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Oilfield filtration:

Optimising cartridge filtration of oilfield brine

Introduction

Oil and gas rich reservoir rocks are perforated at the bottom of a well to initiate production. Perforation is carried out with speciality tools and an arrangement known as a completion assembly, which offset the hydrostatic pressure of the drilling fluid and/or brine to start flow of oil or gas under geo-pressures. Solids are deliberately added to drilling fluids to increase the density and suppress the accidental flow of oil or gas. These solids, if not removed before perforation, are potential agents to block the pores thereby decreasing the permeability of reservoir rocks and hence reduce production rates. Damage to the formation is reduced by displacing drilling fluid from the well with a solids-free brine, known as a completion and/or workover fluid. To ensure greater production the undissolved salt and impurity solids from brine are further removed by filtration.

Cartridge filters are extensively employed in the drilling industry for filtering completion and workover fluids as an aid to modern completion techniques to reduce formation damage. Completion or workover fluids, true solutions of brine in most instances, can increase oil productivity by 850% [1]. Bagsi et al [2] observed high oil production with limited reduction in permeability by using mixtures of KCl, NaCl and CaCl2. Brine densities are increased as well conditions dictate, thereby increasing the percentage of undissolved solids and hence the risk of potential formation damage. Rike [3]

Aprocess improvement study of cartridge filtration of oilfield brine has

been undertaken using statistical experimental design. The findings

are looking to improve overall efficiency and reduce costs at the

wellhead.

presented that brine containing 500 ppm solids can still plug a perforation hole. In another study it was found that the permeability ratio was reduced to 0.39 as a result of using high density CaCl2/CaBr2 brine comprising high percentage of solids [4]. Removing 100% of the solids from a completion fluid is impractical and it has been found that removing solids only larger than 5μm can increase the return permeability by 60% [5].

Because undissolved salt solids are non-uniform in size, a variety of filtering systems and media can therefore be used. The cartridge filters ensure the absolute particle size efficiency by filtering the brine in the size range down to 1 μm in comparison with Diatomaceous earth filters. Modern completion methods use either a two-step filtration, diatomaceous earth (DE) and cartridge filters, or a stand-alone cartridge filter unit. Cantu and Nall [6] compared only the solids removal efficiencies of cartridge and diatomaceous earth filter systems without emphasis on dirt holding capacities and concluded that the filtration end point was reached when the solids concentration became fairly constant at a low level. However, in another study on filtering seawater at a slow flow rate incorporating nominal (first stage) and absolute pleated cartridge filters (second stage) in series, it was found that the service life of an absolute pleated cartridge filter increased by 20 times. Consequently, the selected brine clarity was achieved in 35 times fewer operating hours than for using nominal

filters in the second stage [7]. William and Edyvean [8] indicated that increasing the organic contamination load in seawater decreased particle retention efficiency (dirt holding capacity) and the service life of 5 μm nominal filters. In addition, when the dirt holding capacity was reached, the filter media released particulate matter into the filtrate.

The major disadvantage associated with a mirco-rating cartridge filter is the decrease in filtrate flow rate which is a complex function of the magnitude of flow direction, particle velocities, module geometry, pressure drop, dirt holding capacity (cake or fouling of filter media), point of flow through porous media and the height, count and configuration of pleats [9, 10]. The effects of non-ideal flow behaviour through porous medium that results in medium compression, percentage loss of filtration area and pressure drop has recently been presented by Waghode et al [11]. Moreover, studies regarding calculating minimum external surfaces, cost estimation, filter changes and scaling of disc cartridges to industrial applications are abundant in the open literature [12, 13].

Previous studies based on field trials are mainly concerned with comparison of filter systems in seawater filtration where solids concentrations and sizes are primarily different from mixing brine at oil rigs. Operators switch between two choices of filtration systems. In diatomaceous earth filters the release of fine filter-media particulate results in high throughput but forced many operators to

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Filtration+Separation May/June 2012

Sieving/interception

Depth Filter media

Bridging

Sieving/interception

Filter media

Effluent

Filterate

Mixed Brine

Filtered Brine

Pressuregauge

Mixed Brine

Filtered Brine

Block diagrams of experimental set-up

use a cartridge filter as a guard upstream [6]. Whereas changes in flow rate, pressure differential and particulate accumulation exacerbate the performance of a cartridge filter unit [14].

