Offshore Petroleum Reservoir Research Articles

Evaluation of distinct soft-sediment deformation triggers in mixed carbonate-siliciclastic systems: Lessons from the Brazilian Pre-Salt analogue Crato Formation (Araripe Basin, NE Brazil)

Soft-sediment deformation structures (SSDSs) are ubiquitous in several depositional sedimentary environments and can be triggered by autogenic- and allogenic-related mechanisms. SSDSs identification in the geological record is more frequent in siliciclastic deposits, from which the most accepted models were developed. Given the fact that carbonate rocks have rapid diagenesis, which results in changes in the rheological behavior of deformational processes in short time, these are excellent facies for exploring past deformational mechanisms in a given sedimentary basin. Here, we describe distinct SSDSs developed in mixed carbonate-siliciclastic successions deposited in an Early Cretaceous lacustrine depositional system (carbonate-dominated) subjected to recurrent epicontinental marine ingressions (siliciclastic-dominated). Deformation occur in some siliciclastic intervals but it is more common in carbonate beds. Facies associations analysis combined with detailed outcrop description of the SSDSs allowed the recognition of seismic- and non-seismic-generated structures. Two km-scale, laterally continuous carbonate beds, characterized by deformation-bearing horizons, are interpreted to be the result of seismic shocks. Conversely, most of the recognized SSDSs cannot be confidentially interpreted as seismites. Autogenic triggers and criteria for accessing the origin of deformation mechanisms in both carbonate- and siliciclastic-dominated settings are alternatively proposed. Assessing such criteria is fundamental for differentiatte the timing and range of deformation and are here discussed in the context of the offshore petroleum reservoirs in the Brazilian Pre-Salt successions, where lacustrine carbonates were probably subjected to a similiar superposition of syn-sedimentary seismicity and later events of salt-tectonics and hydrotermalism.

Reservoir Wellbore Stability Analysis and Weak Zones Identification Using the 1D MEM, Swelling Tests and UCS: A Case Study From Mumbai Offshore, India

Reservoir geomechanical studies play a decisive role in identifying wellbore instability and weak zones in offshore petroleum reservoirs. In this study, an integrated workflow developed with 1D Mechanical Earth Models (1D MEM), uniaxial compressive strength and swelling tests using well logs, drilling, and core data of production and exploratory wells from the North-Heera field, Mumbai offshore, India. Lithology-based correlations are utilized for calculating continuous profiles of rock mechanical properties by taking well logs as a primary source of input. Insights of fracture gradients and stresses are generated by interpretation of the pore pressure. UCS created from the well logs shows the low value of 15 MPa and high Poison's ratio of 0.38–0.45. Fracture pressure analysis of the wells specifies the lower fracture initiation pressures at 300 m are 5ppg–15ppg, which are shale and clay formation. Shale swelling tests are performed to know the hydration properties of shales present at shallow depths. The core samples' loading tests' results are very close to the UCS models constructed from the well logs. The combined analysis examines fracture pressures around wellbores, hydration of shales, weak zones identification, and wellbore instability.

Functional microorganisms involved in the sulfur and nitrogen metabolism in production water from a high-temperature offshore petroleum reservoir

Microorganisms with diverse metabolic functions inhabit petroleum reservoir systems utilizing multiple electron acceptors, but current understanding about the composition and diversity of functional microbial community in offshore oilfields is still scarce. In this study, the diversity, phylogenetic distribution and abundance of microbes involved in the sulfur (S) and nitrogen (N) metabolisms in production water of a high-temperature offshore oilfield were described selectively based on the functional gene analysis using polymerase chain reaction (PCR) technique. The detected sulfate-reducing prokaryotes (SRP) were affiliated with δ-Proteobacteria, Thermodesulfobacteria, Nitrospira, Firmicutes and Euryarchaeota, and the key and dominant SRP in the two samples included the genera Archaeoglobus, Thermodesulfobacterium, Thermodesulfovibrio, Thermodesulforhabdus and Desulfomicrobium. The sulfur-oxidizing prokaryotes (SOP) belonged to α-, β- and γ-Proteobacteria, mainly represented by Roseovarius, Rhizobium, Methylobacterium, Thiobacillus and Thauera genera. In addition, the nitrate-reducing and denitrifying communities were composed of α-, β- and γ-Proteobacteria, among which Bosea, Pseudomonas, Marinobacter, Diaphorobacter and Mesorhizobium showed a relatively high abundance. The quantitative PCR results revealed that functional gene abundances of N metabolism (the napA, nirS and nosZ genes) were mostly higher than those of S metabolism (the aprA, dsrA, dsrB and soxB genes), and functional gene abundances in sample X1 were mostly higher than those in sample X2. This work provides basic data for the understanding of diversity, composition and distribution of functional microorganisms in offshore petroleum reservoir ecosystems, and also mitigation of the reservoir souring and control.

