Filtration

The purpose of filtering completion/workover fluids is to prevent damage to the producing formation. If dirty fluid is used, it can result in irreparable damage to the permeability of the formation. Not only do dirty fluids reduce formation permeability, they also reduce the permeability in gravel packs. In order for a fluid to be nondamaging to the producing formation, the solids must be removed. The contaminating solids can come from mud residue left by ineffective displacement or perforation debris. All of these contaminants can and will plug perforations.

Filtering involves the screening or mechanical separation of solids from fluids. Clear brine fluid filtration is, essentially, a protective process. In order to arrive at a satisfactory degree of filtration, an evaluation of the contaminants involved is necessary. The critical factor in the case of solid contaminants in the fluid is the particle size distribution.

Filtration Equipment Sizing

Estimating the properly sized filtration unit is an approximation at best, but some useful guidelines can help. The main objective is to have the longest runtime or filter unit operating time (least amount of downtime) achievable within the limits of the physical equipment size. The two most important criteria are the flow rate and solid content of the fluid. Realistically, deck space may also be a factor in the decision.

Flow Rate. The range of flow rates anticipated should be the primary selection criterion. Flow rates for plate and frame filters are normally between 0.8 bbl/min and 1.0 bbl/min per 100 ft² of filter area. A value on the upper end of that range suggests the need for a larger unit to allow for some flexibility when handling surges in flow.

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TABLE 52. Typical Filtration Equipment

 

TETRA Filtration Equipment
Unit	Filter Area	Flow Rate	Footprint
	*ft2	bbl/min	L x W
SafeDEflo 600	600	6 - 8	22 x 15 feet
SafeDEflow C600	600	8 - 10	16 x 15 feet
SafeDEflo 1100	1,100	8 - 12	24 x 15 feet
SafeDEflo 1300	1,300	12 - 14	26 x 15 feet
SafeDEflo 1500	1,500	14 - 20	26 x 15 feet

Fluid viscosity will also vary with salt composition. A highly saturated summer blend fluid may have viscosity as high as 30 cp. The viscous drag of such a fluid in the filter cake will reduce filter throughput.

 

Solid Removal Rate, _vsol. There are several ways to measure the solid content of a brine, and they have varying degrees of accuracy. Percent by volume (p_sol) will be used in this discussion. The upper limit of suspended solids for efficient filtration is about 2% by volume. Any filter will only operate efficiently at this rate for short periods of time. More realistic levels of solids will probably range from 0.05% to 0.5% by volume. Using volume percent is subject to many assumptions, but this discussion is provided to give only general guidelines.

Use Equation 31 below to estimate the rate in ft³/min that solids will be removed by the filter, using the brine flow rate (Q) in bbl/min and solid content in percent by volume (p_sol).

EQUATION 31.

Example O. Solid Removal Rate

Then, to estimate runtime, locate that solid removal rate on the bottom axis of Figure 22 and move up to the filter curves to see the various runtimes for each filter. We have drawn a line for our example solid removal rate (v_sol) of 0.196 ft³/min. Filter size is indicated in square feet of filter area for four different filter sizes. Clearly, the larger the filter area, the longer the runtime. It is worth noting that if your target is a two hour runtime, you will need to determine your solid removal rate and then select your filter size accordingly. As you can see in Figure 22, solid removal rates between 0.18 ft³/min and 0.45 ft³/min will allow you to achieve a filtration unit operating time of two hours (120 min) if you select the appropriate filter.

FIGURE 22. Approximate Filter Runtime

Opening a filter press to discharge solids and precoat takes between 30 and 60 minutes each time. A runtime of 120 minutes with a 45 minute cleaning and precoat cycle would mean that you could accomplish roughly nine complete cycles in a day, resulting in six to seven hours of downtime for precoating each day.

Larger filter units mean fewer downtime cycles per day. Using the filtration removal rate of 0.196 ft³ from the previous example, runtimes for the different size filter units are shown in Table 53.