Therefore, the current work has focused on the process improvement of cartridge filtration systems for oilfield applications to reduce the time required for filtration and hence reduce cost of filtration through daily rig cost, filter change frequency, enhancing filtrate flow rate, and to meet brine clarity specifications in a minimum number of circulations. (One circulation is the time brine takes being pumped from the surface tank to the bottom of the well and from the bottom back to the surface). Each filter has a characteristic particle retention efficiency that depends on the volume of solids trapped irrespective of the duration of the filtration process. A two level full factorial screening design was employed to get an insight of the contribution of factors on brine turbidity. Numerous studies on the screening of process factors by two level full factorial statistical design are available in the literature. For example, a process optimisation study of inorganic membranes [15], screening of nine chemicals considered as variables on bitumen recovery from oil sand [16] and the optimisation of extractive distillation [17].

Materials and method

The working mechanism of the two types of filter media - depth filters and surface filters

- in cartridge filtration systems used widely in the oil industry is shown in Figure 1 and Figure 2.

A nominal rated depth cartridge filter is typically a precisely wound cotton, polypropylene or polyester fibre on a porous support, which works through the depth filtration mechanism by intercepting or adhering solids to the media fibre. This is characterised by a higher dirt holding capacity but inconsistent removal efficiencies, and is used in situations where brine is relatively clean or as a pre-filtration aid [18]. Absolute rated pleated glass fibre or cellulose cartridges are pleated concertina sheets that work on the principle of surface filtration and are used when absolute particle size removal (2-10 microns) is required. An extra filtration area of 60% is achieved through pleating compared with same rating of depth filters [18]. Pleated cartridge filters cannot withstand higher differential pressures and therefore release more fine particles than nominal filters once the pleated sheets are compressed. Among several available tests to quantify the clarity of brine, the easiest and most accessible method is to report total suspended solids in mg/l or in terms of turbidity units, i.e. Nephelometric Turbidity Units (NTU) [19]. To prevent formation damage and for effective completion there is no widely accepted criteria for total suspended solids in brine returning from the well. However, as an agreed general rule of thumb, no more than 5mg/l or turbidity as low as possible (25 NTU) as an upper critical limit may be used [20].

In this work brine was filtered in two phases. Firstly, freshly prepared brine was filtered and stored in tanks while in the second phase the stored brine was again filtered on returning from the well during circulation. Filtration in the second phase is critical in minimising formation damage – the motivation for this study. The on-field data collected over the past three years reveals that for an efficient filtration process the mixed brine must be allowed to settle for at least three hours, followed by filtration at slow rates i.e. 30-50 gpm. For instance, the mixing time required for 400 bbl brine (28.7 lb/bbl dry NaCl), excluding pit cleaning, would be around 1 hour. Filtering 400 bbl brine by a conventional cartridge unit (consisting of 64 filters) at 50 gpm, taking settling and mixing time into consideration, took around 11 hours. Moreover, during filtration an increase in brine turbidity to an average value of 35 NTU suggested the release of fine and accumulated solids, which was an indication of filter media deterioration and brine quality with time. Therefore conventional cartridge unit and traditional oilfield brine filtration practice is not only known for a slow flow rate and high costs, but also results in poor quality of brine i.e. turbidity.

For these reasons, settling time, cartridge quality and filtrate flow rate were accounted for as factors in the two level full factorial experimental design. This was to investigate significant effects and how to enhance brine quality by developing a process strategy in order to increase the flow rate of an existing filtration system with an emphasis on extended service life of filters. A series of sequential experiments for a different configuration of filtration units was conducted to validate the experimental findings of the screening design at a higher flow rate for both phases of filtration operation. Subsequently, the results were verified through real time data for filtering the brine during well-circulation.

Experimental set up

Figure 3 shows the dual-chamber cartridge filter unit with 32 filters in each chamber. The unit is configured so that filter change

Figure 1: Schematic of a depth filtration mechanism. Figure 2: Schematic of a surface filtration mechanism.

Figure 3a: Parallel configuration. Figure 3b: Series configuration.

