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Advanced Carbonate Reservoir Characterisation

12 – 14 February 2013

Dubai, UAE | The Address Hotel, Dubai Marina

Technical Agenda

Keynote Session: Challenges, Characterisation, and The Way Forward

Session Chairs: Maher Marhoon, Saudi Aramco; Mohammed Badri, Schlumberger

Carbonate reservoirs hold sixty percent of world oil and forty percent of gas reserves. They are prolific reservoirs in terms of their production and flow properties. Although these reservoirs have been extensively characterised with the latest technologies and measurements, there are still major challenges to be addressed by professionals in the industry. Reservoir rock typing (RRT), porosity systems, permeability, mineralogy, saturation, and fracture characterisation have been extensively studied; however, there are still opportunities for developing new approaches based on cross multi-disciplinary skills. Effective data integration and upscaling of measurements require the development of new algorithms and paradigm shift in daily practices. Improving recovery factor in carbonates requires detailed understanding of flow prediction in carbonates in the presence of multi-phase fluids. This session will aim at reviewing current understanding of carbonate reservoirs and outstanding challenges that need to be addressed by industry professionals.

Session 1: Deposition, Diagenesis, and Stratigraphy

Session Chairs: Khalid Alramadan, KFUPM; Prasanta Mishra, Kuwait Oil Company

Carbonate rocks host more than half of the global hydrocarbon reserves and, therefore, have attracted research interest by industry professionals for a long time. It is known that the prediction of petrophysical properties of carbonates and assessment of multiphase flow properties in carbonates are very challenging due to complexity in the depositional processes and various diagenetic changes altering the primary and even the secondary fabric of the rocks. A wide range of the pore and particle sizes are seen, thus making the carbonate rocks heterogeneous and complex, and even in a core piece of 4x6 inches, a wide variation of various rock properties are noticed. Huge volumes of gas have been discovered in the fractured carbonates in the geological formations older than the Jurassic period contributing to the global reserve in the last decade.

Wealth of research has been carried out on the deposition and diagenesis of carbonates. A number of field studies are being taken up to demonstrate the present day carbonate sedimentation of Bahamas, Cocoas Island, and Great Barrier Reef. Each carbonate depositional realm has its own depositional and diagenetic history and unique to itself. It is therefore important to understand the processes that are responsible to bring about the pre and post depositional changes in the rocks. The technological innovations in the field of digital rock physical studies have to play a great role in characterising the tight and fractured carbonates. New micro imaging tools are required for improving the image quality of bore hole image logs taken in the oil-based muds in the deeper carbonate formations. Better seismic acquisition parameters are necessary for improving the quality of data in the sub-salt formations. The sequence stratigraphic studies originally developed for the clastic rocks have been extended to the carbonates but it is still very difficult to apply these concepts in altered, dolomitised, and fractured reservoir. In many of the rocks the microfossil occurrence is very rare, and the absence of key diagnostic taxa of foraminifera and nano fossils makes the task still difficult. The cores are the best source of information for this sequence analysis, substantiated by stable isotope analysis. The classic carbonate rocks of Oman mountains are a source of inspiration to carbonate geologists in the region and the extension of depositional sequences from outcrop to sub-crop clears the understanding of the researcher. Describing the fine scale sequences of third order or higher is still not a very easy task in carbonates without assimilating all the relevant petrographic, bio stratigraphic, isotopic, and well log inputs. Most significant application of the sequence stratigraphic studies in the carbonates would not just confine to the identification of sequences or para sequences but to integrate the findings with the reservoir models to locate the areas of bypassed or leftover oil. Similarly, the fracture network models must be calibrated with the dynamic reservoir models for a close look at the gas reserves. We have to understand carbonate rocks in detail, with the application of all available technologies so that the hydrocarbon reserves and production is sustained for a long time.

Session 2: Advanced Formation Evaluation

Session Chairs: Bruno Lalanne, Total; Mariam Al Saeed, Kuwait Oil Company

This session will deal with the most recent advances and successful history cases of formation evaluation workflows as applied to carbonate reservoirs. Carbonate reservoirs are highly heterogeneous, with trible porosity systems (matrix-vugsfracture) and complex wettability characteristics. These characteristics are key factors during the primary reservoir recovery stage and during redevelopment, or for secondary and tertiary operations stage.

The challenges of modern reservoir characterisation, particularly those pertinent to formation evaluation for advanced reservoir static and dynamic modelling are many, and very complex. Different pore geometries and the absence of clays are probably the most important causes that make up for the differences encountered in the interpretation of well logs for formation evaluation of clastics and carbonates. Carbonate reservoirs change due to different processes like cementation, compaction, dolomitisation, leaching, and dissolution. These processes never really stop in the reservoir. Resistivities measured by well logs in clastics will be lowered by the presence of clay minerals. The presence of these minerals will also make necessary custom made corrections for calculating an appropriate Sw and porosity. Low resistivity values might also be obtained in carbonate formations but for different reasons.

