Ferreira, Joana (University of Lisbon, Faculty of Sciences, Department of Geology) | Azerêdo, Ana C. (University of Lisbon, Faculty of Sciences, Department of Geology and Instituto Dom Luiz) | Bizarro, Paulo (Partex Oil & Gas) | Ribeiro, Maria Teresa (Partex Oil & Gas) | Sousa, Ana (Partex Oil & Gas)
Characterizing highly heterogeneous carbonate reservoirs requires the integration and detailed analysis of petrophysics, facies, diagenesis, geometry, depositional environments and lateral and vertical variability. This is often challenging to conceptual models at oil-field scale, as this thorough analysis is hard to fully reproduce at reservoir-scale models. In order to improve skills and interpretations on both approaches, we addressed a case study from a Middle Jurassic outcrop of Portugal as an analogue for a carbonate reservoir.
The outcrop exhibits three barrier shoreface lithofacies: L1 - oolitic and bio-intraclastic grainstones (divided into: L1a- with planar stratification or unstructured; L1b - with diverse cross-stratification styles); L2 - coarser grained grainstones/rudstones; and L3- coral/algal biostromes. Outcrop analysis was combined with petrographic/diagenetic studies of rock samples. Regarding petrophysical properties, three methods were used to determine the porosity: thin-section impregnation with blue-dyed epoxy resin, rock-slab water saturation and, for a few samples, plug measurements in a Helium gas expansion porosimeter. The plugs were also used to acquire permeability values using a digital gas permeameter.
The results show that the outcrop is a tight reservoir, since most levels have low porosity (~3.5% average) and permeability (mostly <0.1md), though higher values (φ~10-15% and K~160md) occur locally. Most levels are classified as hybrid 1 or diagenetic reservoirs, according to
Normal industry workflows often do not fully consider geological data and conceptual models, and instead rely heavily on geostatistical propagation of well data. The results obtained indicate that there is an improvement in reservoir understanding with an integrated reservoir characterization and modelling process that accounts for actual depositional and diagenetic trends, as well as the distribution of the sedimentary bodies.
The main purpose of this study was to carry out the physical and mechanical characterization of dolomitic limestone from “Camadas de Coimbra” Formation with different degrees of weathering. The study begins with a characterization of the geotechnical properties of the intact stones used in this research. Seven tests (sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability) were carried out. The characterization permitted to understand and evaluate the variation of the mechanical and physical properties throughout the weathering processes. The porosity tends to growth according to the rock weathering degree. Its increase corresponds to a decrease of the ultrasonic velocity and rock density. Considering the totality of the specimens tested, moderate correlation coefficient was found between the longitudinal waves propagation and the point load strength values (r = 0.62). For the longitudinal and transversal waves propagation the correlation coefficient was equal to 0.75.
The main purpose of this work is to contribute to the geotechnical knowledge of several weathering degrees of dolomitic limestone belonging to the “Camadas de Coimbra” Formation. The characterisation of the physical and mechanical properties was carried out.
The study area is located in the city of Coimbra (Central Portugal). Log samples were collected in a prospection campaign performed at Polo I of the University of Coimbra. The rock characterisation was carried out through seven laboratory tests: sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability tests.
Correlations were established based on the test results, in order to define and predict the geotechnical behaviour of dolomitic limestone with different degrees of weathering.
Carbonate reservoir characterization is often a complex task, due to the interplay between primary processes (e.g. depositional environments, facies changes) and secondary processes (e.g. burial, diagenesis, faulting and fracturing, cementation). In order to properly characterize and model such a reservoir, it is paramount to unravel the order by which such processes have affected the rock, leading to the present day petrophysical properties.
In the presented case study (onshore dolomitized carbonate reservoir in Central Asia), a multi-step approach was taken for its characterization and modelling. The characterization phase was focused in understanding the key processes and controls on porosity and permeability. From the core and log data, a detailed sedimentologic and diagenetic study was performed, to identify the depositional environments and facies, as well as the pore system geometry, and its impact on fluid flow. Furthermore, several trends on reservoir quality were identified, related to faults, and associated with depositional cyclicity.