TABLE 53. Filtration Runtime Examples

(10 bbl/min and 0.35% by vol solid content or V_sol 0.196 ft³)

Filration Unit	Runtime/ Cycle	Cycles/ Day	Downtime/ Day
ft2	min	&nbsp:	hr
SafeDEflo 600 and C600	111	9.2	6.9
SafeDEflo 1100	204	5.8	4.3
SafeDEflo 1300	241	5.0	3.8
SafeDEflo 1500	278	4.5	3.3

This brief overview was intended to show how to size a filter and explain the effects of high solid loading. The following paragraphs explore some of the finer aspects of filtration theory.

 

Filtration Theory

Filter Rating Systems. There are several ways to rate the efficiency of a filtration operation. Capabilities are often provided for cartridge filtration units. Cartridge filters either have a nominal rating or an absolute rating.

Nominal Rating. A particle size, in microns (a one-thousandth of a millimeter), may be provided for a particular filter element or cartridge, such as a 10 micron cartridge. This value implies that the filter is capable of removing particles as small as 10 microns. These values are very dependent on test methods; they are meant to provide a helpful guideline, but are difficult to use when comparing one filter to another.

Absolute Rating. The size, in microns, of the largest spherical particle that will pass through a filter element. The efficiency of an absolute rated cartridge is measured in terms of Beta Ratio. A standardized ANSI (American National Standards Institute) test is used to measure the beta ratio—the number of particles of a particular size that are entering a filter, over the number of the same size particles that are leaving a filter in the filtrate (upstream divided by downstream). A beta ratio will always specify the micron size of the particles with which the test was conducted. A beta ratio of B₁₀ = 50 means that there were 50 times more 10 micron particles in the influent than in the effluent from the filter.

Below a certain threshold particle size, each filter will have a different beta ratio for each particle size. A curve can be drawn for a particular cartridge or filter element showing beta ratios at various particle sizes.

Surface Filtration. Surface filtration works largely through direct interception. This means that particles larger than the pore size of the medium are stopped at the upstream surface of the filter. The particle size prevents them from entering or passing through the pores. Surface type media are not perfectly smooth on their upstream surfaces, nor are their pores perfectly uniform in shape or direction.

Typically, when surface type filters are exposed to the flow of contaminated fluid, two effects—the gradual reduction in effective pore size and the building of a cake bed—start to take place almost immediately.

Gradual Reduction in Effective Pore Size. The effective pore size of the medium is gradually reduced, as some of the pores become partially blocked by particles. This reduction makes the filter become more effective in removing fine particles. Pore size reduction can be caused by the retention of extremely small particles within the pores by absorptive forces. It can also occur due to the partial intrusion of soft, deformable particles into the pores, acting under the forces generated by fluid flow, so that those pores are effectively reduced in size. Deformable particles have the ability to conform more closely to the shape of flow passages, thus blocking them to a greater degree than hard particles might. They can also form a slime or gel that can completely clog the filter.

Building of Filter Cake. A cake or bed of filter aid (DE) particles starts to build on the surface of the filter element (filter cloth). This build up of filter aid (filter cake) actually does the filtering. As operating time continues, this filter cake becomes progressively finer due to the same clogging or trapping mechanism noted previously. The filter cake then becomes completely clogged, restricting the flow of fluids through the medium. This effect can be noted by an increase in pressure differential through the filter press.

The pleated paper filter element is a typical example of surface filtration. Dirt retention is directly related to surface area. It is here that the theory of absolute filtration is applied.

The absolute rating of a filter is a frequently misunderstood and misused concept. The largest particle passed by a filter is not a function of the size of the openings in the filter medium; it is a function of the flow through the filter, the particles, the filter medium, the pressure and flow pulsations, and many other factors. The zeta potential, electrostatic charge connection of the particles, fluid, and filter media all interact to determine the size of particles captured under a given set of conditions. Any change in these factors will affect filtration conditions, which will, in turn, affect particle distribution downstream of the filter.

