Acidizing Fluid Research Articles

A Study on Effect Analysis and Process Parameter Optimization of Viscous Acid Acidification in a Porous Heterogeneous Carbonate Reservoir

The homogeneous acid etching of conventional acid in porous heterogeneous carbonate reservoirs leads to a large amount of consumption in the near-wellbore area, which makes the acidification effect often not ideal. In order to improve the acidizing effect of porous heterogeneous carbonate reservoirs, viscous acid is used to increase the stimulation of the target block in this paper. Through systematic experiments, the adaptability of the viscous acid in the four layers of the M reservoir in the target block was evaluated, and the MD and ME layers suitable for acidizing stimulation were determined in combination with physical property analysis. Finally, based on the geological characteristics and experimental data of the preferred layers, a two-scale acid wormhole growth radial model was established, and the construction parameters of acidizing stimulation were optimized. The results show that (1) The preferred viscous acid system has a dissolution rate of more than 95% for the rock powder in the four layers. When the matrix permeability is high, the effect of the acid wormhole is obvious and the permeability increase is higher. (2) The steel sheet corrosion and residual acid damage experiments showed that the acid system was not corrosive to the wellbore, and the reservoir damage rate of the residual acid after the reaction was low. (3) Based on the relationship between reservoir porosity and permeability and the position of edge and bottom water, the MD and ME layers with more potential for acidizing stimulation are selected. (4) The results of the numerical simulation show that the optimal acid pump rate of the MD and ME layers is 1.4 bpm and 1.0 bpm, and the acidizing fluid volume is 255 bbl, which can form effective acid wormholes, and the range of reservoir permeability transformation is the largest. The field application results show that the optimization scheme effectively improves the production of oil wells, verifies the practicability of the scheme, and provides a reference for the process optimization of viscous acid in the same type of porous heterogeneous carbonate reservoir stimulation.

Flowback Additive for Acidizing Fluid to Stimulate Carbonate Gas Reservoirs

Summary Acidizing of oil- and gas-bearing carbonate reservoirs is generally undertaken by using strong mineral acids such as hydrochloric acid (HCl) to enhance permeability. One of the major challenges associated with HCl injection is tuning the reactivity profile to favor the transport of live acid deep into the reservoir while achieving a minimum rock face dissolution. The mineral acid is therefore emulsified in a hydrocarbon phase (e.g., diesel) to retard its reactivity with the rock matrix. The use of emulsified acid is hindered by several limitations such as low emulsion stability at high temperatures, pumping limitations due to high viscosity, the potential of formation damage, and cumbersome mixing procedures at the field scale. In addition, the brines formed as a result of this reaction can be difficult to produce due to higher density and capillary pressures, unfavorable wettability, and low formation pressure. Here, we report on the development of dual-purpose additives that were specifically designed to enhance the recovery of high-density brines and retard the acid/rock reactivity upon addition to the stimulation treatment. Accordingly, seven new additives with fluid flowback properties were developed for use in a single-phase acidizing fluid consisting of HCl (15 wt% and 28 wt%) with the required additives, such as corrosion inhibitor and intensifier, and H2S scavenger. The flowback enhancers (FBEs) were formulated from a blend of water, ester or terpene solvents, alcohols, and surfactants to form optically clear nano- and microemulsions. Surfactant selection was driven by the need to exhibit demulsification properties with condensate, high chemical and thermal stability, compatibility in strongly acidic media, and high-density brines under harsh reservoir conditions. To assess the FBE performance in acidizing formulations (i.e., to serve as both an FBE and retarder), screening studies consisting of static rock dissolution tests and surface tension measurements were performed to downselect FBEs suitable for this application. This was coupled with brine displacement tests in addition to compatibility and stability studies. FBEs that demonstrated superior performance were then selected for further evaluation under reservoir conditions [i.e., core flow matrix acidizing to measure regained permeability and computed tomography (CT) scan for analyzing the wormhole propagation]. The droplet size of the as-prepared nano- and microemulsions was found to be between 10 nm and 850 nm. The FBEs formulated in this study were found to prevent emulsion formation in the presence of condensate and demonstrated remarkable chemical and thermal stability in concentrated acid at temperatures up to 300°F for a duration of up to 24 hours, as confirmed by the consistent low surface tension values (21–29 mN/m). With regard to fluid displacement, column tests performed under ambient conditions revealed quick brine displacement with recovery exceeding 75 vol% in comparison with 16 vol% in the absence of the FBE. Interestingly, the addition of a select FBE from this study to 28 wt% HCl was found to retard the reaction of carbonate dissolution at room temperature. This led us to assess the performance under reservoir conditions utilizing core flow testing. Accordingly, the addition of FBE-F to 28 wt% HCl led to an improvement in permeability by up to 267% as compared with 15% without FBE added. These results are further supported by the CT scan images of the acidized cores, which revealed the formation of a deeper wormhole in the presence of a select FBE.

