Acid injection rates

The rates of acid injection are dictated by allowable injection pressure. As presented by Piot and Perthius, the maximum injection rate that will not fracture the formation may be estimated from Darcy s radial-flow equation, represented as follows  

B is the formation volume factor (very near unity) r is the drainage radius, in ft is the wellbore radius, in ft s is the skin factor (dimensionless) 

Equation (6.1) is a simplified radial-flow equation, accounting for an incompressible injected fluid in a homogenous formation only. It does not take multiphase-flow effects into account. This representation of Darcy s equation should be used only as a guideline for setting the initial treatment injection rate. It is good practice to limit acid treatments in sandstones to several hours at the most. Long HE residence time may be expected to increase precipitation of acid reaction products. Whenever possible, LIF contact time should be limited to 2-4 hours per stage. 

The HCl (or organic acid) preflush should be 50%-100% of the HCl-HF volume. If the solubility of the formation in HCl is less than 5%, then the preflush should be 50% of the HCl-HF volume. If the solubility in HCl is between 5% and 10%, then the preflush should be 100% of the HCl-HF volume. In formations with HCl solubility greater than 10%, larger preflush volume should be considered. 

Low acid volumes should be used in poorly consolidated formations, low-permeability formations, and formations with high clay content. In poorly consolidated sands, excessive HF can cause sloughing or collapse of perforations and, possibly, sand production. Excessive HF in low-permeability formations and in formations with high clay content can cause severe plugging because of reprecipitation of dissolved solids. Formations with native permeability less than 10 mD can be acidized with care. However, hydraulic (propped) fracturing may often be the stimulation method of choice. 

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Figure 8, from reference [14], shows the permeability ratio as a function of the acid injection rate. For brine saturated cores, the final core permeability exponentially increases with the acid injection rate. The same trend is noted in the case of oil saturated cores. 

Limited interval coverage can be overcome to a degree by maximizing matrix acid injection rate. Greater coverage can be accomplished with chemical diversion or, preferably, mechanical placement— including coiled-tubing/injection-nozzle methods. 

To achieve or approach uniform acid distribution, the acid injection rate per unit area to be treated must be varied. Add injection rate per unit area can be described in terms of Darcy s equation ... 

The goal in acid placement is to equalize the acid injection rate per unit area Q/A) across the entire treatment interval, divided into n sections ... 

The MAPDIR method can be combined with real-time evaluation of skin removal. In fact, the calculated skin remaining at any time during treatment is used to determine the maximum allowable differential pressure (dP). Therefore, MAPDIR is used to determine the maximum allowable acid injection rate at any given time, based on the relationship expressed in equation (6.2). 

Diversion of fluids can be achieved by sustaining maximum differential injection pressure at maximum acid injection rates. Again, slightly viscosified fluids are used to reduce friction. 

Fig. 10-2. Partially acid-etched fracture (acid injection rate greater than acid leak-off rate)...

Human tissues can synthesize purines and pyrimidines from amphibolic intermediates. Ingested nucleic acids and nucleotides, which therefore are dietarily nonessential, are degraded in the intestinal tract to mononucleotides, which may be absorbed or converted to purine and pyrimidine bases. The purine bases are then oxidized to uric acid, which may be absorbed and excreted in the urine. While little or no dietary purine or pyrimidine is incorporated into tissue nucleic acids, injected compounds are incorporated. The incorporation of injected [ H] thymidine into newly synthesized DNA thus is used to measure the rate of DNA synthesis. 

OS 44] ]R 4a] ]P 33] 4-Bromobenzonitrile plugs (5 s) were inserted into a continuous phenylboronic acid stream for time intervals of5,15,25, 30,40 and 55 s (25 min reaction period) [6, 7]. For a 5-20 s delay, catalyst saturation occurs owing to the high injection rates rendering the phenylboronic acid concentration too low. In turn, for delay times of 30-55 s, the effective concentration of 4-bromobenzonitrile becomes too low. At 25 s, optimum performance with a yield of 62% was achieved (Figure 4.67). 

Liquid flowing into the chromium treatment module [T-21] is monitored by a pH instrument that controls a feed pump to add the required amount of sulfuric acid from a storage tank. The sulfuric acid is needed to lower the pH to 2.0 to 2.5 for the desired reduction reaction to occur. An ORP instrument controls the injection rate of sodium metabisulfite solution from a metering pump to reduce hexavalent chromium (Cr6+) to the trivalent state (Cr3+). 

So-called "wormholes" can be formed when the injected acid primarily enters the largest diameter flow channels in carbonate rock further widening them (107). Acid only invades the small flow channels a short distance greatly reducing treatment effectiveness. High fluid loss rates, low injection rates, and reduced rates of acid-rock reactions decrease the wormhole length. 