From the above work, a reservoir model was built, to support field development planning and associated uncertainties. A structural and stratigraphic framework was built, and Flow Unit Types (FUT) were defined using seismic, cores, thin sections, logs and mercury injection capillary pressure data (MICP). Property modelling was carried out for porosity and permeability, honouring FUT, depositional and diagenetic trends. In particular, two trends were modelled: a fault-related trend, to introduce the impact of diagenetic leaching related to faults (observed in core data); and a cyclicity related trend, to introduce the impact of preferential fluid flow pathways that occur at or near cycle tops. The uncertainty in the reservoir property models was evaluated with different FUT, driven by depositional and diagenetic concepts.
The results indicate that a significant improvement in reservoir understanding can be achieved with the use of an integrated study and model workflow, focusing on the key control factors that affect the pore system and the distribution of permeability. In this way it was possible to recognize spatial trends and capture the relationship between petrophysical properties, pore architecture and sweep efficiency.
This paper is a study of the potential for underground storage of gas in salt caverns in Portugal, in order to find the volume of gas that can be stored in the national territory. It is emphasized that in Portugal there are plenty of salt dome formations that can provide solutions for storage. For a better understanding, a characterization of the global market for gas will be made, giving greater focus to the current situation of the European Union, and described the methodologies most used for underground gas storage.
The crises between Russia and Ukraine in 2009, have demonstrated the European Union energy system vulnerability as some countries threaten to break natural gas stocks. This scenario is compounded by the high EU dependence of the Natural Gas from Russia. In this context the underground storage of natural gas is presented with a good solution to minimize this problem.
This paper is a study of the potential for underground storage of gas in salt caverns in Portugal, in order to find the volume of gas that can be stored in the national territory. It is emphasized that in Portugal there are plenty of salt dome formations that can provide solutions for storage. For a better understanding, a characterization of the global market for gas will be made, giving greater focus to the current situation of the European Union, and described the methodologies most used for storing gas underground.
2. World Market
In this chapter will be characterized the global market for natural gas (Reserves, Production and Consumption), placing greater emphasis on the state of the EU.
The proven reserves worldwide have grown steadily over the years, in 1980 were at the level of 80 000 bcm, rising to 180 000 bcm in 2006 (GILARDONI, 2008).
2.1.1 European Union
The main reserves of the EU are held by two countries: the Netherlands and the UK.
Until this day the reserves have been declining, reaching 2 500 bcm in 2006, which represents a ratio (R/P) of 13 years.
A field-based, portable geochemical laboratory provided Real-Time geochemical rock analysis from 23 stratigraphic wells drilled in the Lusitanian Basin (onshore Portugal), in order to characterize an unconventional resource play. A dedicated workflow involving field personnel, operations teams and remote data users optimized rock measurements and facilitated data interpretation.
This type of workflow, and the approach to make the most of a novel data set, can benefit drilling operations across the border. The technique was developed to support in particular the characterization of shale intervals on a number of adjacent wells, but both the technology applied and the workflow developed are adapt to single-well operations and conventional reservoirs.
The geochemical analysis system analyzed 500 rock samples from 21 shallow stratigraphic wells, outcrops and existing deep legacy wells throughout the Lusitanian basin, from very shallow to depths of more than 2000 m. The targets were three separate organic-rich intervals of Jurassic age. For each sample the complete analysis was carried out on-site and comprised: elemental composition analysis, mineralogy analysis, independent measurement of Total Organic Carbon (TOC) and Pyrolysis. The organic data were the more crucial ones: TOC results combined with Pyrolysis provided accurate information of the amount of hydrocarbons that can be generated by the rock (S2), the rock maturity (Tmax) and the kerogene type (Hydrogen Index), characterizing the organic matter.
The sum of equipment, analytical technique and workflow utilized represented a novel approach to unconventional resource play evaluation. The organic geochemical analyzers utilized in the project have been specifically developed and built for field utilization, permitting to obtain accurate results on a large number of samples in a short time. These results are usually provided by remote laboratories, resulting in delayed responses and the impossibility to react while operating. In other cases, data sets which generate new information are not utilized due to the lack of a proper decision making process which takes them into account. This work model solves all these issues with an integrated approach and allows to adjust and to optimize an ongoing drilling campaign real-time according to the incoming results as well as to interpret the novel geochemical dataset together with the new well data and existing geologic information early on.
Carbonate reservoirs are commonly heterogeneous and their reservoir quality results from complex interactions between depositional facies and diagenetic processes. The Diagenetic Diagram is a powerful tool that helps in the characterization of the diagenetic processes that have affected the reservoir. From this knowledge, it is possible to significantly improve the understanding of the reservoir's pore system and permeability distributions, which are key factors for development optimization and production sustainability.
