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SummaryIron sulfide (FeS) scales create well deliverability and integrity problems such as decreased production rates and damage to well tubulars. The application of chelating agents for production enhancement has been successful because of its high-temperature stability and its clean characteristic nature without the need for expensive additives. However, chelating agents have not been studied adequately for FeS dissolution. This paper investigates ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacteic acid (DTPA), and N-(2-hydroxyethyl) ethylenediamine-N,N′,N′-triacetic acid (HEDTA) for their FeS dissolution capacities and kinetics at 150°F and 300°F.Chelating agents are expensive chemicals and must be investigated carefully to determine the optimum concentration, pH, treatment time, and dissolver/scale ratio. FeS (0.1 g) composed of mainly troilite was used as the scale. EDTA, DTPA, and HEDTA solutions (10 cm3) were prepared at different concentrations ranging from 0.05 to 0.4 mol/L using deionized water with a resistivity of 18.2 MΩ-cm. The pH of the dissolvers was dependent on the concentration and the degree of neutralization. The ligands were deprotonated at higher pH using sodium hydroxide or potassium hydroxide. A comparative study of the chelating agents with a low pH (3 to 5), moderate pH (5 to 9), and high pH (10 to 14) determined the optimum pH for the scale treatment. The sampling time of the dissolution process set at 1, 2, 4, 8, 20, 30, and 72 hours determined the kinetics of the scale-dissolution process and helped optimize the treatment time. A dissolver/scale ratio of 100:1, 50:1, and 20:1 cm3/g were tested. The iron concentration in the dissolver was quantified using inductively coupled plasma-optical emission spectroscopy (ICP-OES).Two calculated parameters, dissolution capacity and dissolver consumption, determined the effectiveness of the chelating agent in dissolving FeS sulfide. From the bottle tests at 150°F, lower pH solutions were more effective. One hundred percent of the iron from FeS was complexed by 0.3 mol/L dipotassium DTPA (K2-DTPA) after 20 hours of soaking; 0.2 mol/L disodium EDTA (Na2-EDTA) and 0.3 mol/L potassium HEDTA (K-HEDTA) were able to remove, respectively, 69 and 96% of the initial iron present in 0.1 g FeS. The mechanism of dissolution at pH 10), the dissolution of the scale was negligible and was a result of solution complexation after FeS dissociation. The order of the chelating agents in terms of dissolution capacity was DTPA > HEDTA > EDTA at all pH conditions. The kinetics study showed that the optimum treatment depended on the pH of the chelating agent. For pH 5, there was an improvement in the effectiveness of the ligands because of the increase in the system energy and increased activity of the chelating agent. A scanning electron microscopy (SEM) study showed the changes in the morphology of the FeS particles after dissolution with low- and high-pH solutions of the chelating agent.The role of chelating agents in FeS dissolution has not been thoroughly investigated. No study reports the optimum treatment time and dissolver/scale ratio. The role of the pH of the dissolver also needs more attention. This paper fills these gaps in the literature and provides the optimum dissolver composition and treatment time for field operations.

Summary In highly heterogeneous carbonate reservoirs, several acid systems were 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 starts filter-cake formation, which inhibits the wormhole growth. On the 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 addresses 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 through coreflood study and acid-diversion experiments. The system was 15 wt% hydrochloric acid (HCl)-gelled acid emulsified in diesel with a 70:30 acid/diesel volume ratio. Coreflood experiments with Indiana limestone were conducted at 230°F at different injection rates, and the core samples were imaged with a computed-tomography (CT) scan technique after each coreflood experiment. Also, 0.5 pore volumes (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 cm3/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.

Summary Carbonate formations are very complex in their pore structure and exhibit a wide variety of pore classes, such as interparticle porosity, moldic porosity, vuggy porosity, and microporosity. Geologists have defined carbonate pore classes on the basis of sedimentology, thin sections, and porosity/permeability relationships, but the question remains concerning how these pore classes govern the acid flow through porous media. Core samples from six different carbonates, mainly limestone, were selected for the study. The samples were first investigated with thin-section analysis, high-pressure mercury-injection tests, and nuclear-magnetic-resonance measurements for pore-structure characterization, and X-ray diffraction for mineralogy examination. Next, tracer experiments were conducted, and the tracer-concentration profiles were analyzed to quantify the carbonate pore-scale heterogeneity. The heterogeneity is expressed with a parameter f—the available fraction of pore structure contributing to the flow. The data were used to study the flow of acid through carbonate rocks and correlate the pore classes to the acid response. More than 30 acid-coreflood experiments were conducted at 150°F and a hydrochloric acid concentration of 15 wt% on 1.5 × 6-in. core samples at different injection rates on each carbonate rock type. The objective of these sets of experiments is to determine the acid pore volume to breakthrough for each carbonate pore class. The findings of this study help us to connect the results from different characterization methods to the acid flow through the porous media of carbonate rocks. It was also found that the response of the acid depends on the carbonate pore classes. Application to the design of matrix acid treatments in carbonate rocks is discussed.

Summary Carbonate formations are very complex in their pore structure and exhibit a wide variety of pore classes, such as interparticle porosity, moldic porosity, vuggy porosity, intercrystalline porosity, and microporosity. Understanding the role of pore class in the performance of emulsified-acid treatments and characterizing the physics of the flow in the acid propagation are the objectives of our study. The study was performed by use of vuggy-dolomite cores that represent mainly the vuggy-porosity-dominated structure, whereas the homogeneous cores used represent the intercrystalline pore structure. Coreflood experiments were conducted on 6 × 1.5-in. cores by use of emulsified acid formulated at 1 vol% emulsifier and 0.7 acid volume fraction. The objective of this set of experiments is to determine the acid pore volume (PV) to breakthrough for each carbonate pore class at different injection flow rates. In this paper, we examine whether the heterogeneities observable at the thin-section scale have a significant influence on the results of the emulsified-acid coreflood experiments. The heterogeneities were characterized by use of thin-section observations, tracer experiments, scanning electron microscopy (SEM), and resistivity measurements. Thin-section observations provide means to study the size of vugs and their distribution and connectivity, and to explain the contribution of the pore class in the acid propagation. Also, the rotating-disk experiments of emulsified acid with dolomite were related to our coreflood experimental results. The acid PV to breakthrough for vuggy-porosity-dominated rocks, ranging from 0.1 to 0.3, was observed to be low when compared with homogeneous carbonates (intercrystalline pore structure) with PVbt ranging from 2.5 to 3.5. Also, the wormhole dissolution pattern was found to be significantly different in vuggy rocks than that in homogeneous ones. Thin-section observations, tracer results, and the coreflood experiments indicate that the vugs are distributed in a manner that creates a preferential flow path, which can cause a rapid acid breakthrough and effective wormholing rather than those with a uniform pore structure. The rotating-disk results also showed that the reaction kinetics played a role in determining the wormhole pattern.