Effect of Reservoir Fluid Type on the Stimulation of Carbonate Cores Using Chelating Agents

Different fluids have been introduced in the oil industry to be used as alternatives to HCl. Chelating agents such as EDTA (ethylenediaminetetraaceticacid), and HEDTA (hydroxy ethylene diamine triacetic acid) have been used as stand-alone stimulation fluids. These fluids can be used to stimulate water injectors, oil, or gas producers, therefore, the effect of the type of reservoir fluid on the stimulation process should be investigated.In this study, an analytical model was developed to describe the flow of HEDTA and EDTA chelating agents and propagation inside carbonate formations. The analytical model can be used as a pre-design tool before the treatments. The developed model can be used to predict the volume of the chelant solution required to create wormholes in calciteformations at different temperatures. The temperature affects the diffusion coefficient of the chelating agent, wormholing rate, and wormhole shape and size. The dissolving power of different forms of HEDTA chelating agent can be determined using the model. The optimum injection rate based on optimum wormholing conditions was identified for the chelating agents. Also, the model can be used to predict the wormholing rate of different chelating agents in carbonate formations.The analytical model can be used to predict the performance of the chelating agent in carbonate stimulation. The volume of chelating agent required to stimulate carbonate formation per foot thickness was determined using the developed model. The optimum injection rate was determined for different chelating agents using the model and the results were compared with experimental results from previous work and there was a good agreement between the measured and the predicted values. The model can be used to determine the best stimulation fluid based on the temperature and fracture pressure of the target zones.

Chelating agents such as ethylenediaminetetraacetic acid (EDTA) and hydroxy ethylene diamine triacetic acid (HEDTA) have been used as stand-alone stimulation fluids. These fluids can be used to stimulate water injectors, oil, or gas producers. In this study, an analytical model was developed to describe the flow of HEDTA and EDTA chelating agents and propagation inside calcite formations. The analytical model can be used as a pre-design tool before the treatments. The developed model can be used to predict the volume of the chelant required to create wormholes in calcite formations at different temperatures. The temperature affects the diffusion coefficient of the chelating agent, wormholing rate, and wormhole shape and size. The dissolving power of different forms of HEDTA can be determined using the model. The optimum injection rate based on optimum wormholing conditions was identified. Also, the model can be used to predict the wormholing rate of different chelating agents in calcite formations. The analytical model can be used to predict the performance of the chelating agent in calcite stimulation. The volume of chelating agent required to stimulate calcite formation per foot thickness was determined using the developed model. The optimum injection rate was determined for different chelating agents using the model, and the results were compared with experimental results from previous work and there was a good agreement between the measured and the predicted values. The model can be used to determine the best stimulation fluid based on the temperature and fracture pressure of the target zones.

Conventional matrix acidizing treatments rely on hydrochloric acid to stimulate carbonate formations. However, the success of these treatments is often limited because of rapid acid spending at low injection rates and asphaltic sludge precipitation. This study investigated ethylenediamine-tetraacetic acid (EDTA) as an alternative stimulation fluid. Results show that EDTA can effectively wormhole in limestone, even when injected at moderate or non-acidic pH values (4 to 13) and at low flow rates where only face dissolution would occur with HC1. Stimulation with EDTA at low injection rates is consistent with the dependence of the wormhole structure on the Damköhler number for flow and reaction. Sludge tests show that EDTA does not induce the precipitation of asphaltic sludge from crude oil, even in the presence of 3000 ppm of ferric iron. This result is attributed to EDTA being able to form stable chelates with ferric and ferrous iron.

