Rock type based poroperm and continuous permeability predictions in a tight gas formation

Abstract

Porosity from log response such as density provides a continuous representation of pore volume as function of depth in a well, which can be calibrated with core analysis data. Obtaining a continuous log of permeability is not as straightforward as there is yet no means of logging permeability. While DST-derived permeability values are useful in calibrating dynamic models, they only represent an average value over the radius investigated by the test and will not readily correlate to permeability values derived from core especially where there are lateral and vertical permeability variations within the reservoir. It is, however, possible to obtain a depth-continuous permeability estimate by deriving a free regression algorithm known as the poroperm transform function, which defines how the permeability varies as a function of porosity. Such correlations are typically derived empirically from overburden corrected core-derived porosity and permeability data. General porosity-permeability trends are far too scattered to be of use. However, far tighter porosity-permeability trends can be obtained by use of rock typing to identify suitable analogues. The following study highlights how microstructural rock-tying can be used to improve permeability prediction in a set of tight gas sandstone wells. Scanning electron microscopy (SEM) and quantitative X-ray diffraction (QXRD) data were obtained from >200 tight gas sandstone samples from the Southern North Sea for which porosity and permeability measurements had previously been conducted. The SEM and QXRD data were used to derive microstructural and mineralogical rock types respectively. Samples from each rock-types occupy different but overlapping positions on porosity-permeability cross plots. Exponential functions were fitted to porosity-permeability data for each rock type and then applied to the porosity values from wire-line log data to derive continuous permeability estimates. The log porosity curves-being the independent variables of the respective functions, were validated by core observation to avoid error propagation. Continuous permeability curves that honour mineralogical variation were obtained by the use of microstructural rock typing.

Key words: Porosity, Permeability, Rock-typing, Poroperm, Microstructural, Mineralogical.