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Filtration+Separation May/June 2012

by the sum of squares. If the magnitude of variance due to a factor is large compared to error it is accepted that factor’s effect is significant and cannot be ignored. The magnitude of both variances was compared by a statistical test criterion known as Fisher’s F test, given by Equation 1.

errorMSSeffectMSS

F = (Eq: 1)

MSS effect = (Eq: 2)

Where SS effect is known as sum of squares was computed by Yates algorithm d.f is degree of freedom equal to 1.

Since one factor was qualitative, therefore, centre point readings were not considered for computing error. Similarly due to high costs and field constraints, replication was not possible. Therefore, to detect variance, higher order interaction terms or those factors whose sum of squares were relatively small compared to others were summed and consider as errors.

Results and discussion

The present work has been focused to investigate different cartridge filtration strategies in order to find methods intended to increase the dirt holding capacity or service life of filters to attain pre-selected brine clarity at higher flow rates. It was observed from field experience that a decrease in flow rate was connected with the quality of filters and fouling of media. Therefore, a screening test was initiated to study the effects of various factors on filtrate quality in detail.

A factor effect on response is considered significant if its f-value is greater than critical f-value [21]. By comparing factors f-values with critical value, it was evident from the analysis of variance (see Table 4) that flow rate, filter quality and interaction effect of flow rate with filter quality were significant. Although settling time was not yet significant it is important for reducing overall filtration time in the first phase because introducing a more contaminated effluent considerably decreases service life. Since the quantity of solids in a freshly mixed brine is relatively constant, provided that less turbid water for mixing is used. However, settling times at

bottom of the tank. The average brine clarity for all experimental runs after mixing (i.e. composite samples) is shown in Table 1.

Screening

Three factors - settling time of mixed brine, flow rate of filtrate and quality of filters - were investigated at two levels. The quality of the filter refers to the ageing of cartridge filters. Therefore, filters used for two hours were preserved and tested in another experimental run to investigate the ageing effects on dirt holding capacity and filtrate turbidity. Factor levels were chosen from past field experience of the cartridge filtration process. Besides individual effects on brine clarity, interactions of different parameters were also considered for a better insight into the filtration process. Levels were coded + (maximum value) and - (minimum value) for simplicity of analysis as shown in Table 2.

The numeric 1 refers to a new filter and 2 refers to an old filter to represent the discrete quality of the filter media. Old filters were used to compare the dirt holding capacity and thus estimate the service life of new filters in actual test conditions.

The number of experiments according to 2k factorial design was 23 = 8 and was carried out in random order to protect against experimental errors. Clarity of brine in terms of NTU was selected in response. The design matrix constructed according to standard rule order is shown in Table 3.

Analysis of variance (ANNOVA)

The analysis of variance compares the magnitude of variation in response due to a factor with mean error. The effect of variance of a factor between two levels was estimated

in either chamber is possible in continuous filtration. The composite (turbid) brine was pumped through a bag filter (25μm) to each of the chambers in either parallel configuration or series configuration as shown in Figure 3a and Figure 3b. The pressure differential inside each chamber causes brine to flow through the porous cartridge filter media wrapped onto the perforated hollow core connected to a small spherical collecting pan at the bottom. The effluent from both chambers was collectively pumped through the same exit pipe into a clear brine storage tank. It is generally considered appropriate to replace blocked cartridges when the pressure differential reaches 25 psi. However, irrespective of the pressure differential, cartridge change was strongly related to filtrate turbidity, i.e. typically less than 20 NTU. In the first phase brine was filtered to a turbidity of 10 NTU and stored in surface tanks where further settling reduced turbidity to 4 NTU. For the second phase turbidity as low as possible was desirable because pumping turbid brine (> 20 NTU) makes circulation times longer, which leads to a higher consumption of filters.

Analytical methods

The clarity of brine was assessed by a microprocessor-based turbidity meter (HANNA Instruments, Malaysia). The meter passes a beam of infrared light through a sample which is scattered by suspended solids. The amount of scattered light detected by the sensor is converted into Nephelometric turbidity units, where 1 NTU is equal to 0.13 SiO2 mg/l.

Brine of 8.8 ppg, mixed in 32 NTU pit water, from 99% pure NaCl (Baker Hughes Inc, USA) was used in all experiments and the case study.

Turbidity values were averaged from spot samples taken from the surface, middle and

Table 1: Average brine clarity before

filtration.

