Treating Reinjected Oilfield Brine By Microbial Clarification
- N.A. Philippovich (OMV-AG) | J. Winter (OMV-AG)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 2001
- Document Type
- Journal Paper
- 106 - 111
- 2001. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 3.2.6 Produced Water Management, 4.2 Pipelines, Flowlines and Risers, 1.2.3 Rock properties, 1.14 Casing and Cementing, 6.5.2 Water use, produced water discharge and disposal, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3.2.4 Acidising, 4.1.2 Separation and Treating, 1.8 Formation Damage, 2.2.2 Perforating, 4.1.5 Processing Equipment, 4.2.3 Materials and Corrosion, 2.4.3 Sand/Solids Control
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Rising water volumes and processing costs in the oil field have spurred OMV to look for alternatives to traditional water processing. Despite an elaborate central multistep water-treating system, increasing injection pressures, water fouling in the pipelines, and reservoir souring were noticed. In a five-year program involving field measurements, laboratory experiments, the construction of a pilot facility, and test injections, a new strategy was adopted. The central water-conditioning plant (WCP) has been revamped. Instead of biocide addition, removal of organic nutrients by a biological clarification step now produces a clear, well injectable, and stable injection water.
Increasing awareness of environmental problems has made produced-water disposal a critical topic. Water flooding has become common practice for pressure maintenance, recovery of unswept oil, and as a basis for enhanced recovery methods.1 Consequently, the amount of literature covering different aspects of water handling, treatment, analysis, and reinjection is growing rapidly; more specifically, the topic of formation plugging has been dealt with in a number of papers.2-5 Nevertheless, there is still no unambiguous universal method to assess injection-water quality. In fact, very different practical aspects related to composition, solids content, stability, compatibility with formation/formation fluids, and microbiological contamination must be combined to define a parameter like injectivity. Hence, uncertainty about what to measure and the required treatment prevails and theories predicting injector half-lives from quality criteria have not gained general acceptance.6,7 Some authors have used and recommend on-site core testing.8,9 The contribution of bacterial activity to injectivity problems, although discussed early, has been emphasized more recently.10-14 It was understood that because bacterial growth occurs over days and weeks, short-time filtration tests might be inadequate for characterization.
OMV has been operating a water distribution and injection system for more than 30 years. It is currently processing the better part of 107 m3/year of brine through the conditioning plant at Sch nkirchen, some 30 km northeast of Vienna, Austria. The pipeline network totals approximately 80 km of tubulars of various sizes and flow velocities; the volume of produced water is still increasing (Fig. 1).
Status of Water Conditioning
Brine and live oil flow from most fields through pipes to separation facilities where the production is monitored. To keep corrosion at bay, the main lines of the distribution system are made from reinforced cement. The different water streams illustrated in Fig. 2 include surface water runoff and waste water. Fig. 3 depicts the water streams entering the conditioning plant where they are passed through a flotation pool (2), a basin for flocculation by Fe+++ (3), and two sedimentation units running in parallel (4 and 5), before filtration through sand beds (6 and 7). Every hour, 1000 m3 of processed brine leave the plant to injection or disposal wells. Oil is recovered from the floating sludge and injected into transport flow lines to the refinery. Solids are regularly removed from the pools and deposited at a dump. Table 1 shows the composition of the water and some other parameters related to contamination.
Water exiting the WCP must meet the specifications in Table 2.
Formerly, yearly analyses checked these parameters and the concentrations of the ionic species in solution. The frequency of backflushing necessary to regenerate the sand filters in the WCP provides some control over the filtration quality of the water. Another indication is the performance of a diatomaceous earth filter located at injectors in the Hochleiten field 20 km away. Very little information was available before increasing awareness of the problem prompted a comprehensive analysis program in 1993.
These specifications were set somewhat arbitrarily a long time ago, and while a better water quality clearly benefits the reservoirs, the question of how much effort the treatment really needs remained open. Because the water is not just disposed of but also used for pressure maintenance, the degree of conditioning could have been sized to the requirements of the flooded reservoir. However, because of logistical problems (need for separate tubular systems), this approach was rejected from the beginning, and an "overall" quality specification was defined.
Performance and Problems
The system has been working satisfactorily most of the time, but increasing demands and rising costs are a permanent incentive to look for alternative solutions:
The separation of the solids from oil sludge once performed by a filter press needing a cake-building additive is accomplished now through a three-phase centrifuge (Tricanter). This reduces the volume of solids dramatically and recovers more oil.
Tests with a flotation unit instead of sedimentation pools have not led to an improvement and were discontinued.
Over the years, numerous flocculating agents and biocides have undergone testing to increase efficiency and to reduce costs.
However, some problems appeared that seemed intractable by conventional wisdom.
Low-permeability formations in the tens of millidarcies range as well as EOR projects needed a better water quality, otherwise injectivity would be lost instantly.
In all but the most permeable, fractured formations, injection pressure rose continuously. Increased pressure means higher energy expenses but transgressing the frac pressure could compromise the integrity of the formation and modify the flooding pattern. Acid stimulation could restore the injectivity temporarily, but the effectiveness of the treatments seemed to decline progressively. In the end, acidizing was given up altogether, allowing injection to proceed at the maximum allowable pressure until the injector was shut down and another well converted to injection.
In spite of permanently switched biocides, development of bacteria in the whole system was out of control. Fig. 4 shows H2S concentrations increased along the flooding system. Pigged lines yielded a black, foul-smelling sludge attributed to the buildup of biofilms.
Temporary stops caused by power disruptions mobilized huge volumes of suspended material upon restoration of flow in the lines. Filters plugged and had to be cleaned. At irregular intervals, sand filters in the WCP would need flushing back after half the normal water volume, resulting in double downtime and accumulation of untreated water. In severe cases, this could shut down production.
Finally, a polymer pilot failed because of microbiological degradation of the xanthan.
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