Filtration of oilfield waters before repressuring
(Experience in Serbia-Montenegro and Hungary)

         In the gas and oilfields many ancillary processes are kept in operation in order to run the production undisturbed. These processes are to ensure the good quality and to protect the environment. One of such processes is the cleaning, treatment and disposal of waters.

         The specialists of MOL and NIS-Naftagas have been putting their trust in us and we have been able to help them for ten years. The efficient filtration of oilfield water has been elaborated and the gained experience is reported in this paper. A lot of liquid and solid impurities comes with gas and oil from the wells. Their separation is very important.  The major part of liquid impurities is water, which is repressured in most cases deep into the ground after cleaning. The oilfield water may come from one or more layers. The treatment of water and its repressuring (disposal) into the right layer(s) require careful planning and execution.

         Knowing the origin and composition of oilfield water the technology of repressuring can be prepared. An important part of it is the filtration and cleaning requirements. With regard to more and more strict environment protection regulations and limit values, the responsibility for water cleaning technology is getting more and more serious.

         The volume and composition of oilfield waters vary during the lifecycle. It is important to work out such filtration and cleaning technology that can manage the changes in quantity and quality of waters efficiently.

         The oilfield water contains impurities. Majority of them is in liquid or solid state, e.g. quartz sand, clay, pieces of rock, corroded metal particles, asphalt, paraffin, other long carbon-chain compounds, crude oil, etc. These impurities can be dissolved or undissolved (as suspended material) in the water. The separation of suspended materials can be resolved with pure mechanical methods. An efficient sedimentation and mechanical filtration can help us.

         The separation of dissolved impurities is more complicated; it requires chemical treatment in most cases. It is aimed at conversion of dissolved impurities into suspended precipitate that can be sedimentated and/or filtered. Both the sedimentation and the precipitation require huge equipment and sometimes additive agents. These processes are time-consuming.

         In the water to be repressured the impurities are undesirable for many reasons. In the first place the pore size of absorbing rocks is so small; the impurities cannot get through. In the second one these materials are extraneous in that layer and undesirable in respect of environment protection.

         The problem is the absorbing capacity of geological layers rarely good. It is because of small gaps between the grains of porous rocks (some tenth of a micrometre, maximum 1-10 micrometre). In addition the rocks in that layer may already (or still) contain absorbed fluids, the flow to adjacent layers may be good or bad, even the rocks may be dissolved in the repressured water in some extent, etc.

         The aim of this paper is not the giving detailed information about this problem – so much the more it can be very different in each oilfield. We only wanted to point out that how important is the exact, up-to-date and deep knowledge of circumstances. Up to this point the costs were not mentioned. It is easy to see that detailed knowledge of circumstances is a good basis to elaborate alternatives plans and their costs reliably.

         To sum it up the technology consists of collection of separated oilfield water – possible treatment with additives – cleaning and repressuring into the target layer (Fig. 1). Those layers are deep in the ground, usually below 1000 m. This is why high pressure (300 – 400 bar) is required for repressuring. This job can be done with good results using piston pumps. The accurate joints and mechanical valves cannot stand the wearing solid impurities. If they are present, the pumps need very frequent repairs, which are expensive.

         Now the conclusion can be reached; the efficient filtration is motivated by two things, on the one hand the well repair costs in the range of EUR 3‑400,000; on the other hand the very high pump repair costs, in many cases more than EUR 100,000.

         The continuous operation is unimaginable without suitable filtration. There are many well-known methods and possibility to treat the water. But in each case the specific parameters can help to elaborate the long-term and suitable solution with low-cost technology. The first and most important step in the planning process is the setting an aim. The filtering aim that must be suitable to cope with difficulties for long time.

Fig. 1.        Filters in the technology line

1.,     Setting the filtering aim
The characteristics of the absorbing layers have the biggest effects on the setting the filtering aim. First of all we must know what the absorbing capacity of the layer is and what conditions on, what its permeability, etc. An important factor is the starting composition of water and the environmental limit values are also not negligible. The Table 1. shows the data sheet of an oilfield water to be filtered.

Water analysis - inlet of the system                Table 1.

         The above values come from laboratory analysis. Of course the samples were taken and analysed several times and the results varied in time and location, depending on the time and location of taking samples. The above data should be considered as average values. However the standard deviations of values are also must be known for stable and long operation. We have to mention that we have met very different oilfield waters on different locations during our work.

         The required filtering parameters can be specified clearly as parameters of outgoing water. The general requirements are as follows:

-       suspended solids and liquids should be below 2 mg/litre,

-       oil (or CH) should be below 2 mg/litre,

-       size of 96 % of remained impurities should be below 2 μm,

-       filters should work 12 months without maintenance. 