Over time, the absolute rating has mistakenly come to mean attempting to define the largest particle found downstream of a filter under operating conditions. The absolute rating is actually a measurement of the largest glass sphere the filter will allow to pass when exposed to a low pressure differential and nonpulsating flow. Under actual operating conditions, with the infinite array of variables mentioned above, there is no strict relationship between absolute rating and the particle size distribution in the effluent. However, when used as a qualitative measurement for filter selection, the absolute rating is the best guideline available.

Depth Filtration. This type of filtration employs a deep, porous structure. The mechanism of filtering then becomes much more complex. The path through the filter is much longer and more random, providing greater possibility for both direct interception and dirt retention. In general, larger particles will tend to be trapped in the surface layers with the finer particles trapped by succeeding layers. If necessary, the structure of the filter can be density graded. This can be particularly advantageous where the particle sizes or the contaminants are widely distributed. When contaminants are of a more or less uniform size, a surface filter may be equally effective. Depth filtering also gives a higher pressure drop than a surface filter.

It is possible for the overall performance of a depth type filter to be better than that given by the purely mechanical action of direct interception. The inertia of particles that are impinging directly onto the filter medium may generate absorptive surface forces. As a result, the depth filter may trap and retain particles finer than those trapped by pure mechanical filtration alone.

Nominally rated filter elements (cartridges) are typical examples of depth type media. Cartridge filters are produced by using a wide variety of configurations and materials of construction. Filter media include yarns, felts, papers, resin bonded fibers, and woven wire cloths. The filter element is normally designed as a disposable component and is meant to be replaced with a new cartridge when clogged. Cartridge filters are compact, reliable, easy to operate, and require less operator training than most other types of filtration equipment.

In heavily contaminated systems, cartridge filters are most often used downstream of other types of filtration devices for final product clarification. The system most commonly seen in the oil industry utilizes a plate and frame filter press that is precoated with diatomaceous earth (DE) for primary filtration and a downstream cartridge unit, which functions as a guard/polishing filter.

Cake Filtration. Cake type media are generally employed for removal of solids in significant bulk. Usually, the action of fluid flow will cause separate, loose, discrete particles to form into a cake on a supporting screen. The voids between the particles form the pores and flow passages required for filtration. One typical material used to form a cake is diatomaceous earth.

Brine Filtration

Plate and frame filters are large multiplate pressure filters that can treat up to 20 bbl/min. To enhance the filtration process, a filter aid is utilized to precoat the filter cloths that are supported by the plates. The plates and filter cloths merely provide a support for the filter aid which does the actual filtration. A number of precoating materials are available. The most common filter aid in CBF applications is diatomaceous earth (DE). From this, the term DE Filtration was derived.

When this type of filtration system is utilized, certain parameters are controlled that greatly influence the effluent quality of a filtrate. Some of these parameters are porosity, permeability, and pressure differential. Some of the limiting parameters in determining the optimum filtration system for a particular job are: rig space, equipment size and weight, volume capacity, flow rates, desired effluent quality, equipment flexibility, and cost.

In this industry, our goal is to achieve the most efficient and cost effective means of filtration that fits within the specific parameters of a given application. As previously mentioned, the most commonly used filtration system in the area of completion/workover fluids utilizes a filter press as a primary filter with a cartridge filter downstream. The main function of the filter press is to remove all suspended solids; the function of the cartridge filter is to serve as a guard, or polishing filter, and to guard against the loss of DE into the formation should a filter cloth tear.

The filter presses available in the field consist of a series of vertical chambers arranged in parallel. Frame filter presses are produced by stacking flush plates with distance frames separating them. A chamber filter press is produced by stacking a series of recessed plates. In either case, each plate carries a filter cloth, or other suitable filter medium, and the stack of plates is compressed. Most commonly, a hydraulic system is used to expand and retract the ram of the filter press.

The product to be filtered is fed into the press under pressure. The filtrate passes through the filter media and out of the press. At the same time, solids are retained in the form of a cake on the filter media with each chamber of the press performing as a separate unit. Initially, the cloth and DE precoat act as the filter, but as solids and additional DE are collected and built up on the surface, they gradually assume the function of the primary filter medium. Filtering then continues with increasing efficiency until the cake has built to an optimum thickness.

