Acid Fracture Conductivity Behavior of Tahe Carbonate: High Closure Stress, Long-Term Conductivity, and Composite Conductivity

Acid fracture conductivity is a key parameter in acid fracturing designs and production performance prediction. It depends on the fracture surface etching pattern, rock mechanical properties, and closure stress. The fracture surfaces undergo creep deformation under closure stress during production. Preservation of fracture conductivity becomes a challenge at elevated closure stress. In this paper, we investigated acid fracture conductivity behavior of Tahe deep carbonate reservoir with high closure stress and high temperature. A series of acid fracture conductivity experiment was conducted in a laboratory facility designed to perform acid fracture conductivity. Gelled acid and cross linked acid with different acid-rock contact times were tested for analyzing the effect of acid type and acid-rock contact time on the resulting conductivity. Closure stress up to 100MPa was tested to verify the feasibility of acid fracturing for elevated closure stress. Long-term conductivity up to 7-day was tested to determine the capability of conductivity retaining after creep deformation. Composite conductivity of acid fracture with prop pant was also carried out. The study shows that the fracture retained enough conductivity even under effective closure stress of 70MPa. The gelled acid has a much higher conductivity than the cross linked acid for the same contact time. For the gelled acid, contact time above 60-minute does not lead to conductivity increase. Acid fracture with prop pant has a lower conductivity at low closure stress and a higher conductivity at high closure stress than the acid fracture, which shows composite conductivity is a feasible way to raise conductivity at high closure stress. The long-term conductivity tests show that the acid fracture conductivity decreases fast within the first 48-hour and then levels off. The conductivity keeps stable after 120-hour. An acid fracture conductivity correlation was also developed for this reservoir.

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A new method to improve long-term fracture conductivity in acid fracturing under high closure stress

Journal of Petroleum Science and Engineering ◽

10.1016/j.petrol.2018.07.073 ◽

2018 ◽

Vol 171 ◽

pp. 760-770 ◽

Cited By ~ 21

Author(s):

Lufeng Zhang ◽

Fujian Zhou ◽

Jianye Mou ◽

Guoqing Xu ◽

Shicheng Zhang ◽

...

Keyword(s):

New Method ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Closure Stress

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Integrated Acid Fracture Model with Reservoir Simulation Under Non-Isothermal Condition

10.2118/205143-ms ◽

2021 ◽

Author(s):

Tohoko Tajima

Keyword(s):

Reservoir Simulation ◽

Isothermal Condition ◽

Carbonate Reservoir ◽

Temperature Sensitive ◽

Reservoir Model ◽

Coupled Modeling ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Significant Difference ◽

Fracturing Process

Abstract Modeling of acid fracturing process is challenging because of the coupled complex effects of flow through porous media and fractures, chemical reaction in a geostatistical base, wormhole propagation, and reservoir heterogeneity. To avoid the complexity, decoupled approaches are commonly used; the reservoir effect is represented by leakoff with a constant leakoff coefficient, and analytical solutions for heat flux from a reservoir is used to avoid complexity. An acid fracturing numerical model is presented that is coupled with a single-phase black oil reservoir simulator for a vertical well in the carbonate reservoir. The coupled acid fracturing model considers fracture propagation, acid transport, and heat transfer. After simulating acid fracturing, the conductivity of the fracture is calculated using empirical correlations, and the productivity is computed by simulating the flow to the well. Non-isothermal condition is assumed to simulate the flow in both the fracture and reservoir because the acid reaction is temperature sensitive. Leakoff from fracture to reservoir is simulated with a reservoir flow model for pressure and leakoff velocity as functions of time and location. Wormhole propagation from the fracture is considered by using empirical equations for wormhole propagation based on leakoff velocity estimated from the reservoir simulation. The benefits of coupled modeling are evaluated by comparing the conventional acid fracturing model which uses a decoupled approach to the numerical acid fracturing model developed in this study. The results show that the coupling reservoir model improves the accuracy of estimated in fracture conductivity. It has been shown that the analytical equations for heat from a reservoir used in literature overestimates the final acid fracture conductivity. Thus, it is suggested to use fully numerically solve fluid flow and energy balance in a fracture and a reservoir. Complex leakoff due to pressure and temperature change with time and wormhole propagation was implemented in the simulator. The wormhole effect was added and the distribution of leakoff coefficient was reasonable. A comparison of simulation results with and without wormholes showed that the significant difference was not observed in acid concentration, but ideal width distribution was lower with wormholes. It is concluded based on the observation of the study that the leakoff from acid fracture represented by a reservoir model with wormhole propagation is important to correctly understand acid fracture efficiency. Simply using a constant leakoff coefficient can lead to significant error and misleading conclusions.

