HCl/Formic In-Situ-Gelled Acids as Diverting Agents for Carbonate Acidizing

Day 2 Tue, September 15, 2015

2015

Self Cite

The objective of matrix acidizing treatments is to remove the skin damage from the vicinity around the wellbore and enhance the permeability and the well productivity. While this is accomplished in sandstone by using reactive fluids to dissolve the clay and the surrounding cementing materials, carbonate stimulation is successful when the reactive fluids bypass the damage by creating conductive channels or "wormholes". It is rare in both treatments that the acids are used alone without several additives. The presence of any of these additives, which is required to achieve a certain function, may impact significantly the outcome of the acid treatment either positively or negatively. The objective of this study is to examine the effect of the type and concentration of several additives, which are currently in-use in the oilfield, on the calcite dissolution rate with HCl using the rotating disk reactor. Additives such as corrosion inhibitors, synthetic polymers, mono and divalent chloride salts, mutual solvent, dissolved iron, viscoelastic surfactants, and chelating agents were investigated.The results showed that more reliable data was obtained at lower acid concentrations. Higher acid concentrations were associated with significant surface changes and inevitable source of error. The effect of corrosions inhibitors was studied by investigating three corrosion inhibitors (CI-A, CI-B, and CI-C) obtained from three different service companies. The presence of two corrosions inhibitors was associated with a reduction in the dissolution rate (between 7 and 45%), while the presence of the third one (containing formic acid) increased the rate of dissolution up to 38%. The effect of all other additives was further examined in the presence of one of the corrosion inhibitors. The presence of a synthetic polyacrylamide polymer significantly reduced the reaction rate down to 96% compared to a blank acid solution that did not contain the polymer.Monovalent ions (sodium and potassium) showed little effect on the calcite dissolution rate, the presence of divalent ions such as magnesium chloride was associated with a reduction in the rate. Calcium chloride showed an increase in the reaction rate for both cases of the presence and the absence of a corrosion inhibitor.The results also showed that mutual solvent changed the dissolution rate and a minimum was observed at 5 vol%. These results were confirmed by changing the corrosion inhibitor and the same behavior was observed at the same mutual solvent concentration. Dissolved iron reduced the rate of reaction by 17% at iron concentration up to 5000 mg/l. At 17,000 mg/l, the rate was reduced by 39% and a reddish to brown precipitate of ferric hydroxide was observed on the rock samples after the reaction, indicating a ferric precipitate. The reaction of VES-based acid showed a minimum at 125°F, which matched the maximum viscosity at low shear rate. The presence of 5 wt% chelating agents increased the reaction rate of 0.1 N HCl by 12.7 and 25%, when the reactions were conducted with no ...

Section: Materials and Apparatusmentioning

confidence: 99%

Effect of Acid Additives on the Reaction of Stimulating Fluids During Acidizing Treatments

Rabie

Day 2 Tue, September 15, 2015

2015

Self Cite

“…The viscosity was measured as a function of shear rate from 1 to 1020 s −1 at room temperature and 150°F. A pressure of 300 psi was applied to minimize the 19 The viscosity of the tracer solution was measured as a function of shear rate at 150°F at different polymer concentrations. The results showed a non-Newtonian, shear thinning behavior ( Figure 13).…”

Section: Industrial and Engineering Chemistry Researchmentioning

confidence: 99%

Flow of Emulsified Acid in Carbonate Rocks

Zakaria

Ziauddin

2015

Ind. Eng. Chem. Res.

