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| Titel |
Acidization of shales with calcite cemented fractures |
| VerfasserIn |
Kamil Kwiatkowski, Piotr Szymczak, Marek Jarosiński |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138541
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| Publikation (Nr.) |
 EGU/EGU2017-1588.pdf |
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| Zusammenfassung |
| Investigation of cores drilled from shale formations reveals a relatively large number of
calcite-cemented fractures. Usually such fractures are reactivated during fracking and can
contribute considerably to the permeability of the resulting fracture network. However, calcite
coating on their surfaces effectively excludes them from production. Dissolution of the calcite
cement by acidic fluids is investigated numerically with focus on the evolution of fracture
morphology. Available surface area, breakthrough time, and reactant penetration length are
calculated.
Natural fractures in cores from Pomeranian shale formation (northern Poland) were
analyzed and classified. Representative fractures are relatively thin (0.1 mm), flat and
completely sealed with calcite. Next, the morphology evolution of reactivated natural
fractures treated with low-pH fluids has been simulated numerically under various operating
conditions. Depth-averaged equations for fracture flow and reactant transport has been solved
by finite-difference method coupled with sparse-matrix solver. Transport-limited dissolution
has been considered, which corresponds to the treatment with strong acids, such as
HCl.
Calcite coating in reactivated natural fractures dissolves in a highly non-homogeneous
manner - a positive feedback between fluid transport and calcite dissolution leads to the
spontaneous formation of wormhole-like patterns, in which most of the flow is focused. The
wormholes carry reactive fluids deeper inside the system, which dramatically increases the
range of the treatment. Non-uniformity of the dissolution patterns provides a way of retaining
the fracture permeability even in the absence of the proppant, since the less dissolved regions
will act as supports to keep more dissolved regions open. Evolution of fracture
morphology is shown to depend strongly on the thickness of calcite layer – the
thicker the coating the more pronounced wormholes are observed. However the
interaction between wormholes is the strongest when coating thickness is a few times
larger than the initial aperture of the fracture. This leads to formation of favorable
complex networks of wormholes which provide adequate transport of reactive fluids
to fracture surfaces and – at the same time – are capable of supporting fracture
surfaces. As a conclusion, acidization of the reactivated fractures with hydrochloric
acid seems to be an attractive treatment to apply at fracking stage or later on as
EGR.
The results contribute to the discussion on the use of acidization to enhance the gas
production in the shale reservoirs. This communication stresses the importance of the
dissolution of calcite cement in natural fractures in shale formations, which are initially
sealed and become reactivated during fracking. While this research is based on the analysis of
fractures in the Pomeranian shale basin its results are general enough to be applicable to
different formations worldwide. |
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