Ulisses T. Mello

A physical explanation for the positioning of the depth to the top of overpressure in shale‐dominated sequences in the Gulf Coast basin, United States

A one-dimensional model of fluid pressure and porosity evolution is used to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale-dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a “bottleneck” and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, overpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the overpressured shales, compaction propagates downward through the section, releasing fluids from the upper part of the section while continuing to restrain the upward flow of fluids from deeper within the section. As such, overpressures are predicted to be maintained within the deeper regions of a basin for tens to hundreds of millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature-dependent fluid viscosity guarantees that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis of our results, we find that the depth at which the porosity equals 14±4% correlates with the depth to the local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret that the 14±4% represents the threshold porosity for which a shale actually begins to act as a seal. Within the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alternating series of sandstones and shales that grade vertically into massive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandstones play little role in the development of overpressure because of their high permeability except to the extent that the base of the massive sandstones marks the minimum depth possible for the top of overpressure. In contrast, overpressuring is observed to develop within either the shale-dominated sequence or the region of interspersed/interfingering sands and clays. The clay-encompassed sands play only a passive role in the development and maintenance of overpressure because it is the low-permeability clays that control the movement of fluids into and out of the sands.

Role of salt in restraining the maturation of subsalt source rocks

Abstract The presence of salt within a sedimentary basin can potentially modify its temperature distribution and history. In particular, the positive thermal anomaly associated with the top of salt domes has attracted considerable attention in the past. The role of the lesser appreciated negative thermal anomaly associated with the base of salt domes in modifying the maturation history of subsalt source rocks is explored. The finite element method was used to model the transient and steady-state conductive temperature perturbations induced by salt layers, domes and pillows. The results indicate that the modification of the thermal regime due to evolving salt domes may significantly affect the maturation level of source rocks in the vicinity of the domes. Modelling the temperature structure of various salt structures has shown that, in general, the refraction of heat flow induces a dipole-shaped temperature anomaly; a positive anomaly located towards the top of the salt structure and a negative anomaly located towards its base. These dipole anomalies can be strongly asymmetrical, the degree of asymmetry depending on the shape of the salt structure and the proximity of the top of the salt structure to the surface of the basin. However, when the salt structure reaches the surface, the dipole-shaped temperature anomaly collapses to a monopole. Below the salt structure, all sediments, independent of their depth and lithology, are colder relative to a section with no salt. Similarly, salt domes that reach the surface very efficiently drain the heat from below and from the side of the dome. These negative thermal anomalies may extend for a considerable depth beneath the base of the salt dome and may reach values of −85°C locally. Because of the large contrast in thermal conductivity between the highly porous sediments and salt at lower temperatures, the efficiency of a salt dome to channel heat increases the closer the salt dome is to the surface. These results indicate that deep sedimentary basins containing salt are more prospective than basins without salt and/or salt diapirism. In addition to the structural traps associated with salt tectonics, salt domes and tongues connected to their source dissipate heat more efficiently and thus keep deeper regions of the basin relatively colder and potentially within the oil window for a longer time. This cooling effect is maximized when the top of the salt dome remains close to the surface of the sedimentary basin for a significant period of time and may be especially important for continental margins such as Brazil and offshore West Africa, where most of the source rocks lie beneath extensive evaporite deposits. In contrast, it was found that for the Gulf of Mexico basin, pre- and Early Tertiary salt diapirism and sheet emplacement may have caused significant delays in the maturation of subsalt source rocks in the deeper regions of the Gulf basin, but the maturation is likely to be relatively insensitive to the Late Miocene-Pliocene stage of salt mobilization because the time interval has been too short (

Regional slope stability and slope-failure mechanics from the two-dimensional state of stress in an infinite slope

Abstract Rapid estimates of regional submarine slope stability can be obtained using 1-D infinite-slope analysis or empirical 2-D analyses, such as the log-spiral or φ-circle methods. In these methods, slope stability is evaluated along a pre-defined slip surface because the principal stresses in the slope and the slip-plane directions they control are undefined. However, where these pre-defined slip surfaces are not a good approximation of the surface along which a slope failure actually occurs, the analyses cannot explain the physics and observed geometry of the failure. Here we present an alternative, 2-D analytical solution for the state of stress in an infinite slope that incorporates cohesion and constant pore pressure, and yields the principal stresses and possible slip-plane directions along which the slope can fail. As a result, the analysis provides a framework for understanding the general geometry and relative motion of mass movements not addressed by 1-D infinite-slope analysis or the empirical 2-D analyses. We use our 2-D infinite-slope analysis to show that if the compressive stresses in the lower part of a slope are great enough, slope failure will occur along a basal plane, which in turn will permit extensional deformation along a steeper, headwall plane farther upslope. We then discuss how such failure can be facilitated on slopes of low inclination by excess pore pressure. Based on this discussion, we suggest that if pore pressure becomes high enough, slope failure can be initiated at a lower pore pressure and along a lower-angle basal plane than predicted by 1-D infinite-slope analysis.

Uncertainty in Thermal Basin Modeling: An Interval Finite Element Approach

Uncertainty assessment in basin modeling and reservoir characterization is traditionally treated by geostatistical methods which are normally based on stochastic probabilistic approaches. In this paper, we present an alternative approach which is based on interval arithmetic. Here, we discuss a fnite element formulation which uses interval numbers rather than real numbers to solve the transient heat conduction in sedimentary basins. For this purpose, a novel formulation was developed to deal with both the special interval arithmetic properties and the transient term in the differential Equation governing heat transfer. In this formulation, the “stiffness” matrix resulting from the discretization of the heat conduction equation is assembled with an element-by-element technique in which the elements are globally independent and the continuity is enforced by Lagrange multipliers. This formulation is an alternative to traditional Monte Carlo method, where it is necessary to run a simulation several times to estimate the uncertainty in the results.We have applied the newly developed techniques to a one-dimensional thermal basin simulation to assess their potential and limitations.We also compared the quality of our formulation with other solution methods for interval linear systems of equations.