The above-mentioned facts make it clear that our aim is difficult to achieve. The filtering aim is strict – fine filtering size, high volumes and continuous operation (Fig. 1).

         Contrary to other problems the filtering aim is determined by the characteristics of the absorbing rock. A few tenth of micrometre filtering size means serious challenge because of the continuous operation and relatively high volume flow. The filter equipment was designed with enough capacity for a yearlong operation. The free area of filters is very important, as well as the starting pressure drop. The clogging must be avoided for long time; it requires filters withstanding 4-5 bar of ∆p.

         This is the way to meet the requirements: high volume flow continuously. There are some requirements that produce an opposite effect: one of those is the filtering size that should be below 2 μm (the free area of fine filters is small, their structure is sophisticated and can bear a few tenth of bar pressure drop).

         The other problem is the accuracy of the filtering size. Values higher than 80% can be hardly achieved with usual filters in the range of a few micrometre. 

         But the Ecofilt Mikrofilter has such features that help to resolve the problem. Those features are as follows:

-       High-strength structure, withstanding tens of bar  ∆p (pressure drop). This is question of dimensioning and manufacturing only.

-       High filtering accuracy, over 95 % (but even 99 % can be ensured if necessary). It is guaranteed by the manufacturing process.

-       Big free area that guarantees the required long service cycle (long maintenance-free periods)

-       The filtering size can be dimensioned due to the listed features and in spite of unpredictable variations of impurities and volume flow. 

Therefore the filtering aim is: filtering size 2 μm, service cycle 12 months, water flow 10 – 100 m³/h.

2.,     Laboratory analyses, filtering experiments
In spite of the definite filtering aim it is indispensably important to know the impurities deeply in details to determine the suitable filtering capacity. The total concentration of impurities, their chemical analysis and breakdown of concentrations are important to know in order to ensure the long-lasting good results.

         Water samples were taken several times for laboratory analysis. But it was not enough. We put our filters to test operation in a by-pass as shown in Fig. 2.

Fig. 2. Test rig of filters 

         The water samples were taken with the test rig. The pump P-1 was necessary to press the filtered water back to the main circuit. The oilfield water flowed through different filters FU, FSM, F1 and C. The samples of filtered water were taken with valves I1 and I2. The actual pressure was read on manometers marked with PI. The volume was monitored and recorded by flowmeter FI. 

         The test runs usually consisted of two or three phases. The test samples were analysed. The results allowed us to design the right filtering equipment. After the first runs the coarse filters were examined. In the laboratory the filtrate was rinsed out from filters, then the filtrate was analysed. One of the results is shown in Fig. 3.

Fig.  3.   FT-IR analyses of filtrate

         The same method was used in case of oil content, too. The filtrate was rinsed out from coarse filters and oil separators, then analysed.  A result diagram is shown in Fig. 4.

Fig. 4.   FT-IR analysys of oil content before and after filtration
(absorption (ABS)  at 2926cm^-1  is proportional to oil content)

     The black curve represents the oil content in the filtered water, whilst the blue one represents the oil in the water before filtration. The decrease is distinctly visible. The suspended particles were analysed with microscope. The average size was determined. It was usual that these small particles adhered to each other to form bigger size particles in the range of 10 - 1000 μm. The temperature of water has not been mentioned yet, but it is an important factor. The oilfields operate in all four seasons. The oil comes from the depth of the ground; the temperature does not vary too much. However the temperature of separated by-products, among that of the oilfield water, varies in a wide range. The reason of it is in the technology process.

         It is well-known that concentration of dissolved materials depends on the temperature of water. As a consequence, the change in water temperature may have effect on the filtration, too. It was also examined. The dissolved inorganic compounds (e.g. salts of Ca, Mg, Fe) are inclined to precipitation and it may have disadvantageous effect on the filtering process. The low concentration of such compounds is desirable. The concentration of dissolved organic compounds is proportional to chemical oxygen demand (COD) and it is usually high.

         The typical results of test runs are summarized in Table 2, showing the most essential data. The experimental phases are designated with numbers 1-2-3.

         In the first (1) phase of test runs we got closer to the solution. In the second (2) phase the installed additional filter had no effect on the COD value, but reduced the suspended paraffin concentration. In the third phase the filtering size was sharply reduced, so the suspended solids nearly met the standard potable water requirements.

                                                                                                 Table 2.

3.,     Accomplishment
The filtering aim, the test runs and laboratory analyses outlined the way to the right filtering equipment during the design process. First, the high concentration of oil and asphalt with enclosed clay particles requires a coarse oil separator unit to remove such impurities from the water. It would also help the fine filters in the hope of longer service cycle. The oil separator was engineered so (capacity, etc.) that the required service cycle of 1 year can be ensured.

