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Laboratory investigations of compatibility of the Dammam Formation Aquifer with desalinated freshwater at a pilot recharge site in Kuwait
Journal of Arid Environments (1998) 40: 27]42 Article No. ae980428
Laboratory investigations of compatibility of the Dammam Formation Aquifer with desalinated freshwater at a pilot recharge site in Kuwait A. Mukhopadhyay w †, E. Al-Awadi w , M. N. AlSenafy w and P. C. Smith‡ w
Hydrology Department, Water Resources Division, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109, Safat, Kuwait ‡Oil Plus Ltd., Hambridge Road, Newbury, Berkshire RG14 5TR, U.K. (Received 17 September 1997, accepted 26 May 1998) The degree of compatibility of desalinated water with native ground-water and aquifer material of the Dammam Formation at a selected pilot recharge site in Kuwait was investigated. Core flow tests did not indicate any major adverse effect from injection of desalinated water into the aquifer. Geochemical simulation, jar tests and dynamic scaling tests did not suggest any scaling for mixtures of Dammam Formation water and desalinated water. The mercury porosimetry data suggested, however, that injection water should be filtered to remove suspended solids of diameter 1.5 microns and higher to avoid clogging of the pore spaces. q 1998 Academic Press Keywords: porosity; permeability; core flow test; mercury porosimetry; scaling; dynamic scaling test; geochemical simulation
Introduction Kuwait is an arid country without any surface water resource and with limited supply of useable ground-water. The country depends almost exclusively on the desalination of ground-water for its freshwater supply. During the higher demand periods of summer, the desalination plants are left with very little spare capacities, and any major breakdown may cause supply restrictions. The surface storage facility (approximately 10 Mm 3 ) that is currently available for any emergency situation could meet the demand of the current population level (approximately 1.8 million) at an average daily per capita consumption rate of 470 l dayy1 (MEW, 1997) for only 10 to 15 days. Creation of additional surface storage facilities is costly. Storage of water in aquifers through artificial recharge is comparatively cheaper, and technically feasible. Artificial recharge † Corresponding author. 0140]1963r98r010027 q 16 $30.00r0
q 1998 Academic Press
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Figure 1. Universal transverse mercator (UTM) projection of the State of Kuwait, showing the locations of the proposed recharge site, cored test well, and the existing brackish water fields (A, B, C, D, E and others).
of ground-water to create a strategic reserve of freshwater for emergency use is being considered by the Ministry of Electricity and Water (MEW) in Kuwait, and a site has been selected (Fig. 1) to create a reserve of freshwater, with the Eocene Dammam Formation as the target for recharge (Viswanathan & Mukhopadhyay, 1992). Due to the high evaporation rate in the desert environment of Kuwait and the depth of the aquifer, injection through wells is the preferred method of recharge. It has been felt, however, that before implementing the pilot project, the compatibility of the injection water (desalinated freshwater) with the ground-water and the aquifer materials should be investigated, so that proper remedial measures can be adopted to ensure success of the artificial recharge. The results of such a study are reported here.
Petrology and aquifer characteristics of the Dammam Formation at the recharge site The general geology and hydrogeology of the Tertiary succession of Kuwait, of which the Dammam Formation is a part, have been discussed in detail by Mukhopadhyay et al. (1996). The Dammam Formation is a dolomitic limestone of Middle Eocene age,
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180 to 200 m thick that is underlain conformably by the Lower Eocene anhydritic Rus Formation and is unconformably overlain by the Mio-Pleistocene clastic Kuwait Group. A well was drilled at a location within the selected recharge site (Fig. 1) where the top of the Dammam Formation was encountered at a depth of 108 m. The formation was continuously cored from 122 to 271 m using a 2.7-m long double-core barrel with an internal diameter of 101 mm. Except in the ranges of 178 to 183.5 m, where the core recovery was poor (15 to 25%), the recovery in general was 60 to 100%.
Stratigraphy and lithology of the Dammam Formation at the test site Cores recovered from the Dammam Formation were examined in hand specimens and thin sections, and also under a scanning electron microscope (SEM) and by X-ray diffraction (XRD). Based on the lithological characteristics revealed by these examinations, and comparison with the Dammam Formation’s lithology and geophysical log characteristics in the neighbouring Umm Gudair Water Field, the following three-fold subdivision of the sequence penetrated at the study site has been proposed (Fig. 2): Upper Member, 108 to 186 m; Middle Member, 186 to 216 m; Lower Member, 216 to 271 mq. Lithological descriptions of the three members are as follows. Upper member The range from 108 to 120 m mostly consists of chertified dolomite, and possibly represents the karstified zone at the top of the Dammam Formation. The marly, chalky, fossiliferous, very fine-grained, compact dolomicrite and coarser-grained, porous dolomite are the main lithologic constituents of the rest of the member. Vugs, moulds and intergranular porosity are the main types of porosity. Later diagenetic activities have sometimes dissolved the crystal cores, giving rise to intragranular porosity. Remnants of micritic ground mass can be sometimes observed. Partial silicification in the form of spherulitic chalcedony and a few macrocrystalline quartz grains filling the cavities have been observed occasionally. The vugs are often filled with sparry calcite. The allochem grains, mostly represented by fossil shells, float in the ground mass. Pyrite and phosphates are also common. The XRD examination confirms that dolomite is the main mineral constituent of the range. The occurrence of palygorskite fibres covering dolomite grains and spanning across intergranular pores is revealed under SEM. Palygorskite also occurs occasionally as vein fillings. Friable ranges occur below 150 m where recovery of core was poor. Loss of circulating drilling fluid (circulation loss) was also experienced during coring operation below 178 m with very poor recovery (15 to 20%) of cores in the range 178]183.5 m, suggesting that the range may be highly fractured and permeable. Middle member The unit is mainly composed of vuggy and bioturbated laminated dolomite (down to around 206 m) and limestone (below 206 m) with layers of lignite and bituminous material. The laminated dolomite is highly calcitized. Remnants of the original dolomicritic host can be observed in the form of nodules within sparry calcite. The lithotype is rich in phosphate that occurs in the form of pellets and rod-like particles of golden colour, and organic matter that occurs as dark thin laminae. Traces of anhydrite, gypsum and sulphides are also observed. Calcitization, in some cases, is associated with
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Figure 2. Lithostratigraphy of the cored well.
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silicification in the form of precipitation of a radiating mass of chalcedony replacing the ground mass. Calcitization has increased towards the bottom of the range and, below 206 m, the lithology consists mainly of calcite. Vugs, moulds and intergranular porosity are the main porosity types in this range.
Lower member The unit consists of nummulitic limestone with vuggy and moldic porosity. Below 233 m, the nummulitic limestone is associated with bituminous materials. The nummulites are the dominant biogenic allochem. Post-depositional calcitization has produced macrocrystalline and sparry calcite that has invaded the original micritic phosphate and organic-rich ground mass, and cemented the internal chambers of the nummulitic shells. The XRD pattern confirms that the predominant constituent of the lithotype is calcite. Phosphates are common in the range and the presence of traces of pyrite and anhydrite is revealed under SEM. Gas chromatographic examination shows that the bituminous materials occurring below 233 m consist mostly of high alkanes above C 28 . Vugs and moulds are the main porosity types in this range.
