Residual Oil Zones Research Articles

Application of artificial neural network for predicting the performance of CO2 enhanced oil recovery and storage in residual oil zones

Residual Oil Zones (ROZs) become potential formations for Carbon Capture, Utilization, and Storage (CCUS). Although the growing attention in ROZs, there is a lack of studies to propose the fast tool for evaluating the performance of a CO2 injection process. In this paper, we introduce the application of artificial neural network (ANN) for predicting the oil recovery and CO2 storage capacity in ROZs. The uncertainties parameters, including the geological factors and well operations, were used for generating the training database. Then, a total of 351 numerical samples were simulated and created the Cumulative oil production, Cumulative CO2 storage, and Cumulative CO2 retained. The results indicated that the developed ANN model had an excellent prediction performance with a high correlation coefficient (R2) was over 0.98 on comparing with objective values, and the total root mean square error of less than 2%. Also, the accuracy and stability of ANN models were validated for five real ROZs in the Permian Basin. The predictive results were an excellent agreement between ANN predictions and field report data. These results indicated that the ANN model could predict the CO2 storage and oil recovery with high accuracy, and it can be applied as a robust tool to determine the feasibility in the early stage of CCUS in ROZs. Finally, the prospective application of the developed ANN model was assessed by optimization CO2-EOR and storage projects. The developed ANN models reduced the computational time for the optimization process in ROZs.

Methodology for estimating the prospective CO2 storage resource of residual oil zones at the national and regional scale

Residual oil zones (ROZs) are increasingly gaining interest as potential reservoirs for carbon dioxide (CO2) storage. Here, we present a national- and regional-scale methodology for estimating prospective CO2 storage resources in residual oil zones. This methodology uses a volumetric equation that accounts for CO2 storage as a free phase in pore space and as a dissolved phase in oil and does not assume any oil production associated with CO2 storage. Reservoir modeling and the CO2-SCREEN tool are used to demonstrate that CO2 storage in residual oil zones will predominantly take place in the free phase (approximately 92–97%) with some storage as dissolution in oil (approximately 3–8 %). Based on this preliminary demonstration, the CO2 storage efficiency for ROZs using this national- and regional-scale method ranges from 0.61 to 7.1 %. This range indicates ROZs have a similar efficiency potential for storing CO2 as deep saline formations (0.51–5.4 %).

Tectonic controls on residual oil saturation below the present-day fluid contact level in reservoirs of the Persian Gulf

The presence of residual oil below the present-day free water level (FWL) and oil water contact (OWC) is common in many oil fields in the Middle East, particularly those in the Persian Gulf. This residual oil is seen in both clastic and carbonate reservoirs prior to the start of production. The characterisation and modelling of these fields is difficult in practice. Also, these residual oils below the FWL and OWC could become classified as reserves if ways to produce them could be found. However, the first step is to better understand their origin. Therefore, the goal of this study was to investigate the role of geological events on the presence of the residual oil zone (ROZ) below the FWL and OWC.It has been suggested that the presence of residual oil below the present day FWL and OWC is related to the geotectonic history of the region. From the middle Miocene, reverse faulting and overfolding propagated over the Zagros, leading to the amplification of folds and the migration of the Zagros orogeny towards its foreland basin (Persian Gulf). In response to this additional massive loading on the continental margin, the forebulge amplitude was increased, its location migrated towards the uplifted Zagros Mountains, and consequently the Persian Gulf became narrower. This exerted a north to north-east downward tilting of the entire basin, including all the structures and reservoirs previously filled by hydrocarbons. This basin tilting changed the equilibrium of the structures and their fluid contents, and resulted in the hydrocarbons and water attempting to find a new equilibrium. Under these conditions, the early migrated and accumulated oil was flushed out by water (imbibition), and a ROZ was left below the present day FWL and OWCs. The angle of regional basin tilt has been calculated to be 0.836° based on seismic sections.

