EXPLORATION Benzene content of subsurface brines can indicate proximity of oil, gas

Stephen G. Burtell, Victor T. Jones III
Exploration Technologies Inc.
Houston

Over the years geochemists have explored the application of brine benzene analysis as a proximity indicator of petroleum deposits.¹ Such studies have been shown to be highly effective in confirming the approximate distance from a dry hole to a reservoir in the same hydrogeologic system by mapping the plume of dissolved petroleum constituents emanating from the nearby accumulation.²

Major sources of error in these initial studies were due to the difficulty in storing, handling, and analyzing brine samples without loss or fractionation of volatile constituents. With the advent of standardized environmental sampling and analytical techniques, brine analysis for benzene as well as ethylbenzene, toluene, and xylene (BTEX) aromatics is now routine and reliable. This relatively low cost environmental analytical method, assisted by next-day air courier service, can now be effectively applied to domestic and international petroleum exploration drilling programs as an economical formation evaluation tool.

Background

Extensive reservoir fluid compositional studies and research have shown that soluble aromatic hydrocarbons, such as, benzene, toluene, and xylenes make up a large proportion of the dissolved hydrocarbons found in brines associated with oil reservoirs.³

Fig. 1 [13000 bytes] and Table 1 [24000 bytes] show a comparison of oil and co-produced brine from Buccaneer oil and gas field on Galveston Blocks 288 and 296 in the Gulf of Mexico. As can be seen from these illustrations, BTEX aromatic hydrocarbons are the primary volatile liquid hydrocarbons dissolved in the brines due to their high solubility in water. As summarized in Table 2 [44000 bytes], empirical studies by Zarrella et al.² indicate that the benzene content of brines from typical oil reservoirs ranges from about 5 to 20 ppm depending on oil composition and source.

Benzene and related aromatic constituents, which are in chemical equilibrium with adjacent oil accumulations, emanate from the vicinity of the reservoirs, forming a plume of decreasing magnitude with increased distance from the reservoir edge. Research by Zarrella et al.² has confirmed that the distance to an oil reservoir is directly proportional to the log of the benzene concentration of the adjacent brines. This relationship, as plotted on Fig. 2, [9000 bytes] is due to solubility and diffusion factors.

Analyses of benzene concentrations in brine samples from non-productive exploration wells can therefore be used to predict the distance to a nearby petroleum accumulation with a reasonable level of confidence. With tests from the same formation, in two or more nearby wells, results can often be used to provide a much more accurate prediction of the distance and direction to potential undiscovered reservoirs within the stratigraphic horizon tested.

As a data base of local wells is developed, the benzene magnitude to distance relationship can be further refined for a specific basin or formation of interest, improving the quality of the distance predictions.

This exploration method was used extensively by the Gulf Oil Corp. in the late 1960s and 1970s and has been discussed in several books on petroleum geochemistry including Hunt⁴ and Collins.¹

Applications

Brine analysis for dissolved aromatic hydrocarbons can be applied in new field exploration programs, development drilling, and predicting extensions of existing fields.

New field exploration. Brine sample analysis can be used to confirm if, despite an apparent dry hole, the formation has an oil accumulation within a radius of up to about 12 miles (20 km).

This information is extremely valuable when assessing the results of rank wildcat areas and international concessions, when decisions must be made with respect to the general prospectiveness of a lease position, in addition to its obvious use in selecting step-out locations and for evaluation of similar structures in the basin. Results provide a lease evaluation tool by predicting the presence or absence of petroleum within a known radius of an exploration well location.

Development drilling. During development and step-out drilling, brine analyses can be used to help confirm the location of a well with respect to the edge of the reservoir zone. If samples are collected from each well during initial production and testing it may be possible to reduce the number of dry holes by helping to vector step-outs toward areas of higher magnitude benzene content, which would be indicative of more economical reservoir zones.

Field extensions. Extensions to existing fields may be predicted by comparing brine samples from groups of producing wells. As fluids are extracted, formation waters from adjacent areas are drawn to the producing wells. If the brine is drawn from barren areas, the benzene content will continue to decrease over the life of the well.

Wells that draw brine from beneath adjacent, undrained accumulations or untested extension areas in the field often show consistently higher benzene content, since produced brines from these wells were in contact with fresh, unproduced petroleum deposits. This technique is also useful to confirm whether individual wells are influenced by water injection or other secondary and tertiary recovery operations that would alter the magnitude and composition of indigenous formation brines.

