Experimental study on stress monitoring in fractured-vuggy carbonate reservoirs before and after fracturing

Abstract

Natural fractures and caves serve as the main reservoirs of fractured-vuggy carbonate reservoirs, and acid fracturing is the main technical means to improve the well productivity. Only if hydraulic fractures connect the natural fractures and caves are there obvious hydraulic fracturing effects. However, the faults, caves, and natural fractures distributed in the bedrock resulted in the distribution of the complex in-situ stress field, which restricts the direction and geometry of hydraulic fracture, thus deciding whether hydraulic fracture could connect with natural fractures and caves. Therefore, an accurate description of the magnitude and direction of the stress field in different regions of faults and caves is of great significance. In this paper, a series of hydraulic fracturing experiments on artificial carbonate samples with presetting faults and caves were performed. And high-precision strain sensors prefabricated in the rock sample were used to monitor the stress field during fracturing. Four kinds of fault-cave geological combinations are investigated, and the results show that the presence of fault-karst can significantly change the distribution of in-situ stress and result in deflecting of the fracture, which may or may not be beneficial to connection of caves and faults, depending on well placement. There is high stress concentration at the end of the fault, and the maximum and minimum principal stresses are obviously higher than the far-field stresses. The lateral stress of a single fault is very low in the direction perpendicular to the fault plane, and there is little change in the direction parallel to the fault. Due to low maximum and minimum stress, the intersection of intersecting faults will exert a kind of “attraction” to hydraulic fractures. The stress distribution around the cave is lower than that in the distance. The results can provide theoretical guidance for optimizing hydraulic fracturing design in fractured-vuggy carbonate reservoirs.