Settling time

(min)

Brine sample

turbidity (NTU)

0 380-43060 190-220120 130-160

Table 2: Codified values of factors.

S. No Units

Natural values Codified values

Max Min Max Min

Settling time Min 120 60 + -Flow rate Gpm 70 40 + -Filters Quality New Old + -

Table 3: Experimental run order and response.

S. No Std

order

Random

order

Factor A

(Settling

time) Min

Factor B

(Flow rate)

gpm

Factor C

filters

Turbidity

NTU

1 2 1 120 40 1 432 4 2 120 70 1 683 8 3 120 70 2 244 1 4 60 40 1 605 5 5 60 40 2 356 3 6 60 70 1 817 7 7 60 70 2 318 6 8 120 40 2 18

fdSS.

SS

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Filtration+Separation May/June 2012

40.00 46.00 52.00 58.00 64.00 70.00

20

28.5714

37.1429

45.7143

54.2857

62.8571

71.4286

80Warning! I Beams off

A-

A+

Interaction

Turb

idity

A: Settling

B: Flow

InteractionC: Filters

B: Flow Rate

20

28.5714

37.1429

45.7143

54.2857

62.8571

71.4286

80

Turb

idity

40.00 46.00 52.00 58.00 64.00 70.00

40.00 46.00 52.00 58.00 64.00 70.00

20

28.5714

37.1429

45.7143

54.2857

62.8571

71.4286

80

A-

A

Interaction

Turb

idity

A: Settling

B: Flow

process and these results, it was perceived that for the same settling time new filters would have released the same quantity of fine particles into the filtrate at the end of a two hour filtration as released by old filters at the start of the operation. Therefore, for filtration in the first phase, to reduce overall filtration time the settling time may be kept constant at 90 min and the positive effect of higher settling time may be balanced by the phenomena that solids continue to settle as filtration proceeds.

The highest f-value (36.07) of filter quality suggested interception of a wide range of solids that resulted in a constant turbidity at higher flow rates, compared to old filters as shown in Figure 6. The release of fine solids increased drastically when the flow rate for old filters was increased. It was concluded that once the dirt holding capacity was reached, the filters release fine solids as shown in Figure 6. The f-values in variance Table 4 and the results shown in Figures 4, 5 and 6 help deduce that service life depends on filter quality and the flow rate. However, the interaction term BC was more important than flow rate itself because the BC term hypothetically represents a complex function, a decrease in flow rate, that results in the loss of effective filtration area which dictates the service life and hence the overall filtration process. The difference in turbidity achieved with new filters at the end of a 2 hour filtration and at the beginning of operation for old filters was attributed to the BC term i.e. a decrease of effective filtration area that results in a release of fine particles, as shown in Figure 6.

By comparing the dirt holding capacity of filters being investigated it was concluded that the critical service life of 5 μm nominal cartridge filter ranges from 2-3 hours at 70 gpm.

Analysis of experimental findings has revealed that the service life of filters was dependent on flow rate and the volume of solids being introduced. Therefore, to develop a

efficiencies decreased, as shown in Figure 5. The higher turbidity at low settling times suggests the release of particulate solids that is a characteristic of nominal filters.

It may be observed from the results shown in Figures 4 and 5 that the dirt holding capacity and hence the service life depends both quantitatively and qualitatively on the solids (i.e. the size and shape) being introduced. Therefore, based on knowledge about the

different flow rates were studied to gain an understanding of the dirt holding capacity. The difference in turbidity, at the start and end of one hour’s filtration, at both high settling time (A+) and low settling time (A-) for old nominal filters were attributed to the inconsistency of removal efficiency as shown in Figure 4.

Using new filters increased dirt holding capacity while the inconsistency in removal

Table 4: Analysis of variance calculated by Yates` algorithm

Source of

variation SS

effectd.f. MSS

effectf-value Critical f- value

A 144.5 1 144.5 0.44

F 0.1(1,3) = 5.54 [20]

B 2812.5 1 2812.5 8.67

AB 312.5 1 312.5 0.96

C 11704.5 1 11704.5 36.07

AC 480.5 1 480.5 1.48

BC 2380.5 1 2380.5 7.34

ABC 180.5 1 180.5 0.56

Error 973.5 3 324.5

Figure 4: Effect of settling time and flow rate on brine turbidity for old filters.