We conducted tracer experiments on core samples from six different carbonates to study the flow of acid fluid through the porous media of carbonate rocks and correlate the carbonate pore classes to the acid response. Pore structures were characterized by section analysis, high pressure mercury injection tests, and nuclear magnetic resonance measurements; mineralogy was determined by X-ray diffraction. Tracer concentration profiles were analyzed to quantify the carbonate pore-scale heterogeneity. The heterogeneity is expressed using a parameter fthe available fraction of pore structure contributing to the flow. From the results, different characterization methods could be connected to the acid fluid flow through the porous media of carbonate rocks. It was also found that the response of the acid depends on the carbonate pore classes. Applications include the design of matrix acid treatments in carbonate rocks. ■ INTRODUCTIONThe goal of acid-stimulation treatments in carbonate reservoirs is to remove or bypass formation damage by creating highconductivity channels known as wormholes. To propagate deeper wormholes in the formation for better productivity enhancement, retarded acids should be used. Emulsified acids are one of the most widely used retarded-acid systems. HCl acid is commonly used as the internal phase of the emulsion. For the external phase, oil and diesel are the most commonly used hydrocarbons. The external hydrocarbon phase acts as a diffusion barrier to reduce the diffusion of the acid droplets to the surface of the rock, 1,2 which helps in the creation of deep wormholes. Several studies 3−5 examined the reaction of emulsified acid with carbonate. Navarrette et al. 4 indicated that the reaction rate of 28 wt % HCl emulsified acid with limestone was 8.5 times less than that of 28 wt % HCl with limestone. Al-Anazi et al. 6 studied the application of the emulsified acid in tight carbonate rocks using coreflood experiments, and the results showed that the emulsified acid can form deep wormholes in tight carbonate cores. Buijse and van Domelen 7 studied the effect of the injection rate, viscosity, and acid volume fraction on the emulsified acid pore volume to breakthrough in carbonate rocks. Sayed et al. 8 studied the performance of the emulsified acid in low and high permeability carbonate formations using coreflood experiments. It was found that the high permeability carbonate cores consumed more acid volume to breakthrough than did the low permeability cores. Also, an optimum injection rate was observed for the highpermeability cores whereas the acid pore volumes to breakthrough decreased with increases in the injection rate for lowpermeability formations.Hoefner and Fogler 9 studied the pore evolution during the flow and the reaction of the acid in porous media. The initiation of wormholes occurs when the live acid penetrates into pores present in carbonate rocks, and then, pore structure evolution occurs. The process is more efficient when only the large pores in the matrix lead to wormholes. 10 C...

“…A pressure of 300 psi was applied to minimize the evaporation of the sample at high temperature. The specification of the viscometer was given by Rabie et al (2012).…”

Section: Viscosity Measurementsmentioning

confidence: 99%

Application of Novel Polymer Assisted Emulsified Acid System Improves the Efficiency of Carbonate Acidizing

Zakaria

Day 2 Tue, April 14, 2015

2015

In highly heterogeneous carbonate reservoirs, several acid systems have been used to enhance acid diversion during matrix acidizing treatments. Viscosified acid with polymer increases the viscosity of the acid system to improve the wellbore coverage. However, the injection of such acid at low rates had a negative effect on the spending rate and it initiates filter cake formation which inhibits the wormhole growth. On other hand, relatively low viscosity emulsified acid is diffusion retarded which makes it an effective wormholing agent at the low injection rates that occur, for example in low permeability, or damaged formations. None of the studies in the literature address an acid system that uses both advantages.The objective of this work was to investigate the behavior and the performance of a new acid system, polymer assisted emulsified acid, as a self-diverting acid by conducting viscosity measurements thorough coreflood study and acid diversion experiments. The system was 15 wt% gelled acid emulsified in diesel with a 70:30 acid/diesel volume ratio.Coreflood experiments using Indiana limestone were conducted at 230°F at different injection rates and the core samples were imaged using a computed-tomography (CT) scan technique after each coreflood experiment. Also, a 0.5 PV of the polymer assisted emulsified acid was injected to assess the effect of the acid on the permeability of the cores before breakthrough. Finally, two acid diversion experiments at 1 cm 3 /min were conducted into pairs of low and high permeability cores to test the effect of polymer concentration in the acid internal phase on diversion.The viscosity measurements and acid diffusivity measurements showed that increasing the polymer concentration in the acid internal phase of the emulsified acid from 0 to 1.5 vol% significantly enhanced the viscosity of the emulsified acid and reduced the diffusion coefficient by one order of magnitude. Coreflood results showed that the polymer assisted emulsified acid was an effective wormholing fluid at low injection rates while maintaining the high viscosity of the acid system for zonal coverage.Also, it was shown that the emulsion/polymer retention was the main source for permeability damage. However, flowback with mutual solvent removed any remaining damage and permeability enhancement was achieved.Acid diversion experiments are presented that show the self-diverting ability of the polymer assisted emulsified acid into the low permeability cores.