It has been observed in many shale gas operations that salt concentration of flowback water increases with time. Usually, the shape of concentration-time plots is different from one well to another. It is hypothesized that the relationship between salt concentration and cumulative water production provides useful information about the architecture of the fracture network. However, there is no simple analytical model available in the literature for quantitative interpretation of the measured salt profiles, and for evaluating fracturing operations.
In this study, two mathematical models are proposed for history-matching the salt concentration data measured during flowback operations and for investigating the complexity of fracture network. The models describe a relationship between cumulative water production and salt concentration, which in turn is related to the fracture aperture, is mathematically described. The two models describe a relationship between salt concentration profile and aperture size distribution (ASD) of the fracture network. The first model gives the volumetric fraction (as a probability density function) of each fracture aperture. The second model considers a bundle of fractures in series to derive the probability density function.
The salt concentration during the flowback operation of three multi-fractured horizontal wells completed in the three shale members of the Horn River basin is measured. The comparative study of concentration profiles suggests a meaningful relationship between the profile shape and the complexity of fracture network. For wells, with simple hydraulic fractures, the concentration profile reaches to a plateau. However, for wells with more complex fractures, the concentration profile keeps increasing and does not show a plateau. The proposed mathematical models are able to describe and quantify this behavior. Both models have almost the same ASD predictions. The mathematical modeling results show that the ASD is narrower when the concentration-load recovery plots reach to a plateau and therefore the fracture network is simple. However, when the concentration-load recovery plots shows a steady increase; ASD is wider and the facture network is complex.
The presented field study and the proposed mathematical models develop an improved understanding of hydraulic fracture systems. This study provides an alternative approach for evaluating fracturing operations and encourages the industry to manage the flowback operations carefully and monitor the water chemistry.