Injection-Triggered Occlusion of Flow Pathways in Geothermal Operations

Abstract

Reasons for injectivity decline were investigated in a low-enthalpy geothermal aquifer in Klaipeda (Lithuania). It is one of the study sites within the DESTRESS project, which demonstrates different stimulation techniques in geothermal reservoirs. Due to low injectivity, production rates from the Lithuanian field are currently reduced, which lead to negative commercial implications for the site. Productivity from the same wells is measured to be 40 times higher. Injectivity decline in aquifers is often related to clogging processes in spatially correlated highly permeable structures, which control the main flow volume. We subdivided clogging processes into (1) physical, (2) chemical, and (3) biological processes and studied them by analyzing fluid and solid samples as well as operational data. The methods we used are fluid and solid analyses in situ, in the laboratory and in experimental setups, statistical interpretation, and numerical modeling. Our results show that the spatially correlating nature of permeable structures is responsible for exponentially decreasing injectivity because few highly permeable zones clog rapidly by intruded particles. In particular, field operations cause changes of the physical, chemical, and biological processes in the aquifer. Mineral precipitation and corrosion are the main chemical processes observed at our site. Microbial activity causes biofilm while fines migration is caused by changes in physical boundary conditions. Moreover, these processes can affect each other and generate further reactions, for example, microbial activity triggers corrosion in surface pipelines.

A Review of the Hydrochemistry of a Deep Sedimentary Aquifer and Its Consequences for Geothermal Operation: Klaipeda, Lithuania

“…In 2009, as a potential (but ultimately unsuccessful) remedy for declining injectivity, a sidetrack borehole was constructed in borehole 1I at a 3.5°deviation from the vertical between 897 m bgl and 1116 m bgl (total depth). 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].…”

“…The site supplied a district heating network to numerous clients, although since 2013, it only operated from October to April. In 2017, the KGDP was shut down, ostensibly as it could not compete with a nearby waste-incineration plant in terms of economic supply of heat to the district heat network (although reduced injectivity in 1I and 4I could be cited as a contributory factor [11]).…”

“…The injection wells 1I and 4I were very similarly 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]. Test pumping of borehole 4I indicated sandstone permeability of 2.1 to 2.5 mD and a transmissivity potentially as high as 218 Dm [11].…”

“…Over time, the injectivity of the boreholes 1I and 4I has declined dramatically, as described in full in [9,11,16] and summarised graphically in None of these has produced anything other than short-term gains in injectivity. The most successful remediation was the acidisation of both boreholes in 2002 with 20 m 3 18% hydrochloric acid, but even in this case, the injectivity gains were temporary.…”

“…The most successful remediation was the acidisation of both boreholes in 2002 with 20 m 3 18% hydrochloric acid, but even in this case, the injectivity gains were temporary. The fact that productivity is far higher than injectivity when the injection wells are reverse pumped leads Brehme et al [11] to suppose that the declining injectivity is due to a "skin effect" relatively close to the borehole, which can be partially "drawn out" by abstraction.…”