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| Fig. 1: An oil drilling rig in Alberta, Canada. (Source: Wikimedia Commons) |
Oil exploration and production is a highly lucrative, but highly uncertain business. Despite reassuring geological data and perfect drilling execution, sometimes the oil reservoir is dry or uneconomical to produce. There is nothing oil companies can do in the first case except gain a better understanding of the field, perhaps to make analogies to similar fields. With the advancement of enhanced oil recovery (EOR) technology in recent years, some reservoirs that fall under the second scenario are becoming profitable. Downhole catalysis is an example of this new EOR technology that allows for the recovery of high viscosity, high density heavy crude oil after breaking the native compounds to smaller, lower molecular weight oil or gas molecules using catalysts.
This process involves in-situ catalysis to upgrade heavy crude to lighter products. By doing so, the downhole catalysts perform almost the same function as catalytic crackers do in conventional oil refining processes. As in traditional upgrading with catalytic cracking, crude oil is reacted with hydrogen gas on the surface of the catalysts. The biggest difference is that downhole upgrading must also handle impurities that can result in high steam partial pressures from evaporated, naturally-occurring downhole brine, and low hydrogen partial pressures due to relative lack of hydrogen downhole. In isolation, each step of the downhole upgrading process has been proven to be successful but consistent replication of the entire start-up process has been difficult. [1] To date, downhole catalyst upgrading has seen limited commercial application.
Theoretically, producers must first inject catalysts into the wellbore. This can be done by traditional gravel packing method or by proppant injection. A gravel pack is a fairly common method of downhole filtration designed to prevent the flow of formation sand into the wellbore through the perforations by inserting gravel in a downhole mesh screen. [2] Using almost identical technology, it is possible to simply substitute gravel for the desired catalyst. The second method, proppant injection, mimics a step in hydraulic fracturing in which material such as sand is injected into the formation itself, not just the wellbore, in order to keep the induced fracture open. Again, similar technologies can be used with catalyst substituted for sand. Upon placement of the catalyst, there are a few options in order to raise the temperature in the area around the wellbore to induce the reaction. One option includes the placement of electrical heating elements, another involves the cyclic injection of steam into the formation from a neighboring injector well. [3] The former is a relatively straightforward but expensive process that would accompany the gravel pack method. The latter involves a fairly well-understood process that is already implemented in many heavy crude fields worldwide. As the formation begins to increase in temperature, the well is typically shut-in so allow for the breakdown of the heavy crude.
As mentioned, practical application has been limited, but both laboratory and field applications have been promising. In laboratory experiment using heavy crude from the Llancanelo oil field in Argentina with Ni/Mo catalysts, the oil API gravity (a measure of oil density relative to water) increased from 15°C to 24°C when measured at room temperature, meaning that the oil became significantly less dense and thus easier to process downstream. The Llancanelo oil also experienced viscosity reduction from 40°C from 10460 to 10 mPa-sec, meaning it became much easier to induce flow in the lighter hydrocarbons by three orders of magnitude at the measured temperature. [4]
Downhole catalysis is not without its challenges, however. Not all upstream oil companies, especially small independents, have the capital, manpower, or the research capacity to implement a relatively untested technology. They simply have different risk profiles. In terms of the technology itself, catalysts become "spent" it can be expensive to fund downhole electric heaters, and not all reservoir conditions are favorable. Catalysts are designed to only temporarily interact with molecules to lower the activation energy of a certain reaction. However, sometimes molecules can inhibit or poison a catalyst, meaning that the offending molecule does not unbind from the catalyst. Catalyst poisoning rates are increased with pressure, which is especially problematic as the downhole brine creates high steam partial pressures when heating the catalysts. Poisoning reduces the catalytic activity and thus the reaction's speed. To address this issue, the catalysts must be refreshed either by physically replacing the catalysts in place, or by regenerating the catalysts by forcing the bound molecules off the catalyst surface. This process of catalyst recovery and regeneration may be too complex or costly if well geometry is complex. Perhaps catalysis is best used in wells placed in reservoirs that are expected to drain before the catalysts become unusable.
Lastly, pressure can cause more problems than catalyst poisoning: not all reservoirs can withstand the high pressures induced by heating the catalysts. However, this problem is not a new one. Heavy crude producers must take this into consideration if they intend to implement steam flooding technology to induce flow in heavy crude. Diatomite reservoirs, for instance, can undergo rearrangement upon introduction to steam and pressure, so extra care must be taken to not cause damage to the rock formation. Damage to the formation could result in damage to the wellbore near the formation or to above-ground equipment.
Although similar upgrading techniques have been applied to the oil sands, which are treated more like mining operations by oil producers, there is still room for research and improvement on the recovery of high viscosity, high density crude by combining pre-existing thermal stimulation techniques with catalyst technology. In the current economic environment however, it seems that the majority of research on this topic will be confined to university laboratories until the price and demand for crude oil create an environment more favorable to increased industry funding of EOR research.
© Thomas Logan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. G. Moore et al., "A Downhole Catalytic Upgrading Process for Heavy Oil Using In Situ Combustion," One Petro PETSOC-99-13-44, J Can. Petrol. Technol. 38, No. 13, 44 (1999).
[2] R. J. Saucier, "Considerations in Gravel Pack Design," One Petro SPE-4030-PA, J. Petrol. Technol. 26, 205, (1974).
[3] M. B. Abuhesa and R. Hughes, "Comparison of Conventional and Catalytic in Situ Combustion Processes for Oil Recovery," Energ. Fuel. 23, 186 (2009).
[4] I. D. Gates et al., "In Situ Upgrading of Llancanelo Heavy Oil Using In Situ Combustion and a Downhole Catalyst Bed," One Petro PETSOC-08-09-23, J, Can. Petrol. Technol. 47, No. 9, 23 (2008).