Pumping pressure progressively builds up to compensate for the loss of flow rate until, eventually, the cake is fully formed and filtrate flow is negligible. Pumping is stopped and the fluid is displaced from the filter by compressed air (blow down). The press is then discharged; this is accomplished by separating the plates and allowing the cakes to drop out. After being discharged, the press is washed out, closed, and made ready for the next cycle.

Filter Cloths. Filter cloths, often referred to as septums, provide the support for the filter aid. A wide variety of filter cloths may be used, although the most common woven materials are nylon and polypropylene. The smooth surface and good flexibility of these cloths promotes easy cake removal. Weave also affects the tendency for the cake to clog the cloth so that satisfactory performance can only be established on empirical lines. Economical operation may depend upon usage of a relatively expensive filter cloth with proper attention given to cleaning so that individual cloths are not damaged.

Diatomaceous Earth. Diatomaceous earth (DE) is a nonmetallic mineral composed of the skeletal remains of microscopic, single celled aquatic plants called diatoms.

Diatomaceous earth is predominantly amorphous silica (noncrystalline silicon dioxide). It may also contain small amounts of crystalline silica quartz; oxides of calcium, magnesium, iron, and aluminum; and certain trace elements, which occur naturally in the Earth's crust. Physically, the diatom skeletons are intricate structures having many submicron pores. They occur in a large variety of shapes, much like snowflakes. These relatively noncompressible structures occur mainly in particles of 10 to 200 micrometers in diameter. One of the major uses of various grades of processed diatomite is as filter aids.

The primary functions of filter aids are improved filter efficiency, increased filter life, and improved flow rates. In order for a filter aid to perform, it must: (1) form a porous cake, (2) have a high surface area and a narrow particle size distribution, and (3) have an irregular shape.

Automated DE Delivery Systems. Automated DE delivery systems were originally developed in response to the Control of Substances Hazardous to Health (COSHH) regulations regarding airborne crystalline silica. Automated DE delivery systems are designed to reduce the risk of per­sonnel exposure to airborne crystalline silica, which can be found in natu­rally occurring diatomaceous earth.

Automated DE delivery systems have numerous other HSE benefits. For years, diatomaceous earth has been supplied to filtration locations in pallets containing 50-pound paper sacks. To introduce the DE filter aid into the filtration process, filtration personnel were required to handle individual sacks of DE, opening each sack and emptying it into the DE slurry (Add Mix) tank. This process required lifting and carrying the heavy sacks and using cutting devices to open them. This process also created dust and resulted in solid waste requiring disposal. To address these HSE issues and automate the DE introduction process, TETRA recently introduced the SafeDEflo™ diatomaceous earth automated delivery system for use in Gulf of Mexico filtration operations.

The SafeDEflo delivery system utilizes intermediate bulk carriers (IBCs), which hold 1,000-pounds of DE material. The IBCs are filled with DE and then transported to a filtration location where they are paired with a dispensing system that is operated through a pneumatic control panel. The SafeDEflo system provides accurate DE dispensing and a safer, cleaner work area without the need for disposal of sacks, shrink wrap, or pallets.

Precoating

The first step in preparing a plate and frame filter for operation is to build up a precoat of filter aid on the filter cloth (septum).

The purpose of the precoat is:

1. to prevent the filter septum from becoming clogged by impurities, thus prolonging septum life,
2. to produce immediate clarity, and
3. to facilitate the cleaning of the septum at the end of the cycle.

Precoating is accomplished by circulating a slurry of filter aid and filtered or clear liquid between the filter and the precoat tank. Since most of the filter aid particles are smaller than the openings in the septum, they form a thin filter cake by bridging these openings. This filter cake can be upset by air bubbles, sudden changes in pressure, or vibrations. This causes the filtrate to become turbid until the upsetting influences have been corrected.

