Technical Information

    Contents:

 

Summary Technical Specifications

Introduction

TerraMod is a state-of-the-art basin modelling program essential for the effective evaluation of exploration acreage and containing unique features. Providing an analysis of the history of sedimentary basins and, in particular, the evolution of their source rocks and expelled hydrocarbons, it incorporates rigorous calculation routines and automated calibration checks. A unique optimised model that closely fits the specific geology of an exploration area, including thrusted regions, is created.

Other software does not take account of heating rate but TerraMod is very different from earlier basin modelling programs in that it simulates the compaction process in iterative loops, modelling the movement of heat and fluids within the basin through geological time. It thus fully takes account of the pressure and temperature model and incorporates heating rate dependent kinetics, critical for the correct determination of generation volumes in both rapidly and slowly subsiding basins.

The program precisely simulates deposition, fluid dynamics and heat flow including the processes operating during subsidence, uplift, erosion and thrusting. It can be applied to any geological terrain.

The program quantifies compaction through the history of a basin without relying on broad unconstrained assumptions and alerts you to any elements of your model that may not be geologically correct.

The program calculates:

  • hydrocarbon generation and expulsion through time for each of your identified kerogen types and for custom kerogen mixes using any number of independently defined source rocks;

  • oil and gas volumetrics for defined source rock thicknesses and for entire kitchen areas;

  • fully coupled temperature and pressure profiles indicating where under- or over-pressured layers may occur;

  • oil gravity expulsion data for oils produced at any time;

  • timing of active maturation and generation;

  • sealing efficiency of reservoir cap rocks;

  • geohistory and back-stripped subsidence history.

The program contains a project and well databasing system with files that can be configured to your user preferences and retrieved as required.

Your models can be both automatically optimised and checked against calibration data ensuring the best output solution.

The program allows batch simulations and batch plotting which can be run overnight saving you time.

Output plots and reports are of presentation quality suitable for immediate use at in-house meetings, TCM’s or OCM’s.

Online context-sensitive help files guide you in program use and in parameter selection.

Below are details outlining the way in which some of the above parameters are handled. These represent the most commonly asked questions on program operation:

1. Optimisation

Optimisation is carried out to minimise the differences between computed and measured thicknesses by improving porosity estimates and assessments of lithology. Two non-linear equations are being solved for pressure and temperature but the parameters of these equations - hydraulic conductivity, storage coefficient, thermal conductivity and heat capacity are actually dependent on both temperature and pressure. To solve such equations several optimisation runs are required to compute the parameters as functions of the variables and then to calculate the new values of the variables. This process continues until the difference between the previous and current values of the variable is minimised. When there is a rapid change in the value of a variable over a short period of geological time, so that there is a great difference in the initial value and the final computed value of a variable (as in rapidly subsiding or eroding basins), a greater number of optimisation runs are needed. Four runs is the recommended number for normal basins.

 

2. Hydrocarbon Generation

The program can compute hydrocarbon generation using kinetic parameters supplied by the user. If a user defined kinetic file does not exist hydrocarbon generation is solved for Type II and Type III kerogens only. If a kinetic file does exist it solves for this also. A set of kinetic variables defined by mass fraction, activation energy and frequency factor can be input as well as the initial bitumen content for the kerogen and the cracking parameters - activation energy and frequency factor - for conversion of oil to gas.

 

3. High and Low Sedimentation Rates

The program is unique in its ability to handle basins with high and low sedimentation rates. Basins with high sedimentation rates are usually abnormally pressured. This results in fracturing and retardation of compaction as the pore fluids carry an increasing amount of the overburden weight. Retardation of compaction leads to lower thermal conductivities due to the high water content retained in the sediments and to higher geothermal gradients and thus heating rates. High heating rates may result in high hydrocarbon generation rates (provided an adequate amount of organic matter exists in the potential source rock) due to the high expulsion efficiencies for oil. In these basins generation of oil generally takes place at lower vitrinite reflectance values. The converse is true for basins with low sedimentation rates.

 

4. Heat Flow Models

The program uses heat flow as input so that the user may work on the validity of the tectonic model and attendant heat flow history responsible for the formation and evolution of the basin. The amount of heat entering a basin at the base of a sedimentary column correlates with crustal thickness and tectonic activity. In general basins have higher heat flows when tensional forces are in effect and lower heat flows during compressional movements. In TerraMod the heat flow history of a given basin is first determined qualitatively based on regional tectonic patterns. The history is then improved by optimising the model with calibration data.

Heat flow changes lag in their effect as they transmit through the basin and heat flow is only constant as a function of depth if the system is in steady state. If the system is in a transient state heat flow values may change with depth.

 

5. Pressure

The program is unique in the way it computes formation pressure. Other programs determine porosity as a function of depth but TerraMod computes porosity as a function of lithology, compressibility of the sediment, cementation, pressure and temperature. Thus it can calculate whether a sequence has been subjected to a normal, abnormal and/or subnormal pressure history.

 

6. Seal Efficiencies

These are computed as a function of capillary pressure and fracturing in addition to pore pressure. Capillary Pressure is computed as a function of pore size (this is a function of sediment type and compaction) and interfacial tension (this is a function of type of hydrocarbon generated, temperature and pressure).

 

Fracturing is computed as a function of pressure history and the tensile strength of the sediment.

The program calculates sealing efficiency of selected layers through time in terms of the oil and/or gas column height which can be retained below a seal.

7. Intrusion Models

A new intrusion model version is available as a modular upgrade, allowing the program to handle salt movement, including allochthonous salt emplacement and withdrawal, hot sill emplacement and shale movement. The user defines where the intrusion takes place (either as an "into.." or "out of.." event), its temperature and when the event occurred and the program will computes its parameter history and the parameter history of adjacent layers accordingly.