Fracture and vug effects on wormhole pattern during acidizing of triple porosity carbonate rocks

Carbonate reservoir acidization is performed to mitigate formation damage and create desirable and highly permeable pathways for the movement of oil and gas. The most significant characteristic of carbonate reservoirs is the presence of fractures and vugs within their structure, which contribute to their triple porosity. The presence of fractures and vugs complicates the flow equations in the medium, which can significantly impact the success or failure of the acidizing. Therefore, understanding their role in the behavior of fractured carbonate reservoirs is a challenging and important issue. Comprehensive studies investigating the simultaneous effect of fractures and vugs on the acidizing in these types of reservoirs have not been reported. Therefore, this study provides a detailed and more precise examination of the acidization process within a triple porosity carbonate media. To simulate the acidizing, a two-scale continuum model was coupled with the pseudo-fracture method in a triple porosity carbonate media. The flow within the matrix and the vug were modeled using Darcy's law, while the flow within the fracture was modeled using the cubic law (free flow). This modeling approach accurately simulated the overall flow of the acidizing fluid. Generally, increasing the length and decreasing the width of a single horizontal fracture improves the acidizing, whereas a vertical one causes a delay at average injection rate. The presence of cross-fractures enhances acidization at all injection rates, and when a horizontal fracture is present at the system's outlet, it also significantly reduces the PVbt. Conversely, the presence of multiple vertical ones within the reservoir delays the acid reaction time, Increasing the number and size of vug improves the acidizing. When fractures and vugs coexist within the rock, it is the type and location of these fractures that control the wormhole propagation pattern and the extent of variations in porosity inside the vug.

Boosting Reaction Rate of Acids for Better Acid Fracturing Stimulation of Dolomite-Rich Formations

Summary Carbonate reservoirs can be stimulated by injecting acids to boost the rate of hydrocarbon production from low-permeability zones via the creation of conductive pathways. The reaction rate between the acidizing fluid and rock matrix is a key parameter in determining the success of stimulation treatments. Dolomite-rich formations are known to exhibit slower reaction kinetics as compared to calcite. As a result, some acid fracturing treatments fall short of creating an extended fracture or the desirable etching pattern on the fracture faces, thus limiting hydrocarbon flow. Accordingly, the development of an acid package to boost the dolomite dissolution rate will be advantageous to the efficiency of the stimulation treatment in dolomite-rich reservoirs. Accelerating the reaction rate of dolomite with strong mineral acid (i.e., hydrochloric acid, HCl) can be achieved through an additive-driven chemical approach based on the addition of judiciously selected sulfonate-based surfactants. To pinpoint the optimal surfactant(s) type and concentration, static dissolution testing was performed under ambient conditions using outcrop dolomite core samples (Silurian and Guelph). Each core sample was reacted with 28 wt% HCl in the presence and absence of sulfonate-based surfactant additives for a predetermined time. Selected surfactants are used in comprehensive reaction kinetics studies at reservoir conditions using a rotating disk apparatus (RDA). Based on the results of the ambient screening tests, three formulations were found to accelerate the reaction rate by up to 30% as compared to using 28 wt% HCl without the additive(s). The kinetics data collected at a pressure of 3,000 psi and temperatures of 175 up to 300°F showed that the reaction rate of Guelph dolomite can be accelerated by as much as 17–55% with one of the formulations. Coreflood experiments showed an increase in the acid PV to breakthrough (PVBT) when the surfactant package was added to the acid formulation pointing to a rise in the reaction rate of dolomite and the developed acid formulations. The acid formulations showed an improvement in the dolomite and acid reaction rate, which creates the opportunity to apply these formulations in the field to improve the outcome of acid fracturing treatments in dolomite-rich reservoirs.

