The souring of normally sweet production systems is a significant problem which can have implications to continued oilfield operations. Such problems are commonly approached by gathering of field sample and laboratory analysis or by simple test kits. This paper describes an alternative approach which includes the use of specialized field sampling and analysis procedures and portable equipment that can be move from site to site. A case study is presented that illustrates the use of these procedures and equipment.

Keywords: Souring, hydrogen sulfide, microbiology, biofilms, corrosion, MIC, field testing, water injection, oilfield, monitoring

The souring of normally sweet production systems is a significant problem. It can have implications in terms of (1) reduced quality of produced hydrocarbons relative, (2) the reduced productivity of wells, (3) increased corrosivity of produced fluids. In cases where remedial action is not taken, it can also have implications relative to the potential for sulfide stress cracking and selection of materials for downhole, flowlines and surface facilities. Therefore, it is important to be able to properly characterize field situations and make accurate recommendations for remedial actions to minimize the impact of souring and to prevent the occurrence of similar occurrences in other related field operations.

Field souring is often studied by either of two approaches: (1) field sampling followed by transportation of the samples to the laboratory for analysis, or (2) analysis directly in the field using simplified field test kits. In many cases, the field sampling/laboratory approach is difficult due to the remote nature of many onshore and offshore production and injection facilities and the inability to transport, maintain and analyze cultures. Additionally, most simple field tests lack the sensitivity required to properly assess, characterize and differentiate marginal cases. This paper discusses specialized procedures and equipment that have been developed specially for the field or in-plant investigation of souring, scaling, corrosion, fouling and bacterial action in water handling systems.

SYSTEM DESCRIPTION AND FIELD INVESTIGATION  Top

Figure 1 shows schematically the two water sources and systems used for injection in a particular middle eastern oil field for pressure maintenance (i.e., secondary recovery). The first system (Figure 1) obtains its water from a relatively shallow aquifer. It involves a header which collects water from several source wells that is then sent to the deaerator. Following deaeration, the water is pumped into the formation via a network of injection wells. The second system (Figure 1) collects water obtained from oilfield production. The production fluids are collected through a header followed by an oil/water separator. The oil goes to the pipeline and produced water is then re-injected in the producing formation. Both offer potential for reservoir souring and related operational problems from bacterial growth depending on (1) how they are maintained and operated, (2) the composition and quality the injected water and (3) the use of treatment chemical and deaeration.

The field operations under consideration, involved primarily oil production which was originally sweet which then experienced souring. The occurrence of souring appeared to correlate with the time of breakthrough of the injected water into the producing wells. This is often a common occurance.^1-3 It can also be related to bacteria.⁴ However, the exact source and conditions that promoted souring was not directly apparent which necessitated a careful field study. A complete survey of the injected water system was conducted for planktonic and sessile microorganisms and their growth characteristics as a function of water salinity and temperature. The results from the aquifer water system (System 1) were compared to those obtained from the analysis of the produced water re-injection system (System 2).

SURVEY OF PLANKTONIC MICROORGANISMS  Top

The planktonic survey was conducted with three types of growth medium: (1) Modified Hoechst Medium,⁵ (2) Postgate Medium B, and (3) Merck Universal Medium. The first two media were used for detection of sulfate reducing bacteria (SRB) while the third medium was used for general heterotropic (aerobic) bacteria (GHB). Each medium was prepared with the portable culture medium preparation equipment shown in Figure 2. This included a fully equipped carrying case which provided the ability to produce the culture media and to prepare broth bottles and incubate cultures in the field. Separate analyses were made in this study for water samples taken at various locations in both System 1 and 2. The portable equipment also allowed for the incubation of the cultures at two temperatures (38 C and 69 C). A total of 120 bottles were inoculated at selected sample points in the system.

Injection Water Planktonic Survey - System 1 did not show SRB at either the header or in the regions upstream or downstream of the deaerator. In the two injector wells sampled, the SRB populations were 50 - 500 cfu/ml and 5 - 50 cfu/ml. GHB were detected through the injection system in the amounts of 5 - 50 cfu/ml.

In System 2, the degree of SRB infection downstream of the booster pumps up to the injection wells was constant at levels of 5 - 50 cfu/ml. The GHB were found to be present at levels in the range of 5 - 50 cfu/ml to 50 - 500 cfu/ml. The data presented above indicate the there was a low level infection present throughout the water handling system primarily due to GHB. With the level of SRB infection noted, it was questionable whether any souring problems could have been sustained.

