The Merksplas-Beerse geothermal well (17W265) and the Dinantian reservoir

Abstract

The Merksplas-Beerse well (North Belgium) is a low-enthalpy geothermal production well targeting the Dinantian karstic limestones to a total depth of 1761 m. The presence of methane gas in these limestones generated a particular interest in this well. This paper describes the geological profile of this well and the Dinantian reservoir. The Namurian-Visean boundary at 1630 m is determined by the base of the dipmeter draping pattern in the radioactive Chokier shales (base of the Namurian) on top of the karstified Dinantian limestone. The stratigraphic composition of the transitional interval from Dinantian to Silesian correlates closely to the nearby Turnhout well. The two fractured intervals at 1630-1656 and 1739-1747 m respectively were identified in the Dinantian limestones. They are associated with siliciclastic sections in between pure limestones. The reservoir water is a sodium chloride brine of about 74 °C and at a pressure below the hydrostatic. The water is slightly radioactive because of the contact with the Chokier hot shales. A carbon dioxide gas with methane and nitrogen admixture is dissolved in the water. The gas liquid ratio at standard conditions is about one and the bubble point is around 200-400 psi at reservoir temperature. A long duration pumping test shows a high fracture permeability and a productivity index of 5.4 m3/h/bar with a productivity to injectivity ratio of 1.45.

Seismic stratigraphy of Dinantian carbonates in the southern Netherlands and northern Belgium

“…Further basinwards, away from the platforms, sedimentation could have been continuous, with basin sediments deposited over the Dinantian carbonates. The combined observations of a sedimentary hiatus, the stratal onlap patterns, observations of karstic development and presence of deltaic clastics on top of the Dinantian carbonates correspond to a well-known regional unconformity between the Dinantian carbonates and overlying Silesian clastics (Vandenberghe et al, 2000; Schroot et al, 2006; Harings, 2014). This also suggests that the unconformity is more widespread than previously proposed.…”

“…This structure occurs in the Campine Basin, and cross-cutting high angle veins that were widened by karstic dissolution created porosity. This process ultimately produced cavernous porosity (Vandenberghe et al, 2000; Amantini, 2009). The karstic dissolution phase has been related to a regression at the end of the Visean period leading to regional exposure (Vandenberghe, 1984; Dreesen et al, 1987; Schroot et al, 2006) and subsequent karst development at the top of the Visean limestone in the exposed areas.…”

“…The karstic dissolution phase has been related to a regression at the end of the Visean period leading to regional exposure (Vandenberghe, 1984; Dreesen et al, 1987; Schroot et al, 2006) and subsequent karst development at the top of the Visean limestone in the exposed areas. The Merskplas–Beerse geothermal well positioned in these deposits shows porosities of up to 20% (Vandenberghe et al, 2000). A comparable karstic Dinantian limestone reservoir was encountered in geothermal well CAL-GT-01 drilled on the Maasbommel–Krefeld high near Venlo (Netherlands), which encountered heavily karstified sediments including metre-scale cavities (Böker et al, 2012; Jaarsma et al, 2013).…”

“…The Turnhout well (Fig. 5) shows an irregular karstic Dinantian limestone top draped by early Namurian clastics (Vandenberghe et al, 2000). This well is located on the Loenhout build-up, which is a carbonate shoal that developed on an uplifted basement structure (Langenaeker, 2000).…”

“…5). All sites utilise a karstified Dinantian reservoir with porosities up to 20% and two Darcy permeabilities (Vandenberghe et al, 2000; Jaarsma et al, 2013). The occurrence of locally improved reservoir properties contrasts with the notion of the generally tight character of the Dinantian carbonates in the Dutch subsurface.…”