Porosity and permeability Porosity was measured using a helium gas expansion porosimeter and permeability was measured with a gas permeameter. The measured porosity (in percentage) and extrapolated absolute permeability (in millidarcy) are plotted against depth in Fig. 3. It may be seen from the figure that in the range of 123 to 160 m, porosity is in the range of 3 to 35% and the permeability values are in the range of 0.00001 to 80 md (millidarcies). There are very few porosity and permeability measurements in the range of 160 to 190 m due to the difficulty in cutting core plugs from marly and chalky friable dolomite and poor recovery (178 to 183.5 m) while coring. The poor recovery range also coincides with high loss of mud while coring, and the loss continued to a depth of 210 m. The range of 160 to 190 m thus appears to be highly porous and permeable. The range of 190 to 216 m is characterized by porosity in the range of 0 to 33% (most of the porosity is, however, below 20%), and permeability in the range of less than 0.03 to 20 md. Below 216 m, the porosity shows a decreasing trend with depth (from around 20 to 25% at the top of the range to below 10% at a depth of around 260 m). The permeability also decreases simultaneously from around 10 md at the top to below 0.01 md at around 260 m. Examination of Fig. 3 will further suggest that the porosities and permeabilities of the vertical and horizontal plugs are more or less of the same order of magnitude and no systematic difference between the two groups could be found. The transmissivity of the Dammam Formation in the vicinity of the cored well, as determined from the pumping test data, is around 35 m2 dayy1 . Assuming that the whole exposed thickness of the Dammam Formation (122 to 271 m) was contributing to the flow, average hydraulic conductivity of the formation at the site is around 0.24 m dayy1 or a permeability of 320 md. The maximum permeability measured for the core plugs using a gas permeameter is 125 md. Permeability measured on core plugs using Dammam synthetic water and tap water during core flow tests varied in the range of 1 to 120 md. It is, therefore, most likely that most of the flow from the aquifer takes place through some highly permeable zone that may be fractured or vuggy. As discussed earlier, the range of 160 to 190 m may be such a zone.
Figure 3. Measured (a) horizontal (kh_gas, l) and vertical (kv_gas, B) permeability, and (b) horizontal (phi_h, l) and vertical (phi_v, B) porosity of the Dammam Limestone core plugs from the test well.
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Pore size distribution using mercury injection porosimetry and its implication for pretreatment of injection water A pore can be blocked by a particle of the same size as itself. In addition, it can be shown mathematically that three spherical particles with diameters 92% of the pore radius can wedge in a pore to form a stable bridge (other numbers of particles tend to form unstable bridges, although seven particles may also bridge effectively). Thus, to avoid clogging from suspended solids, it is necessary to remove most of the particles of sizes equal to or larger than those which might form such a bridge. The statistical chances of seven-particle bridging may normally be neglected. Based on the abundance of particles of various sizes present in the source water, and the distribution of potentially blockable pores, it is possible to define the filtering requirement of the injection water that will remove suspended solids down to a particular size grade. The treatment should help in containing blocking, and thus loss of injectivity, to within acceptable levels for the life of the water injection scheme. In mercury injection porosimetry, mercury is injected into a core plug under pressure. Measurement of the volume of mercury entering the plug at each pressure stage indicates the cumulative percentage of pores above a specific size, and thus, a pore size distribution is obtained. From this is derived a permeability curve, based on the assumption that a reservoir is made up of a bundle of unconnected parallel capillaries of varying sizes and the fluid flow through these capillaries follows Poiseuille’s equation, as presented by Kestin & Wakeham (1988, p. 111). The equation relating mercury pressure and pore radius holds good for radii of curvature down to 10y6 cm (i.e. 0.01 micron). Below this, the radius of curvature becomes comparable with the diameter of the mercury atom, and the concept of surface tension becomes invalid. The derived permeability curve indicates the degree of loss of permeability when all pores up to a certain size are plugged. A good general guide to this pore size range is the mean hydraulic radius (MHR), which is related to the root mean square of the pore sizes. This concept is based on an extension of the original work carried out by Kozeny (1927). The details of the core plugs used in the study, along with the derived mean hydraulic radius values (MHR) based on porosimetry data for each of the 10 plugs available for this study are presented in Table 1. When determining filtration specifications, it is desirable to define a cut-off permeability above which blockage would cause an unacceptable loss in injectivity. This can be achieved in a number of ways depending on the extent and representivity
Table 1. Porosity, permeability and MHR of core plugs used in mercury porosimetry experiments
Plug reference
Depth (m)
Gas permeability (md)
Porosity (%)
MHR (m m)
10 14 16 17 24 37 40 44 46 48
140.33 146.00 148.63 149.12 157.50 191.50 198.00 204.90 206.00 210.23
9.3 26.2 26.3 19.0 135.5 66.0 0.001 32.3 0.003 46.0
27.2 13.8 0.03 32.7 25.1 33.0 8.6 8.0 3.7 26.2
1.1 1.4 0.1 0.1 4.4 2.2 -0.1 6.7 0.6 2.4
MHR s mean hydraulic radius.
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of the routine permeability data. In this case, with very limited data that does not permit the derivation of detailed statistical parameters of permeability distribution, the median permeability value (derived from the permeability values in Table 1) has been used to define the cut-off point. Utilizing the median permeability value of 22.5 md and relating this to the mean hydraulic radius data in Table 1, a critical pore radius of 1.7 microns is derived. Allowing for the possibility of three-particle bridging, this radius equates to a maximum allowable particle diameter (MAPD) of 1.5 microns. In order to prevent unacceptable losses in injectivity, based on the mercury porosimetry data, a filtration specification of 1.5 microns is therefore recommended. However, there must be some doubt regarding the accuracy of this value, given that the raw data upon which it is based is somewhat limited, and the Poiseuille model assumed here is too idealistic and may not be representative of the actual porosity distribution in the aquifer.
Compatibility of desalinated water with Dammam Aquifer water Chemical character In Kuwait, seawater is desalinated using multistage flash (MSF) technology (Darwish et al., 1995) and is subsequently mixed with 10% brackish water to produce potable quality water that is supplied through the freshwater pipeline. The results of API and dissolved gas analysis of this freshwater and Dammam Formation water collected on-site from the same well that had been cored are presented in Table 2. The particle-size distribution in the desalinated water is presented in Table 3. The data show this water to be relatively clean. As would be expected, the desalinated water contained very low amounts of dissolved salts, but the sulphate content was double the chloride content. Heavy metal concentrations were very low, with only zinc above 0.1 mg ly1 , while nutrients contents were also very low. The Dammam Aquifer water exhibited significant sodium, calcium, chloride, sulphate and bicarbonate contents, with 0.24 mg ly1 iron and 10 mg ly1 nitrate. Heavy metal concentrations tended to be low, and apart from nitrate, nutrient levels were also very low. Except for acetic acid, the volatile fatty acids contents of these two waters were very low, i.e. below the limit of detection of 0.1 mg ly1 . Even the acetic acid contents were below 2 mg ly1 for both the waters, again low values. These waters are thus very low in nutrients that would tend to hinder bacterial activity.
Geochemical simulation Based on the data from chemical analysis, a geochemical simulation of the scaling tendency of the mixtures of Dammam water and desalinated water was carried out using a software system developed by OLI Systems Inc. (1995). The results of these predictions are given in Table 4. These results show that no scale of any type is predicted to form in the aquifer under injection conditions of 358C and 500 psi.
