Successful Implementation of CO2 Energized Acid Fracturing Treatment in Deep, Tight and Sour Carbonate Gas Reservoir in Saudi Arabia that Reduced Fresh Water Consumption and Enhanced Well Performance

Abstract

Abstract Up to three thousand barrels of stimulation fluids are pumped during a single vertical fracture treatment in carbonate reservoir of Saudi Arabia with the objective of creating sufficient conductive reservoir contact for commercial production. The post-stimulation production performance in deep, tight, and sour carbonate gas reservoirs often does not meet expectations due to complex reservoir characterization and poor transmissibility. The post-stimulation cleanup period often takes much longer due to resistance in unloading the pumped fluids. Carbon dioxide (CO2) with 30% foam quality (FQ) has been introduced for the first time during acid fracturing treatment in the tight, sour and high pressure, high temperature (HPHT) carbonate gas reservoir of Saudi Arabia to reduce consumption of fresh water, minimize reservoir damage, reduce the flowback period and eliminate the need for nitrogen lifting with coiled tubing (CT). The water and acid volume were reduced by one third through energizing the fluids system with CO2 during acid fracturing treatment in the candidate well. The Pad fluid was a zirconate crosslinked gel that was optimized for low pH to maintain stability in the presence of CO2. To minimize fluid leak-off and improve diversion effects, a chemical diverting system was included in the CO2 energized acid fracture treatment. A non-emulsifier foaming agent was used in the Pad and acid blends to create the desired emulsion with CO2. The post-fracture flowback period was reduced significantly and a 2.5 fold increase in the gas rate was realized. The addition of liquid CO2 to HCl in quantities sufficient to produce emulsion allows live acid to retard and penetrate much deeper than HCl itself. The CO2 strips hydrocarbon from the rock and allows acid to effectively react to the rock surface. CO2 remains in the liquid or supercritical phase during injection mode and flows back in the gas phase over a wide range of temperature and pressure allowing pumped fluids to recover in fast and safe manner. This paper is focused on laboratory tests of fluid systems, and the design, execution and evaluation of CO2 energized acid fracturing treatment, and a safe flowback strategy used for sour wells with high a concentration of CO2.