Experimental modeling of abandoned shallow oil wells convergence P. Bujok a , M. Porzer a , K. Labus b, ⁎, M. Klempa a , J. Pavluš a a Institute of Geological Engineering, VŠB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic b Institute for Applied Geology, Silesian University of Technology, 2 Akademicka St., 44-100 Gliwice, Poland abstract article info Article history: Received 20 June 2012 Received in revised form 1 February 2013 Accepted 2 February 2013 Available online 20 February 2013 Keywords: Well convergence Physical model Shallow oil wells Well abandonment The paper discusses issues related to inappropriately abandoned production wells in depleted oil field near Hodonín (South Moravia, Czech Republic). The characteristic effects of the well deformation, depending on whether there was a possibility of fluid displacement from the well space were defined on the basis of model experiments. It was proven that in the analyzed conditions, convergence of abandoned wells is not equivalent to their proper liquidation, because the communication of media is still possible, creating a risk of pollution of groundwater and land surface. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The Hodonín oil field under consideration is located in the north- ern part of the Lower Moravian depression (northern part of the Vienna Basin) along the right bank of the Morava River, about 5.5 km south-west from the town of Hodonín. The oilfield consists of several elevated structures, with related accumulations of hydro- carbons. They can be treated as separate deposits, which are part of the so-called Hodonín-Gbel play. The productive series is represented by Sarmatian sandy marls with inserts sand oil horizons, as well as sandstone and lignite beds. Below lie the sandy marls classified as upper Badenian. The overburden consists of sandy marls, clays and sands of Sarmatian age, covered by Quaternary clayey-sandy sediments of fluvial terraces of the Morava River — Fig. 1. The first oil wells in this area were drilled in 1919. They were localized on the basis of the observed leakage of gas and oil in the old riverbed of Moravia. Signs of oil were found at a depth of 218 m below the surface. Since the start of operation in the twenties of XX century, hundreds of production wells have been drilled in the Hodonín region. These wells, gradually excluded from the operation, were systematically plugged, although often after a long time since the abandonment. The plugging technology was often very primitive (removal of casing, filling with clay or soil), or incorrectly performed. Many of the wells were left without plugging. In some oil fields, particularly where the reservoir water have extensive coverage, a grad- ual pressure buildup may occur (MND Group, archival data). This process can lead to restoration of spontaneous connections between the shallow productive layer (situated 100 to 180 m below the surface) and the overburden (surface), through the space of poorly plugged well. Example effects of improper well abandonment are shown in Fig. 2. In order to study the phenomena responsible for the passage of fluids through abandoned wells and mechanisms of well space defor- mation in Sarmatian sediments, a program of simple model experi- ments was designed. The purpose of these models was to provide the view of underground processes occurring after the casing removal in order to abandon oil well in terms of exceeding the rock failure criterion after the thereby induced stress deviation. It should be underlined, that time dependent effects such as creep were beyond the study extent and model resolution. 2. Shallow well convergence and its effects Interactions between rock mass and the space within the abandoned borehole can be considered in two possible scenarios: A) cemented or partly cemented sections of casing were left in the borehole, B) the borehole is open and filled with mud or reservoir fluid. According to the scenario A the destruction of casing due to external pressure is unlikely at least to a depth of several hundred meters (Barree and Mukherjee, 1996; Shuling et al., 2010, 2012). Already at the stage of well design calculations are carried out to prevent the casing destruction due to external or internal pressure. Casing shall be conducted on the basis of these calculations and empirical experience as well. Improperly liquidated production well however, may enable communication be- tween the collector and the surface or zones of lower reservoir pressures. Communication can take place inside the well, or outside the casing, in Engineering Geology 157 (2013) 1–7 ⁎ Corresponding author. E-mail addresses: krzysztof.labus@polsl.pl, klabus@o2.pl (K. Labus). 0013-7952/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2013.02.004 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo