Tag: Fractures
The influence of major faults and fractures on the development of non-matrix porosity system in a pre-salt carbonate reservoir, Santos Basin – Brazil
Faults and fractures are central for characterizing the permeability distribution in carbonate reservoirs since they act as pathways for diagenetic fluids that often favor intense rock dissolution and permeability. Usually, high permeability zones and fractures are not easily recognized in seismic data due to limited resolution and they are often associated with higher concentrations of hydrocarbons or even significant fluid losses during drilling, thus creating a challenge for hydrocarbon exploration. In the Santos Basin, southeast Brazil, the pre-salt carbonate reservoirs from the Barra Velha Formation (BVE) are the main hydrocarbon producers in Brazilian Atlantic margin and well-known for being extremely heterogeneous, exhibiting complex dual-porosity systems. In this study, we built a conceptual model of these fracture zones and non-matrix porosity formation that helped narrowing the understanding of these complex systems. The characterization of faults and fractures was carried out using seismic, well-logs, and borehole image data to understand the influence of these structures in the porosity formation along the Barra Velha Formation. In the study area, three fault sets were defined (F1, F2, and F3) from seismic data. F1 represents to the larger faults, while the F3 fault set represents the smaller faults related to the reactivation of F1; both sets being oriented NE-SW. The F2 fault set is associated with the rift formation and is oriented to NNE-SSW. These three fault sets compartmentalized the studied area into different domains, each exhibiting distinct fracture sets. The natural open fractures were formed during the reactivation of rift faults and are oriented mainly NW, NNE-NNW, NE, and ENE and were identified across the entire study area, but with different intensity values. The fracture intensity closely relates to the distance from major faults where the wells with the highest fracture intensity occurs located 150–590 m from the larger F1 fault set. Scan-lines were conducted throughout the area to determine the fault width, and an average value of 1.2 km was established. Seven non-matrix porosity classes were characterized and classified into stratigraphically concordant and discordant non-matrix pore types at well scale through borehole image interpretation. The Barra Velha Formation exhibit higher occurrence of stratigraphically discordant non-matrix porosity related to fractured zones while stratigraphically concordant non-matrix porosity is mainly controlled by the paleotopography of the study area. Overall, non-matrix porosity formation tends to follow an orientation that suggests a preferential dissolution flow towards NE and ENE directions. Intervals with higher silica content shows a positive correlation with both fracture intensity and stratigraphically discordant non-matrix porosities. This work provides a conceptual model about the fractures and non-matrix porosity distribution in pre-salt carbonate rocks that can help address some of associated structural and stratigraphic uncertainties during field appraisal and development.
Paleokarst features in the Aptian carbonates of the Barra Velha Formation, Santos Basin, Brazil
Seismic data, borehole image logs, and conventional well logs were used to investigate the distribution and characteristics of paleokarst features in the Aptian carbonates of the Barra Velha Formation in a pilot area of the Santos Basin, Brazil. Multiple seismic attributes were used to enhance details on the seismic data and highlight key seismic parameters including strata deformation and geometry, continuity of seismic events, and fault patterns. The study found that karst structures are controlled by faults and fractures along structural highs, which served as a conduit for the flow of dynamic fluids that dissolved the carbonate materials. Several closed, circular depressions and bright spots identified in the northeastern portion of the study area represent possible sinkhole structures. Epigenic and hypogenic processes due to the action of meteoric water, hydrothermal activity, and intra-formation acidity along regional unconformities in the Barremian-Aptian may have been responsible for the dissolution. Limitations of this study are related to the difficulty of integrating multiple datasets with various scales. However, the higher confidence for the occurrence of the karst features is provided by borehole images at the sub-seismic scale. The findings of this study hold significant relevance for the strategic planning of energy development and carbon sequestration initiatives in the Brazilian continental margins, thereby aiding in informed decision-making.
Multi-scale Meshing for Multiscale Three-Dimensional Discrete Fracture Networks
Multi-scale finite element methods require special types of meshes, notably, those that relate coarse elements to sub-meshes contained by them. The geometric description of a Discrete Fracture Network (DFN) in this context, involves the ability of inserting multiple fractures in a pre-defined coarse mesh, while building a sub-mesh around these fractures and tracking fine/coarse element relations. Main steps involve: locating intersections and refining elements at those points, building a data structure that associates each element of a fracture surface to the coarse volume that encloses it, and then generate a sub-mesh of fine elements around the fractures to fill these coarse elements, without altering originally defined nodes in the coarse mesh. This work presents an approach for automatic finite element meshing of fractured reservoirs suited to Multiscale Hybrid-Mixed methods (MHM) [1]. The code is written in C++ and largely relies on two finite element libraries: NeoPZ [3] and Gmsh [2]. Starting with a coarse mesh, fractures are entered as 3D polygons, built from their corner points, and inserted one-by-one. Intersections with one-dimensional elements are computed first and subsequently used to define the intersection with two-dimensional elements (faces). Triangles that result from refining faces to conform to the intersecting fractures are checked for quality, and those with bad aspect ratio (given a tolerance) are coalesced, as fracture surface nodes are snapped into previously defined nodes. The resulting faces are utilized to define volume shells that can be tetrahedralized using Constrained Delaunay algorithms available in the Gmsh library. Element connectivity, transformations between parametric domains of fine/coarse elements, and all other relevant finite element computations are implemented using NeoPZ. Results show that the proposed technique can efficiently construct adequate 3D meshes. While rely-ing on neighbourhood information and consistent element topologies available from NeoPZ’s geometric meshes, enables optimization of multiple algorithms of geometric search that would, otherwise, require a considerable amount of floating-point operations.