Using A Unified Software System To Reduce Exploration Risk Mark Goodwill and Ian Bryant, Schlumberger Information Solutions, 3 Nov 2010

Exploration is risky. Successful exploration involves understanding the risks involved, ranking plays and prospects, managing your drillable inventory and improving your success ratio.

Exploration risks occur both above and below ground. Subsurface risk is often managed by evaluation of the petroleum system as a whole and assigning risk to the individual elements of the system: trap, reservoir, charge and seal. A successful well will require that all four elements have worked together. For example, it is clear that the timing of charge needs to be coincident with or post-date formation of a sealed trap.

These risk elements are best evaluated together using a systems approach, unifying all the appropriate exploration data into a single, easy-to-use software environment, for example, centred around Petrel software. Using a single, unified software system within teams and within companies can help preserve knowledge from play to production and supports thorough, model-centric evaluations of exploration risk elements - trap, reservoir, charge, and seal (Figure 1).

Beyond that, the software system needs to be extensible. No software vendor can provide all the tools that an explorationist will choose to use, so it needs to be as easy and as practical as possible to add additional noncore pieces onto the main software platform. The development of software plugins, for example, using the Ocean software development framework, enables the software system to be extended for regional or play-specific exploration challenges.

In exploration, the datasets are diverse; sometimes the data are very sparse, whereas in other areas the datasets can be massive, with thousands of km of regional 2D seismic lines, merged regional 3D seismic surveys, basin-wide interpreted surfaces and large digital elevation datasets (Figure 2). To make effective use of these massive datasets, the explorationist can now use a software system that is well adapted to modern 64-bit PC hardware and which is sufficiently scalable for prospects to be evaluated within the context of the regional plays. This enables the framework established by regional work to apply consistently to all prospects, facilitating comparison of prospects when assembling a drillable inventory.

Additionally, the creativity of the explorationist should not be constrained by the software. The software system should make it as easy to use hand-contoured maps, outcrop photos or satellite images, as it is to use seismic or well data, or indeed to use gravity, magnetic or electromagnetic data, or any other relevant exploration dataset that the explorationist wishes to incorporate into the play and prospect delineation.

Systems approach to exploration software

A recent survey across 20 of the world's largest exploration companies revealed some interesting statistics about the most common reasons for exploration dry holes (Figure 3). Many more dry holes are attributed to seal and charge failures than to problems with trap definition or reservoir presence.

It may be no coincidence that the great majority of the existing exploration software effort has historically been concentrated on defining trap and reservoir. Exploration software companies have provided good tools for interpreting and modelling trap and reservoir, and oil company explorationists have become efficient users of the available software. Additionally, improvements in seismic acquisition and processing have played a key role in understanding these trap and reservoir elements.

On the other hand, software tools to assess charge and seal have typically been for the expert, full of complex functionality, difficult to use and hard to integrate into the prospect generation workflows.

A systems approach is therefore called for - to access all four of the key risk elements of the petroleum system from basin to play to prospect within a single exploration system software environment.

Trap

Definition of trap geometries often requires a good understanding of a play's structural framework - consolidating all the interpreted horizons and faults into a consistent interpretation.

In the past, creating and interpolating reliable fault interpretations across a series of 2D seismic lines was time-consuming, and the seismic interpreter generally experienced great difficulty in tracking and correlating fault relationships. However, building a structural framework 'on the fly' as an integral part of the seismic interpretation process allows for the construction of clearer horizon and fault relationships, yielding high-quality maps and fault polygons and a more confident understanding of trap geometry.

An appreciation of the uncertainty in the structural definition and the relationship with other parameters such as the velocity model is also an important factor in reducing the risks associated with the trap. Software platforms such as Petrel have uncertainty analysis tools fully integrated into all key steps of the workflows. It is important that these are considered an innate part of the process available to everyone and not just exclusively to the expert user.

Structural reconstruction is another critical task in the understanding of complex traps.

However, in almost all existing software systems, palinspastic work tends to be isolated from the seismic interpretation process, and the geomechanical model is frequently also isolated from the reconstruction. None of this helps with iterating through different interpretations or updating as new data becomes available. A truly integrated exploration software system has both geomechanics and structural reconstruction in the same platform as that used by the seismic interpreter.

Reservoir

The reservoirs being sought today are increasingly difficult to characterise, be it deep-water turbidites, fractured carbonates, deep gas reservoirs or coalbed methane. Software tools are, however, catching up with these plays. Geobodies identified on seismic data can be automatically extracted and integrated into the geological interpretation in a couple of mouse clicks, exploration prospects in fractured reservoir plays can be modelled using curvature attributes in seismic attribute libraries, and vintage well data can be integrated with pressure and core data in next generation petrophysics software such as Techlog.

