As unconventional reserves, oil shale deposits require additional oil recovery techniques to achieve favorable production levels. The efficiency of a shale reservoir development project is highly dependent on the application of enhanced oil recovery (EOR) techniques. There are many studies devoted to discrete investigations of each EOR method. Most of them claim that one particular method is particularly effective in increasing oil recovery. Despite the wealth of such research, it remains hard to say with certainty which technique would be the most effective when applied in the extraction of unconventional reserves. In this work, we aim to answer this question by means of a comparative study. Three EOR methods were applied and analyzed in the same environment, a single target objectan oil field in Western Siberia characterized by ultra-low permeability (0.03 mD on average) and high organic content. Methods involving huff-and-puff injection of a surfactant solution, hydrocarbon gas, and hot water were studied using numerical reservoir simulations based on preceding laboratory experiments. A single horizontal well having undergone nine-stage hydraulic fracturing was used as the field site model. The comparative calculations of cumulative oil production over an 8-year period revealed that the injection of hot (supercritical) water led to the highest oil recovery in the target shale reservoir. Each EOR method was implemented using the best operation scenario. All three cases resulted in an increase in cumulative oil production compared to the depletion mode, though the efficiency was distinctly different. Twenty-six percent more oil was obtained after hot water injection, 16% after hydrocarbon gas, and 12% after a surfactant solution. Simulation of a hot water huff-and-puff operation over a longer period (43 years) led to a level of oil production 3 times higher than depletion. The drawbacks of each EOR method on the shale site are discussed in the results. A possible solution was proposed for preventing the negative effects of heat loss and water blockage incurred from hot water injection. The comparative study concludes that hot water injection should lead to the highest volume of oil recovery. The conclusions drawn are suggested to be relevant for similar shale fields.
The combination of horizontal well drilling and multistage hydraulic fracturing is currently the leading method for developing shale reservoirs. However, oil recovery from these techniques does not exceed 10%, and supporting technologies are being sought. The injection of water or surfactant solutions in huff-n-puff mode is often considered an enhanced oil recovery option for shales, which can be used alone or in combination with other technologies, such as CO2 injection. This study presents experimental and numerical investigations into the huff-n-puff treatment of low-permeability shale cores using water-based compositions. Additionally, an attempt was made to displace oil by applying a nanofluid booster. Computational tomography was used as one of the methods to determine the oil recovery factor. Two core flooding tests with different designs and injection fluids were conducted. The first experiment involved brine filtration in one direction, followed by nanofluid injection. Second core flooding/huff-n-puff test implied injection of a 0.5 wt % surfactant solution. The results showed that the oil recovery factor achieved using brine was approximately 50% and decreased after nanofluid injection until 31%. Besides that, there was no significant effect from the use of the surfactant. At the same time, critical issues were observed regarding a decrease in core permeability. A numerical simulation of the second experiment was performed to restore the relative phase permeability (RPP) in the surfactant–oil system and to study the dependency of the capillary number logarithm on the miscibility coefficient. The simulation showed a good convergence in terms of the recovery factor values, with an error of ∼1.5%.
It is well known that the development of unconventional reserves is quite complicated due to the poor reservoir porosity and permeability. The use of horizontal wells with multi-stage hydraulic fracturing remains one of the promising methods in use today for the development of such reserves. Subsequently, tertiary recovery methods popularly known as enhanced oil recovery (EOR) can then be carried out. In this paper, the compositions of anionic and non-ionic surfactants, potentially suitable for use in unconventional hydrocarbon deposits as EOR agents were investigated (on the example of one of the fields of Bazhenov formation). Also, attention was devoted to the assessment of the feasibility of co-injecting the surfactant solutions with a thermal agent (subcritical water) in a hybrid thermo-chemical EOR process. During the course of the study, 35 samples of industrial surfactants (individual and blends) were investigated. The compatibility of the surfactants with brine water, their stability under reservoir conditions (T>100 °C, P=25 MPa) for more than 14 days, and the effectiveness of the surfactants in reducing the interfacial tension (IFT) at the oil-brine boundary were the key factors in choosing the most appropriate compositions for use in the hybrid EOR. The ability of surfactants to decrease the IFT was investigated using a spinning drop tensiometer while the wettability alteration effect was estimated using a drop shape analyzer. Filtration experiment on oil-saturated core sample and evaluation of surfactant adsorption on rock surface were carried out with the best compositions. The results of the study show that the colloidal systems, represented by mixtures of anionic and non-ionic surfactants, have the best performance. The main components of these surfactant compositions are sodium salts of olefin sulfonates, derivatives of sulfonic acids C15-C20, and ethoxylated alcohols C6-C12. The results of measurements imply that certain compositions alter the initial rock wettability to become more water-wet and reduce the IFT between oil and water to a value of 0.051 mN/m. The adsorption of surfactant molecules on the rock was estimated to be 4 g/kg of rock, and the ultimate oil displacement rate increased due to surfactant injection from 8 % obtained during water flooding to 40.5 %. The possibility of using surfactants within the hybrid EOR technology was proven because the best surfactant mixture showed thermal stability at temperatures above 250 °C. Thus, we can conclude about the possibility of the use of some surfactant mixtures for the development of unconventional oil fields. Also, it is possible to combine the injection of surfactant solutions with the injection of thermal fluid, leading to the generation of synthetic oil in situ, thereby improving the reservoir properties of the rock and recovery of additional oil due to the effect of surfactants. This technology can be possibly applied for the development of unconventional reserves to increase the oil recovery ratio and make the process economically viable.
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