Jar tests Static jar tests were carried out by mixing the two types of water at different ratios (100, 70, 50, 30 and 0 per cent desalinated water) in 1 l bottles and maintaining the
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Table 2. API and dissolved gas analyses of Dammam Formation water and desalinated tap water
Parameter
Dammam Aquifer water Concentration meq ly1 y1 (mg l )
Sodium Calcium Magnesium Potassium Strontium Barium Iron Aluminium Silicon Manganese Zinc Lithium Copper Cadmium Selenium Boron Chloride Bromide Fluoride Sulphate Nitrate Phosphate Ammoniacal nitrogen Temperature (8C) w Bicarbonate w Carbonate w Hydroxyl w Specific gravity at 208C Conductivity (mS) at 258C pH at 258C w TDS (mg ly1 ) CO 2 (mg ly1 ) w O 2 (p.p.b.) w H 2 S (mg ly1 ) w w
728 563 197 22 14.8 0.04 0.24 0.18 9.7 0.02 0.1 0.09 0.01 0.01 -0.06 1.3 1776 8.0 2.0 1200 10.0 -1.0 -0.4 28.3 146.4 Nil Nil 1.0033 6.42 7.14 4679.5 59 30 0.26
31.65 28.15 16.14 0.56 14.8 ] ] ] 0.69 ] ] ] ] ] ] 0.24 50.03 0.1 ] 25 ] ] ] ] 2.4 ] ] ] ] ] 169.76 ] ] ]
Desalinated water Concentration meq ly1 y1 (mg l ) 46.9 33.8 13.9 1.5 0.96 -0.01 0.05 -0.03 1 -0.002 0.22 0.01 0.03 -0.01 -0.06 0.1 58 -1.0 0.2 119 2 -1.0 -0.4 30 18.3 Nil Nil 1.0005 0.563 6.86 297 8.8 41000 Nil
2.04 1.69 1.13 0.03 0.02 ] ] ] ] ] ] ] ] ] ] 1.63 ] ] 2.48 ] ] ] ] 0.28 ] ] ] ] ] 9.3 9.3 ] ]
Measured on-site at the sampling point.
mixtures for 3 days at 358C. Subsequently, any precipitate that formed was filtered and quantified gravimetrically using 0.45 millipore membranes. However, no precipitate was collected on the membranes, supporting the results of the geochemical simulation. Examination of the rare solids collected on the membrane using a binocular microscope, scanning electron microscope and an energy dispersive spectroscope indicated them to be residual salt cubesrcrystals.
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Table 3. On-site particle-size distribution analyses w for desalinated water by Coulter Z1 Instrument, 10 April 1996
Particle diameter, d ( m m) 1.5 2 3 4 5 6 7 8 9 10 12 15 18 20 Turbidity (NTU) Total suspended solids (mg ly1 )
Sampling time (h) 1030 1230 Abundance G d m m in 0.1 ml 524 227 81 39 28 14 3 1 0 0 0 0 0 0 0.12 0.214
533 231 82 42 30 15 3 1 0 0 0 0 0 0 0.13 0.218
w Samples for particle-size distribution analysis had to be dosed with drops of ultrafiltered standard NaCl brine so as to make them conductive enough for analysis using a Coulter Zl instrument.
Table 4. Scaling tendency predictionw for mixtures of desalinated water and the Dammam Aquifer water at 358C, 500 psi using geochemical simulation
Injection water (%)
CaCO 3
0 10 20 30 40 50 60 70 80 90 100
95.5 86.9 77.7 68.2 58.4 48.3 37.5 21.7 10.7 3.9 0.6
Saturation (%) CaSO4 SrSO4 39.0 34.0 29.0 25.0 20.0 16.0 12.0 8.0 5.0 2.0 1.0
68.1 59.4 50.9 42.7 34.8 27.4 20.5 14.2 8.8 4.3 1.1
BaSO4 0 0 0 0 0 0 0 0 0 0 0
w No scale formation is predicted for any case. Scale solids begin to precipitate when saturation is above 100%.
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Dynamic scaling test A more practical test of scaling under field conditions is generally carried out in a dynamic scaling rig where mixtures of two types of water under the pressure]temperature conditions of the aquifer are passed through a capillary coil and the change in differential pressure required to maintain a constant flow rate is monitored. The amount of scale deposited is determined from the difference in weight of the coil before and after the test. The computer simulation suggested that even the raw Dammam water was unsaturated with most of the chemical species that could cause scaling problems (CaCO 3 , BaSO4 , CaSO4 , SrSO4 ), and as this water was diluted with the injected desalinated water the mixtures would be more and more unsaturated (Table 4). The dynamic scaling test was, therefore, run with 100% synthetic Dammam water under a system pressure of 300 psi (expected average hydrostatic pressure in the Dammam Aquifer at the pilot recharge site) at a temperature of 358C (expected prevailing temperature). The differential pressure remained steady at around 1 psi for the entire 4 h of the test, suggesting that no scale was formed during the test.
Compatibility of injection water with core material: core flow tests Twelve horizontal plugs, selected to give as uniform a coverage as possible of the Upper and Middle Members of the Dammam Formation (the target zone for artificial recharge) were used for the core flow test. A Ruska Liquid Permeameter was used for this purpose. The plugs were first cut to size to fit the core holder of the permeameter. They were then saturated with synthetic Dammam water by immersing them in the liquid under a vacuum for more than 72 h. The permeabilities of the core samples were measured for the flow of each 50 cc of Dammam synthetic water for a total flow of 100 cc under a pressure head of 2 bar. The samples were subsequently turned around, and the permeabilities were again measured with the same Dammam synthetic water for another 100 cc flow in steps of 50 cc. Water samples at the outflow end were collected for both forward and backward runs. On completion of the Dammam synthetic water runs, the permeabilities of the core samples were measured using the same procedures as described above, but using filtered desalinated tap water. Samples of the outflow water were again collected for both the forward and backward runs. For sample H-20 (depth: 153 m), due to a very slow flow rate, the measurements were done in steps of 5 cc for a total flow volume of 10 cc of Dammam synthetic water, and for the same volume of flow for the filtered tap water. For samples H-8 (depth: 138 m), H-40 (depth: 198 m) and H-46 (depth: 206 m) no flow could be obtained with either of the fluids. The results of the flow tests are presented graphically in Fig. 4. The three samples (H-8, H-40 and H-46) for which no flow could be obtained, and sample H-20 with very sluggish flow, have not been included in the figure. The figure suggests that in most of the cases there was a maximum of 15% reduction in permeability at the end of the flow test with tap water, though the reduction started from the time of the first flow with Dammam synthetic water. This possibly indicates that the movement of fines has a role to play in this reduction of permeability with flow. The extent of reduction is not, however, alarming and shows a tendency for stabilization, and thus should not have large impact on the artificial recharge. The total suspended solids (TSS) are present in the outflow in variable amounts, suggesting that movement of fines has taken place during the test. The absence of montmorillonite and mixed-layer clay
Figure 4. Variation in the permeability of the core plugs ( }} ) and the total suspended solids in the outflow water (- - - -) during core flow tests. From left to right: the first two points represent forward flow with the Dammam synthetic water; the third and the fourth points represent reverse flow with the Dammam synthetic water; the fifth and the sixth points represent forward flow with desalinated tap water; and the last two points represent reverse flow with desalinated tap water.