The results of considering the specific carbonatic characteristics will be discussed, involving FE special subroutines as applied to issues related to relevant synsedimentary and post-sedimentation diagenetic changes, fracture presence, irregular distribution of porosity and permeability, as well as the problems pertinent to the effects of rock electrical anisotropy on logging apparent resistivity measurements, all cases where the true water saturation, estimation of calcite, dolomite, anhydrite, heavy minerals, and porosity estimates are jeopardised if not properly handled with sound techniques and workflows, established for each reservoir case and well architecture, to take into consideration corrections due to evaluate the reservoir. Accurate estimation of residual oil saturation is highly crucial. It is the key parameter to predict the recoverable oil reserve and to determine the optimal oil-recovery process. In the petroleum industry, coring, logging, pressure transient tests, chemical tracers, and reservoir engineering studies are techniques available today to determine the remaining oil saturation.

Each of these methods has its own benefits and limitations. The remainder of this session will discuss recent advances and best practices available today to determine the residual oil saturation.

Session 3: Quantitative Reservoir Characterisations

Session Chairs: Adrian Immenhauser, Institute for Geology, Mineralogy and Geophysics; Maher Mahoon, Saudi Aramco

Reservoir characterisation is highly dependent on seismic data in addition to other data types such as petrophysical data to define the structural and the stratigraphic frameworks. To utilise the strength of the seismic data which is of high lateral resolution, they have to be integrated in the reservoir characterisation workflow quantitatively. However, there are many challenges in using seismic data to quantitatively characterise reservoirs. Some of those challenges are related to the resolution of the seismic data and others are related to the noise level on them. Traditionally, seismic data were used to identify bright spots or flat spots only. And currently, there are many initiatives to use seismic data to quantitatively derive some reservoir properties such as porosity, rock types, and pore pressure. Complex seismic trace attributes were introduced to the industry in the 1970s with the aim to get more out of the seismic data other than the qualitative interpretation. Since that time, hundreds of seismic attributes were emerged using different ways of computing. The reason for all of those initiatives was to use the attributes as an input in the reservoir characterisation workflow. However, till date, there is no clear workflow to integrate seismic data attributes quantitatively in the reservoir characterisation. Attributes have to be fully understood and related to reservoir properties to be able to integrate it with the different types of data. This session will focus primarily on approaches and methodologies to use seismic data quantitatively to derive reservoir properties. This estimation of reservoir properties will help geoscientist characterize the reservoirs in a more robust way and come up with an understanding that is closer to reservoir properties.

Session 4: Rock Physics

Session Chair: Aiman Bakhorji, Saudi Aramco; Robert Kuchinski, Weatherford

Most research on the effect of porosity, pore type, saturation, mineralogy, and pressure on rock elastic properties has been performed on siliciclastic rocks. The textures and pore structures of such rocks differ substantially from most carbonates. As such, the predictive models used to describe the behaviour of siliciclastic material are generally not applicable to carbonates. Carbonate rock composition derives from a combination of biological and chemical components. This adds complexity and heterogeneity to its structure and petrophysical properties. These complexities and heterogeneities make the studies to understand the effects of porosity, pore type, saturation, mineralogy, and pressure on the elastic properties a challenge. With respect to petroleum geosciences, a general goal of applied geophysicists is to be able to improve the predictive capability of surface seismic observations of the conditions and characteristics of carbonate reservoirs. This session will help understand factors (i.e. pore fluid, porosity, pore type) that influence elastic properties in carbonate rocks.

Session 5: Digital Rocks

Session Chairs: Jonathan Clive Hall, ADCO; Kate Gibbons, Baker Hughes

This session will deal with recent advances in the application of digital rock physics (DRP). DRP technology is reaching a consolidation phase in its development, both in terms of vendors and technologies available worldwide, and in terms of the confidence growing in the results obtained. DRP has been heralded as the quick and cheap replacement to some laboratory SCAL measurements, especially in carbonates with complex pore morphologies and strong heterogeneity. This session will explore to what extent this promise has been realised. The challenges and limitations of the acquisition and interpretation technologies will be reviewed. Novel applications are being devised with before and after treatment formation damage studies benefiting particularly from the technique and set to become a standard part of the workflow for such studies. Also, the study of wormholing and pore morphology changes following CO2 flooding in core monitor wells is illuminating. Pore micro-structures cannot be captured in larger volume samples due to resolution issues and this requires those measurements to be made at various scales. Upscaling/downscaling poses challenges and may be costly in the after measurement modelling required. This may challenge one of the promises of a cheap and quick alternative to SCAL. How do we apply these measurements in uncored wells? Should we rely upon collaborative catalogues of analogue rock samples and should we propose these now? Lastly, some novel mathematical approaches developed in the region using textural classification may unlock parameters required to apply effective media models for electrical, dielectric, acoustic (rock physics applications), fluid flow, and NMR transport properties.

