Fracturing Fluids Impact on Unconventional Shale Rock Mechanical Properties and Proppant Embedment

Singh, V. K. (Core Laboratories) | Wolfe, C. (Apache Corporation) | Jiao, D. (Core Laboratories)

OnePetro 

ABSTRACT: Many of unconventional shale reservoirs are marginally economic due to a combination of high drilling and completions costs and/or low productivity. Technological advances in horizontal drilling and hydraulic fracturing have alleviated these burdens but additional cost reductions and productivity enhancements can be achieved by understanding how the various mineralogy shale rocks mechanical properties and embedment changes during hydraulic fracturing.

In this paper, numerous reservoir shale samples from the Marcellus, Mancos, Eagleford, and Wolfcamp formations were exposed to commonly-used fracture fluids to determine the corresponding changes in the rock mechanical properties. These samples were also subjected to proppant embedment testing to assess the potential reduction in fracture width. The mineralogy of each sample was measured to understand its impact on the changes in rock mechanical properties and corresponding proppant embedment.

The results of this study indicate a reduction in the compressive strength and elastic moduli after exposure to the various fracture fluids. Additionally, there is a significant increase of proppant embedment at given stress levels for different proppant types. This reduction in modulus, combined with increased proppant embedment, can lead to a decrease in fracture conductivity and a subsequent loss of productivity.

1. INTRODUCTION

Increasing demand of fossil fuels have made unconventional shales, which are the source, the seal, and reservoir all in one, gain significant advancement in last few decades, encouraged by emerging technologies in hydraulic fracturing and horizontal drilling. Hydraulic fracturing in the formation is achieved by pumping large volume of fluids into the pay zone followed by slurry of mostly proppant to keep the fractures open and create a conductive path for reservoir fluids once the pressure is reduced.

A successful fracture treatment job requires understanding of shale mechanical properties to create a conductive pathway. An effective achievement of conductive pathway requires the use of two principal materials: fracturing fluid (and additives) and proppant. Shale mechanical properties such as Young's modulus can get affected due to exposure with the fracturing fluids, resulting in weakening of rock frame with time. Any reduction in Young's moduli leads to conductivity loss by reduced fracture width due to proppant embedment as closure stress increases. This reduction in conductivity varies from one formation to other depending upon the mechanical properties, mineralogy, proppant type, fracturing fluid and closure stress. Fracture conductivity reduction in ultra-tight shale reservoirs can have significant economic value and it is desired to optimize the fracture design to improve production and lower completion cost.

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