Capacity assessment and co-optimization of CO2 storage and enhanced oil recovery in residual oil zones

Residual oil zones (ROZs) are increasingly being commercially exploited using CO2-enhanced oil recovery (CO2-EOR) method. In this study, CO2 storage potential, long-term CO2 fate and oil recovery potential in ROZs are characterized based on a reservoir model for Goldsmith-Landreth San Andres Unit in the Permian Basin. The effects of CO2 injection rates, well patterns (five-spot and line-drive), well spacings, injection modes (continuous CO2 injection and water-alternating-gas injection) on the CO2 retention in the reservoir and the oil production are investigated. After the preliminary assessment of CO2 storage and EOR potentials in ROZs, we next develop a novel approach based on a newly developed optimization algorithm-Stochastic Simplex Approximate Gradient (StoSAG) and predictive empirical models constructed using machine learning technique to co-optimize CO2 storage and oil recovery in ROZs. The performance of co-optimization of CO2 storage and oil recovery is compared with the performance of optimization of only CO2 storage.

Characterization of CO2 storage and enhanced oil recovery in residual oil zones

Residual oil zones (ROZs) are reservoirs in which oil is swept over geologic time period and exists at residual saturation. The oil in such reservoirs cannot be commercially exploited using conventional oil recovery methods as the oil exists at residual oil saturation. Instead, enhanced oil recovery methods such as CO2 injection are required. Recently, ROZs have been increasingly studied as potential CO2 storage targets. In spite of increased interest in ROZs, there are significant gaps in the knowledge of parameters and processes that impact CO2 storage and oil recovery. In this work, we identify key geologic and operational characteristics that affect CO2 storage capacity and oil recovery potential by performing Monte Carlo simulations and sensitivity analysis. In addition to CO2 storage capacity, we also characterize the long-term CO2 fate in ROZs. The distinction of CO2 storage in ROZs from conventional oil reservoirs and saline aquifers are also characterized. Furthermore, predictive models based on machine learning techniques are developed to estimate CO2 storage and oil production potentials for ROZs. The applicability of the predictive models is demonstrated for five ROZs in the Permian Basin.

Carbon Dioxide Enhanced Oil Recovery and Residual Oil Zone Studies at the U.S. Geological Survey

The U.S. Geological Survey (USGS) is preparing a national resource assessment of the potential hydrocarbons recoverable after injection of carbon dioxide (CO2) into conventional oil reservoirs in the United States. The implementation of CO2-enhanced oil recovery (CO2-EOR) techniques can increase hydrocarbon production, and lead to incidental retention of CO2 in reservoir pore space allowing long-term storage of anthropogenic CO2. A Comprehensive Resource Database (CRD) containing proprietary data on location, geologic, petrophysical, and reservoir parameters, plus production and well counts for major oil and gas reservoirs in onshore areas and State waters of the conterminous United States and Alaska, was developed to support the USGS assessment. Residual oil zones (ROZs) also can provide potential pore space for long-term storage of anthropogenic CO2. However, ROZs are not included in the upcoming USGS national CO2-EOR assessment because assessment methods for ROZs still are being developed. Additional ROZ CO2-EOR and CO2 retention data and reservoir simulations are needed to calibrate national ROZ assessment estimates.

Modeling oil saturation evolution in residual oil zones: Implications for CO2 EOR and sequestration

Abstract Residual oil zones (ROZs) are extensively developed in carbonate formations in the Permian Basin, West Texas. These ROZs have the potential both for economically-viable CO2 enhanced oil recovery (CO2-EOR) and for significant volumes of associated CO2 sequestration. The accepted model for ROZ formation is based on the hydrodynamic effects of tectonically-controlled increased water flow in aquifers at the base of oil fields. The nature of this process is modelled using a commercial reservoir simulator in this work. These simulations explore the effects of strength of aquifer flow, flow direction, and capillary pressure on the nature and distribution of oil saturations in ROZs. A special emphasis was on understanding the impact of heterogeneity of capillary pressures in ROZ reservoirs. These factors determine the thickness of ROZs, the magnitude of oil saturation, and the slope of water-oil contacts. Understanding the magnitude of oil saturation and how it varies within ROZs is important in determining reserves, and evaluating both EOR and sequestration potential. The geometry of ROZs are established slowly, especially for small regional water fluxes, however oil saturations achieve almost steady states in relatively short time scales. The simulated oil saturation profiles found in this study are in reasonable agreement with the measured profile published for the San Andres Seminole Unit's ROZ. The results support the plausibility of the hydrodynamic model, but do not rule out other models for the origin of ROZs.

