Technical+Background

 **Process Description**


Image courtesy: http://3.bp.blogspot.com/_nzjabESsGNY/RtJxLr2MNTI/AAAAAAAAAC8/K_MclwXT5XY/s400/in%2BSitu.jpeg

**Brief overview of process:**

Toe to Heel Air Injection (THAI) is an integration of injection well and combustion processes with a target to recover about 70- 80 % of heavy oil from unconventional reservoirs. It is a radically new process that uses in-situ combustion and is classed as an enhanced oil recovery process (EOR). Essentially, THAI consists of a vertical injection well located at the ‘toe’ of the horizontal well and a lateral producer that frames the heel to thermally recover the mobilized oil.

What makes the THAI process unique from conventional in-situ recovery processes is that the primary source of energy required for thermal recovery comes by burning approximately 10% of the initial oil in place. The step comprises of a series of steps that is carried out sequentially. Firstly, superheated steam is forced through the vertical well (injector) to bring bitumen’s temperature to its ignition point (400 – 600 degrees centigrade) and consequently, without a delay, compressed air under high pressure is propagated through the same vertical well to stimulate combustion. [1] Under these conditions, controlled combustion (with the consumption of 10% bitumen) of the bitumen can be compared to charcoal fire which produces enough heat but no flames. Heat and combustion gases aid in displacing the heat wave and consequently mobilizing the oil towards the heel of the horizontal producer into the production wells.

The mobilization of oil into the horizontal producer has significant technical implications. THAI employs the use of a catalytic process labeled THAI-CAPRI that allows for the upgrading of the oil downhole due to a radial inflow into the producer. Details related to the catalytic upgrading are discussed below. Some of the pilot projects have also shown evidence of production without employing artificial lift mechanisms such as pumping. The current oil recovery rate is between 70-80% (target rate) and methods to optimize the process further are under development.[1]


 * THAI: A Global Energy Solution **

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**THAI-CAPRI In-situ Catalytic Upgrading:**

**CAPRI** or **CA**talytic upgrading **PR**ocess **I**n-situ is the extension to THAI that aims at achieving further heavy oil upgrading in situ. This process is intended to upgrade the thermally cracked heavy oil that is mobilized into the exposed section of the horizontal producer well. The process itself employs the use of an annular layer of solid catalyst embedded into the perforated horizontal producer. The perforated producer allows for radial inflow upgrading (resembles a downhole catalytic reactor) of the thermally cracked heavy oil from the coke and mobile oil zone (MOZ). The MOZ contains steam, oil and a syngas combustion mixture including CO(g) and unburnt O 2 (g) as a residue from the gasification process.[2]

Image courtesy of: http://www.oilsandsdevelopers.ca/index.php/oil-sands-technologies/in-situ/the-process-2/toe-to-heel-air-injection-thai/

Image Courtesy of: [1]

The radial inflow of the MOZ fluids to the solid catalyst are carried out at high temperatures (400-600 *C) and at high reservoir pressures (generally 30 – 50 bar). This increases the versatility of the THAI-CAPRI processes.

The upgrading of the oil occurs through a series of carbon rejection reactions (based on thermal cracking) and the addition of hydrogen at the surface of the catalyst (process is known as hydroconversion on a hydrotreating catalyst). The source of the hydrogen is a result of the gasification process that was executed in the combustion front of the air and water injection phase of production. The source of hydroconversion on the catalyst is also attributed to several water-gas shift reactions which are currently under research. Although significant information regarding the preparation and operation of the heterogeneous catalyst is patented; a comprehensive attempt is made to study the catalytic process involved.

The following is a list of the reactions used in the catalytic upgrading process as described by Xia and Greaves (2001):

**(1) Pyrolysis – thermal cracking** Heavy oil residue > Light oil + Coke

The purpose of this step is to encourage efficient upgrading of the heavy oil prior to its treatment with the hydrotreating (HDS) catalyst. This step is carried out in the absence of oxygen (pre-combustion phase) and involves the “cracking” of heavier hydrocarbons down to lighter modules that can be efficiently upgraded.

**(2) Oxidation of coke** Coke + O 2 > CO+CO 2 +H 2 O

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**(3) Oxidation of heavy residue** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Heavy residue +O 2 ---> CO +CO 2 +H 2 O

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Steps (2) and (3) are a result of the incomplete combustion and is carried out at combustion front of THAI-CAPRI process. The products (specifically CO2 and H20 will be used subsequently in the gasification and water gas shift carried out during the in-situ upgrading.