Sample collection

Brine samples from drill stem tests, flow tests, or repeat formation testers can be acquired using standard environmental sampling methods by collecting duplicate 1.35 oz (40 ml VOA) sample jars from each test interval.

Samples are collected with no headspace and stored on ice or refrigerated to a temperature of about 5° C. Care should be taken to obtain samples with minimal drilling fluid dilution or contamination from petroleum products used during the drilling operation.

In existing wells on production it is better to obtain a sample, if possible, from the flow line close to the pump rather than from the stand tank. Well fluid samples should be collected into a clean separation funnel and allowed to phase separate for about 5 min. Water samples for analysis are then drawn from the bottom of the funnel and placed into sample jars.

It is very important that good quality brine samples, without appreciable free oil, be collected to reduce the possibility of alteration during sample handling and transport.

Samples can be delivered directly to the laboratory in coolers or sent by air courier on ice in insulated shipping containers. With proper planning and documentation, samples can easily be shipped or hand carried from international locations without significant reduction in sample quality.

Analytical approach

Samples are extracted by US-EPA method 5030, purge and trap followed by analysis using US-EPA method 602, by photoionization detector (PID) gas chromatography for benzene, toluene, ethylbenzene, and xylene content. This standard analytical method, which is routinely applied for ground water contamination studies, can be performed by most environmental testing laboratories.

Typically the method has an analytical detection limit of about 1 microliter per liter (ppb). Samples can often be analyzed within 24 hr of receipt in the laboratory and results sent by fax or e-mail immediately after analysis. Confirmation samples from each interval are recommended to help insure reliable results and interpretation.

If properly preserved during transport and storage, samples can be analyzed up to 14 days from the date of collection with little or no loss of volatile components. Due to loss of volatiles with time, samples stored for extended periods or at room temperature cannot generally be analyzed for reliable results and therefore must be interpreted with caution.

Total dissolved solids (TDS) are also measured on brine samples to help determine solubility factors of organics in the formation fluids. Additional analyses for iodine and other commonly used proximity indicators can also be completed as available sample volume allows.

Data interpretation

As documented by Zarrella et al.² dissolved benzene magnitudes, when plotted versus distance from oil accumulations, plot as a straight line on a semi-log scale (Fig. 2 [9000 bytes].). Test results are posted onto this line to provide an estimate of the distance to an adjacent reservoir.

As a data base is developed for a specific geographical area more precise predictions of distance may be possible. Once a prediction is made it is up to the exploration team to evaluate the results in light of their knowledge of the area's local geologic framework, nearby production, and other exploration test results. Depending on the availability of detailed geologic control from surrounding areas it may be possible to further reduce the area to be searched by eliminating areas around the well that have been previously tested.

Analytical results for benzene can also be compared and confirmed by toluene, ethylbenzene, and xylene data. These compounds, which also have appreciable solubilities in oil field brines (Fig. 1 [13000 bytes] and Table 1 [24000 bytes]), occur in varying proportions in different reservoirs and can therefore serve as independent parameters for evaluation. As noted by Collins¹ typically a benzene to toluene ratio of greater than 1 can be expected for brines adjacent to petroleum accumulations.

While toluene may be present in greater quantities than benzene in the oil (Table 1 [24000 bytes]), benzene is approximately three times more soluble in water or brine as toluene.⁵ Therefore due to diffusion factors the ratio of benzene to toluene in brine adjacent to a reservoir should increase from 1 with distance from the reservoir. The presence and concentration of the other alkylbenzenes further confirms the proximity to a potential reservoir.

Limitations

The benzene content of a brine versus the distance to a potential reservoir is dependent on a number of factors that may limit application of the tool to specific geologic environments. Therefore the following limitations must be considered when applying this method:

The amount of benzene in a particular oil and the associated brines may vary significantly from about 5 ppm to 20 ppm depending on oil composition, salinity, temperature, pressure, and hydrogeologic environment.^6-7 Therefore it is important that these factors be considered when making a prediction of distance to a potential reservoir. As data are accumulated for a specific basin or formation, a more precise proximity curve can be calculated to replace the generalized curve used in Fig. 2 [9000 bytes].

Samples must be reasonably fresh and collected and stored properly to help insure reliable results. Therefore older samples from plugged and abandoned wells are probably not useful for re-evaluation.

The use of the method may be limited in areas where oil pools may be hydraulically isolated by lithologic changes or other permeability barriers. In such cases results can only be applied to the immediate area with hydraulic communication.