Session 6: Geochemistry for Carbonates

Session Chairs: Giles Sermondadaz, Total; Moustafa Oraby, Halliburton

This session will focus on applied carbonate geochemistry. Main fields of discussion include carbonate reservoir diagenesis, source rock (reservoir geochemistry), and rock-fluid interaction

Porosity and permeability in carbonate reservoirs are driven by three main players. First, the depositional architecture and facies of the carbonate body under question (ramp versus rimmed platform); second, the diagenetic reactivity of the carbonates deposited (aragonite versus calcite versus magnesian calcite); third, the subsequent diagenetic history (burial rate and depth, dolomitising fluids, etc.). Seawater geochemistry (aragonite versus calcite seas) has a direct impact of the main carbonate mineralogy/geochemistry at deposition. Subsequently, the geochemistry of both, the carbonate solid and the pore fluid is in constant interaction. Carbonate geochemistry and crystallography defines the stability of a specific carbonate phase. Diagenetic stability defines the response of a given carbonate reservoir to under or oversaturated fluids and hence the creation and destruction of porosity and permeability. Diagenesis has a clear influence on the mechanic behaviour of carbonate bodies and controls, to a significant extent, the fracturing of reservoirs. A second topic deals with source rock (mostly organic) geochemistry, a field of intense research. Organic geochemistry sheds light on the source and type of hydrocarbons and the thermal history. Carbonate aggressive fluids from organic rocks may create extensive hypogenic karst features in the subsurface. Fluid-rock interaction is a research field that is approached both experimentally as well as numerically. Increasingly complex fluid flow models move from 2D to 3D models and acknowledge more realistic rock properties. In summary, this session will deal with a wide range of topics all related to state-of-the-art geochemical research in carbonate reservoir analysis.

Session 7: Data Integration and Reservoir Characterisation

Session Chairs: Aiman Bakhorji, Saudi Aramco; Mahmood Akbar, Schlumberger

Data integration has long been recognised by the industry as key to understanding of reservoir behaviour and planning future operational requirements. Whilst careful acquisition and evaluation of data is critical to the characterisation of various aspects of carbonate reservoirs, our ability to describe and understand the overall behaviour of the reservoir is enhanced by integrating data sets. The complexity of carbonate reservoirs presents distinct challenges—they are highly heterogeneous at a variety of scales from pore geometry, facies distribution through to structural framework. Consequently, it is important to develop robust workflows that respect the uncertainty and scale of the data in order to construct static and dynamic reservoir models. This session will review current industry work flows that integrate data from seismic through to simulation. Presentations will address the challenges faced in integrating data, understanding of uncertainty, dealing with multiple scales, and the importance of integrating dynamic with static data.

Session 8: From Characterisation to Simulation

Session Chairs: Adrian Immenhauser, Institute for Geology, Mineralogy and Geophysics; Robert Kuchinski, Weatherford

Upscaling static reservoir models for fluid flow simulation is generally an exercise that requires a very close focus. This is even more so in heterogeneous carbonate systems, where multiple levels of heterogeneity are at play, and interact with each other. Capturing these interactions properly and effectively into the dynamic simulation model is key to a robust subsurface definition, capable of replicating (or at least approximating) the physics of flow and recovery from these carbonate systems. Parameters of influence exist at multiple levels and scales, and require effective capture in both the static and dynamic domains to ensure simulation models are realistic in their ability to represent current reservoir behaviour and forecast future reservoir performance. However, significant challenges remain today, in effectively translating carbonate reservoir parameters in the static domain, to the dynamic simulation domain. Pore scale parameters such as pore throat size, tortuosity, and secondary porosity, are still only vaguely handled today. Larger scale parameters such as reservoir rock types, permeability, fracture density, fracture width, aperture, etc. although well recognised, pose additional challenges to be upscaled properly, and handled efficiently by today’s reservoir simulators. The presentations should touch upon these and some other challenges currently faced by the industry in realising appropriate translation of these carbonate reservoir parameters from the static to the dynamic model domains (i.e. the upscaling question), while proffering certain food-for-thought questions on areas for possible improvement and additional research. Can we aim for non-upscaling in the future? Is this worthwhile? What are the barriers and current limitations to achieving this?

Session 9: Advances in Reservoir Management and Recovery (Characterisation)

Session Chairs: Mariam Al Saeed, Kuwait Oil Company; Mohammed Badri, Schlumberger

Dynamic reservoir models require the input of vertical permeability, horizontal permeability, porosity, saturation, and pressure. As more measurements are obtained and reservoir models become more complicated, dynamic models are now constructed in millions of cells to capture reservoir variation. History matching is aimed at converging reservoir models with input measurements. Understanding of fluid flow at all scales in carbonate is critical due to their heterogeneity both vertically and horizontally. Fluid displacement and sweep efficiency demand simulation modelling and verification. Furthermore, reservoir models are aimed at providing production forecast requiring time-lapse monitoring of residual oil saturation, pressure, temperature, and fluid fractions. Reducing risk and uncertainty in dynamic reservoir models are of paramount importance to ensure accuracy in reservoir management.