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Acid Fracturing Shales: Effect of Dilute Acid on Properties and Pore Structure of Shale

10.2118/spe-173390-ms ◽

2015 ◽

Author(s):

Weiwei Wu ◽

Mukul M. Sharma

Keyword(s):

Pore Structure ◽

Core Sample ◽

Petrophysical Properties ◽

Acid Dissolution ◽

Eagle Ford Shale ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Microstructure Changes ◽

Fracture Surfaces ◽

Closure Stress

Abstract Many microfractures created during hydraulic fracturing are too small to be filled with proppants and are likely closed during production. However, for some shales that are rich in calcite (calcareous mudstones), such as the Bakken and Eagle Ford shale, dilute acids can be used while fracturing to maintain the conductivity of these microfractures under closure stress by non-uniformly etching the fracture surfaces. The mineralogy and pore structure of the shale and their evolution during acid fracturing are critical factors on the surface surface etching profile and the fluid leakoff. Therefore, understanding how acid dissolution changes the microstructure, petrophysical properties and pore structures of shale is essential in the design and application of acid fracturing in shales. In this paper changes in shale properties and pore structure by acid fracturing were demonstrated and visually observed for the first time with a scanning electron microscope. Acidized sections of a shale core sample were carefully isolated, and its microstructure, pore structure and petrophysical properties were systematically studied and compared with non-acidized sections of the core. Microstructure changes were found to be strongly dependent on mineral distribution, and several patterns were identified: channels developed in carbonate-rich regions; cavities or grooves formed in carbonate-rich islands or carbonate rings; and surface roughness was created in mixed zones of scattered carbonate and inert minerals. Inert minerals such as clay, organic matter stay relatively undisturbed in the structure, while some mineral grains can be dislodged from their original locations by dissolution of the surrounding carbonates. Many macropores with size up to 120 µm were created and mesopores mostly associated with clay gained more accessibility. Significantly increased permeability and porosity was measured in an acidized shale matrix. Brinell hardness measurements show that, as expected, the hardness of the shale was reduced by acidizing. This means that for acidizing to work effectively, it is important to not etch the fracture surfaces uniformly. Doing so will result in a reduction in the fracture conductivity under stress. The microstructure changes introduced by acid fracturing demonstrated in this study will result in the formation of surface asperities which is likely to improve the fracture conductivity of induced unpropped fractures. The acidized shale matrix close to the fracture surface with increased abundance of macropores and accessibility to mesopores may serve as a preferred pathway for fluid flow as well.

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The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

Scientific Reports ◽

10.1038/s41598-021-91475-x ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Viet Cao ◽

Ghinwa Alyoussef ◽

Nadège Gatcha-Bandjun ◽

Willis Gwenzi ◽

Chicgoua Noubactep

Keyword(s):

Contact Time ◽

Diffusive Transport ◽

Iron Hydroxides ◽

Iron Corrosion ◽

Short Term ◽

Batch Studies ◽

Water Decontamination ◽

Co Precipitation

AbstractMetallic iron (Fe0) has shown outstanding performances for water decontamination and its efficiency has been improved by the presence of sand (Fe0/sand) and manganese oxide (Fe0/MnOx). In this study, a ternary Fe0/MnOx/sand system is characterized for its discoloration efficiency of methylene blue (MB) in quiescent batch studies for 7, 18, 25 and 47 days. The objective was to understand the fundamental mechanisms of water treatment in Fe0/H2O systems using MB as an operational tracer of reactivity. The premise was that, in the short term, both MnO2 and sand delay MB discoloration by avoiding the availability of free iron corrosion products (FeCPs). Results clearly demonstrate no monotonous increase in MB discoloration with increasing contact time. As a rule, the extent of MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the vessels (test-tubes). The presence of MnOx and sand enabled the long-term generation of iron hydroxides for MB discoloration by adsorption and co-precipitation. Results clearly reveal the complexity of the Fe0/MnOx/sand system, while establishing that both MnOx and sand improve the efficiency of Fe0/H2O systems in the long-term. This study establishes the mechanisms of the promotion of water decontamination by amending Fe0-based systems with reactive MnOx.

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Modeling Acid Fracturing Treatments with Multi-Stage Alternating Injection of Pad and Acid Fluids

10.2118/203985-ms ◽

2021 ◽

Author(s):

Rencheng Dong ◽

Mary F. Wheeler ◽

Hang Su ◽

Kang Ma

Keyword(s):

Viscous Fingering ◽

High Viscosity ◽

Carbonate Reservoirs ◽

Acid Etching ◽

Fracture Geometry ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Fracture Surfaces ◽

Low Viscosity ◽

Multi Stage

Abstract Acid fracturing technique is widely applied to stimulate the productivity of carbonate reservoirs. The acid-fracture conductivity is created by non-uniform acid etching on fracture surfaces. Heterogeneous mineral distribution of carbonate reservoirs can lead to non-uniform acid etching during acid fracturing treatments. In addition, the non-uniform acid etching can be enhanced by the viscous fingering mechanism. For low-perm carbonate reservoirs, by multi-stage alternating injection of a low-viscosity acid and a high-viscosity polymer pad fluid during acid fracturing, the acid tends to form viscous fingers and etch fracture surfaces non-uniformly. To accurately predict the acid-fracture conductivity, this paper developed a 3D acid fracturing model to compute the rough acid fracture geometry induced by multi-stage alternating injection of pad and acid fluids. Based on the developed numerical simulator, we investigated the effects of viscous fingering, perforation design and stage period on the acid etching process. Compared with single-stage acid injection, multi-stage alternating injection of pad and acid fluids leads to narrower and longer acid-etched channels.

