zero leakage assumption. Norwegian regulations require two environmental surveys post Cessation of Production (CoP). These surveys may however occur in the period between CoP and PP&A, and the closest test stations are generally located 250 meters from the wells. Environmental surveys carried out that far from the well, and possibly prior to PP&A, influence data quality. We argue that the environmental survey data claiming zero leakage, lack sufficient evidence. Thus, based on the reviewed literature outlining the current environmental survey practice, although PP&A well design on the NCS should build on sound principles, we are not able to conclude on the assumption of zero leakage. The interest in risk-based PP&A approaches is increasing globally, and risk-based approaches rely on credible leakage calculations. The failure rates used in these leakage calculations should be based on quality data. The NEA report and other literature indicate that the quality of post PP&A leakage data on the NCS is questionable, and in some cases the data are non-existent. The paper includes suggestions on how to improve the regulatory practice related to environmental surveys.

The Impact of Cascade Large Deep Reservoir on the Migration and Deposition of Cadmium in Lancang River

Metal pollution is a global environmental problem. In order to understand the effects of the cascade hydropower development on Cd in reservoir sediments, this study investigated the migration and deposition mechanisms of Cd in reservoir sediments by using the film diffusion gradient technique (DGT) for the cascade large deep reservoirs of the Lancang River (LCR), Nuozhadu (NZD) and Xiaowan (XW). The results show that: (1) Strong correlations amongst the deposition processes of Cd, Mn and S were found in the sediments of the NZD reservoir; weak correlations between the deposition processes of Cd and Mn were found in the sediments of the XW reservoir. (2) Part of labile Cd in the pore water of the NZD reservoir was supplemented by silt and sand, and another part was inherent in the pore water; most of labile Cd in the pore water of the XW reservoir was inherent in the pore water, and minimal replenishment of sediment was noted. (3) Mn is the main factor controlling the migration of Cd in the NZD reservoir. This paper provides an effective resource for understanding the migration and deposition of Cd in the sediments of large deep reservoirs along the LCR.

Synchronous Cycle of Available Phosphorus, Iron, and Sulfur in the Sediment of Lancang River Reservoirs

Large-scale deep reservoirs associated with hydropower cascade development are known to influence the cycle of phosphorus (P). However, there is scarce information on the fractions and availability of P in sediments of large-scale deep reservoirs constructed due to hydropower cascade development. In this study, we researched the fractions and release mechanism of P in the sediments of large-scale deep reservoirs by analyzing the fractions and availability of P in the sediments of the Xiaowan (XW) and Nuozhadu (NZD) reservoirs in the middle and lower reaches of the Lancang River (China). According to the results, there is a significant difference in the P fractions in the sediments of the XW and NZD reservoirs, but not for the available P in the sediments. Compared to the NZD reservoir, there was less solid bioavailable phosphorus (BAP) in the sediments of the XW reservoir, but the replenishment degree of active solid phase P into pore water was higher in the XW. There was a significant positive correlation between the available P and the BAP; the Fe/P ratio measured by the diffusive gradients in thin films reflects the control of active iron (oxyhydr) oxides over labile P in the sediments. In addition to the reductive dissolution of iron-bound P, the release of P into the large deep reservoirs may be related to factors such as the sulfate reduction and the degradation of organic materials. The P cycling in deep reservoir sediments is mainly controlled by the Fe, and there is a clear spatial distribution of this mechanism in deep reservoirs.

Episodic transport of discrete magma batches beneath Aso volcano

Magma ascent, storage, and discharge in the trans-crustal magmatic system are keys to long-term volcanic output and short-term eruption dynamics. How a distinct magma batch transports from a deep reservoir(s) to a pre-eruptive storage pool with eruptible magma remains elusive. Here we show that repetitive very-long-period signals (VLPs) beneath the Aso volcano are preceded by a short-lived (~50–100 s), synchronous deformation event ~3 km apart from the VLP source. Source mechanism of a major volumetric component (~50–440 m3 per event) and a minor low-angle normal-fault component, together with petrological evidence, suggests episodic transport of discrete magma batches from an over-pressured chamber roof to a pre-eruptive storage pool near the brittle-ductile transition regime. Magma ascent velocity, decompression rate, and cumulative magma output deduced from recurrent deformation events before recent 2014 and 2016 eruptions reconcile retrospective observations of the eruption style, tephra fallouts, and plume heights, promising real-time evaluation of upcoming eruptions.