Summary Recently low-salinity waterflooding was introduced as an effective enhanced-oil-recovery (EOR) method in sandstone and carbonate reservoirs. The recovery mechanisms that use low-salinity-water injection are still debatable. The suggested possible mechanisms are: wettability alteration, interfacial-tension (IFT) reduction, multi-ion exchange, and rock dissolution. In this paper, we introduce a new chemical EOR method for sandstone and carbonate reservoirs that will give better recovery than the low-salinity-water injection without treating or diluting seawater. In this study, we introduce a new chemical EOR method that uses chelating agents such as ethylenediaminetetraacetic acid (EDTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA) at high pH values. This is the first time for use of chelating agents as standalone EOR fluids. Coreflood experiments, interfacial and surface tensions, and zeta-potential measurements are performed with DTPA, EDTA, and HEDTA chelating agents. The chelating-agent concentrations used in the study were prepared by diluting the initial concentration of 40 wt% with seawater and injecting it into Berea-sandstone and Indiana-limestone cores of a 6-in. length and a 1.5-in. diameter saturated with crude oil. The coreflooding experiments were performed at 100°C and a 1,000-psi backpressure. Low-salinity-water and seawater injections caused damage to the reservoir because of the calcium sulfate scale deposition during the flooding process. The newly introduced EOR method did not cause calcium sulfate precipitation, and the core permeability was not affected. The core permeability was measured after the flooding process, and the final permeability was higher than the initial permeability in the case of chelating-agent injection. The coreflooding effluent was analyzed for cations with the inductively coupled plasma (ICP) spectroscopy to explain the dissolution-recovery mechanism. The effect of iron minerals on the rock-surface charge was investigated through the measurements of zeta potential for different rocks containing different iron minerals. HEDTA and EDTA chelating agents at 5 wt% concentration prepared in seawater were able to recover more than 20% oil from the initial oil in place from sandstone and carbonate cores. ICP measurements supported the rock-dissolution mechanism because the calcium, magnesium, and iron concentrations in the effluent samples were more than those in the injected fluids. The IFT-reduction mechanism was confirmed by the low IFT values obtained in the case of chelating agents. The type and concentration of chelating agents affected the IFT value. Higher concentrations yielded lower IFT values because of the increase in carboxylic-group concentration. We found that the high-pH chelating agents increased the negative value of zeta potential, which will change the rock toward more water-wet.

Chelating agents’solutions were introduced as effectivealternatives to strong acids to be used in acid-sensitive situationssuch as high temperature and salinity conditions. However, limitedstudies have been conducted to examine the optimum conditions forimproving the chelating agent performance. In this study, a comprehensivestudy of solubility and physical properties of different chelatingagents’ fluids that are commonly used in the oil upstream applicationswas performed under different conditions. The optimum concentrationranges at which chelating agents are soluble and effective to providethe best acidizing efficiency are determined. Also, more than 340data sets were used to develop new empirical models that can helpin estimating the chelating agents’ properties at wide rangesof concentrations and treatment temperatures. In this work, differentexperimental measurements were conducted using a pressure of 2000psi (13.7 MPa) and a temperature of 120 °C (393.15 K). The conductedexperiments are density and viscosity measurements, solubility experiments,interfacial tension measurements, computed tomography scan, and corefloodingtests. The used chelating agents are diethylenetriaminepentaaceticacid (DTPA), hydroxyethylenediaminetriacetic acid (HEDTA), and ethylenediaminetetraaceticacid (EDTA). Results revealed that HEDTA and DTPA chelating agentshave good solubility at different pH and concentration ranges. However,EDTA showed a limited solubility performance, especially at a concentrationgreater than 15 wt %. Moreover, the developed correlations providedfast and reliable estimations for the chelating agent density andviscosity, and estimation errors of around 1% were achieved. Also,treating the tight carbonate rocks with the optimized chelating agentsolutions showed effective wormholes with a minimum acid volume. Finally,a good match between the actual and predicted pressure drops is achieved,confirming the high reliability of the developed models. Overall,this work can help in designing the stimulation treatment by suggestingthe optimum ranges for fluid concentration and solution pH for wideranges of temperature. Also, the newly developed correlations canbe used to provide quick and reliable estimations for the pressuredrop and the chelating agent properties at reservoir conditions.

Stimulation of high temperature carbonate gas reservoirs using seawater and chelating agents: Reaction kinetics

During matrix acidizing, successful iron control can be critical to the success of the treatment. Iron (III) precipitation occurs when acids are spent and the pH rises above 1, which can cause severe formation damage. Chelating agents are used during these treatments to minimize iron precipitation. In this paper, we studied the effect of iron precipitation in acidizing operations. HCl solutions (5 - 20 wt%) containing 5,000 to 10,000 ppm of Fe3+ were used in these experiments. Biodegradable GLDA (glutamic-N, N-diacetic acid) was studied in the experiments. The effect of varying acid concentration and chelate-to-iron mole ratio was examined. Coreflood experiments were conducted on low permeability Indiana limestone (1 - 5 md) at 200°F. The cores were scanned after treatments using a CT scanner. The core effluent samples were analyzed for total iron and calcium concentrations using ICP-ES. A calcium ion-selective electrode was used to determine the concentration of free calcium ions, i.e. calcium ions not complexed by the chelate, in the core effluent samples. Results showed that the amount of iron recovered depended on both chelate-to-iron mole ratio and the initial permeability of the cores. Calcium is chelated along with iron, which limits the effectiveness of chelating agents to control iron (III) precipitation. Chelating agents are supposed to control iron now that calcium is also chelated, this amount should be accounted for. Acid solutions should be designed considering this important finding for more successful treatments. This paper will discuss the results obtained and give recommendations to enhance the effectiveness of these chemicals in the field.

