A New Evaluation Method for the Fracability of a Shale Reservoir Based on the Structural Properties

Abstract

The fracability of shale reservoirs is one of the key indicators used for evaluating whether or not the shale can be used as a “sweet spot zone.” It has been determined that the structural properties of rock have important influences on the evaluation of the fracability of reservoirs. In the current study, five rock quality designation (RQD) calculation methods were compared and analyzed for the purpose of selecting an RQDI for characterizing rock structures. Focused on the lack of structural factors included in the previous fracability evaluation methods, a new model for fracability evaluation based on the combination of the brittleness index, structural index, and fracture toughness was constructed using a linear elastic fracture theory. The model showed that good fracability not only included higher brittleness but also required less energy to produce cracks. Meanwhile, good fracability also required more discontinuous structural planes. In the current study, a formation with a higher fracability index was considered to be a fractureable interval and a formation with a lower fracability index was a fracture barrier. Finally, the reservoir fracability index was modeled using the Xike 2 well in the north of Guizhou Province as a case study. Subsequently, a fracability logging evaluation method based on the fracability index model was determined, which will potentially provide a new technical tool for future fracturing optimization processes.

Geophysical logging in brittleness evaluation on the basis of rock mechanics parameters: a case study of sandstone of Shanxi formation in Yanchang gas field, Ordos Basin

“…The results calculated through logging are very accurate and could reflect the reservoir mechanics characteristics in a dynamic and continuous manner. So far, it is the most widely used method to obtain rock mechanics parameters (Sui et al 2019). However, because array acoustic logging is costly, there is not enough S-wave logging data for most wells.…”

Baseline Groundwater Quality before Shale Gas Development in Xishui, Southwest China: Analyses of Hydrochemistry and Multiple Environmental Isotopes (2H, 18O, 13C, 87Sr/86Sr, 11B, and Noble Gas Isotopes)

“…It is characterized by large thickness (50-200 m), sapropelic kerogen, and a moderate burial depth (generally less than 1-2 km) [49]. As for its organic chemical characteristics, it is high in organic matter content (w(TOC) of 0.5-3.92%), gas content (saturated adsorption gas content of 1.52-2.77 m 3 /t), and thermal maturity, while it is low in porosity and permeability [50][51][52], indicating the advantages of shale gas accumulation and hydraulic fracturing.…”

Is water saturation a fracability indicator for organic‐rich, yet low‐clay content, tight carbonate source rock reservoirs?

“…Fracability used to be measured by the so‐called brittleness index (BI) that is an intrinsic property and reflects the fact that there is no obvious absorption of mechanical energy in irreversible deformation prior to rock fractures. [ 2 ] It is generally assumed that formations with high brittleness are more susceptible to fractures. During hydraulic fracturing process, a brittle rock tends to form complex fracture networks that are preferable for hydrocarbon production because complex networks give relatively large fracture areas.…”

“…Along the same line of thinking, several investigators have made efforts to develop fracability indicators by combining BI with one or more other factors, including structural index, fracture toughness, mechanical energy dissipation during fracturing, and insitu stress. [2,4,[8][9][10][11] Given the fact that the proposed fracability indicators are empirical in nature and there is no universal agreement even regarding the definition of the closely related BI, [4] it is not surprising that currently there is no widely accepted fracability indicator for different types of unconventional reservoirs. Thus, it may be of practical importance to develop fracability indicators for each type of reservoir.…”