Application of horizontal wells and multi-stage fracturing has enabled oil recovery from extremely low permeability shale oil reservoirs, but the decline in production rate is more than two thirds in the first two years. We are trying to develop chemicals that can be injected into old wells to stimulate oil production before putting the well back in production. The goal of this work is to evaluate chemical blends for such a process at the laboratory scale. The chemical blend contains surfactants, a weak acid, a potential determining ion, and a solvent. Six different solvents were screened: Cyclohexane, D-Limonene, Dodecane, Kerosene, Turpentine, and Green Solvent®. Most of the chemical blends with the solvents extracted about 60% of the oil from shale chips, but the Green Solvent® extracted about 84%. Spontaneous imbibition tests were performed with outcrop Mancos shale cores. Oil was injected into these outcrop cores at a high pressure. NMR T2 distributions were measured for the cores in the original dry state, after oil injection and after imbibition. The aqueous phase from the chemical blend imbibed into the cores and pushed out a part of the oil and gas present in the cores. The surfactant in these blends can change wettability and interfacial tension. The solvent can mix with the oil and solubilize organic solid residues such as asphaltenes. The weak acid can dissolve a part of the carbonate minerals and improve permeability. The synergy can make these chemical blends strong candidates to stimulate oil recovery in shale formations.
The goal of this work is to evaluate the applicability of a novel set of surfactants to enhance recovery from a viscous oil, high temperature, high permeability, clastic reservoir. A large number of novel short-hydrophobe based surfactants/cosolvents were designed and synthesized. As these surfactants do not require expensive aliphatic alcohols for their synthesis, they are likely to be less costly than conventional anionic surfactants. Here only phenol hydrophobe based non-ionic surfactants with varying number of propylene oxide (PO) and ethylene oxide (EO) groups are discussed. These surfactant molecules were investigated for their aqueous stability limits, interfacial tensions (IFT) with a viscous crude oil and oil recovery from sandpack or sandstone cores. Surfactant phase behavior experiments with viscous crude oil showed low IFT (not ultralow) for single surfactant systems. Only one surfactant (Phenol-7PO-15EO) formulation was chosen for coreflood in sandpack and sandstone cores. Water flood recovered about 50% original oil in place (OOIP) and reduced the oil saturation to about 48% in the high permeability sandpacks. The tertiary surfactant polymer flood with Phenol-7PO-15EO increased the cumulative recovery to 99% for sandpacks. The oil recovery was insensitive to injection brine salinity in the range studied. As the permeability decreased, the tertiary oil recovery decreased if the permeability is lower than 7 Darcy. Surfactant-polymer (SP) formulations with this surfactant can be recommended for high permeability sandstone reservoirs with viscous oils, but not for sub-Darcy sandstones.
As exploration for oil and gas continues, it becomes necessary to produce from deeper formations, have low permeability, and higher temperature. Unconventional shale formations utilize slickwater fracturing fluids due to the shale’s unique geomechanical properties. On the other hand, conventional formations require crosslinked fracturing fluids to properly enhance productivity.
Guar and its derivatives have a history of success in crosslinked hydraulic fracturing fluids. However, they require higher polymer loading to withstand higher temperature environments. This leads to an increase in mixing time and additive requirements. Most importantly, due to the high polymer loading, they do not break completely and generate residual polymer fragments that can plug the formation and reduce fracture conductivity significantly.
In this work, a new hybrid dual polymer hydraulic fracturing fluid is developed. The fluid consists of a guar derivative and a polyacrylamide-based synthetic polymer. Compared to conventional fracturing fluids, this new system is easily hydrated, requires fewer additives, can be mixed on the fly, and is capable of maintaining excellent rheological performance at low polymer loadings.
The polymer mixture solutions were prepared at a total polymer concentration of 20 to 40 lb/1,000 gal and at a volume ratio of 2:1, 1:1, and 1:2. The fluids were crosslinked with a metallic crosslinker and broken with an oxidizer at 300°F. Testing focused on crosslinker to polymer ratio analysis to effectively lower loading while maintaining sufficient performance to carry proppant at this temperature. HP/HT rheometer was used to measure viscosity, storage modulus, and fluid breaking performance. HP/HT aging cell and HP/HT see-through cell were utilized for proppant settling. FTIR, Cryo-SEM and HP/HT rheometer were also utilized to understand the interaction.
