Susan M. Agar

Outcrop analog for an oolitic carbonate ramp reservoir: A scale-dependent geologic modeling approach based on stratigraphic hierarchy

Considerable effort has been devoted to the development of simulation algorithms for facies modeling, whereas a discussion of how to combine those techniques has not existed. The integration of multiple geologic data into a three-dimensional model, which requires the combination of simulation techniques, is yet a current challenge for reservoir modeling. This article presents a thought process that guides the acquisition and modeling of geologic data at various scales. Our work is based on outcrop data collected from a Jurassic carbonate ramp located in the High Atlas mountain range of Morocco. The study window is 1 km (0.6 mi) wide and 100 m (328.1 ft) thick. We describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest: (1) stacking pattern of high-frequency depositional sequences, (2) facies association, and (3) lithofacies. Five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data. The surface-based model shows a low-angle profile with modest paleotopographic relief at the inner-to-middle ramp transition. Facies associations were populated using truncated Gaussian simulation to preserve ordered trends between the inner, middle, and outer ramps. At the lithofacies scale, field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation. This study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks. The selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings. This study demonstrates that a scale-dependent modeling approach should be a common procedure when building subsurface and outcrop models.

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

Abstract The introduction reviews topics relevant to the fundamental controls on fluid flow in carbonate reservoirs and to the prediction of reservoir performance. The review provides research and industry contexts for papers in this volume only. A discussion of global context and frameworks emphasizes the value yet to be captured from compare and contrast studies. Multidisciplinary efforts highlight the importance of greater integration of sedimentology, diagenesis and structural geology. Developments in analytical and experimental methods, stimulated by advances in the materials sciences, support new insights into fundamental (pore-scale) processes in carbonate rocks. Subsurface imaging methods relevant to the delineation of heterogeneities in carbonates highlight techniques that serve to decrease the gap between seismically resolvable features and well-scale measurements. Methods to fuse geological information across scales are advancing through multiscale integration and proxies. A surge in computational power over the last two decades has been accompanied by developments in computational methods and algorithms. Developments related to visualization and data interaction support stronger geoscience–engineering collaborations. High-resolution and real-time monitoring of the subsurface are driving novel sensing capabilities and growing interest in data mining and analytics. Together, these offer an exciting opportunity to learn more about the fundamental fluid-flow processes in carbonate reservoirs at the interwell scale.

Numerical simulation of fluid-flow processes in a 3D high-resolution carbonate reservoir analogue

A high-resolution three-dimensional (3D) outcrop model of a Jurassic carbonate ramp was used in order to perform a series of detailed and systematic flow simulations. The aim of this study was to test the impact of small- and large-scale geological features on reservoir performance and oil recovery. The digital outcrop model contains a wide range of sedimentological, diagenetic and structural features, including discontinuity surfaces, shoal bodies, mud mounds, oyster bioherms and fractures. Flow simulations are performed for numerical well testing and secondary oil recovery. Numerical well testing enables synthetic but systematic pressure responses to be generated for different geological features observed in the outcrops. This allows us to assess and rank the relative impact of specific geological features on reservoir performance. The outcome documents that, owing to the realistic representation of matrix heterogeneity, most diagenetic and structural features cannot be linked to a unique pressure signature. Instead, reservoir performance is controlled by subseismic faults and oyster bioherms acting as thief zones. Numerical simulations of secondary recovery processes reveal strong channelling of fluid flow into high-permeability layers as the primary control for oil recovery. However, appropriate reservoir-engineering solutions, such as optimizing well placement and injection fluid, can reduce channelling and increase oil recovery.

The Impact of Hierarchical Fracture Networks on Flow Partitioning in Carbonate Reservoirs: Examples Based on a Jurassic Carbonate Ramp Analog from the High Atlas

