The key objective of the CO2 WAG Pilots is to confirm improved sweep and to enhance oil recovery under CO2 WAG relative to water flooding. Two CO2 WAG Pilots are in progress in a giant Abu Dhabi Oil Reservoir. Each pilot has one horizontal producer and two horizontal injectors along with two vertical pilot observers. A detailed monitoring plan was designed and implemented to monitor pilots’ performance and track CO2 breakthrough and flow path. Injectivity of both water and CO2 was determined in the WAG cycles to investigate any loss of injectivity. The producers are being tested daily for oil rate, water cut, GOR using multi-phase flow parameters (MPFM) while portable test separators are used every quarter to validate these measurements. PVT analysis of produced fluids are being carried out on samples from portable test separators and MPFM sampling point to monitor CO2 content. Different gas and water tracers have been injected to trace the movement and breakthrough of injected fluids into the pilot producers. Carbon and oxygen-isotope analysis for produced and injected CO2 gas is also carried out to monitor CO2 breakthrough. RST logs in the observers demonstrate good sweep across different layers of the reservoir and show that WAG is providing mobility control to CO2. Several data sources were analyzed to determine CO2 breakthrough time and the CO2 flow path. Analysis of CO2 in produced gas has determined the timing of CO2 breakthrough. This is supported by the isotopic analysis of injected and produced CO2 in pilot producers and near-by producers. The tracer analysis results unambiguously identify the source of the produced CO2. Injectivity analysis of both CO2 and water showed injectivity of CO2 was either the same or higher than water injectivity. Moreover, no loss of injectivity was observed between WAG cycles. The pilot has been operated successfully without HSE issues since 2016. Corrosion logs are acquired within the extensive monitoring program along with inhibitor injection to avoid any Asphalting deposition. The paper discusses the performance of the first multi-well CO2-WAG pilots in a giant onshore reservoir in Abu Dhabi which is used to de-risk multiple CO2 WAG full field projects in ADNOC reservoirs. It also highlights the importance of the different reservoir monitoring tools for improved understanding of the pilots which will be used as a basis for implementing CO2 WAG for the full area development.
Continual development of multi stacked super giant brown field in Abu Dhabi, over 60 years, has induced high congestion at surface resulting in challenging wellhead placement, rig movement and overall field connectivity. The conventional surface to subsurface approach has led to an increased number of undrillable wells, which impairs future developments and consequently, the ability to meet production targets and manage the field proactively. This paper proposes an alternative to the surface-subsurface usual development solution that can unlock existing reservoirs potential, while being cost conscious and flexible to cope with business uncertainties and dynamics. Working towards ensuring future sustainability of a brown field has proven its own complexity in developing additional value with minimal cost. Intensive focus is required on the critical factors that play a key role on brown field investment such as subsurface and surface congestion, HSE (Health, Safety, Environment), aging of facilities and it's replacement plan, and environmental issues. One such key factor is the subsurface and surface congestion, which is especially complicated due to different development schemes for each of the stacked reservoirs based on their API gravity and varying reservoir quality. The Shared Earth Model was established due to the absence of a holistic approach to strategically sustain the expansion in the brown field growth till the end of last oil/gas production drop. It is essential to allow all stakeholders to be exposed and visualize the subsurface data openly to collectively decide on the best way forward for each project investment, focusing in sector-by-sector growth. Another objective is locating surface facilities that are of lesser interest to subsurface development and plan their opportunistic removal (when conflicting with the subsurface interests), which will result in a dramatic decrease in undrillable wells by creating space at surface for the drilling location resulting in higher gain by giving priority to oil/gas reservoirs proposed development pattern. There are also challenges to proof quantitively the subsurface congestion and report the long-term risk collectively, which were addressed while developing this system. Allowing each system to talk to each other and work towards digitalization of the entire system for the full field development could not be achieved without this subsurface data platform.
Carbonate reservoir X has varying levels of maturity in terms of development. The South/West is highly matured; development activities have recently kicked-off in the Crestal part while the areas towards the Far North is not fully developed and posed the largest uncertainty in terms of reservoir quality, fluid contacts, oil saturation, well injectivity/ productivity, area potential and reserves due to poor well control. In reservoir X with segmented development areas, patches of bitumen have been found in the Far North. The extent of this Bitumen was unknown. In order to expand the CO2 development concept to achieve production target from the Far Northern flank, an understanding and mitigation of the area uncertainties is crucial. Reservoir bitumen is a highly viscous, asphaltene rich hydrocarbon that affects reservoir performance. Distinguishing between producible oil and reservoir bitumen is critical for recoverable hydrocarbon volume calculations and production planning, yet the lack of resistivity and density contrast between the reservoir bitumen and light oil makes it difficult, if not impossible, to make such differentiation using only conventional logs such as neutron, density, and resistivity. This paper highlights the utilization and integration of advanced logging tools such as nuclear magnetic resonance and dielectric, in conjunction with routine logs, pressure points, RCI samples, vertical interference test and core data to differentiate between reservoir bitumen and other hydrocarbon types in the pore space. The major findings from the studies shows bitumen doesn't form as a single layer but occurs in different subzones as patches which is a challenge for static modelling. When high molecular weight hydrocarbons are distributed in the pore space and coexist with light and producible hydrocarbons, reservoir bitumen is likely to block pore throats. The Bitumen present in this reservoir have a log response similar to conventional pore fluids. The outcome of this study has helped in refining the bitumen boundary, optimize well placement, resolved the uncertainties associated with deeper fluid contacts and provided realistic estimate of STOIIP.