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Figure 5. Comparison of the major cation and anion contents of the outflow water during the forward flow experiment with tap water with those of the Dammam synthetic water and tap water.
minerals in the aquifer eliminates the possibility of permanent damage to the aquifer from clay swelling and dispersion. For samples H-17 (depth: 149 m) and H-37 (depth: 192 m), increases in permeability were noted while conducting the test with tap water. The increases were also accompanied by a relatively larger movement of fines, as shown by the TSS contents. The increases in permeability in these cases may thus be attributed to the clearing of pore throats by the flow of tap water. Chemical analysis of the outflow water collected during the test with tap water suggested that during the forward run the synthetic Dammam water remained in the pore spaces, contaminating the flowing tap water (Fig. 5). By the end of the test, however, most of this synthetic Dammam water must have been flushed out, as the outflows collected during the reverse flow test with tap water were almost identical in quality with that of the tap water (Fig. 6). There were, however, 100% or greater increases in bicarbonate content (from around 18 mg ly1 in tap water to around 29 to 51 mg ly1 in outflow water) and a slight increase in carbonate content (from nil to 0.11 to 0.35 mg ly1 ) that might suggest some amount of dissolution of carbonate in the tap water. This might also explain an increase in suspended solids in the tap water outflow in some cases, due to dislodgement of grains from dissolution of the carbonate matrix materials, and reduction in permeability from the movement of the dislodged particles. The increase in permeability of samples H-17 and H-37 might be due to the fact that these depth ranges represented vuggy dolomite, where dissolution of the matrix materials caused increased communication between vugs and no narrow pore throats are present that could be constricted by the deposition of the dislodged materials. The higher chloride content (58 mg ly1 in the tap water compared to 106 to 140 mg ly1 in the outflow) was more difficult to explain, but might indicate the dissolution of some chloride salts present in the pore spaces. The suspended solids in the outflow water were examined under binocular microscope and were found to consist mainly of carbonate crystal grains with a few elongated wavy grains that might represent palygorskite. Samples H-44 (depth: 205 m) and H-48 (depth: 210 m) also contained dark grey flakes in the outflow water, which might represent bituminous materials present at these depths.
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Figure 6. Comparison of the major cation and anion contents of the outflow water during the reverse flow experiment with tap water with those of the tap water.
The results of the core flow tests suggest that except for some dissolution of the carbonates and movements of fines in the pore space, no major incompatibility problem exists when desalinated water is introduced into the Dammam Limestone Aquifer. The dissolution of carbonate and the movement of fines may be complimentary to each other, and the resulting change in permeability may not be drastic, as indicated by the core flow tests. This conclusion is in line with the preliminary findings of Al-Awadi et al. (1995) from north-western Kuwait. Examination of the core plug samples after the core flow tests under SEM and under the polarized light using a petrographic microscope did not reveal any significant change in the ground mass material or porosity structures through dissolution, deposition or clay swelling. Some suggestion of composite grains of fines blocking some of the pore spaces was seen in one case (depth: 123 m). Otherwise, the texture and porosity structures appeared the same as in the samples before the flow tests. Thus, overall it appears that injection of desalinated water into the Dammam Formation will not result in any drastic change in the aquifer characteristics of the formation.
Conclusions and recommendations Artificial recharge of aquifers is a viable option to manage the limited water resources in an arid environment. The method can store available excess water in aquifers which can then be recovered during emergencies or at times of seasonal high demands. Compatibility of the recharge water with the aquifer material and native water of the aquifer is, however, a prerequisite for the success of the process. A laboratory investigation has been completed to study the extent of compatibility of the Dammam Formation Aquifer of Kuwait with the desalinated water as a prelude to the creation of a reserve of potable water in the aquifer for emergency use. The study has demonstrated that desalinated water has no significant compatibility problems with the Dammam Formation Aquifer and native ground-water. The water does not contain significant suspended solids or active bacteria. The results of mercury injection porosimetry suggest that the injection water should be pretreated to remove suspended particles
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with nominal diameters equal to and greater than 1.5 microns. Although there is some doubt about the reliability of this figure due to scattering of the measured permeability and median diameter data, it is expected that filtering of injection water through appropriate pressure filters before injection will minimize the problem of clogging from suspended solids and biological growths. Clogging from dissolved air may be avoided by (a) maintaining the temperature of the injection water at the same level or higher than that of the aquifer water to prevent formation of gas bubbles from dissolved gases; (b) using air purging valves upstream of the injection; (c) maintaining a full injection pipe all the time down the injection well, thus avoiding cascading; and (d) using special valves at the injection pipe end. A facility for back pumping should also be provided in each well in the form of a submersible pump or air compressor with a downhole airline, in order to clear the injection face from time to time, in case some degree of clogging still occurs after implementation of all the above precautions. No scaling is expected to take place within the aquifer when the injection water comes in contact with the native ground-water. Some dissolution of carbonates may, however, take place, and fines may be dislodged by the dissolution of the carbonates or by turbulence. Their movement with injected water may constrict the pore spaces and reduce the permeability to some extent. However, the extent of this reduction is not expected to be too severe. Back pumping from time to time may also be able to clear pore constrictions from the accumulation of fines. The Upper Member and the upper part of the Middle Member are the most porous and permeable parts of the aquifer. The Lower Member and the lower part of the Middle Member have less porosity and permeability with more organic matter in the ground mass. The latter ranges are, therefore, not very suitable for recharge at the selected site. The recharge wells should be completed in the Upper Member and the upper part of the Middle Member only (the well should not be drilled through the lower part of the formation to avoid contact with the organic-rich zones, if any) to get maximum injectivity during the recharge phase and recovery during the subsequent pumping phase. Creation of a potable water reserve in the Dammam Formation at the selected site should help the water authorities of Kuwait not only to meet the water demand during an emergency, but also in reduction of the load on the desalination plants during high demand periods. The study was made possible by partial funding from the Kuwait Foundation for the Advancement of Sciences (KFAS). The support of the Kuwait Institute for Scientific Research (KISR) in general, and of Dr Jawad Al-Sulaimi, Director, Water Resources Division, and Dr Muhammad Al-Rashed, Manager, Hydrology Department, in particular, were crucial for the successful completion of the project. The permission from the management of KISR for publication of this paper (KISR Publication No. KISR5055) is gratefully acknowledged. The study has drawn heavily on the facilities and manpower of the Petroleum Technology Department and Central Analytical Laboratories of KISR, and the Geology Department, Petroleum Engineering Department and Electron Microscope Unit of Kuwait University. Sincere thanks are extended to all individuals, too numerous to mention by name, in these organizational units who provided help and support in carrying out the study.
References Al-Awadi, E., Mukhopadhyay, A. & Al-Haddad, A. (1995). Compatibility of desalinated water with the Dammam Formation at northwest Shigaya Water Well Field, Kuwait: A preliminary study. Journal of Hydrogeology 3(4): 56]73. Darwish, M.A., El-Refaee, M.M. & Abdel-Jawad, M. (1995). Developments in the multi-stage flash desalting system. Desalination, 100: 35]64.