Pore Structure Sensitivity to External Fluids for Coal Reservoir and Fracturing Initiation/Propagation Mechanism by Using the T1 ∼ T2 NMR Spectrum and Physical Simulation Experiment

Fracture initiation expansion and external fluid sensitivity evaluation have become the main focus of the coal reservoir fracturing effect. Therefore, a study on fracture propagation of hydraulic fracturing and the coupling mechanism between the fracturing fluid and pore structure of coal reservoirs need to be studied. In this paper, fracture conductivity and external fluid sensitivity evaluation were carried out. First, an automatic reservoir fracturing simulation tester and a large-scale computed tomography scanner are used to compare the fracture distribution and calcite content and evaluate the fracturing effects of different fracturing fluids and construction parameters. Second, the variation of porosity and permeability of coal before and after the injection of different external fluids are measured by an overlying pressure porosimeter, high-pressure mercury injection, and two-dimensional nuclear magnetic resonance. The results are as follows. (1) The T1–T2 spectra of dried and saturated water samples indicate adsorbed water and mobile water in the adsorption pore, while mobile water mainly exists in the seepage pore and fracture. Different occurrence states of water have different T1 ∼ T2 spectrum ranges. (2) The experiments of external fluid injection show that different external fluids have different effects on the porosity of the adsorption pore, seepage pore, and fracture. High-viscosity slick water and the acidizing fluid harm the seepage pore porosity, while low-viscosity tricky water and active water improve it. Fracture porosity is not sensitive to the guanidine gel-breaking fluid. (3) Experiments of fracturing initiation show that acid fracturing can reduce the fracture pressure and cumulative injection by 6.9 and 33.9%, respectively. The reaction of the acidizing fluid with carbonate mineral components can significantly increase the acid dissolution ratio and porosity of calcite, which forms the complex fractures and contributes to the increase of fracturing reconstruction volume.

Effect of Mixed Acid Fluid on the Pore Structure of High Rank Coal and Acid Fluid Optimization.

Acidizing technology is an important means to increase production in oil-gas reservoirs. In recent years, acidizing technology has been widely used to increase the permeability of coal seams to enhance gas extraction, where acidizing fluid is the key factor to determine the permeability improvement effect by acidizing technology. In order to clarify the influence of mixed acid fluid on the pore structure of high rank coal and seek the optimal mixed acid fluid suitable for acidizing and permeability improvement of high rank coal in the Jiaozuo coal mine area. Taking the Jiulishan Mine in the Jiaozuo mining area as an example, low field nuclear magnetic resonance (LFNMR) test and static dissolution test were conducted to obtain the T2 spectrum, porosity, movable fluid saturation, pore throat distribution, nuclear magnetic permeability, and dissolution rate of coal samples before and after treatment with distilled water and three mixed acid fluids. On this basis, the influence of mixed acid fluid on the pore structure of high rank coal was analyzed and the optimal mixed acid fluid suitable for high rank coal was selected. The results showed that the pore size, number, and volume of all kinds of pore sizes of coal samples treated with distilled water all decreased, which was manifested by the decrease of effective porosity and nuclear magnetic permeability. After acidification, the proportion of micropore volume in coal decreased significantly, the number and proportion of pore volume of mesopores and macropore-microfractures increased significantly, and the connectivity between mesopores and macropore-microfractures was enhanced, which was characterized by the increase in effective porosity and nuclear magnetic permeability of coal samples. After acidification, the pore-throat ratio of adsorption pores of all coal samples decreased, while the pore-throat ratio of seepage pores increased. By comparatively analyzing the change law of pore structure of coal samples before and after acidizing with three kinds of mixed acid fluids, the optimal mixed acid fluid suitable for acidizing and permeability improvement of high rank coal in the Jiaozuo coal mine area was selected, which was 12%HCL +3%HF.

Combination of experimental, surface and computational insight into the corrosion inhibition of pyrimidine derivative onto Q235 steel in oilfield acidizing fluid under hydrodynamic condition

The paper represents the green synthesis and inhibitive performance of pyrimidine derivative, namely 4-(3-Methoxy-4-hydroxy-phenyl)-2-phenyl-1,4-dihydro-benzo[4,5]I midazo [1,2-a] pyrimidine-3-carboxylicacid ethyl ester (PD) over Q235 steel in 15% HCl under dynamic condition. The impact of KI addition is also examined on the inhibitive performance (ɳ%). The result of EIS suggests the rise in Rct with the rising amount of PD. PDP result reveals the mixed nature of PD inhibitive action. The solo PD performance value is 93% at 400 mg/L. However, addition of KI (0.5 mM) + PD (400 mg/L) results in the increment in performance value value to 97%. The Freundlich isotherm supports the best adsorption process of PD. Density functional theory (DFT) suggests that the neutral form of PD shows a better tendency to donate electrons toward metal atoms than the protonated one. Molecular dynamic simulation (MD) confirmed that the adsorption geometry of PDH+ on iron surfaces exhibits more parallel orientation than PD.