As the industry explores new basins and unconventional resource plays, it is important that the software system is designed to be extensible so that novel plugins for new, niche or evolving workflows can be easily developed and deployed so that exploration companies can differentiate like never before. The Ocean software development framework is one such system.

Charge

Lack of reservoir charge accounts for the failure of approximately 30% of exploration wells. However, in the software systems currently deployed in exploration departments, charge analysis is rarely integrated into the main software workflows - it is often the preserve of domain specialists, sometimes working in isolation from the explorationists responsible for evaluating trap and reservoir elements.

A software system with a module for quick-look petroleum systems modelling would allow geoscientists to test fill and spill scenarios as an innate part of the prospect evaluation workflow. An easy-to-use quick-look tool is valuable in scouting evaluations and ranking plays and prospects. Overall, the aim is improved understanding of the risk of charge failure by making charge modelling more of an everyday task.

Crucially, the role of the basin modelling expert is still maintained - the discipline of full charge risk modelling using the advanced functionality of software tools such as PetroMod becomes more compatible with the everyday work of the exploration finding teams.

Seal

Seal is potentially the largest point of failure in exploration wells. Improved technology for evaluating seal integrity over geological time is needed, complementing charge modelling with structural restoration.

Top seal integrity - and the corresponding risk of hydraulic failure - is controlled by factors such as fluid densities, pressures and column height, all factors that can be modelled by a quick-look petroleum systems module integrated with the main exploration software system.

Many fault-dependent closures are not drilled since they are considered too risky. Therefore, software tools that help evaluate fault seal can be very valuable to convert these closures into drillable prospects. An ideal exploration software system would integrate fault seal analysis and structural restoration into the same unified environment used for defining trap, reservoir and charge, making the mapping of fault juxtaposition, fault geometry and property modelling as innate as picking seismic horizons and well tops.

Case study

An integrated evaluation of the exploration potential of the offshore areas of the Potiguar and Ceará basins of Brazil provides an example of the benefits of this new software system approach.

Fourteen thousand km of 2D multiclient seismic data was interpreted, largely in the unexplored deeper-water portions of these basins off the northern coast of Brazil (Figure 4). Depth conversion control was provided by tying in seven wells from the shallow shelf and a 3D Petrel geological model of both basins was constructed for eight key horizons. This geological framework was then combined with geochemical data and heat flux data to build a 3D petroleum systems model (Figure 5)

The petroleum systems modelling indicated that over 1,700 B bbl of oil have been generated from the multiple source rocks present in these basins, and the models suggest that approximately 40 B bbl remains in known and yet-to-be discovered accumulations. Multiple model runs were made to assess ranges of uncertainty in the resource volumes generated, and each run of the model resulted in the generation of a number of modelled accumulations and their evolution through geological time (Figure 6).

Crucially, as part of the systems approach to understanding and managing exploration risks, the present-day modelled hydrocarbon accumulations were always viewed and interpreted in the same software system as the seismic and well data. The interpretation of trap and the modelling of source were treated together throughout.

The understanding of the risk of charged reservoir presence was further improved by using a software plugin to build a resistivity prediction of each of the major sequences prior to the acquisition of 3D controlled source electromagnetic (CSEM) survey data. The modelled detectability of the resistive bodies embedded in this stratigraphy was used to tune the survey acquisition parameters for optimal illumination with minimised costs.

The five 3D CSEM surveys were acquired in June 2009 and model-based inversion was performed with the same software plugin and displayed within the same software system used throughout the evaluation process (Figure 7).

The subsurface resistivity anomalies detected by the 3D CSEM survey in the basin fill were interpreted to be either hydrocarbon accumulations or igneous rocks. To help distinguish between these alternatives, the results were integrated and viewed together with the results of the charge risk modelling (Figure 8). The modelled hydrocarbon accumulation (green) is co-located with a resistivity anomaly (yellow) at a point where three surface oil seeps (black dots) had been interpreted from satellite data.

Using a unified software system approach - considering trap, reservoir, charge and risk elements within one platform - has substantially improved the drillable prospect inventory of the basin.

Conclusions

Since the aim is to make substantial improvements to the understanding of the four key exploration risk elements, and in particular how each factor influences the others, then stand-alone software applications can no longer be considered appropriate. Failures of source or seal contribute to more than 50% of disappointing exploration well outcomes, so it is particularly important that analysis of these elements becomes innate for explorationists. A unified software system is the only solution appropriate for the holistic consideration of trap, reservoir, source and seal.

The choice of an exploration software system is a bet for the long run - does it allow your explorationists to innovate and to reduce the risk of their prospects?

Notes

Petrel, Techlog Ocean and PetroMod are marks of Schlumberger. 

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