Silurian barrier reef in Lithuania: Reservoir properties and low enthalpy geothermal heat potential

Abstract

The Silurian barrier reef zone in Lithuania extends for hundreds of kilometres in the narrow zone in Middle Lithuania continuing from the Saukenai reef in the north to the Vilkupiai reef in the south. After prolific Cambrian sandstone reservoirs, they are considered a secondary oil target in Lithuania. However, drilling data showed that the detected reefogenic bodies are either “dry" or saturated with water and/or viscous oil thus practically no valuable oil supply has been obtained. This study illustrates a review of the existing geological and geophysical data, acquired during oil and gas explorations since the 1960s, specifically re-analysed for geothermal purposes.The heat production rates of well doublets within the Silurian reefs range from 0.000044 to 0.24 MWh. Within the area of the Kudirka structure, 6 well doublets could be arranged from the existing 16 exploration wells, at the average distance between the producing-injecting wells of around 700 m. Kudirka has the largest reservoir volume and hence the largest reservoir heat potential (250 GWh). At the smallest distance between the doublet wells of 486 m, the thermal breakthrough would be reached after around 30 years of production-injection. Pavasaris reef shows the best geothermal capacity of a well doublet (0.241 MWh). Reservoir properties are especially good in the partially open fracture zone interval. Pavasaris reef has got 2 wells penetrating the reef, thus one doublet could be arranged within the structure. However, the available thermal energy is limited due to hydrogeological closure and small reservoir size (11.2 × 103 MWh). Other reefs (Saukenai, Vilkupiai, North Bliudziai and a reef in the north of the Vadzgirys reef belt) are small isolated structures, thus their heat potential is low. Moreover, they show low flow rates and hence thermal energy that can be extracted at their wellheads in 30 years is well below 1 × 103 MWh. The study indicates that even though the thickness of the reefal build-ups may reach up to c.90 m, the effective thickness of the reservoirs rarely reaches 20 m. Analysis of the well logs and the core sections show a high heterogeneity of the structures, and hence low recovery factors of the doublets. The distribution of effective layers is usually associated with processes such as stylolitisation and secondary fracturing with enhanced dissolution. For measured low flow rates (<9 m3/h), reusing the existing wells shows low economic viability due to the narrow diameters of the boreholes. Therefore, Silurian reefs show low potential for the application of geothermal energy in Middle Lithuania, although small geothermal projects could be achieved in the Kudirka and Pavasaris sites.