Carbon Dioxide (CO2) storage and sequestration in unconventional shale resources has been attracting interest since last couple of years due to the very unique characteristics of such formations have made them a feasible option for this object. Shale formations are found all around the world and the conventional assets are easily accessible, and also the huge move of operators toward developing unconventional reservoirs during past years leaves many of such formations ready for sequestering CO2. Today, the use of long horizontal wells that are drilled on a pad has the lowest amount of environmental footprint in which for storage and sequestration purpose also provides much more underground pore spaces available for CO2.
S. Sina Hosseini Boosari1, Umut Aybar2, Mohammad O. Eshkalak2
1West Virginia University, Morgantown, WV, USA. 2Petroleum and Geosystems Engineering Department, The University of Texas at Austin, Austin, TX, USA.
Received 16 February 2015; accepted 14 March 2015; published 20 March 2015
Copyright © 2015 Authors
In this paper we study the state of the art of the technology of CO2 storage and sequestration and provide different and fresh look for its complex phenomena from a mathematical modeling point of view. Moreover, we hope this study provides valuable insights into the use of depleted shale gas reservoirs for carbon sequestration, which as a result, a cleaner atmosphere will be achieved for the life of our next generations. Also, we present that the depleted shale gas reservoirs are very adequate for this purpose as they already have much of the infrastructure required to perform CO2 injection available in sites.
Geologic formations and abundant reservoirs have attracted much attention within the engineering community for the purpose of reducing carbon dioxide of the atmosphere through storing and sequestration process in recent years. Worldwide energy demand is supported by the significant role of conventional and unconventional resources of oil and gas. This huge dependency has resulted in critical environmental issues such as increased CO2 emission known as the greenhouse gas, and for the purpose of having a clean atmosphere for next generations, CO2 storage and sequestration has gotten a common procedure for this purpose. During the last decade, energy equations are changed due to the advances obtained in drilling and fracturing of unconventional reservoirs both in the US and worldwide. The main simple difference between conventional and unconventional resources is the fact that unconventional are scattered all around the world but conventional exist just in some parts of the world. This strongly means that since unconventional are everywhere on the map then the CO2 sequestration in such reservoirs are very reasonable and economically feasible   .
Shale gas is mostly methane that is trapped within the shale formations. Shale is located thousands of feet below earth surface. Gas-productive shale formations in the continental US are of thermogenic or biogenic origin and are found in Paleozoic and Mesozoic rocks. Shale is a fine-grained sedimentary rock that, when deposited as mud, can collect organic matter. When the organic matter decays over time, petroleum and natural gas products are formed within the rock’s pores. Shales typically hold dry gas, but some formations produce liquid products as well. Conventional gas reservoirs form from the migration of natural gas from an organic-rich source into permeable reservoir rock. Unconventional gas-rich shales, however, generally function as both the source and reservoir for natural gas. Shales have low permeability, which means that trapped gas cannot move easily within the rock. Because of this, a technique called hydraulic fracturing is employed to produce the natural gas. This unique characteristic benefits the sequestration of CO2. Hydraulic fracturing cracks the shale rock through the injection of water, sand, and chemicals at high pressure.
Underground storage and sequestration of carbon dioxide involves the process of injecting highly pressurized CO2 into the formations considering the capability of the rock to permanently keep it from leaking toward the earth surface. There are five applicable options for safely sequestering the carbon dioxide into the geologic formations. Saline formations, basalts, un-mineable coal seams, conventional oil and gas reservoirs and depleted unconventional shale resources are technically studied during the course of this paper. As of now, considering the advances gained through last couple of years in carbon management, scientists have proposed the following statements: 1) estimating CO2 storage capacity +/− 30 percent in geologic formations; 2) ensuring 99 percent storage permanence; 3) improving efficiency of storage operations; and 4) developing Best Practices Manuals   . These technologies will lead to future CO2 management for coal-based electric power generating facilities and other industrial CO2 emitters by enabling the storage and utilization of CO2 in all storage types.
In the U.S., Nuttall et al. (2005) estimated the CO2 sequestration capacity in the organic-rich Devonian black shales in the Big Sandy gas field of Eastern Kentucky to be about 6.8 Gt. To illustrate the importance of shale formations for CO2 storage in Europe, the Dutch resource of shale gas own an estimation of between 48,000 and 230,000 Bcm. If one assumes that technology can be developed to store 1 m3 of CO2 for 1 m3 of CH4 produced, and that only one percent of the resource can be recovered, a substantial 480 – 2300 Bcm of CO2, corresponding to 0.9 to 4.35 Gton CO2, could be stored. In this paper we further analyze the developments, advances and issues related to methods that are in practice and authors will provide alternative technologies that are given a fresh look toward the classical research.
