The effect of rock properties on fracture conductivity in the Marcellus Shale
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Copyright 2016, Society of Petroleum Engineers. The success of fracture stimulations depends on the ability to fracture the rock and to maintain fracture conductivity. Therefore, in order to have a successful treatment, the effect of rock and proppant characteristics should be understood. This paper investigates the relationship between fracture conductivity and rock properties for two different locations in the Marcellus shale. In this work, a series of experiments were performed in order to better understand the mechanical behavior of the Marcellus Shale. Multiple cores from Allenwood and Elimsport locations in Pennsylvania were collected to ensure repeatability in the results. The core samples were cut parallel and perpendicular to the bedding planes in order to analyze the effect of anisotropy in fracture conductivity. Additionally, compressive triaxial tests were performed to obtain the rock mechanical properties such as Young's Modulus, Poisson's Ratio and Ultimate Compressive Strength, and X-ray Diffraction analysis was used to obtain mineralogy composition. The laboratory results were carefully compared with previous fracture conductivity data and surface roughness data from the same Marcellus shale locations where proppant concentration was kept constant to reduce the variables affecting the fracture conductivity behavior. The findings showed that the anisotropy effect is present for this formation and reflected in the fracture conductivity values, where samples parallel to the bedding plane seem to have higher Young's Modulus. It was confirmed that at proppant monolayer concentration the main mechanism for conductivity loss is proppant embedment, where due to the high localized stress, the rock-proppant interaction goes directly to permanent deformation of the rock. It was also observed that a higher Young's Modulus helps to maintain the fracture width which translates in lower rate of conductivity loss with increasing closure stress, therefore, rock mechanical properties have impact on fracture conductivity. The phenomenon observed is less pronounced if a multilayer proppant concentration is used, where the proppant pack characteristics become an important parameter in fracture conductivity. In this paper, we present an analysis of the effect of rock properties in the fracture conductivity behavior based on a series of experimental results from the Marcellus Shale, where the role of permanent deformation was taken into consideration as a parameter causing fracture conductivity lost. This is the first time the effect of rock characteristics in fracture conductivity is analyzed based on experimental work for the Marcellus shale.
