Field researches conducted on the multi-stage hydraulic fracturing measurements manifest that a large proportion of fracturing fluid retains in tight reservoirs, and the flow-back efficiency is usually lower than 30%. The publications have demonstrated that the spontaneous imbibition plays a significant role in large volumes of water loss, which is caused by high capillary pressure of tight rocks. Nevertheless, the imbibition characteristics of tight rocks are quite complex and still need further research owing to the complex pore structure, mineral constituent and strong microscopic heterogeneity.

Comparative imbibition experiments were carried out on 12 core samples, which are seclected from tight sandstone formation from Ordos Basin, tight volcanic formation from Songliao Basin and shale formation from Sichuan Basin. Hence, a new method is proposed to demonstrate the imbibition characteristics and estimate fracturing fluid intake in tight gas formation. Moreover, the authors study the water imbibition capacity and characteristics of tight rocks, which holds diverse mineralogy, pore-size distribution and pore connectivity. The experimental data of tight rock samples in different size and shape can be normalized by using the new characterizing method, which exhibits the imbibition capacity, initial imbibition and late imbibition rate. The imbibed water volume increases with the clay content, and smectite and illite/smectite tends to strong water imbibition. Due to the water absorption on clay surface, the ratio of imbibed water volume to pore volume measured by Helium exceeds 100%. In addition, a general observation shows that each imbibition curve of tight rock can be divided into an initial imbibition region, a non-linear transition region, and a late imbibition region. However, the imbibition characteristics of different tight rocks vary greatly, which could be attributed, at least in part, to the complex pore connectivity, pore-size distribution and mineral composition. Well-developed macropores tend to have well pore connectivity and large time exponent at the initial imbibition region. Well-developed mesopores and micropores tend to have poor pore connectivity and large time exponent at the late imbibition region. These studies can be used to evaluate the fracturing fluid imbibition characteristics, and to understand its relationship with pore connectivity and pore-size distribution in tight reservoir rocks.

You can access this article if you purchase or spend a download.