Advanced Modeling and New Fit for Purpose Fluids Unlock Potential of Hot Carbonate Reservoirs During Acid Fracturing in Uzbekistan (Russian)
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Cited by 3 publications
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Advanced Fracturing Design Simulator-Assisted Modeling Coupled with Application of Enhanced Stimulation Fluids Raises Performance of Acid Fractured Wells
Important factors affecting acid fracturing efficiency include etched fracture geometry, cleanup, and optimum differential etching to retain open channels after fracture closure. A recently applied integrated approach combined improvements in all three factors: new fracture simulation techniques enabled fracture geometry optimization, single-phase retarded acid provided significant increase in half-length, and high retained permeability viscous fluids supported better fracture cleanup. The approach was successfully implemented in several carbonate oil fields and resulted in a substantial productivity index increase. The approach enables acid fracture optimization in three steps. First, the high retained permeability, low-pH pad fluids and polymer-free leakoff control acids are used in combination to enhance formation cleanup after a treatment and to reduce the concentration of polymers in fissures network of naturally fractured carbonate reservoirs. Second, a new single-phase retarded acid is used to achieve longer half-length due to retarded reaction with formation rock and favorable viscous fingering effects. Third, a new acid fracturing simulation model is used to optimize fracture geometry. The simulation technique employs an innovative transport model that includes the viscous fingering effect, advanced leakoff simulation, changing acid rheology upon spending, and a novel calculation approach to mixed fluids' rheology. This combined concept was applied during acid fracturing treatments in moderate permeability wells of carbonate reservoirs with target intervals up to 4,600 m TVD and temperatures up to 125°C. The treatments consisted of guar-free low-pH pad fluid, polymer-free leakoff control acid, and single-phase retarded acid. Treatment optimization was performed using an advanced acid fracturing simulator to properly address the transport processes within the fracture in a low-stress-contrast environment. After the treatments, the pressure transient analysis indicated a strong linear regime for more than 15 hours, indicating effective fracture half-length at least 25% higher than average half-length after acid fracturing in offset wells where the conventional approach had been applied. Post-treatment half-length calculations showed a good match with advanced simulator results and proved the importance of accounting for viscous fingering effects during acid fracture half-length calculations. Calculation of the productivity index from the production data showed at least 15% increase compared to conventional acid fracturing treatments. The post-fracturing production decline rate was at least 20% slower than that of the conventional treatment in offset wells, which can be explained by the longer conductive fracture.
Advanced Fracturing Design Simulator-Assisted Modeling Coupled with Application of Enhanced Stimulation Fluids Raises Performance of Acid Fractured Wells
Important factors affecting acid fracturing efficiency include etched fracture geometry, cleanup, and optimum differential etching to retain open channels after fracture closure. A recently applied integrated approach combined improvements in all three factors: new fracture simulation techniques enabled fracture geometry optimization, single-phase retarded acid provided significant increase in half-length, and high retained permeability viscous fluids supported better fracture cleanup. The approach was successfully implemented in several carbonate oil fields and resulted in a substantial productivity index increase. The approach enables acid fracture optimization in three steps. First, the high retained permeability, low-pH pad fluids and polymer-free leakoff control acids are used in combination to enhance formation cleanup after a treatment and to reduce the concentration of polymers in fissures network of naturally fractured carbonate reservoirs. Second, a new single-phase retarded acid is used to achieve longer half-length due to retarded reaction with formation rock and favorable viscous fingering effects. Third, a new acid fracturing simulation model is used to optimize fracture geometry. The simulation technique employs an innovative transport model that includes the viscous fingering effect, advanced leakoff simulation, changing acid rheology upon spending, and a novel calculation approach to mixed fluids' rheology. This combined concept was applied during acid fracturing treatments in moderate permeability wells of carbonate reservoirs with target intervals up to 4,600 m TVD and temperatures up to 125°C. The treatments consisted of guar-free low-pH pad fluid, polymer-free leakoff control acid, and single-phase retarded acid. Treatment optimization was performed using an advanced acid fracturing simulator to properly address the transport processes within the fracture in a low-stress-contrast environment. After the treatments, the pressure transient analysis indicated a strong linear regime for more than 15 hours, indicating effective fracture half-length at least 25% higher than average half-length after acid fracturing in offset wells where the conventional approach had been applied. Post-treatment half-length calculations showed a good match with advanced simulator results and proved the importance of accounting for viscous fingering effects during acid fracture half-length calculations. Calculation of the productivity index from the production data showed at least 15% increase compared to conventional acid fracturing treatments. The post-fracturing production decline rate was at least 20% slower than that of the conventional treatment in offset wells, which can be explained by the longer conductive fracture.
