Appraising Unconventional Resource Plays: Separating Reservoir Quality from Completion Effectiveness
Authors: Cipolla Craig L. , Lewis Richard E. , Maxwell Shawn C. , Mack Mark Gavin 
Published: May 2016

Appraisal wells in unconventional, very low permeability, resource plays require large hydraulic fracture treatments to assess economic viability. In many cases, drainage area and hydrocarbon recovery are defined by the areal extent and effectiveness of the hydraulic fracture treatment. To increase the drainage area and recovery per well, multiple hydraulic fracture treatments in horizontal and vertical wells are now common, resulting in more complex and expensive completions. Therefore, appraising the completion and hydraulic fracture treatment are just as important as appraising the reservoir. Unlike conventional reservoirs, the complexity and heterogeneity of unconventional resources can make reliable reservoir characterization difficult, which can result in significant uncertainty when evaluating appraisal well performance. Therefore, applying the appropriate technologies for unconventional reservoirs and a holistic approach are essential to properly separate reservoir quality from completion effectiveness.

This paper details technologies and workflows that are essential to the reliable appraisal of unconventional resources, with an emphasis on appraising resources outside of North America. Due to the high cost of appraisal wells in most environments outside North America, operators must assess the viability of unconventional resources using as few wells as possible. The North American model of assessing unconventional reservoirs by drilling and completing a large number of wells may not be economically feasible in areas with insufficient hydraulic fracturing, drilling, and completion infrastructure.

Due to the variability of both hydraulic fracture growth and reservoir characteristics in unconventional reservoirs, properly assessing new plays and subsequently optimizing fracture treatments and completions has historically been a ?trial and error? process requiring a large number of wells and significant capital risk.

However, efficient evaluation of stimulation treatments and completions is now possible by combining microseismic mapping and other hydraulic fracture diagnostics with advanced logs, specialized core tests, 3D seismic, and newly developed ?unconventional? hydraulic fracture models. This holistic approach reduces the number of wells required to assess the economic viability of unconventional resources and reliably separates reservoir quality from completion effectiveness. The application of these unconventional-reservoir-specific technologies, newly developed hydraulic fracture models, and specialized workflows are illustrated using examples from North America.

Introduction

There are significant differences between the evaluation of unconventional resources and conventional plays. The exploitation of unconventional reservoirs requires large hydraulic fracture stimulations that contact a huge reservoir surface area and effectively connect this surface area back to the wellbore. Contacting a large reservoir surface area significantly increases hydrocarbon production rates and recovery, enabling economic development. In fact, the effectiveness of the hydraulic fracture treatment will control both well productivity and drainage area in unconventional reservoirs (Cipolla et al., 2008a). The very low matrix permeability of these reservoirs necessitates a large number of wells to effectively develop the resource base. In recent years the application of horizontal drilling has dominated unconventional reservoir development, accessing much more reservoir volume than vertical wells and reducing the number of wells required to develop the resource. The combination of multi-stage hydraulic fracture stimulation and horizontal drilling has enabled exploitation of vast North American shale resources (Arthur et al. 2008; Jenkins and Boyer 2008) and improved the economics of developing some tight gas resources (Baihly et al. 2009). However, the application of horizontal drilling and the need to perform multiple hydraulic fracture treatments adds to the complexity of the completion and results in much more uncertainty when evaluating well performance and optimizing stimulation designs and completion strategies.