Calibrated rock physics model for quantitative seismic analysis of two-phase fluid saturations

Introduction The project aimed to improve the accuracy of seismic monitoring methods, which are crucial for oil and gas recovery and CO2 storage on the Norwegian continental shelf. Repeated seismic surveys, along with seismic data analysis, are commonly used to detect changes in deep underground reservoirs during hydrocarbon production and CO2 injection and storage. One of the most important aspects to evaluate in reservoir performance is the change in fluid quantities contained in the subsurface geological structures. However, current seismic analysis methods contain high uncertainties in fluid amount estimates. Accurate information in this area is essential for more sustainable and efficient use of resources for oil field development and facilitates the safe reduction of carbon dioxide in the atmosphere. Objective and Methodological Approach The primary goal of the project is to enable more precise and reliable quantification of various fluids within subsurface reservoirs, particularly during different stages of oil and gas production and CO2 storage. These fluids typically create mixtures as a result of processes like water injection aimed at enhancing hydrocarbon production, and CO2 injections intended for long-term storage. Our approach focused on the development of a calibrated rock physics model for partially saturated reservoir rocks. This model plays an important role in seismic inversion, a process essential for interpreting the properties of rock and fluid based on seismic data. To conduct this research, we established a unique laboratory system for testing reservoir rock samples. This system was designed to merge seismic frequency measurements with advanced x-ray imaging techniques, thereby enabling us to closely simulate the actual conditions within a reservoir, including the injection of fluids such as water, oil, and CO2. This also allowed us to estimate fluid quantities, distributions, and the resulting seismic response. The subsequent analysis of the experimental results allows for creating better-performing physical models. Key Findings and Activities 1. Theoretical Advances: We conducted a comprehensive review of relevant rock physics models, selecting the most applicable ones and determining essential parameters for our experiments. 2. Innovative Laboratory System: We developed a novel, state-of-the-art X-ray transparent, seismic frequency apparatus specifically for this project. This system underwent extensive testing, calibration, and validation phases to ensure its accuracy and reliability. 3. Extensive Experiments: Our project included four major experimental campaigns, employing seismic frequency methods. These campaigns were diverse, ranging from experiments on fully and partially saturated sandstones to assess their seismic properties in relation to saturation, wave-strain amplitude, and frequency; to the analysis of the effects of pore fluid capillary pressure on seismic properties; investigations into the mechanisms of seismic dispersion across different lithologies; and comprehensive micro-CT experiments aimed at evaluating the effects of drainage-imbibition and free CO2 saturation on both seismic and ultrasonic properties of sandstones. 4. PhD candidate: An integral component of our project was the education and integration of a PhD candidate. The involvement of the candidate not only significantly enriched our research but also boosted the successful collaboration between NTNU and SINTEF. 5. Dissemination of Knowledge: Our team was committed to actively disseminating our findings. We made substantial contributions, totaling over 40 presentations and papers, at various conferences, workshops, and in leading peer-reviewed publications. Our collaborative efforts were further enhanced by engaging with international and industry partners, notably Equinor and AkerBP, facilitated through regular project workshops hosted by SINTEF. Impact and Conclusive Insights This project represents a significant advancement in seismic monitoring technologies, directly impacting the efficiency and sustainability of resource management in the oil and gas sectors. Our innovative approach and findings provide a valuable framework for future research and practical applications, particularly in enhancing hydrocarbon recovery and CO2 storage. The integration of advanced rock physics models, calibrated with unique laboratory experiment results, marks a pivotal step in subsurface monitoring, setting a new standard for the industry and contributing to global efforts in environmental conservation and responsible resource utilization.

Actual Outcomes: 1. Advancement in the field of Rock Physics: The advancements in rock physics research achieved withing the project led to a solid theoretical foundation for use of more sophisticated and calibrated rock-physics model in seismic data processing workflows. This will lead to more accurate identification and quantification of various fluids in subsurface reservoirs, an advancement in seismic technology. 2. Innovations in Laboratory Systems: The creation of a novel X-ray transparent, seismic frequency apparatus for experimental use in this field has set a new standard for laboratory research in rock physics and seismic analysis. 3. Educational Impact: The training and integration of a PhD candidate within the project not only bolstered research capabilities but also ensured the transfer of knowledge and skills to future generations in this field. 4. Collaboration impact: continuous interaction between NTNU, SINTEF, international partners and Norwegian oil industry has created a unique collaborative environment that jointly solve current challenges within rock physics and seismic monitoring. Potential Impacts: 1. Improved Hydrocarbon Recovery: By enabling a more precise understanding of fluid seismic response in reservoirs, the project results have a potential to enhance the efficiency of hydrocarbon recovery during reservoir production. This is achieved by obtaining more accurate information on expediency of infill drilling targets. 2. Enhanced CO2 Storage and Security: The project has a potential for improving the methods for monitoring CO2 injection and storage, vital for both environmental protection and the safe utilization of carbon dioxide in the subsurface reservoirs. 3. Sustainable Resource Management: The project lays a foundation for more sustainable and efficient use of resources in oil field development, paving the way for future technologies and methodologies that prioritize environmental conservation. 4. Framework for Future Research: The methodologies and findings from this project provide a valuable framework for future research, particularly in enhancing techniques for monitoring subsurface reservoirs. 5. Global Environmental Benefits: The improved capacity for CO2 storage and better monitoring of greenhouse gases highlight the project's potential contribution to global efforts in reducing atmospheric carbon dioxide levels. 6. Industry Standard Setting: The integration of advanced rock physics models, calibrated with unique experimental results, sets new industry standards in subsurface monitoring, potentially influencing future developments in the oil and gas sectors. 7. Collaborative Opportunities: The project's success, facilitated through collaborations with various international and industry partners, demonstrates the potential for future joint ventures that combine academic research with industry expertise.

There is a need for reliable monitoring techniques that would ensure safe and efficient reservoir injection for enhanced oil recovery (EOR) as well as for CO2 storage. Currently, time-lapse seismic methods allow for mapping injected fluid distribution in the subsurface. However, due to a lack of reliable rock physic models, estimates of saturations in two-phase fluid systems are associated with large uncertainties. This project will study the dependence of elastic rock properties and acoustic velocities of reservoir rocks on saturation and fluid pressure. The main focus will be on advanced laboratory experiments for validating and calibrating a newly developed physics-based rock physics model. Different factors that control velocities and attenuation of acoustic waves in two-phase fluid saturated rocks, such as patch size, capillary pressures, permeability, and frequency will be investigated. A custom-built unique low-frequency pressure cell within a high-resolution X-ray tomography scanner will be the primary analysis technique. Partners of the project include SINTEF, NTNU, the University of Edinburgh (UK), and The University of Texas at Austin (USA). In addition, industry represented by Equinor and Aker BP is willing to contribute with technical advice.