Control Over the Fracture in Carbonate Reservoirs as a Result of an Integrated Digital Stimulation Approach to Core Testing and Modeling
The oilfield industry is rapidly changing towards reduced CO2 emissions and sustainability. Although hydrocarbons are expected to remain the leading source for global energy, costs to produce them may become prohibitive unless new breakthrough in technology is established. Fortunately, the digital revolution in the IT industry continues at an accelerating pace. A digital stimulation approach for tight formations is presented, using the achievements of one industry to solve the challenges of another. The fracture hydrodynamics and in-situ kinetics model is incorporated in the advanced simulator together with the detailed multiphysics models based on acid systems digitization, including rheology and fluid- carbonate interactions data obtained from the laboratory experiments. Digitization of fluid-rock interaction and fluid leakoff was performed using a coreflooding setup that allowed pumping concentrated acids in core samples at high-pressure/high-temperature (HP/HT) conditions. Varying the testing parameters across a broad range allowed refining the model coefficients in the simulator to obtain high accuracy in the predicted results. The digital slot concept was used to validate physical models in an iterative experimental approach. The software proved efficient at providing validation of multiphysics models used together with advanced slurry transport in the simulator. The fine computational grid allowed accurate predictions of the fracture geometry, etched width, and channel conductivity, resulting in realistic well productivity anticipations. Since multiple fluid systems of the acid stimulation portfolio were digitized and incorporated into the simulator, it was possible to optimize complex acid fracturing designs in the real field operations that included retarded single-phase and multiphase acid systems, self-diverting viscoelastic acids, and fiber- based diverting systems. Several case studies from multiple areas and reservoirs from Caspian and Middle East areas have demonstrated extremely positive oil and gas production results with reduced acid volumes with the digital stimulation workflow compared to conventionally stimulated offset wells. The digital stimulation workflow brings a new approach to acid fracturing optimization based on an integrated cycle in which high-resolution data from several sources are processed by powerful computing capacities. Starting from digitizing acid reactions with the core samples, through digitized rheology and particle transport in multiphysics models, an advanced numerical simulator tailors an optimum design from a number of acid system options, pumping rates, additive concentrations, and stage volumes to achieve best geometry of etched channels inside a fracture.
New Advances in Single-Phase Retarded Acids for Stimulating Carbonate Reservoirs
Single-phase retarded acid systems provide higher performance and operational efficiency benefits than systems based on unmodified HCl and emulsified acid. When combined with diverting fluid systems and modeling, they have proven to resolve many challenges operators experience with carbonate acidizing. Our work assessed a new advanced single-phase retarded acid. The new fluid can be formulated to reach the dissolving power of up to 28 wt% HCl and is stable at high temperatures. First, static carbonate dissolution tests were conducted to screen the retardation performance of numerous chemicals. The thermal stability of selected retarder packages was evaluated in low-pH environments at temperatures up to 350°F. Wormhole experiments were then performed in Indiana limestone and Silurian dolomite cores at temperatures up to 325°F and with fluids of HCl strength ranging from 15 to 28 wt%. The rate of wormhole propagation was used as a measure to compare the performance of the acid systems. The core flow testing showed that the new single-phase retarded acid is highly effective in forming dominant wormholes in both limestone and dolomite cores in a wide range of acid injection rate, temperatures between 125 and 325°F, and HCl concentrations ranging from 15 to 28 wt%. The new single-phase retarded acid has a low viscosity, close to the viscosity of unmodified HCl, which should result in low friction pressure in field applications. The pore volume to breakthrough data were incorporated into a model and allows predicting the radial performance of the new fluid in the field and examining different treatment scenarios. The results of the simulations show the significant improvement in achieved wormhole depth of the new fluid versus unmodified HCl. The novelty of the new low-viscosity single-phase retarded acid is in the ability to efficiently extend the application envelope of this type of fluid towards the high temperatures and high carbonate dissolving power.
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