8. Thrust Models

Thrusting is handled by selecting thrust as the geological event. One needs to know the time at which the thrusting occurs to input the event at the correct layer number location, the length of time over which the thrusting event operated, its present day thickness and its gross lithology. For thrust sheets the user has to define the layer on which the thrust sits and make an estimate of its initial porosity. Thrust sheets always have a lower initial porosity than uncompacted sediments.

A new thrust model version is available as a modular upgrade, allowing more versatility in constructing a model. In the new version the thrust sheet may not only sit on a layer but can also thrust into existing layers (passive roof duplex model). Furthermore, instead of restricting models to a single thrust sheet with a single gross lithology the user may define up to 10 individual sheets with realistic lithological sequences.

9. Expulsion Efficiency

The program computes expulsion as the sum of three different processes:

Expulsion by pressure in the hydrocarbon phase, which leads to the movement of hydrocarbons (but mainly oil) as separate phases within and through the source rock. This is controlled by the amount of hydrocarbon generation through time. If the amount generated is high then the hydrocarbon phase will force itself through the system resulting in high expulsion rates.

Expulsion of hydrocarbons as a function of their saturation within the source rock. This is not an efficient method and the expulsion efficiencies are low.

Expulsion of hydrocarbons by fracturing of the source rock. The program computes formation pressures as a function of the stresses and loads and then estimates when the system will fail. It also computes tensile strength of the sediments as a function of time also indicating when fractures will form.

Some Advantages of TerraMod

        3D Model

TerraMod is derived from a full 3D model. The mathematics honour 3D principles within the 1D framework.

TerraMod is unique because it is a true basin modelling program. Ninety per cent of the effort and resources of TerraMod simulations go into rigorously defining the geohistory of the well, to define the complete petroleum system. TerraMod is not simply a charge modelling calculator.

TerraMod rigorously models depositional/subsidence history through detailed, iterative compaction routines. The algorithms relate compaction to differential stress and therefore also enable a detailed transient pressure history to be built up within the basin and the well.

TerraMod is specifically developed to model the geological processes operating in a sedimentary basin. The entire philosophy of the software is to enable an accurate and sophisticated geological model to be built up, and to test the ‘geological reasonableness’ of this model before any charge modelling is undertaken. The user is assisted in refining the geological model and through the optimisation process will develop a more detailed model for the geology and evolution of the basin and well being modelled.

Thrust systems and intrusions are geological events that are modelled by TerraMod, as well as the normal depositional/erosional events.

TerraMod models "unsteady state processes" in a rigorous simulation of geological processes. For instance, changes in heat flow, thermal conductivity, porosity, permeability and pressure all inter-dependently vary with time. They are thus "unsteady state." The model algorithms are built around this fundamental principle and therefore work internally by running multiple iterative numerical modelling simulations for the deposition of each layer.

Potential API oil density, and the timing of generation of different oils, can be interpreted from the output results. This is because TerraMod models hydrocarbon potential for individual kerogens of which oil density is a function.

The simulation is fully integrated - not modular - each parameter depends on all others. In particular, temperature and pressure are ‘fully coupled;’ the simulation could not be accurate without this. Further, if compaction, pressure, etc. are not simulated correctly then migration pathways cannot be assessed.

TerraMod’s intuitive work flow ensures that a minimum of initial training is needed and, perhaps more importantly, ensures that when coming back to the software after several months on other projects the user will not have to spend large amounts of time ‘re-learning’ the software.

The integrated database design (based on the widely supported Paradox database architecture) ensures that project and well information are rapidly retrievable in a format that are permanently ‘tied’ to individual wells and projects. Thus, in future years projects can be revisited by new teams who can immediately access project and well details and descriptive information about the modelling objectives and results that are built up during the modelling process. No more ‘re-inventing the wheel!’

Printed graphical output and pre-formatted report templates enable reports to be compiled to a professional standard directly by the user, without the necessity of going through the ‘drafting loop.’

Lithology, Maturity & Pressure:

The lithology parameters for the 70 pre-defined lithologies included within the internal TerraMod database - grain density, initial porosity, compressibility (as a function of porosity), thermal conductivity (as a function of porosity and temperature) and permeability - have been independently laboratory measured. They do not depend on artificial average mixes of pure end-members; for example, a salt with shale intercalations (30% shale) has a thermal conductivity 10% higher than the average of the percentages of each component, thus an average lithology would have an incorrect thermal conductivity.

Maturation and generation algorithms incorporate heating rate rigorously. This is important for two reasons:

a). The relationship between vitrinite reflectance (which is primarily temperature dependent) and hydrocarbon generation is not fixed. Hydrocarbon generation is dependent on heating rate as well as on temperature; for example, in the Vienna Basin peak oil generation occurs at a vitrinite reflectance equivalent of 1.25%Ro, whereas in the South Caspian Basin and Mahakam Delta it occurs at an equivalent of less than 0.7%Ro, and in the Paris Basin it occurs at 0.9%Ro equivalent.

b). Oil expulsion efficiency is partly a function of the pressure of oil which is a function of oil saturation, where for PVT analysis the pressure is heating rate dependent.

The full ‘coupled’ pressure history is calculated from the iterative simulation routines via an iterative loop that inter-dependantly calculates fluid densities, overburden pressure, temperature, porosity, permeability and capillary pressure. The detailed pressure history is crucial for obtaining an accurate compacted subsidence history. A by-product of the simulation is an identification of overpressured and/or underpressured layers; valuable for optimising drilling programmes and understanding expulsion.