Indentaion Hardness for Improved Proppant Embedment Prediction in Shale Formations

Abstract The aim of hydraulic fracturing is the generation and maintenance of highly permeable pathways. To achieve this, a high hydraulic pressure is applied to overcome the breakdown pressure in developed formations. To avoid the closure of the generated fractures due to the acting in-situ stress in the subsurface, proppantsareused. The hydraulic conductivity (Cf) of such a fracture is defined as the permeability of the proppant pack (kpr) times the fracture width (wf). However, this value cannot be considered a constant. The increasing effective pressure leads to an increased stress on the proppants, thus increasing their embedment into the formation surface. Such a situation becomes particularly critical in weak formations and in formations where the interaction between the frac fluid and the formation can lead to a change in the mechanical properties in the fracture face. This paper will present an approach to model the embedment of various kinds of proppant packs with respect to the proppant size distribution and the proppant density. This approach uses the indentation hardness of the surface formation to calculate the embedment. The advantage of this parameter, which can be measured by applying the hardness test according to Brinell, is that it describes the mechanical properties of the surface of the formation. This area, which is highly influenced by the treatment fluid, can show great differences in the mechanical behavior compared to the untreated area. This can lead to an under- or overestimation of the embedment, since the mechanical parameters normally used for calculation are measured on core samples representing the mechanical behaviour of the untreated rock mass. The consideration of the indentation hardness can lead to a more accurate embedment prediction, since it describes the behaviour of the first 0.1-2 mm of a surface, in which the embedment alone can lead to a closure of a fracture. The first series of hardness tests according to Brinell, conducted on different shale types, have proven that the fluid composition affects the indentation hardness of the samples, which will of course affect the depth of the embedment. The measurements are the basis for the embedment modelling which will be presented in this article.