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Kestin, J. & Wakeham, W.A. (1988). Transport Properties of Fluids: thermal conductivity, viscosity, and diffusion coefficient. CINDAS data series on material properties, Vol. I-1. New York: Hemisphere Publishing Corporation. 344 pp. Kozeny, J. (1927). Ubar kapillore leitung des wassers in boden. Silz der Akad. Proceedings of Royal Academy of Science, Vienna, 136(2A): 271. MEW (1997). Statistical Year Book: Water. Kuwait: Ministry of Electricity and Water. 205 pp. Mukhopadhyay, A., Al-Sulaimi, J., Al-Awadi, E. & Al-Ruwaih, F. (1996). An overview of the Tertiary geology and the hydrogeology of the northern part of the Arabian Gulf region with special reference to Kuwait. Earth-Science Reviews, 40: 259]295. OLI Systems Inc. (1995). Environmental Simulation Program, Version 5.0. NJ, U.S.A.: Systems Inc. Viswanathan, M.N. & Mukhopadhyay, A. (1992). Assessment of Artificial Ground-water Recharge in Kuwait: selection of pilot recharge sites, Vol. IV. Kuwait Institute for Scientific Research, Report No. KISR4125, Kuwait.
Laboratory investigations of compatibility of the Dammam Formation Aquifer with desalinated freshwater at a pilot recharge site in Kuwait A. Mukhopadhyay w †, E. Al-Awadi w , M. N. AlSenafy w and P. C. Smith‡ w
Hydrology Department, Water Resources Division, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109, Safat, Kuwait ‡Oil Plus Ltd., Hambridge Road, Newbury, Berkshire RG14 5TR, U.K. (Received 17 September 1997, accepted 26 May 1998) The degree of compatibility of desalinated water with native ground-water and aquifer material of the Dammam Formation at a selected pilot recharge site in Kuwait was investigated. Core flow tests did not indicate any major adverse effect from injection of desalinated water into the aquifer. Geochemical simulation, jar tests and dynamic scaling tests did not suggest any scaling for mixtures of Dammam Formation water and desalinated water. The mercury porosimetry data suggested, however, that injection water should be filtered to remove suspended solids of diameter 1.5 microns and higher to avoid clogging of the pore spaces. q 1998 Academic Press Keywords: porosity; permeability; core flow test; mercury porosimetry; scaling; dynamic scaling test; geochemical simulation
Introduction Kuwait is an arid country without any surface water resource and with limited supply of useable ground-water. The country depends almost exclusively on the desalination of ground-water for its freshwater supply. During the higher demand periods of summer, the desalination plants are left with very little spare capacities, and any major breakdown may cause supply restrictions. The surface storage facility (approximately 10 Mm 3 ) that is currently available for any emergency situation could meet the demand of the current population level (approximately 1.8 million) at an average daily per capita consumption rate of 470 l dayy1 (MEW, 1997) for only 10 to 15 days. Creation of additional surface storage facilities is costly. Storage of water in aquifers through artificial recharge is comparatively cheaper, and technically feasible. Artificial recharge † Corresponding author. 0140]1963r98r010027 q 16 $30.00r0
q 1998 Academic Press
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Figure 1. Universal transverse mercator (UTM) projection of the State of Kuwait, showing the locations of the proposed recharge site, cored test well, and the existing brackish water fields (A, B, C, D, E and others).
of ground-water to create a strategic reserve of freshwater for emergency use is being considered by the Ministry of Electricity and Water (MEW) in Kuwait, and a site has been selected (Fig. 1) to create a reserve of freshwater, with the Eocene Dammam Formation as the target for recharge (Viswanathan & Mukhopadhyay, 1992). Due to the high evaporation rate in the desert environment of Kuwait and the depth of the aquifer, injection through wells is the preferred method of recharge. It has been felt, however, that before implementing the pilot project, the compatibility of the injection water (desalinated freshwater) with the ground-water and the aquifer materials should be investigated, so that proper remedial measures can be adopted to ensure success of the artificial recharge. The results of such a study are reported here.
Petrology and aquifer characteristics of the Dammam Formation at the recharge site The general geology and hydrogeology of the Tertiary succession of Kuwait, of which the Dammam Formation is a part, have been discussed in detail by Mukhopadhyay et al. (1996). The Dammam Formation is a dolomitic limestone of Middle Eocene age,
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180 to 200 m thick that is underlain conformably by the Lower Eocene anhydritic Rus Formation and is unconformably overlain by the Mio-Pleistocene clastic Kuwait Group. A well was drilled at a location within the selected recharge site (Fig. 1) where the top of the Dammam Formation was encountered at a depth of 108 m. The formation was continuously cored from 122 to 271 m using a 2.7-m long double-core barrel with an internal diameter of 101 mm. Except in the ranges of 178 to 183.5 m, where the core recovery was poor (15 to 25%), the recovery in general was 60 to 100%.
Stratigraphy and lithology of the Dammam Formation at the test site Cores recovered from the Dammam Formation were examined in hand specimens and thin sections, and also under a scanning electron microscope (SEM) and by X-ray diffraction (XRD). Based on the lithological characteristics revealed by these examinations, and comparison with the Dammam Formation’s lithology and geophysical log characteristics in the neighbouring Umm Gudair Water Field, the following three-fold subdivision of the sequence penetrated at the study site has been proposed (Fig. 2): Upper Member, 108 to 186 m; Middle Member, 186 to 216 m; Lower Member, 216 to 271 mq. Lithological descriptions of the three members are as follows. Upper member The range from 108 to 120 m mostly consists of chertified dolomite, and possibly represents the karstified zone at the top of the Dammam Formation. The marly, chalky, fossiliferous, very fine-grained, compact dolomicrite and coarser-grained, porous dolomite are the main lithologic constituents of the rest of the member. Vugs, moulds and intergranular porosity are the main types of porosity. Later diagenetic activities have sometimes dissolved the crystal cores, giving rise to intragranular porosity. Remnants of micritic ground mass can be sometimes observed. Partial silicification in the form of spherulitic chalcedony and a few macrocrystalline quartz grains filling the cavities have been observed occasionally. The vugs are often filled with sparry calcite. The allochem grains, mostly represented by fossil shells, float in the ground mass. Pyrite and phosphates are also common. The XRD examination confirms that dolomite is the main mineral constituent of the range. The occurrence of palygorskite fibres covering dolomite grains and spanning across intergranular pores is revealed under SEM. Palygorskite also occurs occasionally as vein fillings. Friable ranges occur below 150 m where recovery of core was poor. Loss of circulating drilling fluid (circulation loss) was also experienced during coring operation below 178 m with very poor recovery (15 to 20%) of cores in the range 178]183.5 m, suggesting that the range may be highly fractured and permeable. Middle member The unit is mainly composed of vuggy and bioturbated laminated dolomite (down to around 206 m) and limestone (below 206 m) with layers of lignite and bituminous material. The laminated dolomite is highly calcitized. Remnants of the original dolomicritic host can be observed in the form of nodules within sparry calcite. The lithotype is rich in phosphate that occurs in the form of pellets and rod-like particles of golden colour, and organic matter that occurs as dark thin laminae. Traces of anhydrite, gypsum and sulphides are also observed. Calcitization, in some cases, is associated with
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Figure 2. Lithostratigraphy of the cored well.
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silicification in the form of precipitation of a radiating mass of chalcedony replacing the ground mass. Calcitization has increased towards the bottom of the range and, below 206 m, the lithology consists mainly of calcite. Vugs, moulds and intergranular porosity are the main porosity types in this range.
Lower member The unit consists of nummulitic limestone with vuggy and moldic porosity. Below 233 m, the nummulitic limestone is associated with bituminous materials. The nummulites are the dominant biogenic allochem. Post-depositional calcitization has produced macrocrystalline and sparry calcite that has invaded the original micritic phosphate and organic-rich ground mass, and cemented the internal chambers of the nummulitic shells. The XRD pattern confirms that the predominant constituent of the lithotype is calcite. Phosphates are common in the range and the presence of traces of pyrite and anhydrite is revealed under SEM. Gas chromatographic examination shows that the bituminous materials occurring below 233 m consist mostly of high alkanes above C 28 . Vugs and moulds are the main porosity types in this range.