Microbial community composition and diversity in production water of a high-temperature offshore oil reservoir assessed by DNA- and RNA-based analyses

Microbial activity is ubiquitous and important in petroleum reservoir systems; however, current knowledge regarding the diversity and functions of microbial communities that are active in offshore petroleum reservoirs remains very limited. In the present work, high-throughput sequencing analysis of the 16S rRNA gene (DNA) and transcripts (RNA) was applied to respectively characterize the genomic and active microbial communities from two production wells in a high-temperature offshore oilfield in China. The results showed that the microbial community compositions in the two wells (X1 and X2) were very different. At the genus level, the dominant members in both genomic and active communities of sample X1 were Bosea, Acinetobacter, Sphingomonas, Candidatus Methanofastidiosum and Methanocalculus. Both genomic and active communities in sample X2 were mainly represented by Pseudomonas, Acinetobacter, Methanothermobacter, Methanosaeta, Candidatus Nitrosopumilus, Candidatus Methanomethylicus, Thermococcus, Candidatus Methanofastidiosum and Methanocalculus. Moreover, using an RNA-based approach, genera with a very low abundance and even undetected genera (based on a DNA approach) were shown to have increased activity. Besides the abundant active members, the comparison between the DNA- and RNA-based datasets also permitted the identification of genera that were relatively more active in the two production water samples. The existence of these active microorganisms suggested they might have potential functions associated with reservoir issues in the offshore oilfield. This study highlights the importance of assessing microbial diversity through a combination of DNA- and RNA-based analyses, and promotes a comprehensive and deep understanding of microbial compositions and their roles in offshore petroleum reservoirs.

Technical and Non-technical Challenges of Development of Offshore Petroleum Reservoirs: Characterization and Production

Offshore oil and gas reservoirs comprise a significant portion of the world’s reserve base, and their development is expected to help close a potential gap in the supply of hydrocarbons in the near future. Continuous advances in technology have helped the oil and gas industry to extend the exploration and production activities to deep and ultra-deep waters in harsher environments. Field development in the offshore environment is associated with numerous significant challenges in different phases including exploration, reservoir description and characterization, development planning, drilling, production, improved oil recovery (IOR) and enhanced oil recovery (EOR), transportation, and decommissioning. These challenges are further complicated by economic restrictions, especially in periods when low or unstable oil prices weaken the incentives of investments due to the high risks and uncertainties involved. Environmental concerns including the adverse effects of seismic surveys, drilling activities, discharge of waste material and produced water, and accidental spills and blowouts add another level of complexity to the design and implementation of offshore projects. Safe, reliable, and efficient development of offshore hydrocarbon reservoirs can be achieved through the proper identification of the challenges and use of modern technologies and innovative methods. This work is an effort to provide a comprehensive review of the history, global distribution, production share, classification, characteristics, and characterization methods of offshore reservoirs, and some of the main aspects and challenges of the experimental and modeling works as well as IOR/EOR planning and implementations from the reservoir engineering and production standpoint. These challenges are categorized based on their area of impact, i.e., characterization and recovery (production) phases. Furthermore, economic and environmental challenges are reviewed. After the recent downturn that caused a decline in offshore investments, current investment trends and the forecasts show that the offshore industry is booming again with more focus on cost saving and operational efficiency. The key technologies that can expedite the growth of the offshore petroleum industry have also been discussed.