A multi-scale and multi-method study (petrography, blue-dye impregnation, selective staining and porosity determination) of Middle Jurassic carbonates from the Lusitanian Basin (Portugal) has been undertaken, to find the best systematic approach to these reservoirs. It has involved thorough diagenetic characterization of each lithotype (lithofacies, texture, porosity, qualitative permeability assessment and diagenetic evolution). The study area was selected based on its excellent and varied exposures of carbonate facies and availability of core.
Methodological and terminological challenges were faced during the study, especially dealing with data coming from several scales (macro, meso, and micro). In order to overcome these challenges, a diagenetic diagram was developed and applied to the selected rocks. It is a tool that allows the integration of data coming from outcrops, hand samples, cores, cuttings, thin sections, and laboratory experiments.
This is carried out in a dynamic, guided, systematic, and rigorous way, enabling the evaluation of the relationship between facies, diagenetic evolution and pore systems. The latter are characterized regarding size, geometry, distribution, and connectivity. This enables the identification and characterization of permeability heterogeneities in the rocks. It was concluded that the main porosity class (i.e. secondary) was created by diagenetic processes.
The proposed method has strong application potential for: detailed characterization and understanding of porosity and permeability in carbonate reservoirs, from a diagenetic evolution and fluid flow perspective (e.g. SCAL and pore system description); definition of diagenetic trends for modeling petrophysical properties and rock types. In this regard, the method is being applied to a Valanginian carbonate reservoir in Kazakhstan, and some preliminary results are presented in this paper. Refining this technique may be helpful for similar carbonate studies, enhancing the results of typical diagenetic studies by improving the characterization of reservoir properties at various scales, thus contributing to a more sustainable exploitation of hydrocarbon reservoirs.
Summary Land seismic acquisition in densely populated areas is often affected by severe operative problems, mainly due to difficulties to access the survey area caused by different permitting issues, and more in general by the presence of obstacles. As a consequence, it is usual to acquire 3D seismic surveys characterized by important discrepancies between the planned and the actual lay out. These variations can typically produce negative effects in the final seismic data quality. The use of cable-free acquisition systems can help to solve this problem. We present here the results of a recent case, where both acquisition systems (wireline and cableless) were deployed at the same time, recording simultaneously two independent datasets over an area of 22.5 sqkm for a total of 4707 cableless receiver points and 2770 shots.
In 2010, natural-gas production rates in the USA for March and April were the highest since 1974 (US Energy Information 2010). This is largely related to recent success in exploiting unconventional reservoirs and successful development and implementation of innovative technologies. In Europe, unconventional reservoirs (in particular shale gas) are receiving considerable attention. Basins with potential shale-gas plays exist, as does the infrastructure to transport gas to the end user. In addition, the gas-price scenario is favorable as well. It will be critical to find US analogs and transfer technologies/processes that will facilitate European shale-play developments into economically successful projects. However, the market and challenges in Europe are significantly different from those in the US. The population density is much higher and environmental regulations will play a different role. In addition, the entire industry is set up different with a lot less drilling rigs and other service providers. For different reasons, costs are somewhat higher than in the US as well. This demands optimization from Day 1. Directional drilling of horizontal wells needs to progress toward sweet spots, and the completion and fracturing process must be optimized starting at the first well. Therefore, it is
important to understand the technologies that were game changers in the US, identify their applicability in Europe, and implement them properly.
This paper discusses two recently developed hydraulic-fracturing processes and a hybrid unit that might have considerable impact on advancing economic recovery of unconventional reservoirs. The first new process is designed for high-pumping-rate, multistage fracturing treatments with low proppant concentration. Highly concentrated proppant slurries are pumped down the coiled tubing (CT), jointed tubing (JT), or combination string, and nonabrasive, clean fluid is pumped down the annulus. In such pumping scenarios, the permanent well tubulars are saved from erosion. The pumping rate can be manipulated in real time to
customize the placement and downhole-proppant concentration instantly. In case of premature screenout, a well could be easily reverse circulated and cleaned for the next stage. The combination of downhole mixing and microseismic fracture mapping provides unprecedented control of fracture execution to facilitate branch fracturing. The second new process is a combination of mechanically activated sleeve completions and fracturing of individual intervals, but with a change in the sequence in which the intervals are stimulated. This new method is proposed with a goal of altering the stress in the rock to facilitate (or enhance) branch
fracturing and connect to induced stress-relief fractures in a single, horizontal well.