Figure 5: Effect of settling time and flow rate on turbidity using new filters. Figure 6: Effects of flow rate and filters quality on turbidity.

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Filtration+Separation May/June 2012

Path 1

Path 2

31 24

18

44.75

35

58.25

81.25B+: 70.00

B-: 40.00

A-: 60.00 A+: 120.00A: Settling Time

C: Filters

67.75

C-:-1.00

C+:1.00B: Flow Rate

Operating Scheme 1

Operating Scheme 2

Operating Scheme 3

Operating Scheme 4

22-28 NTU. This was because brine was introduced simultaneously to both chambers of the filtration unit as shown in Figure 3a. Therefore, it was concluded that turbidity greater than 20 NTU will result in a higher filter change frequency, which was against the objectives. Experimental results showed that an increase in the filtration area increases service life by 4 times.

However, dirt holding capacity for the same rating depends on the volume of particulate solids, and size and shape, irrespective of the time the filters were in operation. A third run by connecting filters as in operating scheme 3 was carried out to extend service life and brine quality, because after reaching the specific dirt holding capacity of the filter the release of fine solids was captured by relatively fresh filters in the second chamber. Moreover, introducing pleated cartridges in operating scheme 4 only ensured superior brine clarity, 9 NTU in 4.5 hours as shown in Figure 8, although service life of nominal filters was greater than the pleated filters by one hour.

Real-time case study as

confirmatory test

Application of the insights gained from the results of sequential tests was difficult in a real situation because replication of the test conditions (solids volume, size, shape and nature) is impossible when brine is circulated through the well. During circulation of brine through the well it picks up a variety of left-over drilling fluid solids, such as sand, debris, dry cement and rust particles from drill pipes, due to the geometry of the well design, turbulence and poor well cleaning operations. Therefore, experimental results of sequential tests were validated and confirmed through a real-time case study. Experience showed that in addition to solids picked up from the well, pumping a filtered brine of more than 20 NTU from rig tanks increases circulation time, filter change frequency and thus the overall cost of the operation.

cartridge filters in parallel configuration were tested at 100 gpm to provide a basis for comparison with different configurations of the filtration system. It was evident from the result of the first test, as shown in Figure 8, that using this configuration leads to frequent filter changes, which reflects high costs and rig time. Therefore, in order to meet the minimum requirements of less than 20 NTU at maximum flow rate, the experimental results indicate the use of a larger filtration area. A filtration area double that of the previous cartridge system was used - 128 cartridges with 64 in each chamber. Pleated cartridge filters offer 60% extra filtration area for the same size of nominal cartridge filters. However, their dirt holding capacity, which depends on the strength of the filter media may be less [17]. Consequently, due to a relatively shorter service life, 5 μm pleated cartridge filters were used in a series configuration upstream in a second chamber. This was to prevent them from premature blockage and as a corrective measure of inconsistent removal efficiency of nominal filters. Three sequential tests comprising a total of 128 filters in different configurations were conducted as shown in Table 5. The turbidity of mixed brine after 90 min of settling time was in the 150-170 NTU range.

Results of four test runs are shown in Figure 8. The dirt holding capacity of operating scheme 1 was reached in one hour while increasing the filtration area - operating scheme 2 extended service life to 4 hours. Although in parallel filtration mode the filter area was increased, a service life of 4 hours gave a relatively higher turbidity,

strategy for improving cartridge filtration an understanding of the process was attempted by representing the effect of choice of factor`s level on response as a cube as shown in Figure 7. At high and low flow rates the effect of reducing settling time for new filters resulted in an increase of 8 NTU turbidity after two hours operation, as can be seen in Figure 7. While past experience showed that loss in filtration area due to accumulation and compression of filter media leads the filtration process to take path 1, while constant addition of solids in brine from the well during circulation takes the process along path 2. Prolonged filtration due to frequent filter change was the consequence of both process paths that results in a shorter service life of filters. Therefore, to meet the minimum requirement of less than 20 NTU at maximum flow rate, it was suggested from knowledge about the process and Fisher’s f-test value (C = 36.07) to use a larger filtration area for intercepting a wide range of solids. BC of 7.34 suggests that the use of a combination of filter media may compensate for the loss of effective filter area.