Reservoir simulation of carbon storage associated with CO2 EOR in residual oil zones, San Andres formation of West Texas, Permian Basin, USA

Residual oil zones (ROZs) are reservoirs in which oil is largely at levels of residual saturation. Such reservoirs cannot be produced by conventional techniques; rather some forms of enhanced oil recovery (EOR), such as CO2 injection is required. As a result, these zones have a potential for CO2 storage associated with EOR activities. In West Texas, the oil production potential of these zones, associated with the San Andres Formation alone, has been estimated as on the order of tens of billions of barrels. A series of numerical simulations of CO2 miscible flooding were conducted on 11 Sub-Volumes cut from a larger static reservoir that represents the range of heterogeneity in permeability and porosity found in San Andres ROZs. This work set out to evaluate the effects of injection strategies and reservoir heterogeneities on the performance of CO2 sequestration. The injection techniques investigated were: continuous CO2 injection and water alternating gas (WAG). Multiple factors were examined, including domain boundary conditions, well patterns, injection rates, permeability anisotropies, and natural fractures. It was found that ROZs could have higher retention fractions (i.e., volume fraction of injected CO2 retained in ROZs) for a combination of inverted five-spot well patterns and large WAG ratios. Based on the results of these numerical simulations, the long-term potential for CO2 storage associated with CO2-EOR of ROZs can be assessed. Our results provide key insights into how future CO2 storage projects associated with EOR in ROZs within carbonate sequences may be implemented.

Formation mechanism of the Donghe Sandstone residual oil zone in the Tarim Basin, NW China

The residual oil zone (ROZ) with near-residual oil saturation is a naturally water-flooded interval adjacent to an oil-bearing reservoir. It has become a promising development target for the technical progress of enhanced oil recovery. The hydrocarbon show, bitumen distribution and grains containing oil inclusions all indicate the presence of ROZs in the Donghe Sandstone Member of the Tazhong4 and Hade Oilfields in the Tarim Basin. Analysis of the two ROZ examples reveals that a stable paleoreservoir and structural adjustment are significant elements of the ROZ-forming process. The ROZ of the Tazhong4 Oilfield is defined as a reactivated fault ROZ (RFROZ) lying between the paleo oil-water contact (OWC) and producing-OWC. Paleo-hydrocarbons leaked through the reactivated faults, and the current oil saturation has a positive proportion to baffle frequency. The ROZ in the paleo trap northwest of the current reservoir in the Hade Oilfield is defined as a migratory trap ROZ (MTROZ). Paleo-hydrocarbons migrated through connected sand bodies. The current oil saturation in the MTROZ has a positive proportion to baffle frequency and negative proportion to petrophysical properties. The microscopic occurrence state of the residual oil in ROZs was analyzed with direct computed tomography (CT) scanning. The displacement experiments coupled with CT scanning aimed to reveal the different occurrence states of the residual oil in the major stage of natural water flooding in the ROZ-forming process. In the microscopic view, the formation of both RFROZ and MTROZ is a process of clustered flow converting to discontinuous flow, and the clustered flow can be scattered to a larger extent in a high permeability zone. The ROZ development feasibility was preliminarily researched. Low-salinity water flooding after the displacement of the formation water in the etching model provided experimental corroboration for remobilizing the residual oil.