Experimental and numerical investigation of mutual solvents for EOR applications

The relationship between residual saturation and wettability is critical for modeling enhanced oil recovery (EOR) processes. The wetting state of a core is often quantified through Amott indices, which are estimated from the ratio of the saturation fraction that flows spontaneously to the total saturation change that occurs due to spontaneous flow and forced injection. Coreflooding experiments have shown that residual oil saturation trends against wettability indices typically show a minimum around mixed-wet conditions. Amott indices, however, provides an average measure of wettability (contact angle), which are intrinsically dependent on a variety of factors such as the initial oil saturation, aging conditions, etc. Thus, the use of Amott indices could potentially cloud the observed trends of residual saturation with wettability. Using pore network modeling (PNM), we show that residual oil saturation varies monotonically with the contact angle, which is a direct measure of wettability. That is, for fixed initial oil saturation, the residual oil saturation decreases monotonically as the reservoir becomes more water-wet (decreasing contact angle). Further, calculation of Amott indices for the PNM data sets show that a plot of the residual oil saturation versus Amott indices also shows this monotonic trend, but only if the initial oil saturation is kept fixed. Thus, for the cases presented here, we show that there is no minimum residual saturation at mixed-wet conditions as wettability changes. This can have important implications for low salinity waterflooding or other EOR processes where wettability is altered.

Elution of copper from vermiculite with environmentally benign reagents

Multiple-point crosses where 20 pairs of regions (ten loosely linked markers) for the study of contiguous exchanges involving two linkage groups, capable of being investigated at a time, were utilized. In order to find out the effect of a chelating agent on interference, crosses were treated with different molar concentrations of ethylene-diamine tetraacetic acid (EDTA). All marker strains were standardized before use by inbreeding with a wild-type of known parentage. Since tests based on Coefficient of Coincidence and on Poisson distribution for finding out the position interference are rather unsatisfactory, a method based onContingency Chi-square test for detecting the intensity and nature of interference is described. Data obtained from 1813 analyzable ordered tetrads show that positive interference is absent in the control crosses. It is present among certain regions in crosses when treated with 4×10−5M and 10×10−5M EDTA but it vanishes again in crosses when treated with 20×10−5M EDTA. Negative interference is present in the control crosses but it varies among pairs of regions on the two linkage groups. The localization and intensity of interference are alterable with EDTA treatment. The data are discussed in the light of certain concepts invoking to explain the mechanism that involves a genetic exchange.

<title>Abstract</title> Background Urinary cell-free DNA (ucfDNA) holds promise for liquid biopsies, yet standardized preprocessing protocols are lacking. This study evaluated the impact of several preanalytical variables on ucfDNA quality: ethylenediaminetetraacetic acid (EDTA) concentration, centrifugation, storage conditions, and thawing methods. Results Untreated urine exhibited complete ucfDNA degradation within 1 day at room temperature (RT). No significant differences in ucfDNA yield were detected among any of the EDTA-treated whole urine samples after 1 day and 3 days of RT storage, whereas the 40 mM group presented significantly lower ucfDNA yields than did the 10 mM group after 7 days of RT storage. In particular, the %cfDNA scores of all EDTA-treated whole urine samples decreased significantly (61.51 ± 1.54 to 32.38 ± 7.04) because of high-molecular-weight DNA contamination from cell lysis. The urine supernatant with 10 mM EDTA optimally preserved ucfDNA over 7 days, but 40 mM EDTA impaired yields and %cfDNA scores. In the thawing experiment with/without EDTA, the frozen urine supernatant showed thawing-method-independent stability, but frozen whole urine thawed at 4°C lost &gt; 60% yield. In the absence of EDTA, nearly all frozen urine samples presented higher %cfDNA scores than did the baseline samples after thawing, and postthawing secondary centrifugation significantly reduced the yields. With 10 mM EDTA, the frozen urine supernatant maintained the baseline-equivalent ucfDNA quality, whereas the whole urine samples presented significantly lower %cfDNA scores, and the frozen urine supernatant subjected to postthawing secondary centrifugation significantly reduced the ucfDNA yield when thawed at 4°C. The 10 mM EDTA-treated urine supernatant stored at -80°C maintained baseline-equivalent ucfDNA for 3 months, whereas samples stored at -20°C degraded ucfDNA after 2 months. Conclusions This work offers practical guidelines for optimizing ucfDNA quality in clinical settings. Without centrifugation, preservatives other than EDTA should be further explored for whole-urine biobanking. The addition of 10 mM EDTA to the urine supernatant is recommended, followed by temporary storage at RT for up to 7 days or freezing at -20°C for less than one month. For long-term storage, ultralow temperatures are necessary. Rapid thawing at RT/37°C can yield good ucfDNA quality.

