Fracture characteristics of reservoirs with different lithologies in marine-continental transitional facies and its influence on fracturing

Abstract

Rock fracture mechanics properties are the intrinsic geo-mechanical basis for determining the effectiveness of reservoir fracturing stimulation. It is of practical significance to clarify the fracture mechanics properties of reservoirs with different lithologies in marine-continental transitional facies to optimize the fracturing stimulation measures and improve the effect of fracture networks. Taking the reservoir in marine-continental transitional facies at the eastern edge of Ordos Basin as the research object， this study carried out the fracture toughness test of various lithologies in the reservoir， clarified the fracture mechanics characteristics of the reservoir with different lithologies and the influencing factors， and finally established a numerical model of fracturing with the logging lithology depositional sequence on the finite element platform， and emphasized the influence of the combination of lithologies on the fracture height propagation. The results show that reservoir lithologies in the marine-continental transitional facies are complex and rich in quartz and clay minerals， and the mechanical deformation shows strong brittleness characteristics. Compared with marine shale， the reservoir rocks in the marine-continental transitional facies are characterized by low fracture toughness. The fracture toughness value shows the characteristics of coal rock clastic sedimentary rock gray rock. Among them， weak structural surfaces such as plant debris and grain layers have a strong deterioration effect by changing the fracture paths and thus affecting the fracture toughness of the corresponding lithologies. The static elastic modulus of reservoir rocks is an important mechanical factor controlling the fracture toughness of different lithologies， and the fracture toughness has a linear correlation with the static elastic modulus. The reservoir rock is characterized by strong interlayer mechanical heterogeneity. The hydraulic fracture height tendency is blunted in the process of moving from the mudstone layer into the sandstone layer， and the interbedded coal layer in the reservoir will significantly reduce the fracture height and increase the probability of non-uniform proppant placement in the reservoir. The influence of reservoir in-situ stress on fracture height morphology is related to the lithology combination， so the selection of horizontal well traversing trajectory in the early stage needs to pay attention to the influence of the lithology combination characteristics above and below the horizontal wells on the fracturing effect of the subsequent horizontal wells.

Keywords

• marine-continental transitional facies
• lithology
• fracture toughness
• influencing factor
• fracturing stimulation