Mapping of paleo residual oil zones on the NCS and the potential for production by CO2-EOR

Existence of paleo residual oil zones (PROZ) on the Norwegian Continental Shelf (NCS) has been observed in a number of exploration wells where thickness of the PROZ vary between a few meters and several hundred meters. In a study for Statoil the existence and extent of resources in PROZs have been mapped, and the potential for CO2-EOR in these zones investigated. The main objective of the study was to reveal the magnitude of the resources, how and how much of the PROZ oil that can be recovered and how much CO2 that can be stored. Knowledge of these issues will be important both for management of the petroleum production on the NCS and for future CO2 storage operations.The study has identified 20 Statoil-operated fields with indications of a PROZ below the oil-water contact (OWC). Estimates of in-place volumes are presented for each of the fields, adding up to a total amount of PROZ oil in the identified fields in the order of 1 GSm3. Simulation of tertiary CO2-EOR on naturally water-flooded oil reservoirs with an underlying PROZ has been performed on generic North Sea oil reservoir models to investigate the potential for oil recovery from PROZs. CO2 storage capacity for the identified fields has been estimated based on results from the simulations.

Integrated CO2-Foam Pilot in a Heterogeneous Carbonate Field

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190204, “An Integrated CO2 Foam EOR Pilot Program With Combined CCUS in an Onshore Texas Heterogeneous Carbonate Field,” by Z.P. Alcorn, SPE, and S.B. Frederiksen, University of Bergen; M. Sharma, University of Stavanger; A.U. Rognmo, SPE, University of Bergen; T.L. Føyen, SPE, University of Bergen and SINTEF; and M.A. Fernø, SPE, and A. Graue, SPE, University of Bergen, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed. A carbon-dioxide (CO2) -foam enhanced-oil-recovery (EOR) pilot research program has been initiated to advance the technology of CO2 foam for mobility control in a heterogeneous carbonate reservoir. Previous field tests with CO2 foam report varying results because of injectivity problems and the difficulty of attributing fluid displacement specifically to CO2 foam. A more-integrated multiscale methodology was required for project design to further understand the connection between laboratory- and field-scale displacement mechanisms. East Seminole Field The East Seminole Field in the Permian Basin of West Texas was discovered in the early 1940s with an estimated 38 million barrels of original oil in place (OOIP). The field was developed throughout the 1960s, producing 12% OOIP through pressure depletion. Water floods began in the early 1970s and continued into the 1980s with strategic infill drilling, reducing the well spacing from 40 to 20 acres. Tertiary CO2 floods began in inverted 40-acre, five-spot patterns in 2013 in the eastern portion of the field. Miscible CO2 injection initially increased oil production and reservoir pressure. However, rapid CO2 breakthrough, high producing gas/oil ratio (GOR), and CO2 channeling was soon observed in peripheral production wells. CO2 performance suffers because of reservoir heterogeneity and unfavorable mobility ratios between injected CO2 and reservoir fluids, resulting in poor areal sweep efficiency, high producing GOR, and CO2 channeling. As seen in other areas of the Permian Basin, tilted fluid contacts, presumably resulting from basin activity or a breach of seal, have created a deeper residual oil zone (ROZ). These zones are thought to have been naturally waterflooded through hydrodynamic dis-placement and have been shown to contain considerable immobile oil (20 to 40% OOIP) that can be mobilized by CO2 flood. Thus, the residual oil saturation in the ROZ is similar to waterflooded zones and establishes it as an economically attractive target for tertiary CO2 recovery efforts.

Low carbon oil production: Enhanced oil recovery with CO2 from North Sea residual oil zones

Residual Oil Zones (ROZ) form when oil has leaked or migrated from a reservoir trap through geological time, leaving a zone of immobile oil. Here we assess the feasibility of ROZ production with CO2 flooding, in a North Sea oil field for the first time. We identify a hydrodynamically produced ROZ, with an oil saturation of 26%, in the Pierce Oil Field of the Central North Sea and adapt established recovery factors for Carbon Dioxide Enhanced Oil Recovery (CO2 EOR) from onshore fields, to estimate oil resource and CO2 storage potential. Our mid case results show that CO2 utilisation increases commercial reserves by 5–20% while storing 15 M t CO2. Based on our calculations CO2 EOR can produce low carbon intensity crude oil from a mature basin and could store more CO2 than is released from the production, transport, refining and final combustion of oil.