Multidimensional insight into the corrosion inhibition of salbutamol drug molecule on mild steel in oilfield acidizing fluid: Experimental and computer aided modeling approach

The study aims to establish detailed surface adsorption of expired salbutamol drug molecule as an emerging anticorrosion additive for mild steel corrosion in oilfield acidizing fluid. To achieve this, wide experimental data from weight loss, potentiodynamic polarization, and DFT studies were considered, statistically analyzed, and modeled. Artificial neural network-genetic algorithm (ANN-GA) and adaptive neural fuzzy inference system-genetic algorithm (ANFIS-GA) were utilized as optimization tools considering multiple inputs and a single output variable (MISO). Maximum inhibition efficiencies of 80 %, 89 % and 84 % were obtained from weight loss, potentiodynamics study and elcectrochemical impedance spectroscopy at concentration of 0.4 g/L-1SB and temperature of 323 K respectively. The results obtained from the polarization test confirmed SB as a mixed-type inhibitor and the kinetic studies fit well in the Langmuir isotherm for surface adsorption. The surface analysis via FTIR, SEM/EDAX, and AFM strongly complements the DFT results. In the soft computing method without (GA) via artificial neural network (ANN) and adaptive neural fuzzy inference system (ANFIS), optimal prediction of 79.74 and 81.24 % was attained at varying operating conditions. The predictive capabilities of the two methodologies were compared in terms of coefficient of determination (R2), chi square (χ2), root mean square error (RMSE) and model predictive error (MPE). Based on the statistical indices obtained, much credence was giving to ANFIS as the best predictive model over ANN. Effect of time and inhibitor concentration are the most significant parameters for the prediction of salbutamol drug as a potential anticorrosion additive for mild steel in oilfield acidizing conditions.

Improving the efficiency of anaerobic digestion: Domesticated paddy soil microbes enhance the hydrolytic acidification of rice straw and pig manure

Improving the efficiency of hydrolytic acidification is critical for methane production from agricultural waste. This study is the first to apply domesticated paddy soil microbes to (DPSM) enhance the hydrolytic acidification of rice straw (RS) and pig manure (PM) to obtain acidizing fluid for anaerobic digestion (AD). At a substrate concentration of 20%, the inoculation of an RS-PM mixture (1:3) with 35% DPSM degraded the volatile solids by 48.1% and yielded 6.8 g/L of volatile fatty acids and 4.7 g/L of acetic acid after seven days of hydrolytic acidification. After 10 days of subsequent AD, the cumulative methane production of the acidizing fluid was 304.96 mL/g COD, similar (P ＞ 0.05) to the control (318.27 mL/g COD). However, the methane production time decreased by 43.4% (from 30 to 17 days), thereby improving the AD efficiency. Inoculation with DPSM is therefore an effective pre-treatment for agricultural waste for methane production.

Anticorrosion activity of two new pyridine derivatives in protecting X70 pipeline steel in oil well acidizing fluid: experimental and quantum chemical studies

Anticorrosion activity of two new pyridine derivatives in protecting X70 pipeline steel in oil well acidizing fluid: experimental and quantum chemical studies

Corrosion damage behaviors of rubber O-rings under simulated acid fracturing conditions

Abstract To explore the damage behavior of O-ring in acid environment, a high-temperature and high-pressure (HTHP) autoclave was used to simulate the service environment of O-ring, and then 168h corrosion test of hydrogenated nitrile butadiene rubber (HNBR) and fluororubber (FM) O-rings were carried out. The corrosion damage behaviors of two kinds of rubber O-rings in the acidizing fluid were studied through determining their tensile strength, elongation at break, hardness, permanent compressive deformation, tensile fracture morphology and sealing property. The results showed that the cross-sectional area and the compression permanent deformation increased, the tensile strength and hardness decreased when the HNBR and FM O-rings under the free state were subjected to acid corrosion. The elongation at break of HNBR decreased, and that of FM rubber increased greatly. Similar with free state, the HNBR and FM O-rings under sealed state also presented the same variation trend. The decrease in the reliability of the O-rings under the sealed state was less significant than that in the free state. In the test, tensile fractures were mostly brittle fractures, HNBR and FM O-rings had obvious corrosion damages such as deformation and swelling. The results could provide a technical basis for the selection of sealing materials, tool optimization design, and construction work in oil and gas fields.