The need for higher oil recoveries and longer production plateaus have led to the large scale implementation of Enhance Oil Recovery mechanisms across carbonate reservoirs in the world, especially in brown fields. The success of these mechanisms relies heavily on the accurate description of geological phenomenon and their characterization in static models. This paper summarizes the challenges of successful development of a mature, highly heterogeneous carbonate reservoir in a brown field in the Middle East with presence of Bitumen in the reservoir intervals, using CO2WAG mechanism. This paper discusses different aspects of Bitumen characterization, beginning with a brief summary of the geological concept behind the preferential Bitumen accumulation within highly cemented intervals using high resolution core & thin section descriptions in the area. The lateral distribution of these intervals was then mapped by integrating core, signatures from logs (reduced porosity), high seismic amplitude signatures in 3D volume and production/injection data from nearby development wells. To capture this phenomenon in the static models, Bitumen was modeled as a discrete property guided by the geological concept. The porosity model includes the impact of Bitumen as the logs capture the degradation. The permeability model was modified by reducing the permeability in cells with Bitumen with a multiplier, since core RCA is subject to cleaning which may result in non-representative measurements. The major findings & conclusions of the project are attributed to the detailed appraisal campaign in this area of the field with focus on identifying and refining presence & distribution of Bitumen using nuclear magnetic resonance logs. MDT data with Vertical interference tests at points above and below the Bitumen confirm no communication. This has impacted the placement of wells within Bitumen area, since CO2WAG mechanism relies on sweep from upward rising CO2 plume which is obstructed by presence of heavy continuous Bitumen accumulations. Improved saturation distribution in models is achieved by using dielectric saturation logs, which results in reduced uncertainty for STOOIP quantification within Bitumen rich regions of the field. An injector-producer pair of Early Production Scheme wells is planned in which will confirm performance with current placement scenarios based on above understanding of Bitumen. The case study identifies and significantly demonstrates the impact of geological phenomenon on the recovery & sweep efficiency of CO2WAG mechanism. Development scenarios must consider the inherent reservoir complexities that are recognized by detailed geological studies, in order to provide representative forecasts that in turn influence the economic viability of the project.
In alignment with most company's vision to reduce greenhouse effects by expanding CO2 carbon capture and utilization for oil fields, CO2 gas is injected into the reservoir to enhance oil displacement and maintain reservoir pressure. Since CO2 gas is miscible in the oil, the gas flood front is not piston-like; the gas is slowly absorbed by the oil changing the oil properties. Detection of the CO2 plume is necessary to monitor CO2/ injected water/ oil distribution (at and away from the well) and to track the effectiveness of the complicated CO2 WAG injection process in terms of improved sweep efficiency. Monitoring CO2 flood front movement requires a logging measurement that can distinguish oil from CO2 in the formation. This measurement must have a sufficiently large dynamic range for the change from no CO2 to a high CO2 saturation to be distinguished within measurement error. This paper highlights the utilization and integration of Reservoir Analysis System (RAS) and Noise tools in CO2 plume detection. It also highlights some of the limitations of the RAS tool in high water transition zone in CO2 WAG environment. As the oil is push away from the injection well, the flood front will be characterized by oil having varying amounts of miscible CO2 gas dissolved in it. The rate and composition of the injection gas determines how much of the oil that will expand. The density of the oil may not change much (since the oil density is close to the CO2 gas density) but the hydrogen density of the oil will decrease considerably as is the case for CO2. Another factor that controls the CO2 plume movement is the stratigraphy of the heterogeneous carbonate reservoir – where the coarsening upwards trend of the facies associations of High Stand Systems Tract (HST) coupled with pervasive cementation, creates non-uniform movement of CO2 plume vertically and spatially. Logging results shows Neutron porosity (PHIRN) and PNC near/far capture ratio are the preferred log measurement curves because they have the most sensitivity to the large decrease in hydrogen index when CO2 replaces oil or is dissolve in oil. Other curves like Ratio of Near to Long at Burst (RLNB), Long Inelastic Rate Sigma (LIRS), temperature and noise logs also play significant roles in CO2 plume indications. The outcome of this study has helped understand the vertical and lateral extent of the CO2 WAG flood front, estimate saturations, evaluate the well integrity, and locate un-swept zones. Additionally, RAS data is hard data used as calibration (blind test) to check the robustness of the dynamic model.
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