Preface

Unconventional hydrocarbon production has drawn more attention than ever recorded in history. The reason is simply the promise of a vast amount of reserve, the potential for cleaner fuels, and advances in technology, which now enable drilling and extraction of hydrocarbons under challenging conditions. Evolution in technologies has also made it possible to exploit unconventional formations more efficiently and economically. Generally, unconventional extraction of oil and gas can only be achieved if the formation is adequately stimulated to encourage outward flow of hydrocarbon fluids. This is necessary because of the formation rock's ultra-low permeability and porosity as well as other uncharacteristic rock and reservoir fluid properties. Sundry methods have been successfully applied in stimulating unconventional reservoirs. Hydraulic fracturing still remains one of the major ways of accomplishing this; the principles adopted in this method, which is very much reflected in most of the other stimulation approaches, is fundamentally the introduction of fluids with special properties into reservoirs. Fluids are injected under different states—e.g., high or low pressure, high or low velocity (injection rate), high or normal temperature, and acidic or non-acidic.

An additional way to enhance reservoir productivity is through the drilling of directional wells. Directional (i.e., inclined and horizontal wells) are considered better than vertical wells in terms of several aspects of reservoir performance. In comparison to vertical wells, the productivity of a reservoir is greatly improved where hydraulic fracturing or other stimulation approaches based on the same concept are implemented through directional wells.

This book sheds light on selected themes that are crucial to the stimulation of unconventional hydrocarbon reservoirs. These are treated under seven areas:


The introductory chapter, "Developments in the Exploitation of Unconventional Hydrocarbon Reservoirs," is a conspectus of key aspects of unconventional hydrocarbon production, and up-to-date developments and challenges based on the

above-named thematic areas. The following are covered: different types of reservoir stimulation approaches; fracturing and reservoir fluids/fluid systems; drivers of fluid behavior with respect to their multiphasic and multicomponent mixing and transport; well test analysis; and environmental impact and health and safety.

Chapter 2, "CO2 Foam as an Improved Fracturing Fluid System for Unconventional Reservoirs," describes the characteristics and applications of CO2 foam-based fracturing fluids. Foam-based fluids have a number of advantages over other types of fracturing fluids. Among other benefits, CO2 foam fracturing fluids are suitable for water-sensitive formations, facilitate water flowback/recovery, reduce the quantity of produced water, and improve proppant placement and distribution. The rheology of CO2 foam is explained, supported by corresponding models and physical experimental studies that describe the foam fluid behavior.

Chapter 3, "Thermodynamics of Thermal Diffusion Factors in Hydrocarbon Mixtures," presents a thermodynamic model of thermal diffusion factors for hydrocarbon mixtures, which relies on the linear transport of intermolecular forces while considering the effects of energy and molecular mass and size. In furtherance, this model is used to examine the behavior of binary hydrocarbon fluid mixtures.

Chapter 4, "Well Test Analysis for Hydraulically Fractured Wells," illustrates how the *Tiab's direct synthesis* (TDS) technique (a well test analysis method) is applied in interpreting pressure tests and assessing the performance of hydraulically fractured reservoirs and their associated wells. The TDS technique can be used for fracture characterization and provides reliable estimates of fracture parameters, for instance, fracture conductivity and half-length.

Chapter 5, "Surface Drilling Data for Constrained Hydraulic Fracturing and Fast Reservoir Simulation of Unconventional Wells," presents a workflow demonstrating the use of surface drilling data in constructing reservoir models. These models are applied in designing hydraulic fracturing operations and for unconventional reservoir simulation. The main surface drilling data considered are rate of penetration (rop), torque (t) and weight-on-bit (WOB).

Chapter 6, "Elastic-based Brittleness Estimation from Seismic Inversion," introduces procedures for determining rock brittleness, fracture density, and hydrocarbon bearing in fractured reservoirs. In the first approach, brittleness is indicated by the parameter *brittleness average* (BA). BA can be used to identify zones of high fracture density, also referred to as the brittle area. It is calculated from elastic properties—Poisson's ratio and Young's modulus—which are in turn derived from seismic data through seismic inversion. Poisson's ratio is determined from P-wave (*VP*) and S-wave (*VS*) velocities, while Young's modulus is expressed in terms of bulk modulus and Poisson's ratio. In the second approach, rock brittleness, fracture density, and the nature of the formation lithology is indicated by the parameter *scaled inverse quality factor of P-wave* (SQp). A related parameter, *scaled inverse quality factor of S-wave* (SQs), is used as an indicator of the presence of hydrocarbons.

Chapter 7, "Human Health Risks of Unconventional Oil and Gas Development using Hydraulic Fracturing," looks at the potential human exposures to emissions (and

attendant health and safety risks) as a result of hydraulic fracturing operations in unconventional oil and gas reservoirs. This is viewed from the perspective of both occupational and public health and safety.