The salinity tolerance of the two source waters (i.e. System 1 and 2 waters) was investigated. SRB growth was found to occur in low (1%) salinity media prepared with the source water from System 1. Only limited growth was noted in the medium prepared with produced water in System 2 (which was of higher salinity) due to the low tolerance of the bacteria to this environment. On the contrary, GHB tend to tolerate the higher salinity common to the produced water. However, their numbers were still greater in the medium prepared with water obtained from the surface sources.The incubations conducted at 69 C showed GHB infections in the range of 2-20 to 5-50 cfu/ml. No generalized SRB growth was found in the media prepared with surface or produced water sampled from the locations indicated above. However, in a few occurrences at 69 C, positive indications for SRB were found (e.g. at one injection well). In this location, the planktonic inoculation showed growth in an SRB bottle with aquifer water of low salinity.

Planktonic Survey of Oil Production Wells - The incubation at 38 C showed a generalized low level growth of GHB. Only very low levels of SRB were detected in the oil production well. At this state in the investigation, it was concluded that planktonic SRB were extremely limited in the production wells. However, a general GHB infection was observed in the system.

SURVEY OF BIOFILMS AND MEASUREMENT OF TOTAL ACCUMULATED SOLIDS  Top

Studies of the amount and nature of accumulated biofilms on metal surfaces were also performed using the portable apparatus shown in Figure 2. The use of this equipment is shown in greater detail in Figure 3. It consists of a tube that contains ten metal coupons, each with an exposed surface area of 2 cm². The system was sampled at various locations and the water flowed through the annulus of the biofilm cell around the coupons. The flow velocities through the cell were controlled in the range of 0.5 to 1.5 m/s.

Growth and Detection of Biofilm Microbes - After exposure, the metal specimens were removed from the biocell and transferred to broth bottles provided with the portable equipment. Figure 3 gives an overview of the biofilm evaluation procedure and how each of the ten coupons were utilized in this evaluation. Two coupons were transferred directly to culture SRB growth at 38 C and 69 C. A third coupon was used to check for SRB growth at 38 C after a longer term exposure of 25 days. Three more coupons were removed to check the sessile population of biofilm from the metal surface by sonication. This treatment has been found to be successful in film removal without producing cell damage. Once the biofilm was dispersed in the solution, a serial dilution was conducted for determination of populations of SRB and GHB. With this technique the cfu per unit volume was related to the number of colonies formed per square centimeter of metal surface area from which the biofilm was obtained. Three samples were cultured; two at 38 C (included replicate) and one at 69 C.

Method for Determination of Total Accumulated Solids on Metal Surfaces - The total amount of accumulated solids and fouling on the coupons was obtained with the next three coupons. These coupons were sonicated in 2 mls of distilled water. The solids content of the water was determined by filtering the water through a 0.45 micrometer cellulose acetate filter. These membranes were dried at 60 C and then the weight increase determined. The final coupon was used for direct examination of the biofilms by scanning electron microscopy (SEM) and for X-ray microanalysis (EDS).

Results of Sessile Study in the Aquifer Water System - The testing and analysis procedure described in the previous section were used to conduct studies at three locations in the aquifer water system (System 1): (1) Header, (2) downstream of deaerator and (3) at a selected injection well.

Header Analysis. The bacterial infection at this location is shown in Table 1. The GHB content was very high (up to 5x10³ - 5x10⁴) with increasing infection after incubation at 69 C versus 38 C. No difference in infection intensity was noted for GHB for the two exposure periods used in this study. By comparison, the SRB showed infection only after incubation at the lower temperature (38 C) and was present in significant numbers which increased with exposure time. The amount of accumulated solids was also very high with the average accumulation rate of 1530 mg/m²/day. However, the SEM/EDS analysis of the surfaces revealed that the majority of the solids in this location were deposited clay materials and not related to the biofilm production.

Analysis Downstream of Deaerator. The bacterial infection at this location was predominantly due to GHB (See Table 2). It was somewhat more intense after incubation at the lower temperature and did not show an influence of exposure time. SRB production was low. The measurement of accumulated solids were much lower than that found at the header location with an accumulation rate of only 262 mg/m²/day. Based on the system design, these results indicate that the lower reading was caused by settling of the clay materials in the line between the header and the deaerator. Therefore, the deaerator tended to act as a trap. The SEM analysis showed a large variety of cells with varying morphologies (e.g. rods, vibros, spilloids) in the solids downstream of the deaerator. Water is the major component in biofilms which allows the hydrated filaments to form films on the exposure metal surfaces.

