Acid Fracturing Optimizing Design and Application in Deep Naturally Fractured Carbonate

Abstract

More than 30% of carbonate reservoirs in the Tarim oilfield and XiNan oil-gas field are naturally fractured-vuggy reservoirs, of which the production is high in most cases and economic returns can be achieved within a short period of time. Acid fracturing is one of the key technology for development of such naturally fractured-vuggy reservoirs. Mud overflow and loss may be emerged at the same time while drilling fractured-vugular carbonate, once it happened, the well control will become complicated. So, the wellbore is drilled a certain distance up or transversal to these huge caves or natural fractures, and acid fracturing will be executed to connected the reservoirs to obtain the hydrocarbon in the reservoir. Acid-fracturing design of fractured-vugular carbonate is based on the relationship between the wellbore and fractured-vugular reservoirs. The acid fracturing design in naturally fractured-vuggy carbonate reservoir is different from the conventional one, it should be optimized based on the spatial positions of the fracture-vuggy body identified via geophysical prospecting, the wellbore and the distance between the two. In the case that the wellbore precisely reaches the top of the fracture-vuggy body, good oil and gas show or overflow is frequently happened. At this moment, drilling ceases and the wellbore are cemented and completed. The small-scale acid fracturing treatment is then carried out, and connectivity between etched fractures and the top of the fracture-vuggy body will lead to high hydrocarbon production. If a certain distance exists between the wellbore and fracture-vuggy reservoirs, 3D seismic imaging will be adopted to calculate the vertical distance between the center of the storage body and the wellbore, which is then used to optimize the injected fluid volume, pump rate and acid fracturing techniques. To create connectivity with the fracture-vuggy storage body relatively far away from the wellbore, acid fracturing involving preflush or multi-stage injection is used. Pressure drops by several ∼ several tens of MPa usually occur as the artificial fracture propagates into the fracture-vuggy body. After fracturing penetrating into the fracture-vuggy body, acid injection starts. During the early acid injection, the pump rate is raised up for longer etched length inside the fracture. It is then lowered during the late acid injection to improve the conductivity of etched fractures. If multiple fracture-vuggy bodies exist near the wellbore, the temporary diverting acid-fracturing will be treated, optimized with respect to the distance between the wellbore and different fracture-vuggy storage bodies, spatial relationship between the maximum principal stress direction and fracture-vuggy storage body and the trend of natural fractures, will be implemented to communicate with multiple naturally fractured vuggy reservoirs. This acid fracturing integrating geophysical prospecting and wellbore trajectory has been applied in hundreds of wells in the Tarim oilfield. Moreover, the success rate of such acid fracturing treatments is increasing with the advancement in the characterization of the fracture-vuggy reservoirs.