Assessing Karstic Network Petrophysics Lessons Learned from Analogue Studies Example from Thermal Pal-Volgy Karst System (Hungary)

Karst features have been proven to be of major importance in most isolated carbonate platform for reservoirs understanding and hydrocarbons recovery. Reservoir characteristics and petrophysical properties are highly impacted by remaining karstic networks. Thereby a thorough quantification of the derived petrophysical properties is necessary. This quantification is generally achieved using analogue data.

This study focuses on Budapest area thermal karst features (Hungary). The Triassic and Eocene carbonates of the Buda Mountains display a long and complex history of diagenetic alteration and tectonic deformation, including different types of karst, mainly dominated by thermal fluid movements. The thermal mixing induced major cavern development (at least since Late Pliocene times), with a large variety of characteristic morphologies and speleothems that reflect an evolution from phreatic convecting cells into pools (with only minor vadose meteoric overprint). The cave passages in Pál-völgy cave and quarry (as well as for neighbouring caves and quarries) are arranged into two main directions in accordance with the faults geometry: NW-SE and NNE-SSW, with subordinate W-E, WNW-ESE passages.

The detailed study of Pál-völgy cave and quarry has allowed the extraction of geometric parameters and the construction of a fine scale deterministic 3D model (meter scale). Three main karst related petrophysical classes are identified, from matrix enhancement, fine-scale dissolution vugs and conduits (decimetre size) to large caves and conduits (meter size). The petrophysical properties of each petrophysical class are measured and upscaled to a classical reservoir grid size (pluri-decametric scale) using a pressure solver.

From the analogue observations, statistics on cave morphologies and distribution have been extracted and used to reproduce the karstic network, using an internal karst modelling tool. By applying equations, derived from Poiseuille and Darcy laws, this newly developed modelling tool allows assessing petrophysical properties associated to cave distribution. These petrophysical values are then compared with the pressure solver outputs to validate the modelling workflow.

The final result is a tool providing a realistic karst distribution, but also quantifying the related petrophysical properties (porosity, permeability and transfer coefficient between matrix and caves) for reservoir assessment purposes.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California