High fluid injection rates are often required. For this reason, friction reducers are often used in acid fracturing. These include polyacrylamide and acrylamide copolymers, guar gum, hydroxyethyl cellulose, and karaya gum (108)... 

Injection rate can have a major effect on the economics of secondary oil recovery. Acidizing or carefully designed hydraulic fracturing treatments can be used in increase injection rates. 

The relative skin (5 - S0) evolution with respect to the cumulative acid volume is given in Figure 3 (solid line). Peaks at about 0.5 and 3.5 m3 are artifacts due to hammering effects following rapid variations of the injection rate. According to Equations 6 and 13, the skin evolution, if the formation were a primary porosity one, should be ... 

Acid fracturing, friction reducers, 15 Acid hydrolysis, lignin, 173 Acid injection into carbonate reservoir, 610-611 Acid-rock reactions, rate, 15,16 Add wormholing in carbonate reservoirs, 608-620 in carbonate rocks, 610-611 Acidity-controlled redox reactions, 141-142 Addization... 

Hyaluronic acid injections temporarily and modestly increase synovial fluid viscosity and were reported to decrease pain, but many studies were short term and poorly controlled with high placebo response rates. 

Chromatography of MBC is carried out on a strong Zipax CSX cation-exchange column (100 cm X 0.21 cm I.D.). The column is first equilibrated under the following conditions column temperature, 60 °C mobile phase, 0.025 TV tetramethylammonium nitrate-0.02 TV nitric acid flow-rate, 0.5 ml/min. The chromatogram shown in Fig.4.28 is of the analysis of benomyl (as MBC) and 2-aminobenzimidazole (2-AB), a minor metabolite of benomyl, present in cucumber in amounts of 0.05 and 0.1 ppm respectively. The limits of detection are in the range 0.05-0.1 ppm (equivalent to 5-10 ng of benomyl injected). UV detection is made at 254 nm. 

Hooijschuur et al. (54) also reported the analysis of the hydrolysis products of sulfur mustard homologues (11), using micro-LC/ESI/MS (triple quadrupole) and micro-LC/FPD in sulfur mode. To improve sensitivity, large-volume injection was used with peak compression by adding suitable coeluting alcohols. LC employed a 0.28-mm ID packed C18 column, eluted isocratically with water-methanol (80 20 v/v)-0.2% formic acid, flow rate 6 xl/min. Spectra were dominated by MH+, [MH — H20]+,... 

FIGURE 12-6. Example of reverse-phase separation of an APC tablet. Column Resolve Ci8 column (5 jam) 3.9 mm x 15 cm. Mobile phase percent methanol noted on chromatogram and remaining percentage is 4% aqueous acetic acid. Injection volume 5 jaL. Flow rate 1 mL/min. Detector 254 nm, 1.0 AUFS. (Note Actual separation will depend upon the quality of the mobile phase and column packing.)... 

Procedure Prepare a series of THI-DNPH Standard Solutions serially diluted from the Stock THI-DNPH Solution. Pipet 1, 2, and 5 mL, respectively, of the Stock THI-DNPH Solution, into separate 10-mL volumetric flasks, and dilute to volume with absolute, carbonyl-free methanol. Prepare a standard curve by injecting 5 p-L of the Stock THI-DNPH Solution, and the serially diluted THI-DNPH Standard Solutions into a 250-mm x 4-mm (id), 10-lm LiChrosorb RP-8 HPLC column (Alltech Associates, Inc., or equivalent) fitted with an ultraviolet detector set at 385 nm. The mobile phase is 50 50 (v/v) methanohO.l M phosphoric acid. Inject 5 pL of sample into the column. Adjustments in the mobile phase composition may be needed as column characteristics vary among manufacturers. At a mobile phase flow rate of 2 mL/ min and column dimensions of 250 x 4.6 mm, elute THI-DNPH at about 6.3 0.1 min. Measure the peak areas. Calculate the amount of THI in the sample from the standard curve. (For THI limits greater than 25 mg/kg, prepare a series of Standard THI-DNPH Solutions in a range encompassing the expected THI concentration in the sample.)... 

Another important factor in the selection of a disposal zone is the injectivity. Can the desired rate of acid gas be injected at down hole conditions The injection rate is a function of the properties of the reservoir, notably the permeability, and the properties of the fluid, notably the viscosity. The injectivity can be estimated by doing an injection test. 

Thus given the injection rate of the test fluid, say water, and its viscosity, and the bottom hole pressure, one can estimate the injection rate for the acid gas at a given bottom hole pressure, given the viscosity of the acid gas. Methods for calculating the viscosity of acid gas were given earlier, and the viscosity of water as a function of pressure and temperature is well known. 

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