Matrix Acidizing: A Laboratory and Field Investigation of Sludge Formation and Removal of Oil-Based Drilling Mud Filter Cake

Summary Hydrochloric and organic acids have been extensively used to enhance well productivity or injectivity in tight carbonate formations (10 to 50 md). The use of these acids, however, can cause instances of complete production loss. This is especially common due to incompatibilities of the acidizing fluid and oil, which can lead to the formation of acid/oil emulsions and sludge formation. Consequently, it is necessary to properly identify and remove such emulsions or precipitations without causing any further damage. Compatibility studies were conducted using representative crude samples and hydrochloric acid (HCl). The experiments were conducted at various temperatures up to 240°F using high-pressure/high-temperature (HP/HT) aging cells for both live and spent acid samples, in which some of the experiments included an antisludge, an iron-control agent, and a demulsifier. In addition, another set of experiments was performed in the presence of ferric ions (Fe3+). The total iron concentration in these experiments was varied between 0 and1,000 ppm. The results showed that commonly used acid systems were not compatible with representative oil field samples. The amount of sludge formed and the stability of formed emulsions increased significantly in the presence of ferric ions and was more severe in the presence of hydrogen sulfide (H2S). Using a field case, this paper will cover the methodology used to ascertain the source of formation damage from acidizing, study the different factors that influence the formation of acid/oil emulsion and sludge formation mechanism, and show how they can be removed. In this example, acid/oil emulsions, sludge formation, and improper drilling mud filter-cake removal were the reasons behind the production loss. However, the methodology can be expanded to cater the many acidizing failure cases faced in the industry worldwide.

Electrochemical and Anticorrosive Behavior of Synthesized Diimine-Based Compound on X80 Steel in Acidic Oil and Gas Well Treatment Fluids at Different Temperatures

A green compound, N,N′-bis(2,4,6-trihydroxybenzaldehyde)-2,2-dimethylpropandiimine, was studied as an acid corrosion inhibitor for X80 steel. Aqueous 15% HCl was utilized for simulating the oil well acidizing fluid at various temperatures. The inhibition performance was assessed using the weight loss method, the electrochemical impedance, the polarization, and the scanning electron microscopy. Polarization investigations showed that the studied diimine inhibitor delays both the cathodic and anodic reactions over chemical adsorption and blocks the active corrosion sites. The adsorption procedure was performed physically as best estimated using the Langmuir adsorption isotherm. It was observed that, the adsorption process was spontaneous and exothermic in the direction of a rise in the bulk phase entropy. The efficiency of the inhibitor was reduced by increasing temperature and raised with an increasing amount of the inhibitor. Impedance data indicated that as the inhibitor amount enhanced the charge transfer resistance of steel also increased while the double layer capacitance decreased. Kinetic and thermodynamic factors such as the activation energy, enthalpy, entropy and the Gibbs free energy of activation and adsorption were computed. Scanning electron microscopy was utilized to examine the steel surface in the presence or the absence of the inhibitor, showing excellent surface protection by the studied inhibitor.

Permeability damage micro-mechanisms and stimulation of low-permeability sandstone reservoirs: A case study from Jiyang Depression, Bohai Bay Basin, China

Abstract According to the characteristics of “structural elements” (framework grain, interstitial material and pore throat structure) of low-permeability sandstone reservoir, the “step by step dissolution and separation” acidification and acid fracturing technology has been developed and tested in field. There are three main mechanisms affecting permeability of low-permeability sandstone reservoir: (1) The mud fillings between the framework grains block the seepage channels. (2) In the process of burial, the products from crystallization caused by changes in salinity and solubility and uneven migration and variation of the syn-sedimentary formation water occupy the pores and throat between grains. (3) Under the action of gradual increase of overburden pressure, the framework grains of the rock is compacted tighter, making the seepage channels turn narrower. The “step by step dissolution and separation” acidification (acid fracturing) technology uses sustained release acid as main acidizing fluid, supramolecular solvent instead of hydrochloric acid to dissolve carbonate, and a composite system of ammonium hydrogen fluoride, fluoroboric acid, and fluorophosphoric acid to dissolve silicate, and dissolving and implementing step by step, finally reaching the goal of increasing porosity and permeability. By using the technology, the main blocking interstitial material can be dissolved effectively and the dissolution residual can be removed from the rock frame, thus expanding the effective drainage radius and increasing production and injection of single well. This technology has been proved effective by field test.