Hydrocarbon reservoirs commonly contain an array of fine-scale structures that are below the resolution of seismic images. These features may impact flow behavior and recovery, but their specific impacts may be obscured by the upscaling process for sector and field-scale reservoir simulations. It is therefore important to identify those situations in which subseismic structures can introduce significant departures from full-field flow predictions. Using exposures of Jurassic carbonate outcrops near the village of Amellago in the High Atlas Mountains of Morocco, we have developed a series of flow simulations to explore the interactions of a hierarchical fracture network with the rock matrix of carbonate ramp strata. Model geometries were constructed in CAD software using field interpretations and LiDAR1 data of an outcrop area that is 350 m long by 100 m high. The impact of water injection on oil recovery between an injector and producer pair was investigated. Simulations were performed by a single medium reservoir simulator using a single mesh to represent fracture planes as well as rock-matrix volumes. The effects of changing scenarios for rock permeability and porosity as well as facture permeability distributions were investigated. First-order results show that the best recovery was achieved by a model with a high permeability, homogeneous matrix combined with a heterogeneous fracture network. The worst recovery scenario was given by a model with low, homogeneous permeability and high fracture permeabilities. The results highlight the importance of the permeability contrasts between the matrix and the fractures for overall recovery and the very significant impact that fractures can have on recovery by creating shadow zones and providing critical connections between permeable layers. The presence of the hierarchical fracture network developed strong fingering even in homogeneous matrix cases and evolving velocity patterns reveal competing fluid pathways among matrix and fracture routes. Insights from these models can help to develop production strategies to improve recovery from fractured carbonate reservoirs and provide an initial platform from which to extend further evaluations of different populations of conductive and baffling structures, spatial variations in wettability and capillary pressures and well positions.

Modelling and simulation of a Jurassic carbonate ramp outcrop, Amellago, High Atlas Mountains, Morocco

Carbonate reservoirs pose significant challenges for reservoir modelling and flow prediction due to heterogeneities in rock properties, limits to seismic resolution and limited constraints on subsurface data. Hence, a systematic and streamlined approach is needed to construct geological models and to quickly evaluate key sensitivities in the flow models. This paper discusses results from a reservoir analogue study of a Middle Jurassic carbonate ramp in the High Atlas Mountains of Morocco that has stratigraphic and structural similarities to selected Middle East reservoirs. For this purpose, high-resolution geological models were constructed from the integration of sedimentological, diagenetic and structural studies in the area. The models are approximately 1200×1250 m in size, and only faults (no fractures) with offsets greater than 1 m are included. Novel methods have been applied to test the response of flow simulations to the presence or absence of specific geological features, including proxies for hardgrounds, stylolites, patch reefs, and mollusc banks, as a way to guide the level of detail that is suitable for modelling objectives. Our general conclusion from the study is that the continuity of any geological feature with extreme permeability (high or low) has the most significant impact on flow.

Fracture analysis in the Amellago outcrop and permeability predictions

Field observations from several outcrops in the Eastern High Atlas Mountains, near Amellago (Morocco), are used to determine fracture-network model parameters, such as the aspect ratio of the fractures represented as rectangles whose longer side is horizontal, the volumetric area of fracture surfaces, the fracture mean size and the fracture density. The fracture orientations can be roughly approximated by Fisher distributions, where the parameters are determined by outcrop measurements. The permeability of the fracture networks can be calculated by application of the Snow equation for infinite fractures or by numerical resolution of the flow equation for fracture networks generated with the parameters deduced from the outcrop measurements. These two permeability estimations are shown to be in good agreement, which suggests that theoretical or semi-empirical solutions may provide reasonable approximations of fracture-network permeability in some carbonate reservoirs when conditioned to appropriate outcrop and subsurface data.

Three-dimensional Printing for Geoscience: Fundamental Research, Education, and Applications for the Petroleum Industry

Three-dimensional (3-D) printing provides a fast, cost-effective way to produce and replicate complicated designs with minimal flaws and little material waste. Early use of 3-D printing for engineering applications in the petroleum industry has stimulated further adoption by geoscience researchers and educators. Recent progress in geoscience is signified by capabilities that translate digital rock models into 3-D printed rock proxies. With a variety of material and geometric scaling options, 3-D printing of near-identical rock proxies provides a method to conduct repeatable laboratory experiments without destroying natural rock samples. Rock proxy experiments can potentially validate numerical simulations and complement existing laboratory measurements on changes of rock properties over geologic time scales. A review of published research from academic, government, and industry contributions indicates a growing community of rock proxy experimentalists. Three-dimensional printing techniques are being applied to fundamental research in the areas of multiphase fluid flow and reactive transport, geomechanics, physical properties, geomorphology, and paleontology. Further opportunities for geoscience research are discussed. Applications in education include teaching models of terrains, fossils, and crystals. The integration of digital data sets with 3-D printed geomorphologies supports communication for both societal and technical objectives. Broad benefits that could be realized from centralized 3-D printing facilities are also discussed.