Amount of Precoat. The amount of precoat is generally 10 to 15 lb of filter aid per 100 sq ft of filter area. The greater amount is used when distribution of flow in the filter is poor or when breaking in new filters. If it is perfectly distributed, 10 lb of filter aid per 100 sq ft of filter area will give a precoat of approximately 1/16 inch in thickness. Precoat slurry concentration depends primarily on the ratio of filter area to filter and piping volume.

Precoat is added to a quantity of working fluid in the precoat mix tank. Vigorous mixing is required to get the light DE adequately mixed. The precoat slurry is then circulated in a closed loop through the filter until the returning fluid is once again clear.

Precoating Rate. The precoat rate depends primarily on the viscosity of the fluid used. The rate should be sufficient to maintain filter aid suspension, but not so fast as to cause erosion of precoat in the filter. For water, a typical rate is one to two gal/sq ft of filter area per minute. Extremely viscous liquids may require rates as low as five gal/sq ft per hour. A general rule for precoating is to precoat at that rate which gives a pressure differential of approximately two psi. For water, an upward velocity of at least four ft/min is required for proper filter aid suspension.

Filtering

After the precoat filtrate has cleared up, the filter is put on stream by:

1. starting the body feed pump,
2. opening the line from the filter feed pump, and
3. simultaneously closing the line from the precoat circulating pump so that flow through the filter is continuous and without sudden fluctuations in pressure.

If the filter is precoated using filtered liquid, it is best to refill the precoat tank immediately by directing all or part of the filtrate to it. Filling the precoat tank at the end of the cycle may cause problems. Since flow through the filter may drop suddenly, the time required for filling may be excessively long.

Body Feed Addition. Insufficient body feed decreases active filtration time. The body feed is completely surrounded by undissolved solids and does not, therefore, increase cake permeability. Cake permeability only increases the cake thickness without adding anything to its permeability. From this point, as body feed is increased, throughput increases slowly and then rapidly for a short span of time. The rate increases and then tapers off; throughput stabilizes and then actually decreases once again, because cake permeability is no longer increased with additional amounts of body feed. Excessive body feed results in a sudden increase in pressure as the cake bridges between the plates. This increase causes a sudden decrease in filter area, and can also result in severe damage to the plates and loss of clarification. In general, proper body feed maintains high permeability of the filter cake and characteristics similar to depth filtration.

Several grades of diatomaceous earth are available as filter aids. In general, the coarser grades provide higher flow rates and lower effluent clarity. The opposite also holds true with the finer grades resulting in lower flow rates and higher effluent clarity.

Rig Practices and Filtration Performance

While a two micron filtration level is generally used as an acceptable standard, the total suspended solids left in the effluent should be of great concern. No fluid containing more than 1,000 ppm should be considered clean. Efforts should be made to reduce solid content to a level as low as practically possible. Strict adherence to sound rig practices, careful fluid management, and competent operators will produce worthwhile results, paying off in improved production rates and extended well life.

In addition to occurring in the well itself, fluid contamination can occur in storage facilities, during transportation, and in surface circulating systems. Utmost care should be taken to minimize the possibility of fluid contamination. It is most important that the entire surface system be cleaned before the clear brine fluid is brought in. It is important that the flowline, shale shaker, sand traps, solid control equipment, return line, pits or tanks, equalizing lines, hopper, mud pumps, mud manifolds, kelly, kill line, choke, and manifold be flushed and cleaned prior to taking on the brine. Although rig time and associated costs are inarguably expensive, it is invariably more cost effective to prevent fluid contamination than it is to remove solids by filtration after they have occurred.

One of the major difficulties in evaluating a filtration process is carrying out onsite determinations of effluent quality. The following components should be considered when analyzing a filtration process for clear brine fluid applications:

1. total suspended solids,
2. maximum size of particles at specified solid content, and
3. cost per barrel.

It is important to understand the different variables that are encountered during the filtration of brines. Some of those variables are flow rate, pressure differential, cycle life, solid content, and fluid viscosity. During the filtration process, the filtration operator has almost absolute control over the variables mentioned above; it is up to the operator to get the maximum efficiency out of the filtration equipment.