A novel acidizing technology in carbonate reservoir: In-Situ formation of CO2 foamed acid and its self-diversion

The serious fingering and nonuniform injectivity profile of acid liquid result from the features of heterogeneity in most carbonate reservoir, leading to a poor effect on the acidizing process. In the present contribution, for the first time, we report a novel diversion acidizing strategy that uses acid-rock reaction between the acidizing fluid itself and carbonate core to produce CO2 at supercritical state. Under the action of a mixture of foaming agent and stabilizer contained in this working fluid, a CO2 foamed acid fluid is in-situ formed in the acidizing layer, which would play a diverting role in carbonate matrix acidizing operations. Experiments on in-situ formation of CO2 microfoams (emulsions) were performed utilizing a visible acid-rock reaction simulation device and a self-made high pressure and temperature foam generator. A series of divided-flow experiments were undertaken by the core displacement setup.Results based on these experiments indicate that with 0.5% SDBS as a foaming agent and 0.15% CMC as a stabilizing agent, the supercritical CO2 microfoams (emulsions) could be created in-situ in the process of acidizing through the carbonate reservoirs. The foamed acid has the accumulating and plugging effects, with divided-flow selectivity not only in the parallel cores of oil saturation and water saturation, but also in the parallel cores of different permeability. This foamed acid fluid could provide better diversion acidizing operations for heterogeneous carbonate formation. Finally, the mechanism of this novel diversion acidizing technology was uncovered in detail. It is suggested that the whole working process is actually an interface effect in the dynamic system of gas/liquid/solid multiphase fluid. Such findings are potentially important for a better understanding on the mechanisms of in-situ formation of CO2 foamed acid and its self-diversion. Looking out to the future, our report attempts to provide a new and efficient technology for the improvement of acid stimulation in the carbonate reservoirs.

Effect of Structural Changes on Corrosion Inhibition Behavior of Synthesized N2O4 Imine Compounds for Steel Pipelines in Oil and Gas Wells

The inhibition properties of synthesized imine compounds N,Nʹ-bis(2,4-dihydroxyhydroxybenzaldehyde)-1,3-Propandiimine, N, N′-bis(2,4-dihydroxypropiophenone)-2,2-dimethylpropandiimine, N,Nʹ-bis(2,4-dihydroxyacetophenone)-1,3-Propandiimine has been investigated for API-5L-X65 steel corrosion in hydrochloric acid by scanning electron microscopy, potentiodynamic polarization and impedance spectroscopy. Aqueous hydrochloric acid was applied to simulate the oil and gas well acidizing fluid. Potentiodynamic polarization studies indicated that compounds retard both the anodic and cathodic reactions through adsorption and blocking the active corrosion sites. The inhibition efficiency increased with inhibitor concentration. Electrochemical impedance spectroscopy data were studied by equivalent circuit and showed that with increasing inhibitor concentration, the resistance of charge transfer enhanced and the capacitance of double layer reduced. The measured data proposed that the inhibition efficacy was increased for inhibitor in the absence of alkyl addition. The geometrical steric hindrance offered by the alkyl groups in the imine compounds plays an important role in its corrosion inhibition properties. Scanning electron microscopy was used to study the steel surface with and without inhibitors.

Feasibility analysis on the pilot test of acid fracturing for carbonate reservoirs in Halfaya Oilfield, Iraq

AbstractThe oil reservoir of Mishrif Formation is of strong heterogeneity vertically and laterally, and its single‐well production rate after conventional acidizing is low and declines fast. Therefore, we intend to carry out the pilot test on diverting acid fracturing so as to improve the stimulation effect of acid fracturing by enlarging the lateral and vertical stimulation range. In this paper, a series of experimental studies (eg, rock mechanics, acid rheology, acid‐rock reaction kinetics, and full‐diameter core etching diversion) were carried out to deal with the reservoir characteristics and stimulation demand of Mishrif Formation, the principal oil reservoir in Halfaya Oilfield in Iraq, the Middle East. It is shown that calcite content of Mishrif Formation reservoir is higher than 95%, indicating the strong acid dissolution. As for rock mechanics, the characteristics of “low Young's modulus and high Poisson's ratio” are presented, and the stress‐strain curves show certain elastic‐plastic characteristics. Additionally, a large number of wormholes appear on the surface of the full‐diameter cores after acid etching, so it is necessary to consider the serious filtration of acidizing fluid near the wellbore. The mechanical strength of rock plates decreases after acid etching, so it is recommended to adopt and optimize the closed acidizing technology so as to avoid excessive etching. Furthermore, the self‐support flow conductivity of etched fractures is weak, so it is recommended to adopt the sand fracturing or the combined stimulation technology of acid fracturing + sand fracturing to improve the reservoir stimulation effects.

