Assessment of pore collapse in North Sea Lower Cretaceous carbonates using downhole elastic strain

The potential for pore collapse is a concern when producing hydrocarbons from fragile sedimentary rocks, like chalks. The reason is that although pore collapse can be a valuable drive mechanism, it can also cause large scale compaction and surface subsidence. This risk is as a rule evaluated from results of rock mechanical testing of core samples, but because a given reservoir typically comprises layers of different porosity, the stress required for fracturing or pore-collapse is then generalized as a function of porosity. Due to different diagenetic history, different reservoirs can have different degree of ce-mentation (induration) for a given porosity, so each reservoir requires a separate stress-porosity relationship. Recent studies of chalk indicate that pore collapse happens at a critical elastic strain irrespective of porosity, pore fluid, and induration. This should not be surprising, bear-ing in mind that the strain of the rock represents an equal strain of the mineral crystals constituting the rock frame, so that onset of pore collapse in chalk is governed by the in situ elastic strain of the calcite. Chalk can be rich in minerals like clay or quartz, but when calcite constitutes the frame, it is calcite that is elastically strained. The simple point is that elastic strain of the calcite crystals is the same as the elastic strain of the rock. A complication with rock mechanical data is that the tested material has been brought to the surface by coring, a process that typically causes tested rocks to contain microcracks (small unloading fractures), so that the tested rocks do not truly represent the rock in situ. This issue points to using downhole deformation data, but these are sparse. A possibility is then to use information from geophysical well logs, because in situ elastic strain can be calculated from well log data, so that the results from rock mechanical testing can be generalized in a simple way. The method for strain calculation is here applied to data from the Valdemar field, which produces hydrocarbons from the Tuxen and Sola formations, a clay-rich Lower Cretaceous chalk interval in the Central North Sea.