Modeling Basin-scale Geomechanics Through Geological Time
Authors: Hantschel Thomas , Wygrala Bjorn , Fuecker Michael , Neber Alexander 
Published: March 2015

Extended Abstract

Geomechanics is commonly applied with reservoir or wellbore scale data to predict near well stresses and strains for well design for appraisal and production wells. The common basic model is a stress-strain simulator based on an "irreversible non-linear elasticity" approach. In this paper, a new simulator type is presented which enables an extension to basin-scale geomechanics in dynamically evolving models through geologic time.

The approach can be applied to scalable petroleum systems models, enabling development problems from exploration to field scale to be analysed. The calculated stresses and strains can be used for improved predictions of fracture orientations and fault properties, and for descriptions of salt movements, all of which are dependent on the evolution of the basin-scale geomechanical framework through geologic time, and not just on the present geomechanical conditions. An application is shown from the Monogas-Thrust Belt of Eastern Venezuela.

Two basic stress-strain models are discussed based on poro-elastic and poro-plastic formulations. The solution for the Biots type effective stress equation is coupled with the standard basin modeling solutions of pore pressure and fluid flow. Some elastic and plastic properties are derived from the basin scale compaction law. Special boundary values are given as positive and negative displacements for compressional and extensional basins, respectively. The poro-plasticity model uses the Capped Drucker-Prager model. The stress formation during failure is ideally-plasticity controlled with a non-associated plastic flow law. In the cap area, associated plastic flow is assumed with work hardening controlled stress formation. The hardening parameter is derived from the compaction law.

This extension to basin scale geomechanics integrates for the first time (i) a dynamic model acting on geological time scales (ii) a coupling of stress calculations with a basin scale fluid simulator (iii) the adjustment of material parameters to larger scale cells (iv) the considerations of plastic and failure effects, such as compaction and fault movements, and (v) the definition of suitable boundary conditions.

Introduction

The main application of Petroleum Systems Modeling (PSM) is "Charge Modeling" which is used to estimate the probability that petroleum of certain quality is available for entrapment in an area or in a specific prospect if a suitable reservoir and seal is present. The considered chain of processes encompasses the generation, expulsion, migration and accumulation of petroleum in a multi-component and three phase numerical model. Three basic physical models are essential: the thermal, the mechanical and the fluid flow model (Hantschel & Kauerauf, 2009). The mechanical model consists of fluid pressure and rock stresses which are closely coupled with each other. In the current PSM simulators, the fluid pressure models are very well developed as (i) it has an own application, the pore pressure prediction, and (ii) it generally controls all separate phase fluid flow.