Grid-free petroleum reservoir characterization with truncated pluri-Gaussian simulation: Hekla case study

A new geostatistical grid-free simulation (GFS) method has been developed recently that represents simulated continuous attributes of the natural phenomena, such as stratigraphic surface boundaries or petrophysical properties, as an analytical function of the coordinates of the simulation locations. Thus, GFS resolves challenges related to model regridding, increasing resolution around already simulated locations and integration of newly available data in a consistent manner. The present paper contains further developments in simulation of categorical variables, such as facies, in a grid-free fashion based on the truncated pluri-Gaussian simulation (TPG) paradigm. The resultant simulation engine allows the entire reservoir system to be represented as an analytical stochastic function: that is, values of any reservoir properties are simulated on demand at requested locations in space. The selection of proper variograms of Gaussian continuous variables for simulation of categorical variables in a TPG framework is proposed through a methodology based on Monte Carlo simulation. The variogram models of the underlying Gaussian continuous variables are obtained by minimizing the difference between numerically computed and target indicator variograms. A local optimization approach is suggested for a fast precise derivation of the variograms. The stable variogram model leads to the closest fit to the experimental variograms of the continuous variables. The automatic establishment of a truncation mask based on multidimensional scaling to convert Gaussian continuous variables to categories is also explained. Finally, the proposed GFS algorithm for petroleum reservoir characterization is demonstrated in its full-scale applicability to the Hekla offshore petroleum reservoir located in the North Sea. The results look promising, and should be beneficial to petroleum reservoir modelling in practice.

Deepwater pipelines – status, challenges and future trends

The demand for energy is steadily increasing and, at least for the coming decades, the world has to rely on oil and gas to address this need. Most of the easiest accessible offshore petroleum reservoirs have been discovered and a great part developed over the last six decades. Thus, development of new oil and gas fields faces a lot of challenges as most of them are in remote areas, in deep waters and/or in areas with extreme environments like the Arctic region. One of the major trends in the offshore petroleum industry points towards deeper waters (e.g. outside West Africa, the Brazilian Pre-Salt developments and in the Gulf of Mexico). This trend also includes increased use of subsea installations instead of platforms, more subsea processing and increased use of pipelines to transport the hydrocarbons to shore or into a pipeline grid. This paper addresses some of the challenges pipeline design, installation and operation may face in deep and ultra-deep waters. The main design challenge is related to the high external pressure that may cause collapse of the pipeline. This potential failure mode is normally dealt with by increasing the pipe wall thickness, but at ultra-deep water depths this may require a very thick walled pipe that becomes very costly, difficult to manufacture and hard to install due to its weight. One approach to overcome this is to improve some of the parameters that determine the collapse resistance by an improved manufacturing process. Other approaches are to ensure a minimum internal pressure is maintained in the pipeline during all phases, or to install a buoyant pipeline that is anchored at a moderate water depth rather that laying on the sea bed.

Postglacial Fault Drilling in Northern Europe: Workshop in Skokloster, Sweden

Postglacial faulting has important implications for predicting the behavior of fault zones during future glaciations. Therefore, PG fault research is expected to contribute significantly in planning the disposal of spent nuclear fuel, CO2 and toxic waste into bedrock that currently is prepared in the Nordic countries. Other fields of applied geoscience which may benefit from PG fault research are mineral explo ration and estimation of mine stability, as some of the faults are located in areas which host gold, copper, and nickel mineralizations in northern Fennoscandia. Major hydropower and tailing dam complexes may also be influenced by PG faults and their current earthquake activity. An improved understanding of the prevailing in situ stress, erosion, uplift, and sedimentation also has implications for the understanding of offshore petroleum reservoirs on the Lofoten-Barents margin.

Microbial communities of a complex high-temperature offshore petroleum reservoir

In this paper, the microbial communities of a complex high-temperature (appr. 85°) offshore petroleum reservoir are described as part of improving the understanding of the biogeochemical processes in the reservoir. Based on molecular biology studies, the results revealed that the microbial communities differed between samples from producing wells and the oil/water production facility of the reservoir, although a few types of thermophilic microbes appeared in both environments. The microbial communities of the different producing wells could be separated into three groups, and the target bacteria related to the genera Halomonas, Arcobacter or Pseudomonas seemed to be associated with these groups. Thus, different microbial communities were detected in this reservoir, and further investigations will clarify if specific target microbes may be used for the identification of different oil qualities or other reservoir characteristics. [Received: November 4, 2007; Accepted: January 10, 2008]

A Design Approach for a "Self-Lifting" Method To Eliminate Severe Slugging in Offshore Production Systems