         It was achieved by providing sedimentation possibility for bigger particles in the oil separator. This technique can keep the pressure drop at relatively low level. The good flow parameters help the efficient separation.

         After the oil separator the water flows into the filter equipment. The continuous operation is a requirement, so the equipment has several filter units connected in parallel. This parallel arrangement makes possible to switch over to a clean unit in case of clogging of an other; so the operation of filter equipment can be maintained up to the required service cycle in extreme circumstances, too. The connecting diagram of the equipment is shown in Fig. 5.

Fig. 5.        Connecting diagram of the equipment

         The ∆p (pressure drop) in each filter unit is monitored by differential manometers. Fig. 6 shows a filter unit drawing. The effluent water quantity is registered by a flowmeter. The long continuous operation is ensured by 4 parallel filtering lines. The selection of the actual working line is done by opening-closing of the relevant valves. Of coarse, the valves can be of manual or automatic operation.

Fig. 6.        A filtering line

         The vessels and pipes are made of carbon steel. In consideration of corrosion control all carbon steel components are hot dip galvanised. There are Ecofilt Mikrofilter (registered trade mark) filter cartridges in the filter vessels. Each cartridge is a cascade filter. The stainless steel base filter layer (guarantying layer) has a coat, a coarse filter layer on it. The coarse filter layer is made of environment-friendly material. The filter cartridges can be maintained easily (Fig. 7).

         The coarse filter layer is required to filter out the most frequent (by concentration and size) impurities. The base filter-guarantying layer sets up a size limit for impurities with accuracy of 98%. This picture (7) shows that moment when the filter cartridges are lifted from the vessel. It is well worth seeing the filtrate on the cartridges. Its black colour means the oily impurities are filtered out by the outer coarse filter layer. When the engineering of filtering process is correct, the majority of impurities is filtered out by the coarse filter layer. It works as a “deep” filter and used to store the filtrate. On the stainless steel base filter layer the quantity of filtrate is very small. The base layer (guarantying layer) is usually not clogged.

Fig. 7     Lifting the filter cartridges

         Fig. 8 shows the picture of the accomplished filter equipment. The collecting pipe and the filtering lines are well identifiable. The differential manometers and the red arms of valves are distinctly visible. In this case we have an indoor filter equipment, and this fact  partly determines the climatic conditions. The outdoor installation arises the question of suitable operating temperature, too.

Fig. 8   Filter equipment        

         The maintenance of filter units is simple in the workshop. The outer coarse filter layer can be removed by pulling. Its maintenance is a replacement or a washing in solvent. In our experience the washed coarse filter can be re-used 4 – 5 times. The filter cartridges can be cleaned in solvents with soft brush. It is 100% effective because of structure of filter. After cleaning the filter cartridges with the coarse filter layer can be mounted back into the vessel. In our experience the lifetime of filter cartridges is 10 years in highly polluted service circumstances.

         The filter unit mounting is in similar way as you can see it in Fig. 7. When the gaskets are in good working order, the filtration can be continuously ensured with the required parameters and efficiency.

4.,     Results
For many years of operation of filters the effluent water quality has been monitored. The good work of filters confirmed by the water analysis: the concentrations of suspended solids and oil have been below the required levels. Also, in our long-term experience it is proved that the required filtering size and service cycle (1 year) can be maintained. For example here is our latest case: one of our filter equipments required service after 17 months of continuous operation.

         The Ecofilt Mikrofilter equipment was tested in extreme circumstances, too. When some vessels were cleaned, it was used to filter stiff clay. It was capable to operate 7 days (In normal conditions the concentraion of clay is 1 - 10000 mg/m³ in the oilfield water). 

4.1    Economic benefits 

         The new installation was checked by relevant economic experts. We can report on the results that many years have passed, but the Ecofilt Mikrofilter cartridges still work, no replacement was necessary up to this moment. The filter units are serviced as scheduled, washed or new coarse filter layers fitted on the base filter layer. The spare part demand is very low: only new coarse filter layers were required and supplied.

         The maintenance and repair of injection pumps have been reduced down to one sixth of the original. Earlier – before the accomplishment of this filter project – 3 pumps were repaired annually. After the use of new filtration of oilfield water 1 pump have to undergo general overhaul in every second year. In addition the repair costs of wells have been sharply reduced. The reason for it is the much lower frequency of repairs.  All above-mentioned are based on the right filtering aim and carrying out our plans to the full.

         Hereby we would like to express our acknowledgments which are due to specialists of both MOL and NIS-Naftagas for the opportunity and correct co-operation. 

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