Reflux of acidizing fluid for enhancing biomethane production from cattle manure in plug flow reactor

The performance and microbial community succession of a 36 L working volume plug flow reactor was evaluated for treated cattle manure at an organic loading rate of 2.67 g-TS/L/day, a temperature of 38 °C ± 1 °C, and a hydraulic retention time of 18 days. A reflux of acidizing fluid effectively enhanced anaerobic digestion performance and promoted optimization of microbial community structure. The average biogas volume production rate was 1.08 L biogas/L reactor, which was 116.5% higher than the control without reflux of acidizing fluid. The specific qmethane production yield and methane content reached 0.204 L/g VS and 70%. Moreover, methane yield achieved 0.34 m3/kg removal COD with a COD removal of about 70.56%. The bacteria genera Christensenellaceae, Bacteroidales, vadinBC27, Ruminococcaceae and Treponema_2 were further enriched. Methanosarcina became the dominant methanogen in the whole PFR operation process. This study offers new opportunities for producing renewable energy from enhanced cattle manure biodegradability.

Simulation of effective fracture length of prepad acid fracturing considering multiple leak-off effect

For the purpose of improving the accuracy and effectiveness of acid fracturing design for carbonate reservoirs with developed natural fractures, an acidizing fluid flow and reaction model taking the multiple leak-off effect of natural fracture, wormhole and matrix into account was established according to the liquid phase reaction equilibrium principle and the local reaction equilibrium principle after the dynamic change of fracture geometry in the process of fracture created by prepad fluid was simulated in the classical pseudo-three dimensional mathematical model of fracture propagation. Then, the acid fracturing stimulation of a case well on site was taken as an example. The newly developed model was used to simulate the filtration process of acidizing fluid in fractures and the dynamic etching morphology of acidic rocks during the acidizing fluid injection of prepad acid fracturing. The effective length of etched fractures was determined by analyzing the concentration change of acidizing fluid along the direction of hydraulic fracture length and the threshold concentration of residual acidizing fluid comprehensively, and then it was compared with the interpretation result of pressure buildup test. And the following research results were obtained. First, in the process of acid fracturing in fractured–porous reservoirs, the acidizing fluid filtration velocity along the direction of fracture length is not constant and the filtration velocity curve fluctuates in a serrated shape. And the acidizing fluid filtration velocity where etched wormholes meet natural fractures is commonly higher than that in matrix. Second, acidizing fluid is lost seriously and the effective distance of acidizing fluid gets short significantly during the acid fracturing of fractured–porous reservoirs. Third, acid fracturing in the verification well is remarkable in blockage removing and stimulation, and its well test interpretation results are consistent with the simulation interpretation results provided by the newly developed mathematical model. It is indicated that this newly developed model is reliable. In conclusion, the mathematical model of prepad acid fracturing which considers multiple leak-off effect is more suitable for acid fracturing simulation of fractured–porous reservoirs.

Comparative Analysis of Corrosion Inhibition: Between Jatrophacurcas, Palm and Diesel Oil based Emulsified Acids for Acid Stimulation Operations

The recent environmental concerns have raised the need for a green acidizing fluid, which is able to replace diesel oil as an oil phase in emulsion based acidizing fluids. In order to do so it must be able to provide not only retardation in acid reaction against the rock surface, but also provide suitable inhibition characteristic that can reduce the amount of corrosion which occurs due to acidizing. In this paper emulsified acids made from triglyceride oils namely; Jatropha curcas oil and Palm oil have been tested for corrosion inhibition in comparison with Diesel oil based emulsified acid and 15wt.% HCl. The emulsion made with Jatropha curcas oil showed capability to replace diesel oil as a viable and an environmental friendly substitute for oil phase in emulsified acids.