2. Shale Gas Resources in the U.S. and Worldwide
In 2000 shale gas provided only 1% of U.S. natural gas production; by 2010 it is over 20% and the U.S. government’s Energy Information Administration predicts that by 2035, 48.9% of the United States’ natural gas supply will come from shale gas. These trends obviously demonstrate that we can expect that shale gas will greatly expand worldwide energy supply.
Further, China is estimated to have the world’s largest shale gas reserves. Some researches show that shale gas production in the USA and Canada could help Europe to be less dependent to Russian and Persian Gulf countries. Figure 1 shows that unconventional resources (Shale Gas, Tight Gas, Coalbed Methane) containing hydrocarbon are almost all over the world. The most common form of unconventional natural gas is shale gas. In this study, the main focus will be on storage and sequestration in shale gas.
Figure 1. Worldwide shale gas resources.
3. Shale Reservoirs Modeling and Simulation
Reservoir simulation and modeling of unconventional resources have been given much more attention over the past years. Many numerical and analytical models have been developed and extensive reservoir studies have been conducted. Commercial reservoir simulators are also improved to handle and capture fluid flow behavior and natural gas production from unconventional assets, such as shale. However, developing an unconventional reservoir model that accounts for all pressure dependent phenomena and integrates all physics incorporated in gas flow for tight formations is still a challenging target for the petroleum industry. Among analytical and semi- analytical methods, works done by   have provided comprehensive progress in the modeling of shale gas reservoirs. Many authors, such as   , have used numerical simulation techniques in order to model different aspects of unconventional shale reservoirs.
Analytical reservoir models are widely employed because of their relative simplicity compared to numerical approaches. The aim of analytical solutions is to provide a simple solution which covers the fundamental physics of the phenomenon. In order to accomplish these goals, the analytical solution must have some simplifying assumptions. Having constant and/or homogeneous rock and fluid properties (density, compressibility, permeability, and viscosity) are the common assumptions for analytical solutions. The purpose of these assumptions is to linearize the partial differential equations arising from modeling of fluid flow in a reservoir; in our case, the diffusion equation. There are two methods in the literature customarily preferred for analytical fluid flow solutions: one is proposed by   , which has solution in time domain; while the other techniques are introduced by  and use Laplace transformation for solutions. Following these two solution techniques, researchers have proposed analytical solutions for hydraulically fractured unconventional reservoirs  . Analytical reservoir models are applicable for quite accurate, simple and robust simulations to evaluate production performances of hydraulically fractured unconventional reservoirs.
Eshkalak et al. investigated the geo-mechanical properties of Marcellus shale. They generated a common source for securing these rock mechanical properties, geomechanical well logs, and studied various characteristics, such as minimum horizontal stress, young, bulk shear modulus, as well as poison’s ratio that play an important role in defining the stress profiles of an unconventional reservoir.
Moreover, having an access to rock geomechanical properties will enhance the understanding of the parameters, such as conductivity and pressure dependency of permeability  –  .
In Figure 2, the natural gas production for past years and projections for following years are given. The shale gas is the only form of natural gas whose production amount is increasing. All other types of natural gas are either decreasing or remaining constant. It can be easily said that in the future the USA and world natural gas production is the shale gas, the Y axis is trillion cubic feet  .
Figure 2. Natural gas production outlook.
Generally, two-phase fluid flow of water and gas in a dual-permeability model is considered in constructing the geologic model of shale gas reservoirs. The dual-permeability model considers the intercommunication between the inter-granular void spaces in contrast to the dual-porosity model. Also, this model considers flow in two domains including the matrix and fracture. This model allows the transfer of both gas and water between the matrix and fracture domains, gas velocity in the matrix and fracture domains is calculated with the Equations (1) and (2):
where vg is gas velocity, Kg is gas permeability, Dg is gas diffusivity, Pg is gas pressure, Cg is gas con centration, and µg is gas viscosity. Subscripts m and f represent matrix and fracture domains. Velocity of the water flowing in matrix and fracture are determined with Equations (3) and (4), respectively:
where, vw is water velocity, Kw is water permeability, Pw is water pressure, and µw is water viscosity.
3.1. Flow in Matrix
The equations of gas transport thus are simplified for matrix domain as shown in Equation (5):
where Z is the gas compressibility factor, R is the gas constant, T is temperature, M is gas molecular weight, and qg is gas mass flow rate per unit matrix-block volume. Subscript m and f represent the exchange between matrix and fracture. For the water phase, the same equation is shown in Equation (6):
where Φm is matrix porosity, sw is water saturation, and Bw is water compressibility factor.
3.2. Flow in Fracture
After some manipulation and simplifications, the gas flow governing equation in fracture becomes as the following Equation (7):
For the water phase, Equation (8) represents the related formula.
Equations (9) to (12) represent the auxiliary relations used in the solution method.