Influence of Water–Oil Saturation on the Fracture Process Zone: A Modified Dugdale–Barenblatt Model

Abstract

The wetting and nonwetting fluid saturations in porous reservoirs always change during long-term injection and production. The fracture process zone (FPZ) is a prominent feature in the rock fracture process. If the FPZ properties are influenced by pore fluids, the process of hydraulic fracturing will change greatly. The existing models do not consider the role of pore fluid when characterizing the FPZ. In this paper, a modified Dugdale–Barenblatt (D–B) model with capillary pressure is proposed. The model reflects the fact that the FPZ length decreases nonlinearly with the increase in capillary pressure, and it reveals the mechanism of capillary pressure on the equivalent fracture cohesion in the FPZ, which affects the FPZ length. Three-point bending tests were carried out on sandstone under various fluid saturations through digital image correlation (DIC), acoustic emission (AE), and scanning electron microscope (SEM). It was found that the FPZ length of the water–oil-saturated samples was 30–50% smaller than that of water-saturated/oil-saturated samples due to the capillary pressure effect, and the modified D–B model was well consistent with the experiments. The AE behaviors of different saturated samples were not the same: The cumulative AE signals changed abruptly at 90% of the peak load for the water–oil-saturated samples and at 50% of the peak load for water-saturated samples. This demonstrated that the effect of capillary pressure was more obvious than the weakening effect of microstructural damages. The significant influence of capillary pressure on FPZ requires continuous recognition in hydraulic fracturing design.

A Review of the Hydraulic Fracturing in Ductile Reservoirs: Theory, Simulation, and Experiment

“…The pore fluid also influences the size of rock FPZ. Nie et al [181] found in the experiment that the rock FPZ is affected by the fluid in the pores. The FPZ of the watersaturated specimen is 30% longer than that of the dry specimen, and the capillary force can reduce the FPZ length.…”

An Improved Model for Evaluating the Brittleness of Shale Oil Reservoirs Based on Dynamic Elastic Properties: A Case Study of Lucaogou Formation, Jimusar Sag

“…The digital image correlation method was used to monitor the fracture process in real time, and the displacements of the specimens were calculated by an open-source program [47]. Based on the existing methods, the fracture process zone length of reservoir rocks was identified by the characteristics of horizontal displacement [48][49][50]. And the typical fracture process zone lengths of Lucaogou Formation rocks with different brittleness B 4 were characterized in Figure 11, where the length of the fracture process zone is larger when the rock brittleness evaluated by the improvement model is smaller.…”

Improved technique for toughness testing of shale rocks

“…Distilled water and saline solution of KCl caused a 70% decrease, the decrease with alcohol and kerosene was 25%. Different authors have indirectly measured that the size of the FPZ is larger for samples saturated in certain aqueous solutions [42,68]. At the same time, the importance of FPZ (r c .)…”

“…A key to further reduce scatter and uncertainty in fracture testing of shale rocks is therefore to ensure a similar initial microcrack density. References in the literatur [42,68,70] point to two basic techniques: ultrasound measurements and micro-tomography. Propagation speeds of mechanical waves are modified by matrix stiffness; detection thresholds may be the key in the applicability of tomography techniques.…”