Results indicate that the dual polymer fracturing fluid is able to generate stable viscosity at 300°F and 100 s-1. Results show that the dual polymer fracturing fluid can generate higher viscosity compared to the individual polymer fracturing fluid. Also, properly understanding and tuning the crosslinker to polymer ratio generates excellent performance at 20 lb/1,000 gal. The two polymers form an improved crosslinking network that enhances proppant carrying properties. It also demonstrates a clean and controlled break performance with an oxidizer.
Extensive experiments were pursued to evaluate the new dual polymer system for the first time. This system exhibits a positive interaction between polysaccharide and polyacrylamide families and generates excellent rheological properties. The major benefit of using a mixed polymer system is to reduce polymer loading. Lower loading is highly desirable because it reduces material cost, eases field operation and potentially lowers damage to the fracture face, proppant pack, and formation.
Initial rate and decline are the two main parameters defining the economics of unconventional shale oil development. To improve economics, companies drill longer horizontal wells with more than twenty equidistant stages, different completion strategies and various additives such as surfactants and nano surfactants. This procedure evolves to factory mode in which tasks are optimized in timing and performance without attention to the matrix aspects of improving the recovery. Here, we report the design of a mutual solvent injection pilot in the Vaca Muerta unconventional reservoir during the completion of four unconventional shale oil wells. Reducing
Vaca Muerta has been long regarded as a water wet shale because of the limited water backflow post-fracking job. Alternating water injection was implementing assuming that the well productivity is driven by spontaneous imbibition, but this strategy has been unsuccessful as capillary pressure hysteresis drives this mechanism. We started studying Vaca Muerta from the rock microstructure using energy-dispersive spectrometry and focused gallium Ion Beam ablation FIB SEM images. The microstructure varied widely from millimeters in the same plug which could be expected because in shale rocks millimeters represent more years of deposition than in a conventional reservoir. We identified intercalations of massive water wet zones and strongly oil wet zones in the Vaca Muerta kitchen zone. The oil wet intercalations have high porosity and adsorption isotherm indicating 100 to 1000 times more permeability than the water wet zone. The water wet intercalations are highly saturated with water, and on the contrary, the oil wet intercalations are highly saturated with oil. The pilot designed consisted of four wells in which we will test different injection concentrations but keeping the total mass constant. In this manner, we will estimate the volume contacted by the solvent.
The laboratory protocol indicates a large percentage of macro and meso-pores. We implemented the dimethyl-ether injection which changes the interfacial tension, viscosity and wettability and we obtained the modified relative permeabilities which were the injection of dimethyl ether at 30% concentration along with the hydraulic fracture stimulation stages doubled the initial oil production rate.
The pilot consisted of five wells in which we will test different injection concentrations but keeping the total mass constant. In this manner, using the numerical simulation, we will estimate the volume contacted by the solvent.
Intercalations of high porosity high permeabilities zones in which the injection of a mutual solvent that reduces viscosity and could change wettability in oil wet/water-wet Vaca Muerta improving matrix connectivity.
Feustel, Michael (Clariant) | Goncharov, Victor (Clariant) | Kaiser, Anton (Clariant) | Smith, Rashod (Clariant) | Sahl, Mike (Clariant) | Kayser, Christoph (Clariant) | Wylde, Jonathan (Clariant) | Chapa, Amanda (Clariant)
Transportation of waxy crude oils and mitigation of wax deposition are major challenges especially in regions with cold climate. A viable solution for minimizing organic precipitation and fouling in pipelines or storage tanks is the use of inhibitors and dispersants, however, often those pour point depressants (PPDs) have their own challenges due to their own high product pour points. To overcome these issues a series of high active winterized polymer micro-dispersions were developed. Composition and physical properties of several light to heavy waxy crudes were fully explored based on SARA analysis, wax content and paraffin carbon chain distribution. Performance of candidate chemistries from four major classes of polymeric paraffin inhibitors were studied using industry standard methods. Selected high performing chemistries were processed into micro-dispersions using solvents and surfactants under high shear/ high pressure blending. The new polymer micro-dispersions (MDs) were characterized by their pumpability and stability at cold climates. Series of pour point measurements, rheology profiles and wax deposition tests were carried out for performance comparison of MDs to standard polymers in solution. Processes developed here were versatile to convert polymers from all classes of chemistries into micro-dispersion. Binary and ternary polymeric dispersions were also created showing synergistic effects on the pour point reduction and inhibition of wax deposition of the selected challenging crude oils. The performance of the new polymer micro-dispersions was found comparable to superior with standard polymers in solution. Hence, it was possible to create pumpable inhibitors for extreme cold climates without compromising on performance. The systematic approach used here allowed development of more customized solution based on crude characteristics and desired performance. Micro-dispersions were found stable for long term storage in temperatures ranging from -50°C to +50°C. Multiple global field trials are on-going with very positive results demonstrating early success in lab-to-field deployment. Based on lab and field data, this paper demonstrates that highly active micro-dispersed polymers can perform at significantly lower dosage rate when compared to winterized polymers in solution. Cold storage stability and pumpability eliminated the needs for heated tanks and lines reducing operation and capital expenditures.
