Comparative study on acid fracturing and propped hydraulic fracturing for the highpressure tight carbonate formation
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abstract
Acid fracturing is one of the methods for well stimulation in carbonate formations. Compared to propped hydraulic fracturing, acid fracturing takes a simpler procedure, and therefore, usually with a lower cost. On the other hand, fracture conductivity is more difficult to maintain, especially in formations that have high closure stress. In this study, a procedure is developed to evaluate the feasibility of acid fracturing by comparing conductivity generated from an unpropped fracture, a propped fracture and acid fracture. The target formation features extreme closure stress and low permeability at a depth of 5000-7000 meters. The efficiency of acid fracturing is challenged by the severe condition of the formation. The laboratory experiments were performed using the core samples from three wells from the productive interval of the target reservoir. The acid fracturing treatment conditions that have been used in the field were simulated during the laboratory experiments. The fracture conductivity was measured for unpropped, acid-etched, and propped fractures at different closure stresses using an apparatus consisting of a modified API conductivity cell and a load frame The fracture face surface of the intact and acidized core samples was scanned with a profilometer to characterize the change in surface profile to calculate the volume of dissolved rock caused by acid etching, and the initial fracture conductivity under zero closure stress. Conductivity decline as a function of closure stress was recorded and examined during the study. The results of the comparative study on fracture conductivities showed that the final propped fracture conductivity was higher than the acid fracture conductivity for all of the core samples. Also, the rate of conductivity decline with increasing closure stress was lower for the propped fracture conductivity compared to the acid fracture conductivity. The conclusion was made that the propped hydraulic fractures would retain a greater conductivity under the formation closure stress compared to the acid fractures. Hence, the hydraulic fracturing with proppant would be a more effective stimulation treatment compared to the acid fracturing for improving the wells productivity for the studied reservoir.
