Tracer technology has evolved significantly over the years and is now being increasingly used as one of the effective monitoring and surveillance (M&S) tools in the oil and gas industry. Tracer surveys, deployed as either interwell tests or single-well tests, are one of the enabling M&S technologies that can be used to investigate reservoir connectivity and flow performance, measure residual oil saturation, and determine reservoir properties that control displacement processes, particularly in improved oil recovery (IOR) or enhanced oil recovery (EOR) operations.
As part of a comprehensive monitoring and surveillance program for a GAS-EOR pilot project, an interwell gas tracer test (IWGTT) was designed and implemented to provide a better understanding of gas flow-paths and gas-phase connectivity between gas injector and producer pairs, gas-phase breakthrough times ("time of flight"), and provide pertinent data for optimizing water-alternating-gas (WAG) field operations. Additional objectives include the detection and tracking of any inadvertent out-of-zone injection, and acquisition of relevant data for gas reactive transport modelling. Four unique tracers were injected into four individual injectors, respectively, and their elution were monitored in four "paired" updip producers.
In addition to the reservoir connectivity and breakthrough times between the injector and producer pairs, the results showed different trends for different areas of the reservoir. The gas-phase breakthrough times are slightly different from the water tracer breakthrough times from a previous inter-well chemical tracer test (IWCTT). Residence times for the tracers indicate different trends for three of the injector-producer pairs compared to the last pair. These trends reflect and support conclusions regarding reservoir heterogeneities also seen from the previous IWCTT, which were not anticipated at the beginning of the GAS-EOR pilot.
This paper reviews the design and implementation of the tracer test, field operational issues, analyses, and interpretation of the tracer results. The tracer data has been very useful in understanding well interconnectivity and dynamic fluid flow in this part of the reservoir. This has led to better reservoir description, improved dynamic simulation model, and optimized WAG sequence.
Chen, Hsieh (Aramco Services Company: Aramco Research Center-Boston) | Kmetz, Anthony A. (Aramco Services Company: Aramco Research Center-Boston) | Cox, Jason R. (Aramco Services Company: Aramco Research Center-Boston) | Poitzsch, Martin E. (Aramco Services Company: Aramco Research Center-Boston)
Full field inter well tracer programs have become more and more ubiquitous for effective reservoir surveillance. Novel tracer materials with much higher detectability and lower costs have been actively screened. One of the biggest challenges in deploying novel material types, however, is their elevated irreversible retention to reservoir rocks. Herein we benchmarked traditional inter well tracer chemicals and then the sensitivity of ever-increasing irreversible retention that might be associated with unconventional materials.
Using field-scale reservoir simulations with a Langmuir-type tracer irreversible retention model, we rigorously test the limits for tracer irreversible retention in order to have successful inter well tracer test (IWTT). Specifically, we studied the tracer breakthrough peak concentrations as a function of tracer irreversible retention as well as inter well spacing in synthetic waterflood patterns. Custom reservoir simulator functionalities were built to perform the simulations. Additionally, coreflood experiments on common oil field tracers were conducted to acquire independent irreversible retention values and compared to the modeling results.
For the reservoir simulations, we first tested the ideal tracer case with no irreversible retention and found perfect agreement with the standard Brigham-Smith model. We then tested for tracer breakthroughs with increasing irreversible retention values and found that the tracer breakthrough peak concentration drops off dramatically. With the consideration that the limit of detection (LOD) of contemporary analytical instruments are at the part per trillion (ppt) level, the simulation results suggested that the tracer irreversible retention should be less than 10 μg/g-rock (mass of adsorbed tracer / mass of rock) in order to have meaningful IWTT with a well spacing of 2000 ft and an injection tracer mass up to 100 kg. Finally, two field tests using fluorobenzoic acid (FBA) based tracers deployed in the highly saline and retentive carbonate reservoirs in Saudi Arabia were compared. The irreversible retention number of the FBA based tracers was estimated to be less than 5 μg/g-rock from the model. Corresponding coreflood experiments for FBA tracers in high temperature and salinity carbonate cores show 0 +/− 10 μg/g-rock irreversible retention number within error ranges, verifying the prediction of our simulation results.