Using XML to improve the productivity and robustness in application development in geosciences

In this paper, we describe an approach to apply Extensible Markup Language (XML) technologies to improve the robustness of geological and geophysical applications as well as to increase the efficacy in the application development process. Geological and geophysical applications are often data centric, I/O intensive and their development is incremental. Therefore, significant amount of development resources is devoted to the design and reengineering of the container data structures that store data. This process is time consuming, mechanical and error prone. Normally, ad hoc parsers are necessary for reading inputs, as well as numerous filters, or adapters to transform the data for integration with other legacy applications. Most of this can be avoided by using XML-related technologies. XML has a type system schema that can be used to define input parameters and constraints. The XML parser can validate the input data using the constraints defined in the schema. Exporting results in XML format allows the use of Extensible Stylesheet Language Transformations (XSLT) to transform XML output to any other format necessary for integration with legacy applications. Additionally, XML-data binding code can be automatically generated in specified languages such C++ and Java. We used this approach to develop applications for seismic ray-tracing and basin modeling with great success, and the major benefits of this approach were the significant gains in productivity during the developement and application robustness.

Integrating Mathematical Optimization and Decision Making in Intelligent Fields

In this paper a decision-making approach that can be applied to problems that are relevant to the oil and gas industry is presented. This methodology is supported by state-of-the-art mathematical optimization algorithms, and is based on the formal integration of the decisions in question with well-studied optimization procedures. The integration of the methodology with the application adds to its robustness. Two different types of problems are formulated and solved. The first kind is based on deciding which wells have to be shut in during a given production interval whilst simultaneously optimizing the controls for each selected well. The second category involves deciding for a group of wells which ones have to be injectors or producers, and at the same time searching for optimal well locations. In all the results obtained we can systematically see that the set of decisions proposed by the integrated approach mean substantial improvement in field production. For example, in the first class of problems studied, the production oil target is satisfied, and up to 50 percent of produced water is saved with respect to the reference case. The huge amount of information available, for example, in Intelligent/Smart Fields or Closed-Loop Reservoir Management can be utilized for rigorously making solid decisions. In this work we put an emphasis on integration of real-life decisions with a realistic simulation-based mathematical optimization framework. This framework can be also useful for establishing a common language for decision makers and researchers within a given organization, and as a consequence endowing the decision-making process with agility and robustness. It should be stressed that ultimately it is human interpretation and intuition that drives the making of crucial decisions. Automated tools should be understood as an additional (and hopefully valuable) source of information for making these important decisions.

Time‐dependent reservoir characterization of the LF sand in the South Eugene Island 330 Field, Gulf of Mexico

One of the most important features of time‐dependent reservoir characterization is the added additional constraints that can be incorporated to update the initial reservoir model. Uncertainty of the model may be gradually reduced as the iteration of time‐dependent reservoir characterization continues.

Techniques for including large deformations associated with salt and fault motion in basin modeling

Abstract Modeling large deformations such as non-vertical fault displacement and salt motion have been a major obstacle for the improvement of regional basin modeling studies. Because salt has a large thermal conductivity and is practically impervious, and faults can act as conduits or seals during the evolution of sedimentary basins, they are critical in making accurate predictions of the generation, migration and accumulation of hydrocarbons within salt-bearing basins. To model numerically the evolution of salt structures is not a trivial task and one of the major difficulties in modeling the motion of salt and faults is the management of numerical meshes that are severely corrupted with large deformations. In this study, we use a topological framework for the representation of complex geological structures that makes it possible to model geological processes with large deformation within sedimentary basins and the lithosphere. This framework greatly facilitates the automatic meshing and remeshing required during modeling because meshes, like lithology and physical properties, are treated as attributes of subregions of the model. In this context, we developed a series of techniques to classify fault blocks in order to model the displacement of multiple faults simultaneously in the correct order. In addition, we show that this framework allows the decomposition of the basin model along geological discontinuities and makes it suitable for parallel computation of the solution of differential equations governing generation and migration of hydrocarbons.

Enabling high-resolution forecasting of severe weather and flooding events in Rio de Janeiro

Safe operation of many cities is affected by relative extremes in weather conditions. With precipitation events, local topography and weather influence water runoff and infiltration, which directly affect flooding. Hence, the availability of highly focused predictions has the potential to mitigate the impact of severe weather on a city. Often, such information is simply unavailable. The initial step to address this gap is the application of state-of-the-art weather models at an urban scale calibrated to address this mismatch. The generation of operational forecasts at such a scale for the Rio de Janeiro metropolitan area suggests a horizontal resolution of approximately 1 km and a vertical resolution in the lower boundary layer of tens of meters. Forecasting impacts from storm-driven flooding events requires the development of a coupled hydrological model that operates at a street scale with resolution of approximately 1 m, capturing local terrain effects and simulating surface flow and water accumulation, especially for overland flow and ponding depth. This coupled approach has enabled operational prediction of storm impacts on local infrastructure, as well as measurement of the model error associated with such forecasts.

4-D seismic reservoir simulation in a South Timbalier 295 turbidite reservoir

This case history describes a combination of time‐lapse (or 4-D) seismic with 3-D elastic seismic modeling, reservoir characterization, and fluid‐flow simulation to better understand drainage patters of oil, gas, and water into wells and to identify bypassed pay.