Geocellular rendering of cave surveys in paleokarst reservoir models

Bjarte Lønøy,Jan Tveranger,Christos Pennos,Fiona Whitaker,Stein-Erik Lauritzen

doi:10.1016/j.marpetgeo.2020.104652

Bjarte Lønøy, Jan Tveranger + Show 3 more

Open Access

https://doi.org/10.1016/j.marpetgeo.2020.104652

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Abstract

Infilled and collapsed cave systems are an important component of many paleokarst reservoirs. Incorporating these features into industrial reservoir models commonly relies on geostatistical modelling methods that often fail to capture key aspects of connectivity, geometry and volume of the paleokarst features realistically. The present work investigates the implementation of realistic cave geometries in geocellular models using survey data from an active karst cave as a starting point. The proposed method utilizes cave survey data to generate a dense equally spaced point-cloud representing the cave system. The point-clouds are used for geometric modelling and subsequent geocellular discretization of the karst system. The volumetric and geometric accuracy of this novel reservoir modelling method is compared to that from two established methods by benchmarking against the cave survey data. Additionally, the interlinkage between grid cell resolution, applied filter cut-off and geocellular rendering are evaluated. This study demonstrates that our proposed novel methodology can provide an excellent geometric and volumetric geocellular rendering of karst systems using cave survey data as input. Employing a combination of cave network maps and forward modelling of collapse and infill may enable model rendering of these features that more closely echoes processes controlling cave and karst breccia formation and geometric characteristics. In turn, this could offer better constraints to forecast paleokarst reservoirs architecture and properties.

Highlights

Active epigene and hypogene karst systems are the precursors of paleokarst reservoirs and can be used as analogues for geometric con figurations of paleokarst formed under given stratigraphic, tectonic and environmental constraints
The results show that all methods are applied to the same cave survey data, the volumetric and geometric representations of the cave system in the geocellular model will differ depending on the al gorithm used
The uncertainty in karst pore volume can be evaluated using fractal distributions (e.g. Curl, 1986; Pardo-Igúzquiza et al, 2018) and sedimentary thickness mapping (e.g. Lønøy et al, 2019a), and can be included in a reservoir model using stochastic modelling. This is outside the scope of this study but shows that it is difficult to establish the actual volume of a karst system and that several factors need to be considered in paleokarst reservoir modelling

Summary

Introduction

Active epigene and hypogene karst systems are the precursors of paleokarst reservoirs and can be used as analogues for geometric con figurations of paleokarst formed under given stratigraphic, tectonic and environmental constraints. Boasting some of the most productive wells in oil history (Viniegra and Castillo-Tejero, 1970; Fournillon et al, 2012), the recovery factor (RF) from karst-related reservoirs is generally very low when compared to conventional carbonate- and organic build-up res ervoirs (Sun and Sloan, 2003; Montaron, 2008; Montaron et al, 2014). Production from these reservoirs is often associated with issues such as rapid water breakthrough, bypass flow and drill-bit drops

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