Enhanced oil recovery in unconventional plays has been a focus of many E&P operators to increase recovery factors. Conventional displacement-based secondary (e.g. water flooding) and tertiary EOR methods are not viable options due to their low injectivity in these ultra-low permeability formations. As such, huff-n-puff (HnP) EOR techniques involving field gas injection may be the most effective EOR methods to increase the recovery factors from these shale formations. Several numerical reservoir simulation studies have showed the efficiency of CO2 HnP process in shale and tight formations (Yu et al, 2014); however, these studies show little to no field results to support the simulation predictions. This paper describes the conceptual development of the reservoir simulation models to investigate the viability of gas injection HnP in unconventional reservoirs and the results of applying those methods in Eagle Ford field test sites. Single-well and multi-well models with multi-stage hydraulic fractures were constructed and history matched using their primary production period performance. Sensitivity studies were conducted on well communication behavior/impacts, injection gas compositions, injection rates, injection/production cycling, and reservoir fluid types to optimize the pilot project well location(s) and to inform development strategies. Optimal cases from the simulation study were successfully applied to multi-well pads in the Eagle Ford formation across multiple fluid types. The simulation and the field application results were summarized and compared to provide detailed insights of unconventional HnP EOR. This study indicates the importance of confinement of the gases to afford optimal recovery factors during unconventional gas HnP EOR. An optimization engine was used in this study to optimize key operational parameters, such as injection pressures and slug sizes, to maximize recovery and efficiency. The resulting EOR designs were successfully implemented in field operations. Field recovery factors are within 10% of those predicted by simulation, indicating the value of numerical reservoir simulation prior to field trials and subsequent future development.
First, I would like to express my profound gratitude to my academic supervisor Dr. Sepehrnoori for the continuous guidance and support during these two years of graduate school. I am truly grateful for his interest and willingness to guide me since the very first day of my graduate studies. I would also like to express sincere gratefulness to Dr. Wei Yu for his dedication, expertise advises, and insistent valuable guidance. I will always appreciate how he shared part of his extensive expertise in unconventional reservoirs, and reservoir modeling with me. Also, I would like to thank Erich Kerr and his company for supplying the field data that made this research possible. I gained great practice and understanding through their suggestions for my research work. Their invaluable support received weekly through my entire career will always be appreciated. There are not enough words to express how convinced I am that this would not have been possible without the support of my family. I thank my father Mario and my mother María for covering part of my tuition, and also my sister Carla Adriana for their continuous encouragement to pursue my dreams. I would like to thank Carolina for her caring love and support. I am indebted to my parents, my sister and my girlfriend who never stopped encouraging during both the good and the not so good times. I would also like to thank Reza, Sutthaporn, Fabio, Jorge, and Esmail for their collaboration and assertive comments for my work and my research. Finally, a very special acknowledgment is extended to the Fulbright Commission of Ecuador, the Department of State of the United States, and the Cockrell School of Engineering for being the sponsoring entities which provided me the scholarships to fund and make this accomplishment possible.
Shale field operators have vested interest in optimal spacing of infill wells and further fracture optimization, which ideally should have as little interference with the existing wells as possible. Although proper modeling has been employed to show the existence of well interference, few models have forecasted the impact of multiple inter-well fractures on child wells production to optimize further hydraulic fracture designs. This study presented a rigorous workflow for estimating the impacts of spatial variations in fracture conductivity and complexity on fracture geometries of inter-well interference. Furthermore, we applied a non-intrusive embedded discrete fracture model (EDFM) method in conjunction with a commercial black oil reservoir simulator to investigate the impact of well interference through connecting fractures by multi-well history matching, based on a robust understanding of fracture properties, real production data and wellbore image logging. First, according to updated production data from Eagle Ford, the model was constructed to perform four (parent) wells history matching including five inner (child) wells. Later, fracture diagnostic results from well image logging were employed to perform sensitivity analysis on properties of long interwell connecting fractures such as number, conductivity, geometry, and explore their impacts on history matching. Finally, optimal cluster spacing was recommended considering interwell interference. The simulation results show that well interference is present and reduces effectiveness of the fracture hits when the connecting fracture conductivity, primary fracture conductivity, and number of connecting fractures increase. Because of these interwell long fractures, the bottomhole pressure behavior of the parent wells tends to equalize. Furthermore, the EDFM application is strongly supported by complex fracture propagation interpretation from image logs through the child wells in the reservoir. Through this study, three possible scenarios are shown with robust history matching of the model considering more than 20 complex dominant long interwell fracture hits and more than 2000 hydraulic fractures. The model became a valuable stencil to decide the well location and spacing, the completion staging, and to optimize the hydraulic fracture treatment design as well as its sequence so that it can be expanded to other areas of the field. The simulation results were applied to the field successfully to afford significant reductions in offset frac interference by up to 50% and reduce completion costs up to 23% while improving new well capital efficiency.
The Austin Chalk formation has seen numerous periods of active development since the early 1980’s and has recently become a new focus as an unconventional resource. Due to the low permeability, fracturing, with proppant or acid, is required to economically produce the reservoir. This study presents the results of a series of conductivity tests performed to evaluate the stimulation efficiency of proppant fracturing and acid fracturing in the Austin Chalk. The study uses both downhole core and outcrop samples. The downhole cores were from an Austin Chalk well in Webb County, Texas. The outcrop samples were collected from similar Austin chalk sub-intervals that the core samples were from and were collected from a road cut in the Del Rio, Texas area. Testing samples were prepared for the fracture conductivity tests based on API RP-61. Fracture surfaces were created either by tensile fracturing the sample, or by saw-cutting. For acid fracture conductivity, 15% HCl was injected to create acid-etched surfaces, and the effect of acid contact time was investigated. For propped fracturing, proppant size and concentration were set as study parameters. The fracture surfaces were scanned before and after each experiment in order to characterize changes in the fracture surfaces, such as acid etching and proppant embedment. Proppant size distribution was measured by sieve tests before and after conductivity measurement to evaluate proppant crushing. Closure stress up to 6000 psi was applied during the conductivity measurements, and some experiments stopped before reaching this upper limit because the sample lost mechanical integrity under a lower closure stress. The study results show that because the mechanism of creating conductivity is different for acid fracturing and propped fracturing, the conductivities from the two stimulation methods behave differently. The initial conductivity varies but the level of conductivity at low closure stress generated from acid and proppant are similar. The conductivity from propped fracture exhibits a semi-log decline trend, with a low decline rate and sustained conductivity at high closure stress. The conductivity from the acid fracture experiments strongly depends on acid injection conditions and mechanical integrity of the rock after acidizing, indicating that design of the stimulation is critical for a successful treatment. Acid fracture conductivity in general declined more rapidly with increase in closure stress than did proppant conductivity. The observations from the experimental study revealed that both acidized and propped fractures have potential to create sufficient conductivities in the low permeability Austin Chalk formation.
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