In preparation for a field pilot of cyclic solvent injection (CSI) on two depleted cold heavy oil production with sand (CHOPS) wells, a series of oilsands coreflood experiments were conducted to evaluate the effectiveness of various commercially available solvents and make a solvent recommendation for the pilot. Oil recovery and solvent recovery were the key performance indicators used to compare CSI effectiveness of each solvent blend. The operating pressure for each test was kept relatively constant for each solvent blend tested. Tested solvents included blends of methane/propane, carbon dioxide/propane, methane/ethane, 100% ethane, and nitrogen. Sensitivities for depletion rate and blowdown pressure are also presented. Overall the 100% ethane test performed the best with the highest oil recovery and solvent recovery in the fewest cycles. Due to the lack of commercial ethane supply and the industry experience with methane/propane in Husky Edam’s CSI pilot, a methane/propane blend was recommended for the field pilot in Manatokan East near Bonnyville Alberta Canada.
Wax deposition in wells from shale oil production is a major challenge. Crystal modifiers and dispersants may mitigate wax deposition via steric hindrance and wettability alteration, respectively. Interaction of reservoir brine coproduced with oil and the chemicals may increase mitigation efficiency. This study investigates quantitative effect of brine on inhibitor performance, at three different flow rates, representing different field conditions.
Two dispersants and two crystal modifiers at low concentrations (500-1000 ppm) are selected among eight chemicals after screening. In our work, pressure drop in flowlines is a measure of mitigation efficiency. Three light crude oils are investigated; they are from shale formations which have high wax content. Brine effect on chemical performance is investigated from high flow rate to low rates. To select chemicals and provide a guide for flow experiments, we include optical microscopy with polarized light, rheology, and particle size measurements as part of the study.
Crystal modifiers are observed to delay pressure build up from deposition significantly compared to dispersants. For dispersants, the effect of brine is found to improve efficiency. At the intermediate flow rate, brine is found to increase efficiency of two dispersants, AO4 and PARA, by 70% and 21%, respectively, in terms of total fluid transported before blockage. For crystal modifiers, there is practically no increase in flow throughput by brine. Dispersant and brine have a synergistic effect, leading to decrease in adsorption of wax crystals on the surface of pipeline. Crystal modifiers do not have an appreciable effect on surface wettability in the presence of brine and no significant improvement is observed in inhibitor efficiency. The observation is the same for all four crystal modifiers in the screening tests at three different flow rates in the flow experiments. The eight chemicals used in this work were provided from different companies. The basis of their selection was on reduction of viscosity in vials from the addition of small amount of chemicals. We find that viscosity reduction alone is not a measure of effectiveness of chemicals. All the chemicals supplied by various companies reduce viscosity of the shale oil sample at the conditions of wax formation. However, deposition in dynamic flow tests do not relate to bulk viscosity reduction.
Our investigation includes a comprehensive set of particle size and flow measurements of low dosage crystal modifiers and dispersants in wax mitigation over a range of flow rates. There is a significant synergistic action of dispersants and brine. Chemical inhibitor performance is a strong function of flow rate and metal surface.
Non-thermal-solvent and thermal-solvent based heavy oil recovery processes are technologies in which solvent is used as either the main or the secondary agent, in conjunction with heating, for bitumen viscosity reduction. In these processes a hydrocarbon solvent is injected into the reservoir and produced back with the recovered bitumen. A fraction of the injected solvent is retained in the reservoir at an equilibrium state as gas and liquid phases. Since the cost of injected solvent in these processes is a major portion of the operating cost, recovery of the retained solvent from the reservoir at the end of bitumen depletion stage results in recovery of significant capital and thus improvement of the process economics.
Imperial-ExxonMobil have been optimizing the existing and developing new recovery technologies to improve the efficiencies, economics and environmental performance of heavy oil production operations. Recent focus has been on developing solvent based recovery processes through an integrated research program that includes fundamental laboratory work, advanced numerical simulation studies, laboratory scaled physical modeling, and field piloting. The research program aims at in-depth investigation and understanding of process physics and mechanisms to allow evaluating and optimizing process performance.
