Experimental Study on Improving the Fracture Conductivity of Acid Etched Fracture for Ultra-Deep and High Stress Carbonate Reservoirs

Abstract

ABSTRACT With the development of carbonate reservoirs moves to the ultra-deep layer, the etched fracture would be closes quickly in the high stress. In this paper, high channel sanding method with acid fracturing technology are combined, the strength and spread ways of the different type proppant are optimized, and the fracture conductivity of continuous sanding and channel sanding are tested. The experiment result shows that high strength ceramic proppant can improve the conductivity of acid erosion fractures under high pressure. In the stress of 50MPa, fracture conductivity by high channel sanding with 86MPa ceramic could be increased by more than 51% than that of conventional acid etched fracture. The flow conductivity increase amplitude reaches more than 700% in the condition of 90MPa. By changing the sanding mode, the quantity of proppant can be saved, the pump risk can be reduced, and the stable and solid support channel can also be formed to improve the conductivity of acid etched fractures in ultra-deep carbonate reservoirs. INTRODUCTION With the gradual development of the Carbonate reservoir to the ultra-deep layer, the depth of the reservoir has reached 8,800 meters, and the temperature has been raised from 120°C to 180°C. The increase of reservoir depth and temperature brings many new challenges to reservoir stimulation. When the effective closure stress of the reservoir increases from 10 MPa to 55 MPa, the conventional acid etched fracture conductivity decreases by 95% (Yao et al., 2015). With the increase of closure stress, the insufficient support strength is the main reason for the rapid decrease of the fracture conductivity. In order to increase the fracture validity period after acid fracturing, the main objective is to improve the long-term etched fracture conductivity. The existing acid fracturing is to dissolve calcareous minerals on the fracture surface, leaving the unreacted rock skeleton as the supporting point. Due to the uneven distribution of each point and its low compressive resistance, it will be crushed gradually under high closure stress. Many scholars (Yi et al., 2006, 2008, 2010, Gao et al., 2015, Wang et al., 2012,2016, He et al., 2014) tried to improve the fracture conductivity by using cross-linked acid carrying sand or compound acid fracturing, and achieved certain effect in the oilfield application. However, with the increase of reservoir depth, it becomes more and more difficult to add sand into the carbonate reservoirs. In the meantime, proppant concentration cannot be effectively increased, making it difficult to achieve the effect of high concentration proppant placement (Zhuang et al., 2014). In recent years, many researchers try to use high channel pulse sand-adding method in low permeability reservoir (Zhong et al., 2012, Wu et al., 2014, Liu, 2015, Fu et al., 2016).