Porosity and permeability Porosity was measured using a helium gas expansion porosimeter and permeability was measured with a gas permeameter. The measured porosity (in percentage) and extrapolated absolute permeability (in millidarcy) are plotted against depth in Fig. 3. It may be seen from the figure that in the range of 123 to 160 m, porosity is in the range of 3 to 35% and the permeability values are in the range of 0.00001 to 80 md (millidarcies). There are very few porosity and permeability measurements in the range of 160 to 190 m due to the difficulty in cutting core plugs from marly and chalky friable dolomite and poor recovery (178 to 183.5 m) while coring. The poor recovery range also coincides with high loss of mud while coring, and the loss continued to a depth of 210 m. The range of 160 to 190 m thus appears to be highly porous and permeable. The range of 190 to 216 m is characterized by porosity in the range of 0 to 33% (most of the porosity is, however, below 20%), and permeability in the range of less than 0.03 to 20 md. Below 216 m, the porosity shows a decreasing trend with depth (from around 20 to 25% at the top of the range to below 10% at a depth of around 260 m). The permeability also decreases simultaneously from around 10 md at the top to below 0.01 md at around 260 m. Examination of Fig. 3 will further suggest that the porosities and permeabilities of the vertical and horizontal plugs are more or less of the same order of magnitude and no systematic difference between the two groups could be found. The transmissivity of the Dammam Formation in the vicinity of the cored well, as determined from the pumping test data, is around 35 m2 dayy1 . Assuming that the whole exposed thickness of the Dammam Formation (122 to 271 m) was contributing to the flow, average hydraulic conductivity of the formation at the site is around 0.24 m dayy1 or a permeability of 320 md. The maximum permeability measured for the core plugs using a gas permeameter is 125 md. Permeability measured on core plugs using Dammam synthetic water and tap water during core flow tests varied in the range of 1 to 120 md. It is, therefore, most likely that most of the flow from the aquifer takes place through some highly permeable zone that may be fractured or vuggy. As discussed earlier, the range of 160 to 190 m may be such a zone.
Figure 3. Measured (a) horizontal (kh_gas, l) and vertical (kv_gas, B) permeability, and (b) horizontal (phi_h, l) and vertical (phi_v, B) porosity of the Dammam Limestone core plugs from the test well.
32 A. MUKHOPADHYAY ET AL.
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Pore size distribution using mercury injection porosimetry and its implication for pretreatment of injection water A pore can be blocked by a particle of the same size as itself. In addition, it can be shown mathematically that three spherical particles with diameters 92% of the pore radius can wedge in a pore to form a stable bridge (other numbers of particles tend to form unstable bridges, although seven particles may also bridge effectively). Thus, to avoid clogging from suspended solids, it is necessary to remove most of the particles of sizes equal to or larger than those which might form such a bridge. The statistical chances of seven-particle bridging may normally be neglected. Based on the abundance of particles of various sizes present in the source water, and the distribution of potentially blockable pores, it is possible to define the filtering requirement of the injection water that will remove suspended solids down to a particular size grade. The treatment should help in containing blocking, and thus loss of injectivity, to within acceptable levels for the life of the water injection scheme. In mercury injection porosimetry, mercury is injected into a core plug under pressure. Measurement of the volume of mercury entering the plug at each pressure stage indicates the cumulative percentage of pores above a specific size, and thus, a pore size distribution is obtained. From this is derived a permeability curve, based on the assumption that a reservoir is made up of a bundle of unconnected parallel capillaries of varying sizes and the fluid flow through these capillaries follows Poiseuille’s equation, as presented by Kestin & Wakeham (1988, p. 111). The equation relating mercury pressure and pore radius holds good for radii of curvature down to 10y6 cm (i.e. 0.01 micron). Below this, the radius of curvature becomes comparable with the diameter of the mercury atom, and the concept of surface tension becomes invalid. The derived permeability curve indicates the degree of loss of permeability when all pores up to a certain size are plugged. A good general guide to this pore size range is the mean hydraulic radius (MHR), which is related to the root mean square of the pore sizes. This concept is based on an extension of the original work carried out by Kozeny (1927). The details of the core plugs used in the study, along with the derived mean hydraulic radius values (MHR) based on porosimetry data for each of the 10 plugs available for this study are presented in Table 1. When determining filtration specifications, it is desirable to define a cut-off permeability above which blockage would cause an unacceptable loss in injectivity. This can be achieved in a number of ways depending on the extent and representivity
Table 1. Porosity, permeability and MHR of core plugs used in mercury porosimetry experiments
Plug reference
Depth (m)
Gas permeability (md)
Porosity (%)
MHR (m m)
10 14 16 17 24 37 40 44 46 48
140.33 146.00 148.63 149.12 157.50 191.50 198.00 204.90 206.00 210.23
9.3 26.2 26.3 19.0 135.5 66.0 0.001 32.3 0.003 46.0
27.2 13.8 0.03 32.7 25.1 33.0 8.6 8.0 3.7 26.2
1.1 1.4 0.1 0.1 4.4 2.2 -0.1 6.7 0.6 2.4
MHR s mean hydraulic radius.
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of the routine permeability data. In this case, with very limited data that does not permit the derivation of detailed statistical parameters of permeability distribution, the median permeability value (derived from the permeability values in Table 1) has been used to define the cut-off point. Utilizing the median permeability value of 22.5 md and relating this to the mean hydraulic radius data in Table 1, a critical pore radius of 1.7 microns is derived. Allowing for the possibility of three-particle bridging, this radius equates to a maximum allowable particle diameter (MAPD) of 1.5 microns. In order to prevent unacceptable losses in injectivity, based on the mercury porosimetry data, a filtration specification of 1.5 microns is therefore recommended. However, there must be some doubt regarding the accuracy of this value, given that the raw data upon which it is based is somewhat limited, and the Poiseuille model assumed here is too idealistic and may not be representative of the actual porosity distribution in the aquifer.
Compatibility of desalinated water with Dammam Aquifer water Chemical character In Kuwait, seawater is desalinated using multistage flash (MSF) technology (Darwish et al., 1995) and is subsequently mixed with 10% brackish water to produce potable quality water that is supplied through the freshwater pipeline. The results of API and dissolved gas analysis of this freshwater and Dammam Formation water collected on-site from the same well that had been cored are presented in Table 2. The particle-size distribution in the desalinated water is presented in Table 3. The data show this water to be relatively clean. As would be expected, the desalinated water contained very low amounts of dissolved salts, but the sulphate content was double the chloride content. Heavy metal concentrations were very low, with only zinc above 0.1 mg ly1 , while nutrients contents were also very low. The Dammam Aquifer water exhibited significant sodium, calcium, chloride, sulphate and bicarbonate contents, with 0.24 mg ly1 iron and 10 mg ly1 nitrate. Heavy metal concentrations tended to be low, and apart from nitrate, nutrient levels were also very low. Except for acetic acid, the volatile fatty acids contents of these two waters were very low, i.e. below the limit of detection of 0.1 mg ly1 . Even the acetic acid contents were below 2 mg ly1 for both the waters, again low values. These waters are thus very low in nutrients that would tend to hinder bacterial activity.