It is well known that the efficiency of regular acid treatments goes through a maximum at an optimum acid injection rate where it gives the maximum wormhole propagation rate (ratio between wormhole length to pore volume of acid required to breakthrough). This rate is a strong function of the acid-rock reaction: diffusion of the acid to the rock, surface reaction, and mass-transport of the reaction products. Adding a polymer and other additives to the acid may affect the wormhole propagation rate. However, literature survey didn’t highlighted in details how the wormhole propagation rate will change when polymer and crosslinker will be existing with the regular acid. Therefore, the objective of this study is to determine the wormhole propagation rate of gelled and in-situ gelled acids as a function of the injection rate. The effects of temperature, lithology, and initial permeability on the wormhole propagation rate were considered in this work. A coreflood study was conducted using calcite cores at temperature range 100 to 250 °F. Injection rate was varied from 1 to 20 cm3/min using 5 wt% HCl regular, polymer gelled, and in-situ gelled acids. Calcium, cross-linker, and acid concentrations were measured for the core effluent samples. The wormhole propagation rate of gelled acid required was higher than regular acid where gelled acid required less acid volume to breakthrough. However, the wormhole propagation rate of in-situ gelled acid required was lower than regular acid where in-situ gelled acid required a higher pore volume to breakthrough. Three regions were obtained for the wormhole propagation rate of in-situ gelled acid as a function of the injection rate: Region I, where acid injection rate is low, and in-situ gelled acid wasn’t able to breakthrough from the cores at injection rate less than 5 cm3/min. Region II, where the acid volume required to breakthrough decreased as injection rate increased in the range between 5 and 10 cm3/min. Region III, where the acid volume injected increased with the injection rate at injection rate higher than 10 cm3/min. The optimum injection rate wasn’t affected by temperature, or initial core permeability. The volume required to breakthrough decreased as the temperature was increased, and as the initial core permeability increased.

Location and magnitude of formation damage due to iron precipitation during acidizing carbonate rocks

As an important enhanced oil recovery (EOR) method, flue gas flooding enhances reservoir energy, reduces the crude oil viscosity, and improves the flow capability of the crude oil. This technique significantly optimizes crude oil extraction performance, achieving a higher oil recovery factor while simultaneously facilitating flue gas sequestration and efficient utilization of resources. In this study, a series of high-temperature and high-pressure PVT experiments are conducted to measure the relationship between the dissolved gas–oil ratio and saturation pressure. Core flooding experiments are then used to examine the effects of the injection rate and core fractures on flue gas flooding under immiscible conditions. Additionally, the multiple relaxation time lattice Boltzmann method (MRT-LBM) is employed to simulate the flue gas-oil two-phase flow, taking into account the impact of miscible effects on the flue gas flooding process. Experimental results show that as the dissolved gas–oil ratio increases, the saturation pressure gradually increases but the rate of increase slows down. With the increasing flue gas injection rate, gas channeling occurs earlier, leading to a reduction in the oil recovery factor. Fractures intensify the occurrence of gas channeling, resulting in a further decrease in the final oil recovery factor, which is only 24.93%. Simulation results show that a lower injection rate and a higher miscible effect both contribute to improving the oil recovery factor. Miscibility facilitates the mixing of gas and crude oil, allowing the gas to carry and mobilize the oil more effectively, creating a wider gas distribution area. This promotes the flowing of flue gas into a broader range of pores around fractures, continuously expanding the gas-swept region and achieving a higher sweep efficiency and oil recovery. This study provides a theoretical basis for optimizing flue gas flooding technology and offers a scientific reference for simulating the oil–gas multiphase flow under complex reservoir conditions.