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**(4) Carbon Rejection (Upgrading step)** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">CHx > CHx1 + C (x1>x)

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">This step is carried out at high temperatures as well in the presence of the HDS (hydrotreating/ hydrodesulfurization) solid catalyst. The purpose of this step is to saturate the carbon hydrogen bonds into more desired and value added feedstock to cater the process industry.

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**(5) Gasification of hydrocarbon** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">CHx --> C+x/2H2 <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">C+H 2 O(g) ---> CO + H 2 <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">C+CO 2 > CO

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**(6) Water-gas shift** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">CO +H 2 O<>CO 2 +H 2

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Steps (5) and (6) are used to produce hydrogen required for another upgrading step that is carried out simultaneously with the carbon rejection step. The details of this step is outlined in step (7)

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**(7) Hydrogen Addition (Reductive Upgrading step)** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">CHx +H 2 --> CHx1 (x1>x)

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">This step essentially serves the same purpose of as step (4) however it employs the reduction of the light oil components to upgrade. The hydrogen used in this step results from the products of the gasification reaction in step (5).



<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**CoMo - HDS (hydrotreating/ hydrodesulfurization) Solid Catalyst**



Image courtesy of petrobank.com

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Although significant research on optimizing the CAPRI process has been underway for the past decade, the surface characteristics and catalytic behavior of HDS is not fully understood in the THAI-CAPRI application. The catalyst consists of an alumina base (Al2O3) impregnated with cobalt and molybdenum (CoMo) - sometimes in combination with nickel NiMo. The hydrotreating process occurs after the adsorption of the molecule on the surface of the catalyst (light oil components) on a coordinatively unsaturated molybdenum atom (CUSMO).The catalyst process during the radial inflow face undergoes physisorption, chemisorption, catalyzed reaction and consequently desorption into the producer.[3]

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Characteristics of HDS Catalyst:
<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">(1) **Selectivity** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The higher the unsaturation of the Mo complex, the catalyst has proven to be more selective towards hydrotreating and the addition of hydrogen at the adsorption sites

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">(2) **Stability** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">The catalyst specifically used for this type of application is highly stable in a wide range of operating conditions (under 600*C and 30-50 bar). Although this catalyst is used for a high temperature, high pressure application (HTHP) it can also be used in fairly low temperatures and high pressure conditions (LPHP).

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">(3) **Activity** <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">A limitation of the catalyst involves extending the catalyst lifetime as metal contaminants with heavy metals, and exceeding amounts of sulfur severely deactivates the CoMo surface sites consequently decreasing the extended lifetime of the catalyst. This could potentially limit the extended use CAPRI in sour settings. Currently, several studies are being carried out on regenerating and boosting the activity of the HDS catalyst. [2]

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">In essence the principles of the heavy oil upgrading process is indifferent from many of surface upgrading counterparts however, the unique economic and environmental contributions which is brought as a result of the innovative THAI-CAPRI will be discussed later in this report.

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Click Here for Next Page: Environmental and Economic Impact


 * References:**

[1] Cobb,K;Scitizen:Will Toe-to-heel air injection extend the oil age? []

[2] <span style="background-color: #ffffff; font-family: 'Times New Roman'; font-size: 12px; line-height: 17px; text-decoration: none; vertical-align: baseline;">Shah A, Wood J, Fishwick R, Greaves M, Rigby S. In-Situ Up-Grading of Heavy Oil/ Natural Bitumen: CAPRI Process Optimization. Department of Chemical Engineering; University of Birmingham. IOR, Department of Chemical Engineering, University of Bath [Online] 2007,11, 383-393 http://archivos.labcontrol.cl/wcce8/offline/techsched/manuscripts%5Cqrc251.pdf (acc. Sep 28, 2011).

[3] Portele; Grange; Delmon; Online Wiley Library. [Online] **<span style="color: #404040; font-family: verdana,arial,helvetica,sans-serif; font-size: 11px;">2005, **//<span style="color: #404040; font-family: verdana,arial,helvetica,sans-serif; font-size: 11px;">12 //, Article Archive. http://onlinelibrary.wiley.com/doi/10.1002/bscb.19911001125/abstract (accessed Nov 11, 2011)