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Using Advance Acid Fracturing Design to Increase the Production Efficiency in a HPHT Reservoir: A Success Story from Southern Mexico

10.2118/207332-ms ◽

2021 ◽

Author(s):

Frank Figueroa ◽

Gustavo Mejías ◽

José Frías ◽

Bonifacio Brito ◽

Diana Velázquez ◽

...

Keyword(s):

High Temperature ◽

Laboratory Tests ◽

Reaction Rates ◽

Gas Production ◽

Carbonate Reservoir ◽

Fluid Distribution ◽

Acid Etching ◽

Southern Mexico ◽

Acid Fracturing ◽

Fluid Systems

Abstract Enhanced hydrocarbon production in a high-pressure/high-temperature (HP/HT) carbonate reservoir, involves generating highly conductive channels using efficient diversion techniques and custom-designed acid-based fluid systems. Advanced stimulation design includes injection of different reactive fluids, which involves challenges associated with controlling fluid leak-off, implementing optimal diversion techniques, controlling acid reaction rates to withstand high-temperature conditions, and designing appropriate pumping schedules to increase well productivity and sustainability of its production through efficient acid etching and uniform fluid distribution in the pay zone. Laboratory tests such as rock mineralogy, acid etching on core samples and solubility tests on formation cuttings were performed to confirm rock dissolving capability, and to identify stimulation fluids that could generate optimal fracture lengths and maximus etching in the zone of interest while corrosion test was run to ensure corrosion control at HT conditions. After analyzing laboratory tests results, acid fluid systems were selected together with a self-crosslinking acid system for its diversion properties. In addition, customized pumping schedule was constructed using acid fracturing and diverting simulators and based on optimal conductivity/productivity results fluid stages number and sequence, flow rates and acid volumes were selected. The engineered acid treatment generated a network of conductive fractures that resulted in a significant improvement over initial production rate. Diverting agent efficiency was observed during pumping treatment by a 1,300 psi increase in surface pressures when the diverting agent entered the formation. Oil production increased from 648.7 to 3105.89 BPD, and gas production increased from 4.9 to 26.92 MMSCFD. This success results demonstrates that engineering design coupled with laboratory tailor fluids designs, integrated with a flawless execution, are the key to a successful stimulation. This paper describes the details of acidizing technique, treatment design and lessons learned during execution and results.

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An innovative concept on deep carbonate reservoir stimulation: Three-dimensional acid fracturing technology

Natural Gas Industry B ◽

10.1016/j.ngib.2020.09.006 ◽

2020 ◽

Vol 7 (5) ◽

pp. 484-497

Author(s):

Jianchun Guo ◽

Bo Gou ◽

Nan Qin ◽

Junsheng Zhao ◽

Lin Wu ◽

...

Keyword(s):

Three Dimensional ◽

Carbonate Reservoir ◽

Acid Fracturing ◽

Reservoir Stimulation ◽

Innovative Concept

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Prediction of Stimulation From Acid Fracturing Treatments Using Finite Fracture Conductivity

Journal of Petroleum Technology ◽

10.2118/6123-pa ◽

1977 ◽

Vol 29 (09) ◽

pp. 1186-1194

Author(s):

E.J. Novotny

Keyword(s):

Fracture Conductivity ◽

Acid Fracturing

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Analysis of Creep Characteristics of Shallow Mined-Out Areas Roof under Low Stress Conditions

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.105-107.832 ◽

2011 ◽

Vol 105-107 ◽

pp. 832-836 ◽

Cited By ~ 1

Author(s):

Shu Ren Wang ◽

Hui Hui Jia

Keyword(s):

Creep Deformation ◽

Thick Plate ◽

Plate Theory ◽

Technical Problem ◽

Stress Conditions ◽

Creep Model ◽

Term Strength ◽

Creep Characteristics ◽

Over Time

Under low stress conditions, when the load exerting on the mined-out areas roof is less than the rock long-term strength, the rock roof will generate some creep deformation. In order to prevent the roof of the mined-out areas suddenly collapse, and to ensure the operator and construction equipment above the mined-out areas safety, it is an important security technical problem to reveal the creep characteristics of the shallow mined-out areas roof. Taking the mined-out areas of Antaibao Surface Mine as background, considering the rheological properties of rock roof, and assuming the roof was a rectangular thick plate, the creep characteristics of mined-out areas roof were analysed by applying the thick plate theory and Kelvin creep model. The regression equation of the roof deflection increment over time was given, and the creep characteristics of the shallow mined-out areas roof were revealed also.

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