CO2 EOR and Associated Storage in Residual Oil Zones: Modelling the Evolution and Significance of Oil Saturation in Residual Oil Zones

Residual oil zones (ROZs) are reservoirs in which oil is largely at levels of residual saturation. Such reservoirs cannot be produced by conventional production techniques. ROZs in carbonate reservoirs in the Permian Basin of West Texas were initially interpreted (from wireline logs), as being productive oil zones. If these zones were completed for production, they produced largely water. Over the last decade it has been demonstrated that CO2 injection can make these zones economic to produce. As a result, ROZs in the Permian basin and elsewhere have become attractive targets for CO2-EOR (CO2 enhanced oil recovery) and have a large potential for CO2 sequestration consequential to the EOR activity. The viability of CO2 EOR in ROZs is currently being demonstrated by the results of CO2 injection into the ROZs at the Seminole, Wasson Denver Unit, and Goldsmith oil field. The recoverable oil from ROZs in both the San Andres and Canyon Reef formations of Permian Basin, have been estimated as 12 billion barrels. A key question is the capacity of ROZs to sequester CO2. The current project is the first study of ROZs based on extensive studies of cores, wireline logs, and production data from several ROZs in the San Andreas Formation. Understanding the magnitude of oil saturation and how it varies within ROZs is important to modelling both EOR and sequestration. The commonly accepted model for the formation of ROZ is based on the hydrodynamic effects of tectonically-controlled increased water flows in aquifer at the base of oil fields. In this work, the nature of this process was modelled using a commercial reservoir simulator. These flow simulations were designed to understand how the effects of strength of aquifer flow, flow direction, and capillary pressure on the nature and distribution of oil saturations in ROZs. A special emphasis was on understanding the impact of reservoir heterogeneity on the variation of capillary pressures throughout ROZs. Heterogeneities in capillary pressures appear to dominate the distribution of oil saturation within the ROZ and will also strongly influence the performance for both oil production from CO2 injection, as well as associated CO2 storage. Finally, we discuss the implications of our results to the understanding of both CO2 EOR and storage in ROZs.

Residual Oil Zone “Fairways” and Discovered Oil Resources: Expanding the Options for Carbon Negative Storage of CO2

Recently completed geophysical and reservoir engineering-based investigations have identified the presence of large Residual Oil Zone (ROZ) “fairways” in the Permian and numerous other U.S. and international oil basins. These ROZ “fairways” offer a third geologically viable option, in addition to saline formations and mature oil fields, for storing CO2 captured from industrial sources.The benefits of pursuing ROZ “fairways” as a third geologic option for storing CO2 are many. First, the ROZ “fairways” offer a carbon negative option for storing CO2, where the carbon content of the injected CO2 significantly exceeds the carbon content of the by-product oil recovered as part of injecting CO2. Second, the “fairways” are extensive and continuous, with an established upper seal and have a massive volume of reservoir space for secure, long-term CO2 storage. Third, for much of the ROZ resource, the production of by-product oil will cover the costs of installing and operating the CO2 storage system while also providing revenues for covering a portion of the costs of capturing CO2. Finally, the ROZ “fairways” are located outside and beyond the structural closure of existing oil fields, providing new, accessible geologic settings for storing CO2.In summary, the identification and characterization of ROZ fairways adds a significant new option for efficiently and economically storing CO2. Our discussions with scientists from numerous other countries, namely Australia, Colombia, Kuwait, Lithuania, Norway and Saudi Arabia, confirm that ROZ “fairways” are not unique to U.S. oil basins, but are likely a major international phenomena that can support expanded application of CO2 capture, utilization and storage.