Geochemical simulation Based on the data from chemical analysis, a geochemical simulation of the scaling tendency of the mixtures of Dammam water and desalinated water was carried out using a software system developed by OLI Systems Inc. (1995). The results of these predictions are given in Table 4. These results show that no scale of any type is predicted to form in the aquifer under injection conditions of 358C and 500 psi.
Jar tests Static jar tests were carried out by mixing the two types of water at different ratios (100, 70, 50, 30 and 0 per cent desalinated water) in 1 l bottles and maintaining the
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Table 2. API and dissolved gas analyses of Dammam Formation water and desalinated tap water
Parameter
Dammam Aquifer water Concentration meq ly1 y1 (mg l )
Sodium Calcium Magnesium Potassium Strontium Barium Iron Aluminium Silicon Manganese Zinc Lithium Copper Cadmium Selenium Boron Chloride Bromide Fluoride Sulphate Nitrate Phosphate Ammoniacal nitrogen Temperature (8C) w Bicarbonate w Carbonate w Hydroxyl w Specific gravity at 208C Conductivity (mS) at 258C pH at 258C w TDS (mg ly1 ) CO 2 (mg ly1 ) w O 2 (p.p.b.) w H 2 S (mg ly1 ) w w
728 563 197 22 14.8 0.04 0.24 0.18 9.7 0.02 0.1 0.09 0.01 0.01 -0.06 1.3 1776 8.0 2.0 1200 10.0 -1.0 -0.4 28.3 146.4 Nil Nil 1.0033 6.42 7.14 4679.5 59 30 0.26
31.65 28.15 16.14 0.56 14.8 ] ] ] 0.69 ] ] ] ] ] ] 0.24 50.03 0.1 ] 25 ] ] ] ] 2.4 ] ] ] ] ] 169.76 ] ] ]
Desalinated water Concentration meq ly1 y1 (mg l ) 46.9 33.8 13.9 1.5 0.96 -0.01 0.05 -0.03 1 -0.002 0.22 0.01 0.03 -0.01 -0.06 0.1 58 -1.0 0.2 119 2 -1.0 -0.4 30 18.3 Nil Nil 1.0005 0.563 6.86 297 8.8 41000 Nil
2.04 1.69 1.13 0.03 0.02 ] ] ] ] ] ] ] ] ] ] 1.63 ] ] 2.48 ] ] ] ] 0.28 ] ] ] ] ] 9.3 9.3 ] ]
Measured on-site at the sampling point.
mixtures for 3 days at 358C. Subsequently, any precipitate that formed was filtered and quantified gravimetrically using 0.45 millipore membranes. However, no precipitate was collected on the membranes, supporting the results of the geochemical simulation. Examination of the rare solids collected on the membrane using a binocular microscope, scanning electron microscope and an energy dispersive spectroscope indicated them to be residual salt cubesrcrystals.
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Table 3. On-site particle-size distribution analyses w for desalinated water by Coulter Z1 Instrument, 10 April 1996
Particle diameter, d ( m m) 1.5 2 3 4 5 6 7 8 9 10 12 15 18 20 Turbidity (NTU) Total suspended solids (mg ly1 )
Sampling time (h) 1030 1230 Abundance G d m m in 0.1 ml 524 227 81 39 28 14 3 1 0 0 0 0 0 0 0.12 0.214
533 231 82 42 30 15 3 1 0 0 0 0 0 0 0.13 0.218
w Samples for particle-size distribution analysis had to be dosed with drops of ultrafiltered standard NaCl brine so as to make them conductive enough for analysis using a Coulter Zl instrument.
Table 4. Scaling tendency predictionw for mixtures of desalinated water and the Dammam Aquifer water at 358C, 500 psi using geochemical simulation
Injection water (%)
CaCO 3
0 10 20 30 40 50 60 70 80 90 100
95.5 86.9 77.7 68.2 58.4 48.3 37.5 21.7 10.7 3.9 0.6
Saturation (%) CaSO4 SrSO4 39.0 34.0 29.0 25.0 20.0 16.0 12.0 8.0 5.0 2.0 1.0
68.1 59.4 50.9 42.7 34.8 27.4 20.5 14.2 8.8 4.3 1.1
BaSO4 0 0 0 0 0 0 0 0 0 0 0
w No scale formation is predicted for any case. Scale solids begin to precipitate when saturation is above 100%.
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Dynamic scaling test A more practical test of scaling under field conditions is generally carried out in a dynamic scaling rig where mixtures of two types of water under the pressure]temperature conditions of the aquifer are passed through a capillary coil and the change in differential pressure required to maintain a constant flow rate is monitored. The amount of scale deposited is determined from the difference in weight of the coil before and after the test. The computer simulation suggested that even the raw Dammam water was unsaturated with most of the chemical species that could cause scaling problems (CaCO 3 , BaSO4 , CaSO4 , SrSO4 ), and as this water was diluted with the injected desalinated water the mixtures would be more and more unsaturated (Table 4). The dynamic scaling test was, therefore, run with 100% synthetic Dammam water under a system pressure of 300 psi (expected average hydrostatic pressure in the Dammam Aquifer at the pilot recharge site) at a temperature of 358C (expected prevailing temperature). The differential pressure remained steady at around 1 psi for the entire 4 h of the test, suggesting that no scale was formed during the test.
Compatibility of injection water with core material: core flow tests Twelve horizontal plugs, selected to give as uniform a coverage as possible of the Upper and Middle Members of the Dammam Formation (the target zone for artificial recharge) were used for the core flow test. A Ruska Liquid Permeameter was used for this purpose. The plugs were first cut to size to fit the core holder of the permeameter. They were then saturated with synthetic Dammam water by immersing them in the liquid under a vacuum for more than 72 h. The permeabilities of the core samples were measured for the flow of each 50 cc of Dammam synthetic water for a total flow of 100 cc under a pressure head of 2 bar. The samples were subsequently turned around, and the permeabilities were again measured with the same Dammam synthetic water for another 100 cc flow in steps of 50 cc. Water samples at the outflow end were collected for both forward and backward runs. On completion of the Dammam synthetic water runs, the permeabilities of the core samples were measured using the same procedures as described above, but using filtered desalinated tap water. Samples of the outflow water were again collected for both the forward and backward runs. For sample H-20 (depth: 153 m), due to a very slow flow rate, the measurements were done in steps of 5 cc for a total flow volume of 10 cc of Dammam synthetic water, and for the same volume of flow for the filtered tap water. For samples H-8 (depth: 138 m), H-40 (depth: 198 m) and H-46 (depth: 206 m) no flow could be obtained with either of the fluids. The results of the flow tests are presented graphically in Fig. 4. The three samples (H-8, H-40 and H-46) for which no flow could be obtained, and sample H-20 with very sluggish flow, have not been included in the figure. The figure suggests that in most of the cases there was a maximum of 15% reduction in permeability at the end of the flow test with tap water, though the reduction started from the time of the first flow with Dammam synthetic water. This possibly indicates that the movement of fines has a role to play in this reduction of permeability with flow. The extent of reduction is not, however, alarming and shows a tendency for stabilization, and thus should not have large impact on the artificial recharge. The total suspended solids (TSS) are present in the outflow in variable amounts, suggesting that movement of fines has taken place during the test. The absence of montmorillonite and mixed-layer clay
Figure 4. Variation in the permeability of the core plugs ( }} ) and the total suspended solids in the outflow water (- - - -) during core flow tests. From left to right: the first two points represent forward flow with the Dammam synthetic water; the third and the fourth points represent reverse flow with the Dammam synthetic water; the fifth and the sixth points represent forward flow with desalinated tap water; and the last two points represent reverse flow with desalinated tap water.