Analysis at Water Injection Wells. The infection at the aquifer injection wells was essentially similar to that indicated for the location downstream of the deaerator. A large variety of cells types were observed in the biofilm after drying which included mainly rods and vibros.

Results of Sessile Study for Produced Water Injection System - Two locations were examined in the produced water injection system (System 2). One of these was on a by-pass at the booster pump and the second was on the site of a selected water injection well. With the exception of the system and selection of the sampling sites, the analysis procedures were the same as used for the aquifer injection system as discussed previously.

Analysis at Booster Pump. The coupons exposed at the booster pump became colonized with a large number of microorganisms. The extent of the infection is shown in Table 3 for two exposure times and for incubation at 38 C and 69 C. The SRB infection was extensive after incubation at the lower temperature reaching level in the range of 5x10³ - 5x10⁴ cfu/cm². The GHB infection while less extensive than that of the SRB, it was comparable to that observed in the aquifer system. The rate of solids accumulation at the booster pump was 171 mg/m²/day. The EDS analysis showed evidence of iron sulfide resulting from the hydrogen sulfide produced by the SRB infection along with the deposition of silicate minerals.

Analysis at Injection Well. The coupons exposed at the injection well site exhibited the same degree type and extent of bacterial infection as found to occur at the booster pump by-pass. It was characterized by extensive SRB infection and a lesser but prevalent GHB population. The average accumulation rate of solids at the injection well site was 363 mg/m²/day. The main difference found at this location was the growth of thermophillic SRB (TSRB) during incubation at high temperature (68 C).

MICROBIAL GROWTH IN POROUS MEDIA  Top

To simulate bacterial growth within the formation, a special sand pack assembly was used (See Figure 4). The sand pack unit consisted of a series of tubes (4 mm diameter; 40 mm length) filled with sand having an average particle size of between 30 and 150 micrometers. At the inlet and outlet of the column, a stainless steel filter element with a lattice opening of 32 micrometer was installed. A special procedure utilized in this study was the ability to transfer the sand from the sand pack unit into sterile vials that contain 2 ml of sterile water using an aseptic piston. After sonication, a tenfold serial dilution was performed allowing presentation of the subsequent bacterial growth in terms of cfu/ml or as cells per sand pack volume of 2 ml. The significance of this procedure is that it simulates bacterial growth in the formation while using sand that is basically easier to work with than core samples. Therefore, if high bacterial counts and plugging of the sand pack are observed, it suggests similar tendencies in the formation.

After a period of 12 days of exposure to the header environment, each sand pack was transferred to a bottle containing 1.8 ml of SRB medium. They were then sonicated and the population enumerated as previously described. During the exposure period, the flow through the sand pack system was found to decrease considerably. However, complete plugging was not observed. The results are presented in Table 4. Infections of both GHB and SRB were detected. SRB growth was in the range of 500 - 5000 cfu/ml for two sand packs with one being as high as 5x10³ - 5x10⁴. By comparison, the inflection of GHB was consistently higher in the range 5x10³ - 5x10⁴ to greater than 5x10⁵. The morphology of the GHB in the porous media was examined by SEM and found to be primarily filament shaped as compared to those found in the previous studies which had a greater variety of morphologies. This work indicated that resident strains of both SRB and GHB were able to colonize a porous medium such as found in the formation.

SUPPORTING WATER STUDIES  Top

Analysis of Suspended Solids - These tests were performed on-line in both Systems 1 and 2 using the portable equipment incorporating the flow through system and a micropore (0.45 micrometer) cellulose filter. The produced water system showed considerably higher levels of suspended solids than did the surface water system. By comparison, the produced water had levels of suspended solids ranging from 165 to 350 mg/liter whereas the surface water system had levels from only 0.7 to 4.8 mg/liter.