High-level Screening for Williston Basin Residual Oil Zones Using Location-independent Data

Residual oil zones (ROZs) are naturally waterflooded, traditionally noneconomic reservoirs in which potentially large amounts of CO2 could be stored in association with enhanced oil recovery. To date, ROZ exploration throughout the United States has primarily consisted of manual matching of hydrodynamics with structural data in preconceived, specific locations, assessing well- or field-scale analyses, followed by field tests. To locate potential ROZs more quickly en masse, Schlumberger Petrel software was used to build a 3-D geocellular model of the Williston Basin, and Schlumberger PetroMod was used to populate petrophysical properties and simulate hydrocarbon generation, migration, and accumulation.

Evaluation of Technology and Policy Issues Associated with the Storage of Carbon Dioxide via Enhanced Oil Recovery in Determining the Potential for Carbon Negative Oil

Numerous legal and regulatory frameworks in the U.S. and globally are acknowledging the opportunity for greenhouse gas (GHG) emissions reductions offered by combining the long-term storage of CO2 in association with carbon dioxide enhanced oil recovery (CO2-EOR), to support carbon capture and storage (CCS) as a climate mitigation technology. These include the U.S. Environmental Protection Agency (USEPA), the International Organization for Standardization (ISO), the State of California, and the Intergovernmental Panel on Climate Change (IPCC), among many.In this paper, a brief overview is provided of the literature on GHG lifecycle analyses (LCA) applied to CO2 storage in association with CO2-EOR. Then, various techniques for performing LCA related to CO2-EOR operations are summarized. Moreover, since most past LCA analyses of CO2-EOR projects were generally based on historical CO2-EOR operations, LCA based on assumptions from these operations are likely not to represent future emissions reduction opportunities where CO2 storage is a co-objective with increased oil production. Thus, the paper examines different CO2-EOR development options that could greatly increase the amount of CO2 injected, and ultimately stored, to recover incremental oil via the application of CO2-EOR, and speculates on how even greater storage efficiencies with CO2-EOR can be realized.Assuming historically-based values for CO2 utilization, most life cycle analyses of CO2 storage in association with CO2-EOR show that the emissions associated with producing, processing, transporting and/or utilizing the incremental oil produced are greater than the CO2 injected and stored in association with CO2-EOR. However, current CO2-EOR operations are achieving much higher utilization values, and assuming the wide-scale application of “next generation” technologies applied to existing and potential new resource targets, even larger utilization values are realizable.Recent work by Advanced Resources, for example, shows that “next generation” CO2-EOR applied to the main pay zone (MPZ) of oil reservoirs uses, on average, about 0.45 metric tons per barrel of oil produced, while CO2-EOR applied to the residual oil zone (ROZ) underlying and in between existing oil fields uses, on average, about 0.50 metric tons per barrel of oil produced. Many projects are likely to achieve even higher values of CO2 utilization for CO2-EOR. These utilization values are over double that assumed in most traditional LCA analyses applied to CO2-EOR operations. And even greater utilization values are realizable. For comparison, emissions associated with the production, transport, refining, and ultimate combustion of the incremental oil produced are estimated to be on the order of 0,42 to 0.43 metric tons per barrel.Given these values for CO2 utilization with CO2-EOR, accounting for emissions associated with CO2-EOR operations, with activities downstream of CO2-EOR operations (i.e., crude oil transport and refining), and even accounting for the combustion of fuels created from the barrel of crude, the amount of CO2 injected and stored in the reservoir during CO2-EOR can generally be greater than that associated with the emissions associated with the incremental oil produced.Going forward, it will be important that legal and regulatory frameworks for verifying and accounting for GHG emissions reductions acknowledge the importance of CO2 storage with CO2-EOR in achieving GHG emissions goals.

