Conductivity of hydraulic fracturing in tight carbonate intra-platform shoal reservoirs

The carbonate intra-platform shoal is a typically tight oil and gas reservoir with low porosity and low permeability. A common way to stimulate this type of reservoir is through acid fracturing. Hydraulic fracturing, however, is another relatively newer technique for the stimulation of tight carbonate reservoirs, which form self-propped and propped fractures. In order to explore the applicability and efficacy of hydraulic fracturing in such reservoirs, a series of experiments were conducted on intra-platform shoal carbonates from the Ordovician Yijianfang and Yingshan Formation in the Tarim Basin of China. The results show that under low closure pressures, self-propped fractures provide a relatively high conductivity for fluid flowing as long as the closure (confining) pressure is relatively low, with almost 50,000 mD-ft, five times greater than propped fractures. Thus, the generation of self-propping fractures is a viable stimulation mechanism for tight carbonate reservoirs. However, with increasing closure pressure, the conductivity of self-propped fractures declines rapidly, decreasing to 15 mD-ft when the closure pressure increased to 4000 psi. At higher pressures (e.g. higher than 4000 psi), fracture conductivity is greater for propped fractures. The roughness of the fracture surfaces has an apparent controlling effect on fracture conductivity under various degrees of displacement. However, under high closure pressures the fracture conductivity is not significantly related to the degree of displacement. The conductivity of propped fractures is positively correlated with the size and concentration of the proppants. The inhomogeneous distribution of proppants on the surface of fractures intensifies the damage of self-propped fractures under high closure pressures. Although fracture conductivity declines sharply with increasing confining pressure, a series of additional fractures are formed in the surrounding rocks due to widening of pre-existing micro-fractures and stylolites. These types of fractures can intercommunicate with the newly created fractures, thereby enhancing the stimulation effect. In general, self-propped fractures are not suitable for high closure pressures. However, proppants compensate for this shortcoming.