Sequential test approach for

well circulation

Four experiments were conducted to predict cartridge filter behaviour for the critical filtration phase when brine is constantly circulated through the well where left-over solids differ considerably in shape and size compared to brine mixed on the surface. For this purpose a sequential approach was adopted and a test incorporating 64 five micron nominal

Table 5: Operating configurations of cartridge filtration unit.

S.

No

Configuration

(Operating

scheme)

1st chamber

(No. and type of

filter)

2nd chamber

(No. and type of

filter)

Total No.

of filters

1 Parallel 32-nominal 32-nominal 642 Parallel 64-nominal 64-nominal 1283 Series 64-nominal 64-nominal 1284 Series 64-nominal 64-absolue pleated 128

Figure 7: Effect of all factors on response as cube representation. Figure 8: Sequential test results for different operating configurations.

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Filtration+Separation May/June 2012

Sample Point 1

Sample Point 2

Well

Return Brine Filtered Brine<10NTU

Filtration Unit

Operating Scheme 2

Operating Scheme 3

Operating Scheme 4

NT

U

Time hrNo. of Circulation

Re

turn

NT

U

128 Parallel Nomial Cartridge

64 Pleated Cartridge Series Upstrem

dirt holding capacity is reached. Therefore, the selection of the NTU at which filters might be changed is critical in minimising circulation time in a cost effective way. A better understanding of the results of filtration in operating scheme 2 was important to assess filter change time. The estimated service life of nominal filters was 2.5 hours under operating scheme 2 at actual well conditions as shown in Figure 10. From an economic point of view and the understanding gained from experimental findings, it was suggested to replace filters only in first chamber. This was because after reaching critical service life of 2.5 hours the release of filtered solids not only reduced the life of the filters in the second chamber but also deteriorated the brine quality that ultimately resulted in longer circulation times. The turbidity (NTU) of brine returning from the well was plotted against number of circulations. Because filtered brine of 4 NTU was pumped from the rig storage tanks, only annular volume (typically 560 bbl) was considered for the estimate of the circulation cycles.

A sharp decrease in return NTUs above 60 may be observed in Figure 11 due to filter

bbl return brine from the well was swept-out before commencing filtration until the return-brine turbidity was in the range of 100-160 NTU. Higher NTUs dictate initiation of a well cleaning programme. Similarly to ensure the integrity of continuous filtration and service life, bottom-up volume was swept out since brine is non-viscous and solids tend to settle at the bottom at a slow rate. The results of tests performed on real-time well circulation are shown in Figure 10.

Operating scheme 3 showed a critical filter change time of 3 hours, compared with 4 hours when pleated cartridge filters were used in operating scheme 4, as shown in Figure 10. An increase of one hour of operation was attributed to the additional filtration area of pleated cartridge that resulted in a clear brine of 5 NTU turbidity over 4 hour’s filtration. A sudden increase in turbidity after 4 hours filtration in operating scheme 4 indicated blockage and replacement of filters in the first chamber. However, filter change at this stage may result in unnecessary costs for brine filtration in the first phase. It is observed that filters release particulate solids once their

The schematic of the set-up for brine filtration during well circulation is shown in Figure 9. Filtered brine from the storage tank (4 NTU) was pumped into the well and the return was sent back to an adjacent tank. The returned brine was continuously filtered and pumped to the filtered brine storage tank and subsequently to the well again. The clarity of the brine was assessed at regular intervals of time at two points, as shown in Figure 9. Sampling point 1 ensured the clarity and level of decrease in solids from the well while sampling point 2 helps detect the appropriate time for filter replacement.

Based on process know-how in order to reduce the solids-laden brine returns, it was recommended to pump brine of 4 NTU from rig tanks. However, restricting brine clarity to 4 NTU considerably increases costs though the filters may be used further. Therefore, in this case study in order to evaluate circulation time for different operating schemes, filters were changed immediately when a turbidity of 10 NTU from the filtration unit was reached, as shown at sampling point 2 in Figure 9.

Unlike filtering freshly prepared brine, turbulent flow regimes at the drilling-bit constantly stir solids that are carried to the surface. Therefore, precautionary measures were taken to replicate test conditions to some extent. For example, more or less 50

Table 6: Summary of experimental results and observations.