Residual-Oil Zone: Paleo-Oil Characterization and Fundamental Analysis

Summary In most reservoirs around the world, paleo oil exists below the free-water level (FWL) and is considered residual oil to natural/geological waterflood. This nontrivial resource of residual oil will not flow by primary- or secondary-recovery means but requires carefully designed enhanced-oil-recovery (EOR) methods to mobilize it. To date, there is no detailed analysis of paleo oil in the literature simply because it is difficult to obtain a reservoir sample. This study provides a comprehensive paleo-oil analysis for samples obtained from reservoir sponge cores. The oil in the sponge core was extracted, analyzed, and compared with main-pay-zone (MPZ) oil. Critical data have been unveiled on paleo-oil characterization through fundamental studies on oil quality, fingerprint, filling history, available hydrocarbon components and compounds, and molecular-level characterization. It was found that the global composition and overall quality of paleo oil are very similar to those of the MPZ oil. However, the differences between the two oils were only apparent when the study was extended further to include molecular-level analysis and available hydrocarbon components and compounds. These differences may define the appropriate EOR methods to mobilize this oil and explain trapping mechanisms caused by fluid properties. Gas-chromatography (GC) studies revealed that paleo-oil extracts have the same pristane/phytane ratio as the MPZ oil, suggesting that they are of the same origins and share the same source rock. Further analysis showed a good match of the terpane biomarkers between paleo-oil extracts and MPZ oil but with slightly lower maturity levels. Paleo-oil quality was compared with MPZ oil by use of pyrolytic oil-productivity index (POPI) analysis which indicated the same °API range as the MPZ oil and the same light volatile, thermally distilled, and cracked components. Paleo and MPZ oils were also analyzed with low-field nuclear magnetic resonance (NMR) to qualitatively test the similarity of the oil components and to measure their apparent viscosities. Both oils have shown very comparable viscosity measurements and NMR signatures. The simulated distillation analysis showed that lighter components in paleo oil are less abundant than in MPZ oil, whereas medium-to-heavy components are relatively similar. A Fourier transform ion cyclotron resonance (FT-ICR) study, which zoomed into the heavier components, revealed that paleo oil has less aromaticity than MPZ oil and lacks aromatic sulfur and disulfur compounds, a negligible amount of nitrogen compounds, and no resin-type components. This study provides in-depth information about oil extracted from the residual-oil zone (ROZ), which does not flow by primary or secondary recovery means. To our knowledge, there is no available information in the literature that explains the components, compounds, quality, and behavior of this oil because it is difficult to obtain reservoir samples. These data shed light on a possible trapping mechanism caused by fluids in place. The study also used several methods and tools to confirm the conclusions and to ensure repeatability of results.

CO2 storage in Residual Oil Zones: Field-scale modeling and assessment

Residual Oil Zones (ROZs) are formed as the result of secondary tectonic activities which trigger extensive oil remobilization after the primary petroleum migration. The ROZs are attractive targets for CO2 Enhanced Oil Recovery (CO2-EOR) and storage: first, because in many cases, the thickness of the ROZ ensures an overall recoverable volume of oil comparable to that of the Main Pay Zone (MPZ) and second, because the ROZ has favorable containment and capacity for large-scale and long-term EOR-storage projects. This study investigates one of the underlying theories of the ROZ formation, called the Altered Hydrodynamic Flow Fields (AHFF). The impact of the AHFF process on the formation of ROZs is specifically investigated using a field-scale simulation model for a Permian Basin San Andres reservoir. The simulation model is tuned and verified by validating the ROZ's characteristics, such as the thickness of the ROZ, the shape of the saturation profile, and the tilt of the OWC. The tuned Permian Basin San Andres reservoir model is used to simulate the primary recovery and the secondary waterflooding phases in the MPZ. Depending on the CO2 availability and ROZ development strategy, six different development scenarios are specified beyond the waterflooding phase. The corresponding simulations are performed to find the optimum EOR-storage strategies for the MPZ-ROZ through the extensive comparison of key performance parameters, including the cumulative oil recovery, CO2 storage and, net CO2 utilization. The results confirm the technical viability of CO2-EOR and storage in the ROZ. The most favorable expansion strategy in terms of oil production and CO2 storage is the simultaneous development of the MPZ and the ROZ from the beginning of EOR-storage process. Most importantly, the study demonstrates that the volume of the utilized CO2 has a substantial effect on the success of the EOR-storage. While other expansion strategies such as sequential development also provide reasonable oil production response and CO2 storage potential, early project expansion into the ROZ without sufficient investment in CO2 resources is shown to be detrimental to the economics of the project. Finally, several important technical considerations for CO2 storage are qualitatively discussed, including assessment of the ROZ storage capacity, saltwater disposal requirements, and reduced risk of CO2 leakage in the ROZ.

