A 3D numerical model for chimney formation in sedimentary basins

We propose a 3D model for pipe and chimney formations in tight rocks in sedimentary basins. It is an adaption of a model for hydraulic fracturing in an anisotropic stress field by fluid injection (fracking). The trigger for chimney formation is high overpressure in permeable units, such as reservoirs or aquifers. The permeable units serve as a source of high-pressure fluid that drives the chimney formation. The numerical model is based on cells that “fracture” when the fluid pressure exceeds the least compressive stress and random rock strength. The locally highest points in the reservoir rock become the most likely places for chimney formation. Fracturing implies that cells have their permeability changed from their initial value to a value that represents an average permeability of an open fracture network. Chimney growth appears as chains of cells (branches) emanating from the base of the cap rock. These chains of cells grow towards the surface. The branches have an enhanced permeability during ascension because the fluid pressure in the fracture network is greater than the least compressive stress. The fluid pressure keeps the fracture network open. When the branches reach the hydrostatic surface, the fluid pressure drops below the least compressive stress and the fracture network closes. The model produces pipe structures and chimneys as accumulations of branches that reach the surface. The degree of random rock strength controls how pipe-like the chimneys become. The chimney, which is formed by branches of the fractured cells, drains the reservoir for overpressured fluid. Chimney formation stops when the overpressure in the reservoir is reduced below the least compressive stress at the base of the caprock. The fracture permeability of the chimney branches controls how many branches are produced, and thereby how wide the chimney becomes. A “low” permeability produces wide chimneys with many branches, and a “high” permeability gives narrow chimneys made of just a few branches. The model is demonstrated in a setting similar to the chimneys observed in the cap rock over the Utsira aquifer in the North Sea. By using the proposed model, the permeability of such chimneys is estimated to be of the order of 10 micro-Darcy.