It is often the large IOCs that bring their technological expertise to bear on developing difficult, sour gas fields. With the relative number of contaminated fields increasing, unproduced gas heavily contaminated with CO2 amounts to roughly 21% of the world’s total reserves.

Figure 1. Percentage of total recoverable gas with contamination greater than 15% plotted as a function of the year of discovery. (Images courtesy of Shell)
Among the many reasons for the industry’s heightened interest in contaminated gas is the fact that many recently discovered gas reservoirs are contaminated and that significant quantities of recoverable gas lie in reservoirs that also contain CO2 and H2S. In addition, there is a new emphasis on existing but undeveloped resources.

The absence of an overview that addresses the global contaminated gas resource base led Shell to conduct its own assessment.

Data source and criteria

The study that was undertaken used the IHS Energy Data Information Navigator (EDIN) database on worldwide reported gas reservoirs. Although the database has limitations, it is extensive and is considered reliable.

The following criteria were used to select gas fields from the database.

• Fields had to contain more than 0.5 Tcf of total recoverable gas in place;
• For associated gas, the gas-to-oil ratio had to exceed 1,000; and
• Fields for which there exist no specific reports for CO2 and H2S concentrations were excluded. (Recommended specifically by IHS; a missing entry does not necessarily mean a zero value.)

The total amount of gas (methane) before any of these criteria were applied was 5,890 Tcf. Applying the first two criteria reduced the amount to 4,840 Tcf. Applying the third criterion reduced the amount to 2,220 Tcf.

The results

To address the questions that motivated the study, it was necessary to discover the volume of recoverable gas at particular levels of contamination. For each discovery year, the Shell team calculated the percentage of total recoverable volume lying in reservoirs where total contamination is greater than 15%. With this number in hand, it was possible to calculate the fraction of contaminated gas discovered in any given year.

Figure 1a shows the results. The graph shows a rise in the levels of contamination in the period since the 1960s, with the trend especially pronounced in the 1970s and early 1980s. Since that point, the amount of contaminated gas discovered dropped and now hovers at slightly less than 20%.

The trouble with this type of graph is that it shows that the total amount discovered in a given year varies considerably from one year to another. To correct for this, the team defined a weighting factor, which is the fraction of gas discovered in any year against total gas discovered. This factor was used to calculate the normalized fraction of contaminated gas.
The results, shown in Figure 1b, reveal an even more pronounced trend. After a spike in the ’70s and early ’80s, the level of contaminated gas discovered has decreased significantly, stabilizing at a slightly higher level in comparison to the one prior to the spike. In both Figure 1a and 1b, a five-year moving average also more clearly showcases the underlying trends.

Another way of presenting this data is to look at volume divided by type into gas containing CO2 and gas containing H2S (Figure 2). The categories in the graphic correspond to the different techniques required to treat the gas (i.e., bulk removal of contaminants, scavenger removal, etc.). While a treatment decision is not based solely on the amount of CO2 and H2S, the categories allow for first-order classification.

The results indicate that only about 5% of the world’s gas is extremely contaminated (>15 % contamination by either CO2 or H2S). At least 62% (categories C and 1) of the contaminated gas is eminently treatable using existing solvent processes.
Figure 2. Available world gas categorized based on the level of CO2 and H2S contamination. (ALL = % of world gas volume in category, PP = % of pre-production world gas volume in category).


The only ambiguous category is Category 3 — CO2<15 and H2S =1-15 (category 3) — which corresponds to 30% of gas. Even this category will never have contamination levels greater than 30%. By definition, most of this gas exhibits contamination levels closer to 15% than 30% and is treatable with traditional methods. Although this gas can be treatable (like C and 1), there is a question of whether treatment can be carried out economically in a world where greenhouse gases and sulfur mountains are no longer acceptable and where additional costs have to be borne to solve the disposal issue.

Preproduction

Figure 3 categorizes gas that is not yet in production (i.e., fields in development or fields where there are no plans for production). This segment of the market contains about 267 Tcf (for which data is available in the IHS EDIN database). Roughly 5.5% of the gas meets the criteria.

Even though there are some noticeable differences between the overall picture and the one for gas fields not yet in production, a similar trend persists. About 68% of the gas fits into the easy-to-treat categories. The most noticeable difference is the fact that the high CO2 category (with low H2S) contains significantly more gas (about 21%). This might justify current efforts geared at producing reserves with high CO2 contamination.

Large reservoirs

A previous Shell study identified a number of fields that could be commercially viable using bulk CO2 removal technology. The study found that there was a substantial amount of contaminated gas in large fields that previously had not been considered because suitable technology was not available.

Figure 3 shows the distribution of contaminated gas as a function of the field size. There is no correlation between the level of contamination and the size of the field. In fact, according to our definition, the three largest fields, which account for about 61% of the gas, are not contaminated. This does not, however, negate the fact that there are large individual contaminated fields that, until now, have been unexploited because of the lack of a suitable technology.
Figure 3. Percentage of total gas volume lying in contaminated reservoirs as a function of the size of the field. (The upper box above each column represents the number of fields. The lower box shows the percentage of total gas in each category.)


Reading the results

There are large quantities of gas in contaminated environments, and many contaminated fields have never been logged because they were not considered to have development potential. New technology for contaminated fields is thus of interest.
Within the reported data, there are considerably more contaminated fields that are not producing, presumably because no economic method is available. In the high CO2 category, for example, the amount of gas that is not being produced is six times greater than the amount in production. The good news is that higher oil prices change the playing field; so what was uneconomic in the past is not necessarily uneconomic in the present.

New technologies

The picture changes as new technology becomes available. And improving recoverability via new technology is now the focus of Shell’s contaminated gas effort.

One new technology is condensed contaminant centrifugal separation, or c3-sep. This consists of a two step process. The first step is for pressurized gas to be expanded isentropically to the two-phase thermodynamic region on the p, T diagram where droplets of CO2-rich liquid form.

In the second step, the droplets are separated. Although the droplets are too small to separate using traditional cyclone technologies, they can be removed by a new centrifugal device called a rotational particle separator — a rotating bundle of straws that accelerates agglomeration of the droplets. This results in larger, easily removed droplets downstream of the device.

The innovative physics and chemistry of this technology enables contaminants to be compactly separated in the liquid phase, meaning that expensive compression is not required and the contaminants can easily be sequestered by pumping them back into the reservoir from which they came.