Air injection-based enhanced oil recovery processes are receiving increased interest because of their high recovery potentials and applicability to a wide range of reservoirs. However, most operators require a certain level of confidence in the potential recoveries from these (or any) processes prior to committing resources. This paper addresses the challenges of predicting field performance of air injection projects using laboratory and numerical modelling. Laboratory testing, including combustion tube tests, ramped temperature oxidation and accelerating rate calorimeters can supply data for simple analytical models, as well as providing important insights into potential recovery-related behaviours. These tests are less suited to providing detailed kinetic data for direct and reliable use in numerical simulators. Indeed, the oxidation reactions are sufficiently complex that, regardless of how powerful the thermal reservoir simulator is, its predicting capability will strongly depend on the engineer's understanding of the process and ability to model the most relevant oxidation behaviours of the particular oil reservoir under study. It is proposed that the optimum design cycle for air injection-based processes is to perform laboratory testing that would aid in the understanding of the process and in the design and monitoring of a pilot-scale field operation. Analytical models and simplified, semi-quantitative reservoir simulation models would be employed at this stage. If this evaluation stage is successful, a pilot operation would be initiated and the data gathered during the pilot, as well as laboratory oil property and compositional data, would then be used to history match and tune a model for predictions of the full field operation. Introduction This paper has been written in response to questions which many reservoir engineers express when evaluating the feasibility of air injection as an enhanced oil recovery process for their fields. Questions such as, "What laboratory tests are available? What type of data is provided by each test? How do we use the lab results to predict field performance?" are not uncommon, and, although there are not straightforward answers, a discussion on the usefulness of different lab tests is presented to clarify some of the related concepts. This document has also been written in response to the concerns and comments expressed by many reservoir simulation practitioners when matching combustion tube tests and other supporting oxidation experiments, and trying to predict field performance of an air injection project based on kinetic parameters obtained from such tests. Questions such as, "How do we use the lab data in the reservoir simulator? What are the limitations of thermal reservoir simulation when predicting field performance of air injection projects?" are addressed to provide additional feedback and promote further discussion. Additionally, this manuscript describes some of the combustion behaviours which have been observed by the In Situ Combustion Research Group (ISCRG) at the University of Calgary while performing combustion tube tests and supporting cracking/oxidation experiments, and gives some recommendations to improve the modelling of the combustion process using thermal reservoir simulators.
In-situ combustion (ISC) is a promising enhanced oil recovery process for the vast heavy oil accumulation of the Orinoco Belt in Venezuela. Combustion tube tests were performed to assess the feasibility of the process in a reservoir of the area. Given the successful laboratory results, it was decided to proceed with the design of a pilot test. Along with the basic design calculations, a simulation model was built to aid in selecting optimum well locations and operating strategies of the pilot. This would also be used for history matching of the actual operation and further optimization. One of the features of the model is the inclusion of the foamy oil behavior exhibited by the oil. For the modeling of the combustion process, a kinetic model developed in-house by PDVSA Intevep using thermo-gravimetric and scanning calorimetry experiments from an analog field, was employed. The first stage of the study involved the characterization of the oil into the same pseudo-components utilized by the kinetic model. A match of relevant PVT data was done for this purpose. In the second stage, the field scale model was history matched with the new fluid model, which included the foamy oil behavior. The best agreement with field measured data was obtained with a dispersed-gas foamy oil model with velocity dependence of the reaction that converts the low-mobility dispersed gas into high-mobility free gas. The following stage consisted of the history match of the combustion tube test, which was partly achieved with an assisted-history-matching tool. In the last stage, the results obtained from the combustion tube match were applied to the field model. In order to determine the most appropriate locations of production and injection wells, several pilot configurations were studied combining vertical and horizontal wells. A sensitivity analysis was completed using operational parameters such as injection rates and distance between producers and injectors wells. Based on ultimate recovery, the best pattern configuration was selected along with the optimum operational parameters. This paper illustrates the application of a workflow for modeling ISC from laboratory experiments to the field scale.
As much of the oil in the Akal field of the Cantarell complex is contained in the low permeability oil wet matrix, foam injection has been proposed as a method to control fluid mobility in the fracture, with the added benefit of transporting surfactant into the matrix so that additional oil can be liberated through a reduction of interfacial tension between oil and water. Presented in this paper is the work flow undertaken during an extensive study of all available laboratory experiments and pilot single well foam injection tests. Laboratory experiments ranged from simple water plus surfactant imbibition tests and surfactant flooding tests, to more complex foam flooding in split core experiments and co-injection of surfactant and gas for generation of foam in-situ. There were three field pilot single well foam injection tests that were included in this analysis that were of the huff-and-puff design. This extensive analysis was done with the aid of numerical simulation that resulted in the development of a novel foam regeneration model that handles both mobility control and interfacial tension reduction effects. It is shown that with identical foam parameters, this model matches all laboratory core flood studies as well as the field pilot tests, showing that this foam model is capable of predicting foam performance in both laboratory and field settings. The foam components can be chosen to be defined as either gaseous or aqueous components and this choice is shown to affect the impact of capillary pressure on foam flow into the matrix. Also discussed in this paper are details of how the foam behaves when injected into a gas saturated zone where the foam combines with in-situ gas, resulting in higher foam qualities than was injected. It is demonstrated that foam mobility control as a function of foam quality is an important aspect for matching field performance. The significance of correct foam density calculations is also discussed using field scale models. The work done to match the many laboratory and field scale foam tests resulted in a significant improvement of the understanding of foam degradation, regeneration, permeability blockage, and flow in porous media and the phenomena responsible for generating incremental oil.