Corrosion - Corrosion rates were measured with a sidestream corrosion cell coupled with a field portable electrochemical potentiostat with capabilities for conducting linear polarization resistance measurements per ASTM G59. The results for both systems are given in Table 5 along with measured levels of dissolved iron, oxygen and hydrogen sulfide. Carbon dioxide was very low in both systems with concentrations of 1 to 5 ppm. The surface water system upstream of the deaerator indicated high levels of oxygen and correspondingly high corrosion rates (up to 0.6 mmpy). However, downstream of the deaerator, levels of dissolved oxygen were in the range 5 to 90 ppb with much lower corrosion rates (<0.1 mmpy). The concentration of dissolved hydrogen sulfide and iron were very low for all points in the system monitored. The produced water system downstream of the booster pump, had characteristically very low oxygen levels with between 1.5 and 2.5 ppm hydrogen sulfide with corrosion rates consistently around 0.1 to 0.2 mmpy. These data indicated that the systems were being operated within specification.

Scaling Tendencies and Water Compatibility - Table 6 shows a comparison of the water compositions from the aquifer and produced water systems relative to connate water. Both types of injection water showed a positive scaling tendency. Water^[5] from both system^[5] shows levels of sulfate sufficient to sustain SRB growth.

SUMMARY AND DISCUSSION  Top

The planktonic surveys of both System 1 and 2 show only low bacterial infection levels under most conditions. In the aquifer water injection system only showed detectable SRB at the injection wells. The infection is somewhat greater in the produced water system, but still SRB were only detectable in one well. TRSB were seldom detected. Therefore, according to this initial survey, the potential for reservoir souring was considered to be low. On the contrary, the sessile bacteria survey shows a generalized SRB infection. Due to the general presence of GHB throughout both systems, it is likely that they would tend to deplete any oxygen within the biofilm and produce organic acids which could promote SRB growth. Additionally, the relatively high rates of solids accumulation from clay materials in the surface water system and extent of biofilm growth also support conditions for SRB growth. The porous media study showed high levels of infection after relatively short periods of exposure. Therefore, the potential for massive growth in the porous reservoir media has been shown. A subsequent analysis of an injection well backflush indicated an infection with SRB bacterial consistent with the determinations found for the injected water system.

A generalized SRB growth was not found in the oil producing wells. The fact that TSRB were so limited supports a conclusion that the reservoir was only locally infected. This suggests that the formation is highly inhomogeneous consisting of both tight and porous zones. The latter promoting the formation of channels that also include fractures in the formation (see Figure 5). In all of the abovementioned zones the temperatures are less than 50 C which is ideal for mesophilic SRB growth. Therefore, it appears that conditions are correct of substantial sessile growth which is accelerated by the high surface area of the formation. The hydrogen sulfide is likely to be more mobile than the sessile bacteria that produced it. This mechanism accounts for the presence of low level hydrogen sulfide in the production wells without a generalized infection of SRB.

Based on the evidence in this situation, the remedial actions prescribed included the use of a conventional biocide applied unconventionally, (i.e. with very high concentrations) were expected to reduce the production of hydrogen sulfide. Additionally, the optimization of chemical treatment for reducing scaling and corrosion was also expected to minimize the conditions for SRB colonization and growth in the water injection system. Based on subsequent field monitoring, it appears that the proposed souring model was confirmed.

REFERENCES  Top

1. Stetter, et.al., "Hyperthermophillic Archaea are Thriving in Deep North Sea and Alkakan Oil Reservoirs", Nature, Vol. 365, Oct. 1993.
2. Fielder and M.J. Littlewood, "Hydrogen Sulfide in the Thistle Field, Norwegian Society of Chartered Engineers, Oil Field Chemicals", Fagernes, March, 1988.
3. Al-Ruwehy, et.al., "A Study of Microbiological Growth and Reservoir Souring in the Lekhwair Field", 7^th Middle Eastern Corrosion Conference, NACE International, Bahrain, Feb. 1996.
4. Salinetro, et.al., "Growth and Control of Sulfidogenic Bacteria in a Laboratory Model Seawater Flood Thermal Gradient", SP 25198, New Orleans, March 1993.
5. Surinach, et.al., "An Examination of Sulfate Reducing Bacteria", U.K. Corrosion/84, I. Corr. S&T, London, Nov. 1984.
6. Surinach, "A New Concept of Treating Surfaces Exposed to Oilfield Water Systems", SPE 16262, Soc. Petroleum Eng., San Antonio, 1987.

FIGURE 1 - Oilfield Water Injection System

FIGURE 2 - Portable Microbiological Survey Instruments

FIGURE 3 - Method for Sessile Bacteria Studies

FIGURE 4 - Biofilm Monitoring Unit - PS Biofilm

FIGURE 5 - Schematic of Reservoir Souring Modeling