Operating

scheme

Filtration

phase aFlow

rate

(gpm)

Service

life

(hours) b

Turbidity

(NTU)

Filter

changes

No. of filters

replaced c

Total

time of

filtration

(hours) d

1 Surface 50 3 20 128 91 Surface 70 2 18-28 128 71 Surface 100 1 25-30 192 52 Surface 100 4 18-20 128 33 Surface 100 5.5 18-20 128 34 Surface 100 4.5 5-10 128 32 Circulation 100 2 40-50 256 143 Circulation 100 4 20 256 104 Circulation 100 4 18 192 7

Figure 9: Experimental set-up with brine-circulation

through the well.

Figure 10: Results of the real-time case study.

Figure 11: Return brine turbidity versus circulation.

(One circulation equals 4 hours at a 100 gpm flow rate)

a Surface phase refers to filtering 400 bbl freshly mixed brine, while circulation refers to filtering brine during well

circulation.

b Corresponds to the average duration when filter are changed once turbidity reached 10-20 NTU.

c Total number of filters used during filtration.

d Total time included time required for filtration and filter replacement.

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Filtration+Separation May/June 2012

changes in operating scheme 3. On the basis of past experience and service life estimated from Figure 10, nominal filters in the first chamber were changed in the first circulation and subsequently of the second chamber at 1.5 circulations once 10 NTU at the filtration unit was observed. Clearly pleated filters in operating scheme 4 achieved the required return turbidity in a fewer number of circulations with only one filter change of nominal cartridges. Moreover, the filter change frequency and therefore the time required in replacing filters and associated costs was also reduced when using pleated filters compared to nominal filters. Therefore it was concluded that operating scheme 4 was the most suitable configuration for brine filtration during circulation through the well, while operating scheme 3 was the promising option for filtering mixed brine on the surface. The results are summarised in Table 6. Operating scheme 1 was taken as reference to compare the experimental results for brine filtration in the surface phase, while operating scheme 2 provided a reference for the circulation phase.

Conclusions

The work focused on the process improvement of a cartridge filtration system for oilfield applications to reduce the time required for filtration and hence cost of filtration as a daily rig cost, filter change frequency, enhancing filtrate flow rate and to meet brine clarity specifications in a minimum number of circulations. Three factors - settling time, cartridge quality and filtrate flow rate - were investigated through two level full factorial design to study the effects of factors on brine quality and a systematic approach for increasing flow rate for brine filtration was proposed.

The results of a slow flow rate filtration process studied through experimental design were used to improve flow rate by undertaking sequential tests to meet minimum requirements of brine clarity at a low cost and less rig time. Both 5μm nominal depth filters and absolute pleated cartridge filters were investigated under various configurations and the experimental results were verified through real time data of brine circulating through the well.

It was observed that nominal filters in a series combination may extend service life to 5 hours and is a suitable option for filtering mixed brine on the surface provided larger volumes are mixed in tanks to save mixing and settling time. For brine circulated through the well, a batch of 64 absolute cartridge filters installed upstream of 64 nominal filters not only reduced filter cost and change frequency but also resulted in the required turbidity of returned brine in 1.5 circulations - 6 hours.

Acknowledgments

The authors thank Scomi KMC Sdn Bhd Malaysia for providing technical and

engineering assistance and permitting this study. Gratitude is extended, also, to Chemical Engineering Department, UET Peshawar Pakistan for supporting this research.

References

[1] Pasztor, A.J, and Snover, A.J., How to Treat Metal Contamination From Heavy Clear Brines,” Oil And Gas Journal, July 18, 1982

[2] S. Bagsi, M, V Kok and U. Turksoy, “Determination of Formation Damage in Limestone Reservoirs and its Effect on Production”, J. Pet. Sci Engg. 28 (2000) 1-12

[3] Rike, James L, “The Relationship Between Clean Fluids and Effective Completions”, Proceedings of SPE Annual Technical Conference and Exhibition, September 1980, Dallas, Texas

[4] A. M. Hemeida and A. A. Gawish, “Formation Damage Tests of Some Completion Fluids”, Oil and Gas Business Journal, 2008 (Http://Www.Ogbus.Ru/Eng/)

[5] Bennion, D.B., Thomas, F. B., Douglas W. B., Effective laboratory coreflood tests evaluate and minimize formation damage in horizontal well Technology”, Presented at 3rd conference on horizontal wells November 1991, Houston, Texas.