38 A. MUKHOPADHYAY ET AL.
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Figure 5. Comparison of the major cation and anion contents of the outflow water during the forward flow experiment with tap water with those of the Dammam synthetic water and tap water.
minerals in the aquifer eliminates the possibility of permanent damage to the aquifer from clay swelling and dispersion. For samples H-17 (depth: 149 m) and H-37 (depth: 192 m), increases in permeability were noted while conducting the test with tap water. The increases were also accompanied by a relatively larger movement of fines, as shown by the TSS contents. The increases in permeability in these cases may thus be attributed to the clearing of pore throats by the flow of tap water. Chemical analysis of the outflow water collected during the test with tap water suggested that during the forward run the synthetic Dammam water remained in the pore spaces, contaminating the flowing tap water (Fig. 5). By the end of the test, however, most of this synthetic Dammam water must have been flushed out, as the outflows collected during the reverse flow test with tap water were almost identical in quality with that of the tap water (Fig. 6). There were, however, 100% or greater increases in bicarbonate content (from around 18 mg ly1 in tap water to around 29 to 51 mg ly1 in outflow water) and a slight increase in carbonate content (from nil to 0.11 to 0.35 mg ly1 ) that might suggest some amount of dissolution of carbonate in the tap water. This might also explain an increase in suspended solids in the tap water outflow in some cases, due to dislodgement of grains from dissolution of the carbonate matrix materials, and reduction in permeability from the movement of the dislodged particles. The increase in permeability of samples H-17 and H-37 might be due to the fact that these depth ranges represented vuggy dolomite, where dissolution of the matrix materials caused increased communication between vugs and no narrow pore throats are present that could be constricted by the deposition of the dislodged materials. The higher chloride content (58 mg ly1 in the tap water compared to 106 to 140 mg ly1 in the outflow) was more difficult to explain, but might indicate the dissolution of some chloride salts present in the pore spaces. The suspended solids in the outflow water were examined under binocular microscope and were found to consist mainly of carbonate crystal grains with a few elongated wavy grains that might represent palygorskite. Samples H-44 (depth: 205 m) and H-48 (depth: 210 m) also contained dark grey flakes in the outflow water, which might represent bituminous materials present at these depths.
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Figure 6. Comparison of the major cation and anion contents of the outflow water during the reverse flow experiment with tap water with those of the tap water.
The results of the core flow tests suggest that except for some dissolution of the carbonates and movements of fines in the pore space, no major incompatibility problem exists when desalinated water is introduced into the Dammam Limestone Aquifer. The dissolution of carbonate and the movement of fines may be complimentary to each other, and the resulting change in permeability may not be drastic, as indicated by the core flow tests. This conclusion is in line with the preliminary findings of Al-Awadi et al. (1995) from north-western Kuwait. Examination of the core plug samples after the core flow tests under SEM and under the polarized light using a petrographic microscope did not reveal any significant change in the ground mass material or porosity structures through dissolution, deposition or clay swelling. Some suggestion of composite grains of fines blocking some of the pore spaces was seen in one case (depth: 123 m). Otherwise, the texture and porosity structures appeared the same as in the samples before the flow tests. Thus, overall it appears that injection of desalinated water into the Dammam Formation will not result in any drastic change in the aquifer characteristics of the formation.
Conclusions and recommendations Artificial recharge of aquifers is a viable option to manage the limited water resources in an arid environment. The method can store available excess water in aquifers which can then be recovered during emergencies or at times of seasonal high demands. Compatibility of the recharge water with the aquifer material and native water of the aquifer is, however, a prerequisite for the success of the process. A laboratory investigation has been completed to study the extent of compatibility of the Dammam Formation Aquifer of Kuwait with the desalinated water as a prelude to the creation of a reserve of potable water in the aquifer for emergency use. The study has demonstrated that desalinated water has no significant compatibility problems with the Dammam Formation Aquifer and native ground-water. The water does not contain significant suspended solids or active bacteria. The results of mercury injection porosimetry suggest that the injection water should be pretreated to remove suspended particles
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with nominal diameters equal to and greater than 1.5 microns. Although there is some doubt about the reliability of this figure due to scattering of the measured permeability and median diameter data, it is expected that filtering of injection water through appropriate pressure filters before injection will minimize the problem of clogging from suspended solids and biological growths. Clogging from dissolved air may be avoided by (a) maintaining the temperature of the injection water at the same level or higher than that of the aquifer water to prevent formation of gas bubbles from dissolved gases; (b) using air purging valves upstream of the injection; (c) maintaining a full injection pipe all the time down the injection well, thus avoiding cascading; and (d) using special valves at the injection pipe end. A facility for back pumping should also be provided in each well in the form of a submersible pump or air compressor with a downhole airline, in order to clear the injection face from time to time, in case some degree of clogging still occurs after implementation of all the above precautions. No scaling is expected to take place within the aquifer when the injection water comes in contact with the native ground-water. Some dissolution of carbonates may, however, take place, and fines may be dislodged by the dissolution of the carbonates or by turbulence. Their movement with injected water may constrict the pore spaces and reduce the permeability to some extent. However, the extent of this reduction is not expected to be too severe. Back pumping from time to time may also be able to clear pore constrictions from the accumulation of fines. The Upper Member and the upper part of the Middle Member are the most porous and permeable parts of the aquifer. The Lower Member and the lower part of the Middle Member have less porosity and permeability with more organic matter in the ground mass. The latter ranges are, therefore, not very suitable for recharge at the selected site. The recharge wells should be completed in the Upper Member and the upper part of the Middle Member only (the well should not be drilled through the lower part of the formation to avoid contact with the organic-rich zones, if any) to get maximum injectivity during the recharge phase and recovery during the subsequent pumping phase. Creation of a potable water reserve in the Dammam Formation at the selected site should help the water authorities of Kuwait not only to meet the water demand during an emergency, but also in reduction of the load on the desalination plants during high demand periods. The study was made possible by partial funding from the Kuwait Foundation for the Advancement of Sciences (KFAS). The support of the Kuwait Institute for Scientific Research (KISR) in general, and of Dr Jawad Al-Sulaimi, Director, Water Resources Division, and Dr Muhammad Al-Rashed, Manager, Hydrology Department, in particular, were crucial for the successful completion of the project. The permission from the management of KISR for publication of this paper (KISR Publication No. KISR5055) is gratefully acknowledged. The study has drawn heavily on the facilities and manpower of the Petroleum Technology Department and Central Analytical Laboratories of KISR, and the Geology Department, Petroleum Engineering Department and Electron Microscope Unit of Kuwait University. Sincere thanks are extended to all individuals, too numerous to mention by name, in these organizational units who provided help and support in carrying out the study.
References Al-Awadi, E., Mukhopadhyay, A. & Al-Haddad, A. (1995). Compatibility of desalinated water with the Dammam Formation at northwest Shigaya Water Well Field, Kuwait: A preliminary study. Journal of Hydrogeology 3(4): 56]73. Darwish, M.A., El-Refaee, M.M. & Abdel-Jawad, M. (1995). Developments in the multi-stage flash desalting system. Desalination, 100: 35]64.
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