Experimental investigation of shale imbibition capacity and the factors influencing loss of hydraulic fracturing fluids

Abstract

Spontaneous imbibition of water-based fracturing fluids into the shale matrix is considered to be the main mechanism responsible for the high volume of water loss during the flowback period. Understanding the matrix imbibition capacity and rate helps to determine the fracturing fluid volume, optimize the flowback design, and to analyze the influences on the production of shale gas. Imbibition experiments were conducted on shale samples from the Sichuan Basin, and some tight sandstone samples from the Ordos Basin. Tight volcanic samples from the Songliao Basin were also investigated for comparison. The effects of porosity, clay minerals, surfactants, and KCl solutions on the matrix imbibition capacity and rate were systematically investigated. The results show that the imbibition characteristic of tight rocks can be characterized by the imbibition curve shape, the imbibition capacity, the imbibition rate, and the diffusion rate. The driving forces of water imbibition are the capillary pressure and the clay absorption force. For the tight rocks with low clay contents, the imbibition capacity and rate are positively correlated with the porosity. For tight rocks with high clay content, the type and content of clay minerals are the most important factors affecting the imbibition capacity. The imbibed water volume normalized by the porosity increases with an increasing total clay content. Smectite and illite/smectite tend to greatly enhance the water imbibition capacity. Furthermore, clay-rich tight rocks can imbibe a volume of water greater than their measured pore volume. The average ratio of the imbibed water volume to the pore volume is approximately 1.1 in the Niutitang shale, 1.9 in the Lujiaping shale, 2.8 in the Longmaxi shale, and 4.0 in the Yingcheng volcanic rock, and this ratio can be regarded as a parameter that indicates the influence of clay. In addition, surfactants can change the imbibition capacity due to alteration of the capillary pressure and wettability. A 10 wt% KCl solution can inhibit clay absorption to reduce the imbibition capacity.

A critical review of water uptake by shales

“…The process of water absorption by the clay minerals in the shale is referred in short as clay hydration. Several authors (Makhanov et al 2014;Ge et al 2015;Yang et al 2016) have reported that imbibed volume in some samples can be greater than the initially measured pore volume, which they suggested was due to absorption of water molecules by the semi-permeable clay membranes. Yang et al (2016) reported that this imbibed volume can be up to 400% of the pore volume measured before imbibition.…”

Modelling shale spontaneous water intake using semi‐analytical and numerical approaches

“…Here, we model the capillary imbibition phase in a shale sample and compare the results with the experimentally measured data points. Although the characteristics of water imbibition curves in different tight rocks vary significantly (attributed to different matrix and pore space structures), generally, the imbibition curve is divided into the following three regions: (1) initial linear capillary imbibition (which is the focus of this work); (2) transition region, which occurs at higher water saturations where the role of capillary driving force gradually diminishes; and (3) diffusion region, which occurs in longer periods . Roychaudhuri et al considered spontaneous water imbibition into cubic samples of 1 cm 3 taken from the Marcellus shale member of the Appalachian Basin to investigate the role of capillary imbibition in the fracking fluid intake process.…”

“…During shut‐in periods, the pressure gradient (viscous force) gradually diminishes and the capillary force takes over the water intake via a slow countercurrent imbibition process . Recent core‐scale observations confirm that the capillary imbibition plays a remarkable role in fluid migration from fracture to matrix and is mainly controlled by the rock and fluid properties, fracture interface, and shut‐in‐time length …”

Thermogravimetry as a tool for measuring of fracturing fluid absorption in shales

“…Spontaneous imbibition refers to the process of adsorption with no pressure driving of the phases into the rock. Some experimental investigations were also performed on shale imbibition capacity towards hydraulic fracturing fluids [10,11]. Currently, it is well known that the imbibition of fracturing fluids is mainly controlled by the capillary pressure, while the effects of clay absorption have not been studied thoroughly [10].…”

“…Some experimental investigations were also performed on shale imbibition capacity towards hydraulic fracturing fluids [10,11]. Currently, it is well known that the imbibition of fracturing fluids is mainly controlled by the capillary pressure, while the effects of clay absorption have not been studied thoroughly [10]. Because different mechanisms seem to be facilitating imbibition in the shale, there is not so far a verified answer for the geomechanisms behind those observations [11].…”