Abstract
The Klaipeda Geothermal Demonstration Plant (KGDP), Lithuania, exploits a hypersaline sodium-chloride (salinity c. 90 g/L) groundwater from a 1100 m deep Devonian sandstone/siltstone reservoir. The hydrogen and oxygen stable isotope composition is relatively undepleted (δ18O=c. -4.5‰), while the δ34S is relatively “heavy” at +18.9‰. Hydrochemical and isotopic data support the existing hypothesis that the groundwater is dominated by a hypersaline brine derived from evapoconcentrated seawater, modified by water-rock interaction and admixed with smaller quantities of more recent glacial meltwater and/or interglacial recharge. The injectivity of the two injection boreholes has declined dramatically during the operational lifetime of the KGDP. Initially, precipitation of crystalline gypsum led to a program of rehabilitation and the introduction of sodium polyphosphonate dosing of the abstracted brine, which has prevented visible gypsum precipitation but has failed to halt the injectivity decline. While physical or bacteriological causes of clogging are plausible, evidence suggests that chemical causes cannot be excluded. Gypsum and barite precipitation could still occur in the formation, as could clogging with iron/manganese oxyhydroxides. One can also speculate that inhibitor dosing could cause clogging of pore throats with needles of calcium polyphosphonate precipitate.
Highlights
The small country (65,000 km2) of Lithuania contains a varied geological sequence [1], comprising a largely continental or epicontinental sedimentary sequence, which dips and thickens towards the north-west, overlying metamorphic and metasedimentary “basement” rocks of the Precambrian Baltic Shield
Middle Devonian dolomite dolomitic sandtone and marlstone D2-D1 geothermal complex (Parnu strata) D2-D1 geothermal complex (Kemeri sandstones with silt/clay and some gypsum toward base)
The potential saturation and precipitation of minerals suggested by the hydrogeochemical modelling have been confirmed and complemented by empirical observation of (i) filter residue captured near the production and injection wellheads from bag and cartridge (1 μm mesh) filters installed on the fluid circulation system (Table 8, Figure 8)
Summary
The small country (65,000 km2) of Lithuania contains a varied geological sequence [1], comprising a largely continental or epicontinental sedimentary sequence, which dips and thickens towards the north-west, overlying metamorphic and metasedimentary “basement” rocks of the Precambrian Baltic Shield. Well 2P was constructed in 1997 to 1123 m bgl (1116 m bsl), with the Kemeri sandstones being encountered between 985 and 1113 m bgl (Figure 2); these are described as light grey, fine-medium-grained sandstones with some silt and clay and with some gypsum in the lower. Middle Devonian dolomite dolomitic sandtone and marlstone D2-D1 geothermal complex (Parnu strata) D2-D1 geothermal complex (Kemeri sandstones with silt/clay and some gypsum toward base). The injection wells 1I and 4I were very constructed (same screen and gravel pack types) with the exception that the diameters were slightly larger, the final screen and casing string being installed in the Kemeri sandstones at 9 5/8′′ nominal diameter, within a 15′′ annulus [14, 15]. In sandstone core recovered from this sidetrack, a typical porosity of 26% was recorded by mercury porosimetry, and 80% of this pore space was found to have an aperture of between 3 and 40 μm [11]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call