A Method to Obtain a Permeability - Constrained and Consistent Saturation Height Model

Abstract

Abstract The process of differentiating between rock volumes based on petrophysical properties and geological indicators is commonly referred to as rock typing. A rock type can be identified by a given porosity – permeability (k-Phi) transform and Saturation Height Model (SHM) in petrophysical space. Rock typing is a useful method by which geological interpretations are combined with petrophysical measurements and translated into physical space for use in subsurface modelling. Efforts here involve utilizing k-Phi transforms as an input to SHM, thereby streamlining the rock typing process and allowing for compatibility with existing rock typing workflows. A fundamental part of building realistic subsurface models includes populating a geologic framework with petrophysical properties. From a petrophysical point of view, critical properties with significant impact on the modelling outcome (hydrocarbon volumes recoverable and producibility) are hydrocarbon saturation, permeability and relative permeability. Hydrocarbon saturation is an expression of the rock capillarity translated into a mathematical expression from capillary pressure measurements or well log saturations. Permeability is commonly predicted from porosity, via transform equations used to differentiate reservoir rocks of different quality. Previous work has shown that permeability and SHM used for subsurface modelling are generally consistent. This implies that the number of input parameters to the SHM can be reduced, which can be done by integrating permeability and saturation data (from logs and core measurements). The number of parameters used in the predictive SHM is reduced from six to four. Here, we propose to constrain the entry pressure (responsible for hydrocarbon entry height) by using routine core analysis data. This approach aiming to look at the plug and log scale has significant benefits when a SHM is derived from well log saturation data or when a limited range in properties is sampled by capillary pressure measurements. The workflow allows the use of other permeability sources (like Drill Stem Test if representative of matrix) as starting point in the process of building a SHM via a simplified Brooks-Corey function. The function can be looked at as a hybrid between the Leverett's J and Brooks-Corey function with entry pressure dependency on the permeability to porosity ratio higher than for Leverett's J (0.7 vs 0.5). The result of linking k-Phi transforms to saturation height modelling allows for compatibility with multiple rock typing approaches that utilize different sorts of parameters to define k-Phi, including Lucia's classic rock fabric numbers, a useful benchmark.