Possible occurrences of transition zones and residual oil zones below oil-water contacts in mature oil fields, southeast Saskatchewan

A transition zone (TZ) from oil to water formed by capillary force beneath traditionally defined producing oil-water contact (OWC) has been well-known for years. Recent research and production activities in the Permian Basin and the Williston Basin in United States suggests that a residual oil zone (ROZ) may exist below traditionally defined OWC under certain geologic and hydrodynamic conditions. If this ROZ is sufficiently thick and extensive, it may be economically recoverable using tertiary recovery techniques. We have investigated possible occurrences of TZ/ROZ in the Williston Basin, southeast Saskatchewan, using well-log interpretation based on Archie’s equation. Out of 33 pools examined, we have evaluated three areas, the Bellegarde Tilston pool area, an unnamed Souris Valley pool, and the Rosebank Alida pool area, as possibly having thick (up to 47 m) low-oil-saturation TZ/ROZs below the OWCs. Evidence includes oil stains, core-based oil saturation, fluorescence on drill cuttings, and oil shows and sulfur water reported in drill stem tests. The oil in place within the possible TZ/ROZs is estimated to be approximately half the amount of the oil within their associated main pay zones. Tilted OWC and other findings may suggest that the identified oil-bearing zones are ROZs related to hydrodynamics. The hydrodynamic flows may also have created some other ROZs that still remain undiscovered.

4D reservoir characterization using well log data for feasible CO2-enhanced oil recovery at Ankleshwar, Cambay Basin - A rock physics diagnostic and modeling approach

In recent years, rock physics modeling has become an integral part of reservoir characterization as it provides the fundamental relationship between geophysical measurements and reservoir rock properties. These models are also used to quantify the effect of fluid saturation and stress on reservoir rocks by tracking the changes in elastic properties during production. Additionally, various rock physics models can be applied to obtain the information of rock properties away from existing drilled wells, which can play a crucial role in the feasibility assessment of CO2-enhanced oil recovery (EOR) operation at field.Thus, the objective of this study is to develop a rock-physics model of the Ankleshwar reservoir to predict the reservoir response under CO2-EOR. The Ankleshwar oil field is a mature field situated in Cambay Basin (Western India) that witnessed massive peripheral water flooding for around 40years. Since the field was under water flooding for a long term, reasonable changes in reservoir elastic properties might have occurred. To identify potential reservoir zone with significant bypassed (or residual) oil saturation, we applied the diagnostic rock physics models to two available wells from the Ankleshwar oil field. The results clearly indicate transitions from clean sands to shaly sands at the base, and from sandy shale to pure shale at the top of the reservoir pay zone, suggesting a different seismic response at the top when compared to the base of the reservoir in both the wells. We also found that clay content and sorting affects the elastic properties of these sands, indicating different depositional scenario for the oil sands encountered in the Ankleshwar formation. Nevertheless, the rock physics template (RPT) analysis of the well data provides valuable information about the residual oil zone, a potential target for CO2-EOR. Further, a 4D reservoir characterization study has been conducted to assess the seismic detectability of CO2-EOR, and we observed that the injected CO2 changes the Vp, Vp/Vs and P-wave impedance (PI) dramatically, suggesting a reasonable change in the seismic response of the reservoir, even if we consider patchy saturation model. Thus, it is highly advisable to conduct 4D/time-lapse seismic monitoring in this field for optimized production by CO2-EOR and the reservoir management.