This paper broadens the scope of the extensively used Brigham-Smith tracer behavior model by incorporating tracer irreversible retention effects. More accurate design and interpretation of inter well tracer tests may be achieved through the new insights presented. Better waterflood management can then be established because of the reduced uncertainties from the more precise tracer data. In addition, this study set an unambiguous standard for the tolerable irreversible retention limits for any new materials targeting inter well tracing applications.
This paper sheds light on the design of a Partitioning Interwell Tracer Test (PITT) for a normal 5-spot chemical EOR pilot targeting the Sabriyah Mauddud (SAMA) carbonate reservoir in Kuwait. This pilot is currently going through the water pre-flush phase which will be followed by chemical injection in the near future.
Due to recent improvements in the synthesis of partitioning tracers, water-based partitioning tracers can now be utilized to evaluate variations in oil saturation pre and post EOR applications as well as interwell connectivity for chemical EOR pilots. A PITT is planned to take place in support of a normal 5-spot chemical EOR pilot in the SAMA reservoir. Four unique passive tracers will be injected into the pilot injectors prior to better understand reservoir conformance. This will be followed by the co-injection of four passive-partitioning tracer sets to evaluate oil saturation before and after chemical injection.
A fit-for-purpose facility set-up has been installed to perform quasi-simultaneous injection of different tracer packages into the four EOR pilot injectors and a robust sampling strategy was developed to analyze the produced fluids from the pilot producer and sampling observation well in addition to other offset producers surrounding the pilot area. Periodic lab analyses involving techniques such as Gas Chromatography coupled with Mass Spectroscopy (GC-MS) will be performed to analyze tracer breakthrough times, peaks, dispersion and partitioning-passive tracer relationships. The findings from this PITT will be used to develop a comprehensive understanding of oil distribution and inter-well connectivity within the pilot area to assess the techno-economic feasibility of multi-pattern chemical EOR deployment.
This PITT in a major carbonate reservoir is one of the first reported field cases to evaluate variations in oil saturation and interwell connectivity for subsurface chemical EOR applications in a normal 5-spot pattern pilot area.
This paper presents a systematic approach to evaluate limited crossflow between layers of a stratified reservoir using interwell chemical tracer test. Previous studies either entail no crossflow between layers or assumes an established vertical equilibrium across transverse direction (owing to viscous/capillary/gravity driving mechanisms); however, they fail to detect the presence of limited crossflow between layers through a ‘bridge’; ‘bridge’ in this study is defined as apathwaythrough which crossflow takes place with an adjacent layer. Obviously, crossflow may take place through several bridges along a layer.
A new formulation is developed to study and detect the limited crossflow. The crossflow is detected through monitoring of the fraction of total injected fluid that arrives at the producer from each layer as a function of time. We employed both numerical simulation and field examples to verify the proposed method. A transition period is identified during which thefraction of injected fluid flowing through each layer significantly changes because of crossflow. Our results indicate that tracer can be used to evaluate number of bridges. The distance between injector and the bridge location can significantly change the interwell tracer results. Identification of crossflow between layers and evaluation of transition zone significantly improves the current efforts to understand reservoir complexity and sweep efficiency. The outcome of this research can help design more successful EOR processes.
The residual oil saturation obtained from SWCTT is critical for designing enhanced oil recovery (EOR). However, a key assumption in conventional SWCTT is that only single phase (water) is mobile. In reality, this is often not the case, and significant error can occur if the conventional SWCTT analysis method is used when multiple phases flow at the same time. The objective of this study is to improve the accuracy and precision of SWCTT interpretation in multi-phase flow condition.
In this paper, we propose an innovative procedure of modified SWCTT and the method of moment (MoM), aiming at the two-mobile-phase condition. In the development of the algorithm, a ratio parameter is introduced to adjust the calculated swept volume difference between the conservative tracer and partitioning tracer. In addition, a mixture injection of oil and water is required, instead of pure water injection in SWCTT.
The proposed approach is verified through numerical simulation on synthetic cases with known input parameters. The simulated models consist of a radial flow regime with a single vertical well in the center. The input oil saturation varies from 0.1 (immobile oil saturation) to 0.9. Our results show that the saturation estimated from modified MoM matched the simulation input data, which indicates that our approach is able to capture the saturations under two-mobile-phase condition. Moreover, the modified MoM can also be applied in single-mobile-phase condition and improved accuracy of conventional MoM.
