Extensive experimental work has indicated that low-salinity waterflooding is an enhanced oil recovery technique that improves oil recovery by lowering and optimizing the salinity of the injected water. Most of the low-salinity waterflooding studies focused on the injection brine salinity and composition. The question remains, how does the salinity and composition of the reservoir connate water affect the low-salinity waterflooding performance? Therefore, in this work different connate water compositions were used to investigate the role of reservoir connate water on the performance of low-salinity waterflooding. In this paper, nine spontaneous imbibitions experiments were performed. Two sandstone types (Bandera and Buff Berea) with different clay contents were used. The mineralogy of the rock samples was assessed by X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. This work describes the experimental studies of the spontaneous imbibition of oil by low-salinity and high-salinity brines using 20 in. length outcrop samples. The main objectives of the spontaneous imbibition study was to investigate the role of the composition of the reservoir connate water (Na+, Ca2+, and Mg2+), the effect of rock permeability, and test the effect of temperature (77 and 150°F) on the performance of the low-salinity waterflooding recovery. The volume of the produced oil was monitored and recorded against time on a daily basis. Imbibition brine samples were analyzed at the end of each experiment. Results demonstrate that the spontaneous imbibition oil recovery ranged from 38 to 69% OOIP for high permeability Buff Berea cores (164-207.7 md), while oil recovery of the low permeability Bandera cores (31.1-39.2 md) ranged from 20 to 51.5% OOIP at 77°F and 14.7 psia. The oil recovery decreased when the average pore-throat radius decreased. The reservoir connate water composition has a dominant influence on the oil recovery rate. The changes in the ion composition of reservoir connate water (Ca2+, Mg2+, and Na+) showed a measurable change in the oil production trend. Reservoir cores saturated with connate water containing divalent cations of Ca+2 and Mg+2 showed higher oil recovery than for cores saturated with monovalent cations Na+. In all cases, a measurable ion exchange was observed, while there was no significant change in the pH of the imbibition brine during the experiment. The ions exchange effect was more pronounced than the pH effect in the low-salinity waterflooding performance for Buff Berea and Bandera sandstone. As the temperature increased from 77 to 150°F, an additional oil recovery up to 15.4% of OOIP was observed by spontaneous imbibition for Buff Berea cores.
Iron precipitation is a serious problem in acidizing treatments which can damage the formation permeability by restricting the flow channels. Solutions introduced in the past included the use of buffers, reducing agents, and chelating agents. Laboratory and field experiences concluded that chelating agents are the most effective remedy for controlling the iron precipitation. However, substantial limitations hinder the effectiveness and the application of several of the current chelating agents in the industry. These limitations include poor thermal stability at elevated temperatures, higher cost, low solubility in acidic medium, tendency of precipitating calcium products, and serious negative health and environmental impacts.This work introduces sodium gluconate as an efficient and environmentally friendly iron chelating agent. The chemical exhibits an excellent sequestering property for iron over a wide range of pH values, with a chelation power comparatively closer to those reported for EDTA and NTA. The salt is readily dissolved in the acids with a solubility of 600 gm/l.The objectives of this study are to: (1) investigate the ability of sodium gluconate to prevent iron precipitation at different iron loading, (2) examine the efficiency of the new chelant in hydrochloric acid solutions (5, 10, and 15 wt%) at a temperature of 200 and 250°F and chelant to iron molar ratios of 1:1, 2:1, and 4:1, and (3) determine the optimum chelant to iron molar ratio that results in the highest chelation capacity. The effectiveness of the new chemical was examined by determining the percentage increase/ reduction in core permeability.In stirred reactor experiments, iron precipitation was observed at a pH above 2 with the absence of the chelant at iron loading at 5,000 and 10,000 ppm at 77°F. The presence of Na gluconate at an equimolar ratio of the iron loading prevent the precipitation over the investigated range of iron load and temperature.Coreflood results showed that at low injection rates of 0.5 cc/min, 200°F, and 5 wt% HCl with 10,000 ppm Fe 3ϩ the core was damaged and the acid was not able to enter the core due to iron precipitation on the core inlet. At a higher injection rate (2 cc/min) the presence of 5 wt% Na-gluconate enhanced the permeability by 74% (versus 50% when no chelant was added) after injecting 0.75 PV of the acid with a load of 5,000 ppm Fe 3ϩ . At a load of 10,000 ppm of iron (III), the permeability increased to 165% compared to 70% for a controlled experiment. The results of another set of experiments at 250°F and 15 wt% HCl with a load of 10,000 ppm indicated that a 1:1 molar ratio of chelant to iron concentration is the optimum for the maximum chelation capacity compared to the results obtained for other experiments in which the molar ratio of sodium gluconate to iron was 2:1 and 4:1. These results were confirmed by fluid analysis, which showed a significant increase in both ion concentrations at this molar ratio in the effluent samples.
