A Parametric Study on Completion Design in Shale Reservoirs Based on Fracturing-to-Production Simulations
Authors: Cohen Charles-Edouard , Kamat Sumant , Itibrout Tarik , Onda Hitoshi , Weng Xiaowei , Kresse Olga 
Published: March 2015

Advances in horizontal drilling and new practices in hydraulic fracturing have changed the paradigm of shale reservoirs in the last decade. Nevertheless, completion and stimulation engineers still face serious challenges due to the complex physics involved during hydraulic fracture propagation including hydraulic fracture interaction with natural fractures, stress shadow effects, and proppant transport in complex fracture networks. One of the main questions is how to optimize the number of stages and the placement of perforation clusters accounting for these complex physical phenomena and the wells' economics. To answer this question, it is necessary to analyze how the completion design and the fracturing process are related to the short and long term production.

This paper investigates the relation between the production and the completion design. A state-of the-art, fracturing-to-production simulation workflow is used to carry out a parametric study on completion design. The fracturing simulations are performed with the unconventional fracture model (UFM) that models the hydraulic fracturing process in a complex formations with pre-existing natural fractures including interaction with natural fractures and between hydraulic fracture branches (stress shadow effects). The resulting complex fracture networks are then explicitly gridded to build an unstructured grid that is then passed to a numerical reservoir simulator to run the production simulations and accurately model multiphase reservoir flow around complex hydraulic fracture networks. The base case of this study represents a synthetic reservoir model replicating properties of the Marcellus shale. One of the main parameters investigated is the number of perforation clusters per stage for both a constant pumping rate and for a constant average rate per perforation cluster. We also investigated the influence of the number of stages on production, for a given lateral length and a given total treatment volume. The results from this study provide new understanding of the impact of completion design on production and illustrate its use to find optimum completion design based on modeling. For example, some results show that for a constant average rate per cluster a clear optimum can be found as function of the number of cluster per stage, while this task can be more challenging with a constant total pumping rate.

Introduction

Economic production from shale gas reservoir depends greatly on well spacing and proper completion design of the wells. Among the most important considerations for completion design are defining the number of stages, the number of perforation clusters per stages as well as the spacing between perforation clusters as well as deciding the type of treatment fluid, proppant and the quantities to use. This design has a significant impact on the economics of the well due to the usually large number of stages required to complete a shale gas or a shale oil well. It is not uncommon to have ten to twenty stages for a single well, with three to eight perforation clusters per stage. Each stage may involve two to three hours of pumping of more than 200,000 gallons of fluids and 250,000 lbs of proppants, in addition to time spent to perform the perforation work and the placement of packers to isolate between stages. Therefore, the time for completion and stimulation of a well in a shale reservoir can easily extend beyond a week with intense and large scale operations and logistics. This adds significantly to the capital expenditure of the well, and impacts its economic viability. In addition, the completion design can significantly impact the production outcome from the well, by defining where the well connects to the heterogeneous reservoir and influencing both the hydraulic fracture propagation and the proppant placement. In this context, the optimization of the completion design is a serious challenge but critical for the development of shale resources.