[6] Cantu, L.A and Nall, A.E, “Field Evaluation of D.E. and Cartridge Filters for Completion/Workover Fluid Filtration”, SPE Annual Technical Conference and Exhibition, 1987, Dallas, Texas

[7] Lee, J.M., Frankiewicz, T., and Walsh, J. 2006. “Treatment of Seawater With Cartridge Filtration--A Field Trial”. SPE Proj Fac & Const1 (4): 1-5. SPE-101941-PA.

[8] William, C. J. and Edyvean, R. G. J., “The Effect Of Organically Loaded Feedstreams On The Operation Of Cartridge Filters”, Wat. Res. 30(4,) 1996, 972-980.

[9] Wakeman, R. J., Hanspal, N .S., Waghode, A. N. and Nassehi, N., “Analysis Of Peat Crowding And Medium Compression In Pleated Cartridge Filters”, Chem. Engg Rech Design 83 (A10), 2005, 1246-1255 [10] Ni Mhurchu, J., Dead-end and Cross Flow Microfiltration of Yeast and Bentonite Suspensions: Experimental and modeling studies incorporating the use of Artificial Neural Networks”, PhD Thesis, Dublin City University, School of Biotechnology, November 2008.

[11] Waghode, A. N., Hanspal, N. S., Wakeman, R. J. and Nassehi, V., “Numerical Analysis Of Medium Compression And Losses In Filtration Area In Pleated Membrane Cartridge Filter”’, Chemical Engineering Communications, 194 (8) 2007, 1053-1064

[12] Giglia, S., Rautio, K., Kazan, G., Backes, K., Blanchard, M., Caulmare,J., “Improving the accuracy of scaling from discs to cartridges for dead end microfiltration of biological fluids”, Journal of Membrane Science, 365 (2010) 347-355

[13] Zarenezhad, B., “A Correlation for Prediction of the Required Minimum External Surface Area of Gas Cartridge Filters in the Petroleum Industry”, Petroleum Science and Technology, 29 (5) 2011 522-528

[14] Juhasz, H., “Filter performance: Myth and reality. Practical Aspects of Filter Testing”. Proceedings of the 3rd World Filtration Congress, The Filtration Society, September 1982, pp. 596-603.

[15] Ling, W. S., Tye Ching Thian,T. C. and Bhatia, S., Process Optimization Studies For The Dehydration Of Alcohol–Water System By Inorganic Membrane Based Pervaporation Separation Using Design Of Experiments (DOE). Separation and Purification Technology, 71, 192 (2010).

[16] Fong, N., Ng, S., Chung, K. H., Tu, Y., Li, Z., Sparks, B. D., and Kotlyar, L. S., “A Two Level Fractional Factorial Design To Test The Effect Of Oil Sands Composition And Process Water Chemistry On Bitumen Recovery From Model Systems”, Can. J. Chem. Eng. 82 (2004). 782-793.

[17] Biglari, Mazda , Langstaff, Andrew and Elkamel, Ali., “The Application of Response Surface Methodology for the Optimization of an Extractive Distillation Process”, Petroleum Science and Technology, 28 (17) 2010, 1788-1798

[18] K. Sutherland, “Filters and Filtration Handbook”, 5th ed, Butterworth-Heinemann, Linacre House, Jordan Hill, Oxford OX2 8DP, UK

[19] TETRA Inc., Engineered Solutions Guide for Clear Brine Fluids and Filtration, 2nd ed, 2007, Tetra Technologies Inc., Texas, USA.

[20] Pourciau, R. D., Fisk, J. H., Descant, F. J., “Completion and Well-Performance Results, Genesis Field, Deepwater Gulf of Mexico”, presented at the SPE Annual Technical Conference and Exhibition, October 2003 Denver, Colorado, USA,. SPE Drilling and Completions

[21] Montgomery, D. C., (2000), Design and Analysis of Experiments, 5th ed. John Wiley & Sons, New York, NY

Contacts:

Afzal Ahmad

Scomi KMC Sdn Bhd, Malaysia

E-mail: eefzal@yahoo.com

Iftekhar Ali Khan, Muhammad Imran Ahmad and

Hayat Khan

Department of Chemical Engineering, University

of Engineering and Technology