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.