Summary Recently, exploitation of offshore petroleum reservoirs has moved to ever-increasing water depths. Production from fields in water deeper than 1800 m is now a reality. The use of long deepwater risers that conduct production from multiple wellheads on the seafloor to the surface predisposes the system to severe slugging in the riser for a wide range of flow rates and seabed topography. Considering the length of the deepwater risers, the problem is expected to be more severe than in production systems installed in shallower waters. Severe slugging could occur at a high pressure, with the magnitude of the pressure fluctuations so large as to cause a shorter natural-flow period with subsequent consequences, such as premature field abandonment, loss of recoverable reserves, and earlier-than-planned deployment of boosting devices. In this study, a novel idea to lessen or eliminate severe slugging in pipeline/riser systems has been investigated. The principle of the technique is to transfer the pipeline gas to the riser at a point above the riser base. The transfer process will reduce both the hydrostatic head in the riser and the pressure in the pipeline, consequently lessening or eliminating the severe slugging by maintaining the steady-state two-phase flow in the riser. In this paper, a steady-state model is presented. It can be used as a design tool for the self-lifting concept by determining the operation envelope for a successful self-lift operation; given the location of the gas takeoff or injection points, it can predict the acceptable range of pressure losses in the gas bypass for continuous steady-state operation of the self-gas-lift system. The model was found to perform very well when compared to the acquired experimental data.

Applicability ranges for offshore oil and gas production facilities

In the early stages of the selection process for the hardware to exploit an offshore petroleum reservoir, it is important to be able to identify rapidly which production facility type(s) are likely to deliver the greatest value. This paper explores key features and constraints of the ten common fixed, floating and subsea facility options. Both shallow and deepwater are considered, along with regional variations. It is shown that facility applications may be categorised in a very simple matrix form, with the water depth and well count being particularly important drivers of facility choice.

Severe Slugging Attenuation for Deepwater Multiphase Pipeline and Riser Systems

Summary Exploitation of offshore petroleum reservoirs has recently moved to ever-increasing water depths. Production from fields in water deeper than 1800 m is now a reality. The use of long deepwater risers that conduct production from multiple wellheads on the sea-floor to the surface predisposes the system to severe slugging in the riser for a wide range of flow rates and seabed topography. When one considers the length of the deepwater risers, the problem is expected to be more severe than in production systems installed in shallower waters. Severe slugging could occur at high pressure, with the magnitude of the pressure fluctuations so large as to cause a shorter natural flow period with subsequent consequences, such as premature field abandonment, loss of recoverable reserves, and earlier-than-planned deployment of boosting devices. In this study, a novel idea to lessen or eliminate severe slugging in pipeline/riser systems has been thoroughly investigated. This idea was first proposed by Barbuto1 and later developed independently by Sarica and Tengesdal.2 The principle of the technique is to transfer pipeline gas to the riser at a point above the riser base. The transfer process will reduce both the hydrostatic head in the riser and the pressure in the pipeline, consequently lessening or eliminating severe slugging by maintaining steady-state two-phase flow in the riser. An experimental study has been conducted with a 7.62-cminside-diameter (ID) riser (14.63 m high) and pipeline (19.81 m long) system. A broad range of data was collected from the facility in both the severe slugging and stable regions. It was found that the severe slugging models currently available do not predict the region accurately for larger-diameter pipes. Data acquired with the external gas bypass have proved the proposed elimination technique.

Prediction of Reservoir Variables Based on Seismic Data and Well Observations

Offshore petroleum reservoirs are usually explored by acquisition of vast amounts of seismic reflection data and observations along a small number of wells drilled through the reservoirs. The seismic data are indirect measurements of reservoir characteristics and have good spatial coverage but low precision. The well observations are sparse but precise. Information from these sources of data are integrated to characterize the reservoir variables of interest—reservoir porosity in the current study. Critical parameters in the acquisition procedure are unknown and are simultaneously estimated. This process, termed seismic inversion, constitutes a spatial, multivariate, ill-posed inverse problem. The problem is cast in a Bayesian framework with a focus on sampling from the posterior model. High dimensionality, nonlinear components, and unknown parameters in the likelihood model, along with a complex design of the conditioning data, complicate the problem. A fast sequential sampling algorithm containing several parts subject to analytical evaluation is developed under relatively weak, realistic assumptions. Samples of spatial reservoir porosity characteristics can be used to predict and assess uncertainty in the petroleum volume contained in the reservoir. Moreover, porosity is highly correlated with permeability, which represents the fluid flow characteristics. Porosity and permeability are the critical factors in determining and optimizing the future production from petroleum reservoirs. The study is based on data from the Troll field in the North Sea.