Case studies

Summaries of several case studies performed by Gulf Oil were presented by Gulf Research & Development Co. scientists Pirkle, Hager, and Jones⁸ and Jones.⁹ Over a period of years various Gulf Oil divisions kept records of the predicted distances to petroleum reservoirs and actual distance to subsequent discoveries.

Table 3 [21000 bytes] includes a list of petroleum discoveries near to 24 benzene anomalies from West Texas, Oklahoma, New Mexico, Mississippi, Gulf of Mexico, and Alberta. As can be seen from these results, the distance to a potential new field discovery can be reliably predicted from benzene analysis in a wide variety of environments, reservoir types and basins.

In the Alberta basin of Canada, where this tool was applied extensively for many years, records of the benzene content of Nisku and Leduc formation brines in exploration wells versus the distance to production proved very useful. As summarized in Fig. 3 [18000 bytes], this observed relationship correlates extremely well with predicted benzene magnitudes as calculated using a diffusion model for benzene migration from typical Nisku reservoirs.

This model for the Nisku formation was tested in the Stettlers region of Alberta, where anomalous benzene magnitudes were identified in two non-productive exploration wells located about 4 and 5 miles from the nearest known Nisku production. Brine benzene contents for these wells of 4.4 and 5.0 ppm plot well off the calculated magnitude to distance correlation line (Fig. 4).

These results were used to predict the presence of an undiscovered accumulation at a radius of 1 to 2 miles from the dry holes. Later drilling confirmed this prediction with the discovery of a new Nisku field a distance of about 2 miles from the test wells. This field has an area of about 12 sq miles with a 20 ft thick oil pay. Fig. 4 shows the new field boundaries with respect to the original test wells and confirms the dissolved brine benzene content in these wells now plot near to the predicted correlation line.

Conclusions

Analysis of dissolved benzene and BTEX hydrocarbons in formation water samples from exploration and development wells provides a reliable means of predicting the presence of and distance to petroleum deposits in adjacent, hydrologically equivalent areas.

When evaluated in light of prevailing geologic conditions, this information is invaluable in both frontier and development areas, since the results can not only reduce the area to be searched but can significantly increase the information gained from an exploration test.

References

1. Collins, A.G., Geochemistry of oilfield waters, Elsevier Scientific Publishing Co., New York, 1975, pp. 314-315.

2. Zarrella, W.M., Mousseau, R.J., Coggeshall, N.D., Norris, M.S., and Schrayer, G.J., Analysis and significance of hydrocarbons in subsurface brines, Geochimica et Cosmochimica Acta, Vol. 31, 1967, pp. 1,155-66.

3. Weisenburg, D.A., Bodennec, G., and Brooks, J.M., Volatile liquid hydrocarbons around a production platform in the northwest Gulf of Mexico, Bulletin Environmental Contamination Toxicology, Vol. 27, 1981, pp. 167-174.

4. Hunt, J.M., Petroleum geochemistry and geology, W.H. Freeman & Co., San Francisco, 1979, pp. 448-450.

5. Callahan, M.A., Slimak, M.W., Gabel, N.W., May, I.P., Fowler, C.F., Freed, J.R., Jennings, P., Durfee, R.L., Whitmore, F.C, Maestri, B., Mabey, W.R., Holt, B.R., and Gould, C., Water related fate of 129 priority pollutants, Volume II: Halogenated aliphatic hydrocarbons, halogenated ethers, monocyclic aromatics, phthalates esters, polycyclic aromatic hydrocarbons, nitroamines, and miscellaneous compounds, published by U.S. Environmental Protection Agency, Washington D.C., 1979, pp. 71-1 to 71-9 and 80-1 to 80-7.

6. Bennett, B., Dale, J.D., Bowler, B., and Larter, S.R., A rapid method for the assessment of subsurface oil-water partition behavior for phenols and BTEX in petroleum systems, presented at the 211th American Chemical Society meeting, Division of Geochemistry, New Orleans, March 1996.

7. Dale, J.D., Aplin, A.C., Larter, S.R., Bennett, B., and Macleod, Controls on the distribution of alkylphenols and BTEX in oilfield waters, presented at the 211th ACS meeting, Division of Geochemistry, New Orleans, March 1996.

8. Pirkle, R.J., Hager, R.N., and Jones, V.T., Second derivative absorption spectroscopic determination of benzene and toluene at the wellsite, presented at the 187th ACS national meeting, St. Louis, April 1984.

9. Jones, V.T., Overview of geochemical exploration technology, 1984, Short Course on Geochemical Exploration Technology, Rocky Mountain Association of Geologists, Denver, January 1984.

The Authors