The success of matrix acidizing treatments, whether in carbonate or sandstone formations, depends significantly on the selected acid or acid mixtures. Limitations are applied on all existing acidizing fluids including hydrochloric acid and organic acids. These limitations include: low dissolving power, product solubility, stability, biodegradability, and the inevitable cost of additives necessary to mitigate corrosion problems. This work proposes a new mixture of lactic and gluconic acids which offers favorable technical characteristics and excellent health and environmental profile. After formulated, the acid was tested and optimized for the maximum calcium product solubility. The new acid is noncorrosive, nonvolatile, nontoxic, and can be used at a higher pH with significant sequestering power, and it is readily biodegradable (98 % at 48 h). The solubility of calcium salt of this acid is approximately 400g/l (compared with 300 g/l for calcium acetate, 166 g/l for calcium formate, and 79 g/l for calcium lactate). Interestingly, sodium salt of the acid mixture was reported as a corrosion inhibitor for steel alloys. The objectives of the work are to: (1) examine the dissolving capacity and reactivity of the proposed acid through solubility and reaction rate studies over a temperature range of 80-300°F using the rotating disk reactor, (2) investigate the effectiveness of the new acid to create dominant wormholes and determine the optimum injection conditions in calcite cores. Acid capacity reactions with Pink Desert limestone powder showed that 1:1 of 1 M lactic:gluconic acid mixtures was the optimum molar ratio that resulted in dissolving the maximum calcium amount for the reaction at 25°C and 500 rpm, while the reaction of lactic acid alone at the same acid concentration showed a white precipitation of calcium lactate in the collected samples. Reaction rate experiments on the rotating disk reactor showed that the rate of reaction of the proposed acid at 1:1 molar ratio is confined by the reaction rate of the two individual acids (lactic and gluconic acids). However, the reaction of lactic acids resulted in white precipitates on the surface of the rock disks used in the experiments. Coreflood study showed the ability of the new acid mixture to stimulate Indiana limestone cores at various injection rates, acid concentrations, and over temperature range between 150 and 300°F. The results also confirmed that 1:1 molar ratio of the two acids is the optimum for the minimum acid pore volume required to breakthrough. 20 wt% of the proposed acid was the optimum acid concentration associated with the minimum acid pore volume. Above this concentration, little impact was noted and the reduction in the pore volume leveled off.
Perforation is the most common way to establish effective communication between the reservoir and the wellbore in wells completed with casing. Although perforation with shaped charges has become the dominant method for completion, conventional shaped charges do not always provide deep and clean-enough tunnels that result in the required productivity and/or facilitate subsequent stimulation treatments. Therefore, the objectives of this work are to utilize the Perforation Flow Cell (PFC) and the Perforation Treatment Cell (PTC), both developed by GEODynamics, to investigate the impact of using reactive liner shaped charges on the outcome of matrix acidizing treatments and to compare the performance of reactive and conventional charges for different charge types and loads. Cream Chalk cores with 7 in. diameter × 24 in. length were perforated with two weights of reactive and conventional charges (15 and 23 g) under simulated downhole conditions using two designs of charges; deep penetration (DP) and good hole (GH). All cores were initially saturated in odorless mineral spirit (OMS) and the same fluid was used to flush the core before and after the acidizing step. Porosity and pre-shot (initial) permeability were measured. After perforation, post-shot permeability was reported and the cores were CT-scanned to visualize and measure the geometry of the perforation tunnels. 15 wt% HCl was used for the acidizing step at 200°F and the effluent samples were periodically collected to measure Ca, Mg, Fe, Al, and metal ions that are present in the core, tubulars, cement, and shaped charge material in the perforation assembly. Cumulative acid pore volume and acid injectivity were reported. CT scans were performed again after acidizing to assess the wormhole morphology obtained with various types of charges. Experimental results showed that reactive charges create tunnels with more effective (open) length when compared to the equivalent conventional charge, especially at the tip of the tunnel. As a result, stimulation treatments were enhanced and less acid pore volume and time to breakthrough were required. These results were confirmed by chemical analysis that showed higher calcium and metal ion concentration in the effluent samples when the conventional charge was used. CT-scanning after treatment showed a dominant wormhole created from the tip of the perforation tunnel when the reactive charges were used compared to multiple and deviated/branching wormholes with the conventional charges.
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