As much of the oil in the Akal field of the Cantarell complex is contained in the low permeability oil wet matrix, foam injection has been proposed as a method to control fluid mobility in the fracture, with the possible added benefit of transporting surfactant into the matrix so that additional oil could be liberated through a reduction of interfacial tension between oil and water (if this effect is significant for the surfactant in question). Presented in this paper is the work flow undertaken during an extensive study of all available laboratory experiments and pilot single well foam injection tests. Laboratory experiments ranged from simple water plus surfactant imbibition tests and surfactant flooding tests, to more complex foam flooding in split core experiments and co-injection of surfactant and gas for generation of foam in-situ. There were three field pilot single well foam injection tests that were included in this analysis that were of the huff-and-puff design. This extensive analysis was done with the aid of numerical simulation that resulted in the development of a novel foam model that handles both mobility control and interfacial tension reduction effects, and is capable of simulating foam degradation, foam regeneration, and trapped foam phenomena. Previous foam models available in commercial numerical simulators were not capable of simulating all of these foam effects together. It is shown that with identical foam parameters, this model matches all laboratory core flood studies as well as the field pilot tests, showing that this foam model is capable of predicting foam performance in both laboratory and field settings. The foam components can be chosen to be defined as either gaseous or aqueous components and this choice is shown to affect the impact of capillary pressure on foam flow into the matrix. Also discussed in this paper are details of how the foam behaves when injected into a gas saturated zone where the foam combines with in-situ gas, resulting in higher foam qualities than was injected. It is demonstrated that foam mobility control as a function of foam quality is an important aspect for matching field performance. The significance of correct foam density calculations is also discussed using field scale models. The work done to match the many laboratory and field scale foam tests resulted in a significant improvement of the understanding of foam degradation, regeneration, permeability blockage, and flow in porous media and the phenomena responsible for generating incremental oil.
PEBI ("perpendicular bisector") grids have been shown in the past to have the potential to reduce computational times for the simulation of relatively straight-forward processes. It is the purpose of this paper to investigate PEBI-based gridding for a much more complex thermal process in a full field setting using commercial simulation products. The goals are to study both computing efficiency and accuracy by comparing results obtained by modelling a field using the more conventional corner point-based gridding with local grid refinement to those obtained using a PEBI gridding approach. The field in question has been produced for about 20 years, with an operational history that includes cyclic steam stimulation in portions of the field. Many vertical faults have been mapped in the reservoir, and over 140 wells, many of them deviated or horizontal, have been drilled. A simulation study had been done using approximately 170 000 active corner point cells. It was found that the inclusion of foamy oil behaviour was necessary to get a good match of the production history. It appeared though that this simulation was lacking in accuracy near horizontal wells. This was probably due to the inability of the model to align its locally refined grids properly with many of the wells, and the difficulty in getting reasonable refinement levels where cyclic steam stimulation was being carried out, without making the problem considerably larger. A second simulation study was carried out using a PEBI-based grid. The more flexible aspects of this gridding system allowed construction of a better aligned grid, especially near the horizontal wells and near faults. Moreover, the characteristics of the PEBI-based grid also allowed efficient grading of grid cell sizes, so that particularly fine-scale gridding could be used near wells, while still maintaining an overall model size that was about half that of the corner point model. All processes modelled in the original simulation were replicated in the PEBI-gridded model, including the foamy oil and thermal aspects. The PEBI gridded model ran in about a third of the time of the original corner point model, and showed accuracy improvements which were attributable to the better placement of grid cells. These results showed that the simulator's ILU-based sparse matrix solver technology was very capable of computing in an unstructured grid environment, even while using cells near wells that were smaller than those used in the corner point model. Thus, PEBI-based gridding can be used to efficiently model complex processes in a full field setting. These grids can demonstrably improve accuracy and are much more adaptable for modelling near wells and in a complex geological setting. A three-fold run time improvement was noted for the field in question when comparing to a more conventional, corner point gridded model. Introduction PEBI ("perpendicular bisector") grids were introduced into reservoir simulation as early as 1989(1). These early grids used stacked layers with a PEBI grid in each layer. The construction of these two-dimensional PEBI grids could be based on first laying down a collection of nodes (points) in the reservoir, and then constructing a cell around each node that consists of all points in the reservoir that are closer to that particular node than any other. The resulting (several-sided) cells form what is known as a Voronoi tessellation(2), and the cells become the "control volumes" for the discretization. The associated PEBI grid is the triangulation that consists of the nodes and connecting segments that join pairs of nodes wherever the nodes' associated cells meet at a common face. The procedure of building the triangulation as a dual grid to a Voronoi tessellation shows why the inter-nodal segments between connected nodes are perpendicular to their associated common faces, and why the faces intersect the mid-points of the segments; hence the term, "perpendicular bisector". Of course, the preceding discussion ignores numerous details concerning node layout, boundaries, and many other issues, and is only meant for illustration, but it does give a brief indication of how things might work.
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