Experimental Measurement of Vertical and Horizontal Permeability of Caprocks from the Krechba Field, Algeria and the Controls on their Permeability

Caprock properties play a crucial role in determining the seal capacity of a structure and so are important during exploration, appraisal and field development. Less attention has been paid to caprocks than reservoirs since if petroleum is present, then the seal must be working. However, injection of CO2 to underlying reservoirs will alter the reservoir conditions from those against which the caprock was previously effective. Increased knowledge of the petrological and petrophysical characteristics of caprocks is required in order to lay a foundation to predict the effect of the altered conditions caused by CO2 injection. Vertical (kv) and horizontal (kh) permeability were measured experimentally across a range of effective pressures for an unusually coarse grained, heterogeneous caprock (siltstone) to a natural gas reservoir and current CO2 storage reservoir, from the Krechba Field, Algeria. Permeabilities as low as 10-23m2 were recorded and were in the range of, or lower than typical fine grained siliciclastic caprock lithologies. The permeability was analysed in conjunction with mercury injection porosimetry data, and textural and mineralogical data from traditional light microscopy, backscatter secondary electron microscopy (BSEM) and cathode luminescence (CL) techniques as well as new QEMSCAN techniques to elucidate the controls on permeability. As predicted and measured by previous experimental work on fine grained siliciclastic lithologies, permeability is effectively controlled by porosity, pore size distribution and clay fraction. Permeability generally decreases with decreasing porosity and poresize distribution and increasing clay content. However, scatter in the trends was caused by heterogeneity of the sample leading to large kv and kh ratios. Primary depositional features led to layers of relatively low and high permeability in the samples, with kv controlled by the lowest permeability layer, and kh controlled by highest permeability layer. Thus kh and kv in heterogeneous rocks is not a simple relationship between porosity, pore size distribution and clay fraction, and is dependent upon their distribution.