Constitutive Relations for Reactive Transport Modeling: Effects of Chemical Reactions on Multi-phase Flow Properties

The relationship between flow properties and chemical reactions is the key to modeling subsurface reactive transport. This study develops closed-form equations to describe the effects of mineral precipitation and dissolution on multi-phase flow properties (capillary pressure and relative permeabilities) of porous media. The model accounts for the fact that precipitation/dissolution only takes place in the water-filled part of pore space. The capillary tube concept was used to connect pore-scale changes to macroscopic hydraulic properties. Precipitation/dissolution induces changes in the pore radii of water-filled pores and consequently in the pore size distribution. The updated pore size distribution is converted back to a new capillary pressure–water saturation relation from which the new relative permeabilities are calculated. Pore network modeling is conducted on a Berea sandstone to validate the new continuum-scale relations. The pore network modeling results are satisfactorily predicted by the new closed-form equations.

Fundamental controls on flow in carbonates: an introduction

The challenges posed by carbonate reservoirs today have been recognized for decades. ‘Old’ problems include the prediction of heterogeneous carbonate rock properties in the subsurface, the variable wettability of carbonate rocks, and the uneven sweep and early water breakthrough that commonly result from the presence of fractures, karst and/or other heterogeneities providing high-permeability flow paths (Bulnes & Fitting 1945; Craze 1950). While recent shifts in the industry have diverted attention to unconventional resources, conventional carbonate reservoirs remain the largest class of oil resources for the planet. Their recoveries, however, remain relatively low overall (Montaron 2008; Burchette 2012). While new technologies offer a deeper understanding of near-wellbore properties and processes, there remain large uncertainties in the distributions of flow velocities and the nature of flow paths between wells. These uncertainties arise from our inability to delineate geological features over a wide range of length scales, to represent them appropriately in flow simulations, to quantify their impacts on flow, as well as their impacts on evolving multiphase fluid distributions. While we wrestle with suitable proxies for geological heterogeneities that cannot be deterministically resolved in subsurface data or explicitly represented in field-scale reservoir models, many in the industry are forced to rely on long-established simulation technologies that have not kept pace with developments in modelling. In the absence of fully deterministic characterization and representation of the reservoir in our flow-simulation models, we need to mitigate the risks associated with uncertainty. Experiments designed to improve our ability to predict in the subsurface and to explore the range of possible responses can help. Clearly, there is an opportunity to pursue carefully designed subsurface experiments to test hypotheses for flow behaviour. Shallow subsurface experiments have already shown that, even between very closely spaced wells, it can be extremely difficult to define precise flow paths …

Summary of the AAPG–SPE–SEG Hedberg Research Conference on “Fundamental Controls on Flow in Carbonates”

A joint AAPG–Society of Petroleum Engineers–Society of Exploration Geophysicists Hedberg Research Conference was held in Saint-Cyr sur Mer, France, on July 8 to 13, 2012, to review current research and explore future research directions related to improved production from carbonate reservoirs. Eighty-seven scientists from academia and industry (split roughly equally) attended for five days. A primary objective for the conference was to explore novel connections among different disciplines (primarily within geoscience and reservoir engineering) as a way to define new research opportunities. Research areas represented included carbonate sedimentology and stratigraphy, structural geology, geomechanics, hydrology, reactive transport modeling, seismic imaging (including four-dimensional seismic, tomography, and seismic forward modeling), geologic modeling and forward modeling of geologic processes, petrophysics, statistical methods, numerical methods for simulation, reservoir engineering, pore-scale processes, in-situ flow experiments (e.g., x-ray computed tomography), visualization, and methods for data interaction. The conference was organized into four thematic sessions on the first two days (fundamentals, measurement and detection of flow on laboratory to field scales, uncertainty and prediction, and novel modeling and simulation techniques); a field trip on the third day was preceded by a dedicated poster session that introduced the geology of the area, whereas the ice breaker featured guest lectures on innovation and complex adaptive leadership, as well as a panel discussion. Given the challenge of cross-disciplinary communication, delegates were encouraged to adopt a beginners mind, challenging the status quo and exploring basic questions that the establishment might have overlooked. Stepping back and slowing down to promote effective conversations among different disciplines was emphasized upfront. Several delegates noted that technical jargon was a significant barrier to novel thinking in the way that it impeded effective communication among disciplines during the meeting. Cross-disciplinary interactions were encouraged by several further mechanisms, representing a shift from more common Hedberg Conference formats. Overall, the …