This paper reviews the design and implementation of a full-field interwell tracer program for a giant onshore oil field in Abu Dhabi. The field is under peripheral waterflooding in order to maintain reservoir pressure and provide a mechanism to sweep the oil. However, the existence of strike-slip fault planes juxtaposed across the reservoirs added a variable to the complexity of waterflood management. To improve the understanding of reservoir heterogeneity and reduce the uncertainties associated with major faults, a full-field water-tracer program has been designed.
Water injection is extensively applied as an oil recovery method. This generates a large volume of produced water that must be separated from the production stream and disposed with technical, economic and environmental consequences. The optimized use of water resources in the oil production is linked with the swept efficiency or drainage of that water in the reservoir, which is the aim of this article. The inaccessibility of petroleum reservoirs associated with the lack of tools able to effectively characterize the rock formation channels make tracers a very important qualitative and quantitative evaluation technique to date and indispensable to the management of such water. The hydrodynamic characterization of channels traversed is possible by previously marking the fluid with a water soluble conservative tracer, which, by definition, flows through the formation dissolved in water without interacting with the rock-fluid system, i.e., without chemical reaction or adsorption (conservative or ideal tracer). This work builds a connection between laboratory experiments and field data. Laboratory tests were performed in a linear (1-D) physical model (with target-field rock) and in a 5-spot physical model (outcrop rock), using the target field water and with temperature of the target field (1-D test) or room temperature (2-D test). The field application was performed in a reduced pilot consisting of an inverted 5-spot unit (1 injection well and 4 production wells). The mathematical simulation of 1-D laboratory data was made using the traditional convection-diffusion equation. Field and laboratory 5-spot data were adjusted using a mathematical model (2-D convection-diffusion equation) and its analytical solution through the Generalized Integral Transform Technique (GITT) for an ideal 5-spot pattern. This study suggests that working with a tracer pulse, instead of a slug, is the best strategy and facilitates the interpretation of lab and/or field results. Furthermore, it is possible to estimate the swept efficiency using the mathematical interpretation of a pulse. The collected information allows the interpretation of the hydrodynamic characteristics of the studied field area and serves as a basis for future EOR applications of polymers and/or surfactants.
The interest in low-salinity-water injection (LSWI) compared with seawater injection or high-salinity-produced-brine injection is increasing in both laboratory and field tests. The single-well chemical-tracer test (SWCTT) is also becoming increasingly popular as an in-situ test to assess the reduction in oil saturation caused by an enhanced-oil-recovery (EOR) process. Hence, accurate modeling of SWCTTs is essential. In this paper, modeling and simulation of the SWCTT of LSWI in a carbonate reservoir is investigated by use of the UTCHEM reservoir simulator, a nonisothermal, 3D, multiphase, multicomponent, chemical compositional simulator developed at the University of Texas at Austin (UTCHEM 2011). Both radial- and Cartesian-grid models are set up for a field-scale pilot by use of measured rock and fluid data of a Middle Eastern reservoir. Tracer reactions and the empirical LSWI model implemented in UTCHEM are used to estimate residual oil saturation (ROS) as a result of LSWI. Two approaches are used to estimate ROS to LSWI, including analytical and numerical methods. Results show that both approaches give consistent values for ROS for homogeneous radial- and Cartesian-grid models. The two approaches were inconsistent for the multilayer radial model, which highlights the necessity of the use of numerical approaches for layered reservoirs. The Cartesian-grid model was used to investigate the effect of heterogeneity on SWCTT results, where a new numerical approach is proposed for estimating ROS. This finding validates the approach used and the implementation of both tracer reactions and the LSWI model in UTCHEM. The proposed approach can now be used to estimate ROS of the SWCTT for reservoirs with different degrees of heterogeneity, which provides a clear insight into reservoir performance before planning multiwell demonstration pilots.
The single-well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs. Most often, residual oil saturation is measured; less frequently, connate water saturation (Swc) is the objective. Either saturation is measured where one phase effectively is stationary in the pore space (i.e., is at residual saturation) and the other phase can flow to the wellbore. Recently, the SWCT method has been extended to measure oil/water fractional flow at measured fluid saturations in situations in which both oil and water phases are mobile.