In this paper, development of a new method for recovery of the retained solvent from the reservoir at the end of the bitumen depletion stage is introduced. This method takes advantages of solvent vapor-liquid thermodynamic equilibrium to strip the retained solvent from the reservoir. A stripping gas is injected and circulated in the bitumen depleted chamber to vaporize and recover the retained solvent to the surface. The reservoir modeling results show that this method is very effective and efficient in accelerating recovery of the retained solvent. The physical modeling experimental data confirms the effectiveness of this method. Field pilot data from a solvent assisted recovery process are presented which demonstrate solvent recovery efficiency using continuous steam injection.
Sabet, Nasser (University of Calgary) | Mohammadi, Mohammadjavad (University of Calgary) | Zirahi, Ali (University of Calgary) | Zirrahi, Mohsen (University of Calgary) | Hassanzadeh, Hassan (University of Calgary) | Abedi, Jalal (University of Calgary)
This work focuses on modeling the miscible viscous fingering in porous media accounting for asphaltene precipitation and deposition. The mass balance equations for solvent and asphaltene are defined, and the highly nonlinear system of equations is solved numerically through hybridization of compact finite difference and pseudo-spectral methods. We explain how asphaltene precipitation and the resulting formation damage influence the growth of viscous fingers.
To conduct our analysis, we use the experimental data for the amount of asphaltene precipitation at different solvent mass fractions and also oil viscosity at various asphaltene and solvent contents. This data is measured in our lab and is used as input for the nonlinear numerical simulations. For these simulations, the conventional finite difference schemes cannot be applied as they suffer from the excessive computational time and most importantly, numerical dispersion. Therefore, we employ hybrid techniques to benefit from the high accuracy of spectral methods and capture the nonlinear dynamics of fingerings on very fine grids.
Hydrocarbons such as light n-alkanes are widely used as diluents in the production and upgrading of heavy oils. The addition of a diluent to heavy oil or bitumen alters the chemical forces acting within the mixture, leading to the precipitation of asphaltenes. It is hypothesized that precipitation of asphaltene from oil changes the viscosity behavior of the mixture, influences the dynamics of viscous fingering, and therefore affects the oil recovery. Moreover, asphaltene deposition alters the porosity and permeability of the porous media and might modify the flow paths, leading to possible formation damage. Our results show that asphaltene precipitates are mostly accumulated in the contact interface between the solvent and oil. The major asphaltene deposition occurs along the growing fingers leading to permeability reductions up to 30% in the studied cases.
Scaling groups for hybrid steam-solvent recovery processes are presented in this paper. A brief discussion of the derivation of the scaling groups is given first. Then an examination of the comparative behavior of these scaling groups at different scales is provided using reservoir simulation for the example of a high solvent load steam-butane gravity drainage process (i.e., steam-butane hybrid (SBH)).
Scaling groups were derived for hybrid steam-solvent recovery processes by inspectional analysis using governing equations for multi-phase flow in porous media. The effects of key mechanisms in these processes (diffusion, dispersion, advection and capillary pressure) were examined within the context of the derived scaling groups using reservoir simulation of SBH at three different geometric scales, ranging from the laboratory scale through a semi-field scale to the field scale, for one specific set of operating conditions. The scaling groups were used to analyse and interpret the numerical results.
The scaling groups were characterized according to the physical mechanisms from which they were derived. The intent of this analysis was to determine which of the mechanisms tend to be most important to the SBH process at different geometric scales. It is clear from a cursory examination of the scaling groups that all of the scaling groups representing the behavior of the SBH process cannot be satisfied when the geometric scale is changed from the laboratory scale to the field scale. The results of the study also indicate that the Pujol and Boberg scaling criteria for thermal processes seem to provide a reasonable approach for scaling SBH, when they are adapted to include the effects of dispersion. The influence of capillary pressure was secondary to other mechanisms involved in the process. It was evident from the simulations that the influence of dispersion was much more pronounced than diffusion for the solvent loading that was considered. Further, it was found that mechanical dispersity must be scaled with length to scale this mechanism appropriately in the reservoir simulator that was used in this study (CMG STARS™). As a final observation, the influence of capillary pressure was secondary to other mechanisms involved in the process.
Few studies on scaling high solvent loading hybrid steam-solvent processes have been undertaken. Using reservoir simulation to study scaling groups for these processes is a novel approach to this subject. Understanding the scalability of hybrid steam-solvent processes from the laboratory scale to the field scale would improve the capability of laboratory experiments to represent the performance of these recovery processes at the field scale.