3.1 Characterization of crude drill cuttings
3.1.1 Heavy metals and hydrocarbons content
From the results shown in Table 2, the harmful elements that exist in the studied crude cuttings are mostly the heavy metals and hydrocarbons. The heavy metals are namely; Lead (Pb), copper
(Cu), chromium (Cr), Cadmium (Cd), Zinc (Zn) and manganese (Mn). As it can be seen from this table, there are some samples that have elements of amounts exceed the maximum tolerated values for hydrocarbons and heavy metals (Khodja, 2008). Thus, the current drill cuttings are conventionally toxic (Lake and Menzies, 2007) and can not be landfilled and hence, further treatments are needed.
3.2 Characterization of Ordinary Portland Cement (OPC)
The mineralogical, chemical and particle size composition of cement has an important role in the properties of solidification/stabilization (S/S). Below are some results:
3.2.1 Mineralogical analysis
Analysis by X-ray crystallography of the cement sample revealed the mineral phases Hatrurite Ca3SiO5, Brownmillerite Ca2FeAlO5 and Gypsum Ca(SO4)(H2O)2 (see Fig.2a).
3.2.2 X-ray fluorescence analysis
The chemical analysis results of the major elements expressed in percentage by mass (%) are given in table 3.At the end of these two analyzes we can conclude that the sample is made up of minerals which are part of the constitution of a cement, namely hatrurite and brownmillerite. In addition, the presence of gypsum is noted in very low concentrations, a level of (CaO) at 64.33%, and the presence of sulfur (SO3) is also noted at 1.83%.
3.2.3 Granulometric distribution
The composition and the grain size of the cement has a big role in the mechanical and retention properties of Hydrocarbon (HC). The particle size analysis revealed the presence of two particle size ranges for Portland cement. The grain size population varies between 3.90 and 451.55 µm with a median size of 15.68 µm. The Fig 2.b shows the distribution histogram of the different particle size phases of the cement. The laser particle size measurement results for OPC were detailed in table 4.
3. 3 Solidification/Stabilization parameters improvement:
3.3.1 Effect of Cement
In all experiments, sample 5 in Table 5 was chosen for the test because it has a higher oil level
(Oil %=9.61). This crude sample is considered as a reference sample (RS). Table 5 depicts the hydrocarbons concentration with the different ratios of cement-to-drill cuttings (Ccement). The ratios were (0.02:1) ,(0.04:1) ,(0.06:1) ,(0.09:1) ,(0,11:1) and ( 0,13:1). As it can be seen, the hydrocarbons concentration is inversely proportional to the concentration of Ccement. The cement concentration was effective in reducing hydrocarbon concentration from the drill cuttings up to a (0.09:1). Increasing the cement concentration beyond (0.09:1) has no significant effect on the hydrocarbon reduction. Therefore, economically, the ratio of (0.09:1) cement-to-drill cuttings can be chosen as the optimal amount, to reduce the hydrocarbons concentration from the drilling cuttings obtained from Hassi Messaoud oil field. Increasing the cement concentration beyond (0.09:1) resulted, Theoretically, addition of cement results in formation of a more protected film hence forming a solid matrix which in turn strengthen the encapsulation process. Consequently, large hydrocarbons amounts are removed with addition of an optimized concentration of cement (Zhang and Bishop, 2002).
3.3.2 Sand addition effect
Table 5 illustrates the hydrocarbons percentage for different concentrations of sand. It is noticeable that the addition of sand reduces hydrocarbon ratio within the selected sample (RS). The hydrocarbon ratio reduced gradually from 9.61% to 5.21% after adding (0.04:1) of sand-to-drill cuttings. This was expected since adding sand to the mixture gives good cohesion, because the sand contributes to a good granular gradient, and from there, a balanced distribution of cement and additives inside the mixture (Belferra et al., 2016). The ratio (0.04:1) is considered as optimal concentration of sand for the reasons: after this amount of sand (0.04:1), the decrease in the hydrocarbon content is very low (0.13%).
3.3.3 Sodium silicate effect
To determine the effect of sodium silicate concentration (Csi) on hydrocarbons removal, different sodium silicate-drill cuttings ratios were tested. As it can be seen from Table 5, the reduction of hydrocarbons percentage due to Csi addition was significant until the ratio (0.006:1) of sodium silicate: drill cuttings. After this ratio, there is almost no change in hydrocarbons content. The Sodium silicate that was mixed with Portland cement improved mechanical properties such as the strength of materials (Wang et al., 2019).
3.3.4 Activated carbon effect
The effect of adding activated carbon on hydrocarbon concentration reduction is shown in Table 5. It is evidenced that the hydrocarbons content decreases until 5.88% after the ratio (0.013:1) of (activated carbon: drill cuttings). However, the hydrocarbons content starts to increase beyond this ratio of activated carbon: drill cuttings. This explains that the activated carbon considered like adsorbent of hydrocarbons (Arafat et al., 1999). The adsorption of hydrocarbon on activated carbon is much faster for until (0.013:1). It can be seen that the Table 5 contains an optimum concentration of activated carbon that should not be exceeded to avoid the release of hydrocarbons.
3.3.5. Organophilic clay effect
Table 5 shows the effect of adding organophilic clay for hydrocarbons removal. A gradual degradation of hydrocarbons is noticed with increasing the ratio of (organophilic clay : drill cuttings) until it reaches (0.013:1) which corresponds to a hydrocarbon content reduction of 6.25%. After this ratio, the rate of hydrocarbons starts to increases again. Therefore, this ratio value is considered as the optimal value of organophilic clay and should not be exceeded so as not to have the opposite effect for this specific type of drill cuttings. This phenomenon is justified by two reasons; firstly, the clay is fragile, and this property affects the solidification of the treated samples matrix, when a certain concentration is exceeded, the contaminants will be released. Secondly, (0.013:1) of clay decreases the percentage of hydrocarbons to 6.25%, which promotes the dispersion of the non adsorbed oily particles in the treated samples. the importance of the adsorbent of organophilic clay, either as a pre-treatment or as an additive, actually promotes the immobilization of the contaminants in the solidified-stabilized waste (Paria and Yuet, 2006).
3.4 Combination of Optimum ratios
In the previous section, the optimum parameters of solidification/stabilisation treatment were determined. The optimum concentrations results were for the cement-to-drill cuttings mass ratio is (0.09:1). For the additives-to-drill cuttings mass ratio are (0.04:1), (0.006:1), (0.013:1) and (0.013:1) for the sand, sodium silicate, Organophilic clay and activated carbon, respectively. At this stage, we aim to combine the last optimum results. Several mixtures where formulated in order to obtain the optimum formula. This involved the use of the obtained optimal concentrations of the tested parameters and the following mixtures were tested:
Cuttings + optimal of cement + optimal of sodium silicate = Mixture A
Cuttings + optimal of cement + optimal activated carbon = Mixture B
Then, the optimal quantity of the sand is added to the afore mentioned mixtures:
Cuttings + optimal of cement + optimal of sodium silicate + optimal of sand = Mixture C
Cuttings + optimal of cement + optimal of activated carbon + optimal of sand = Mixture D
Cuttings + optimal cement + optimal organophilic clay+ optimal sand = Mixture E
Table 6 summarizes the results of these combinations on the hydrocarbon content within the studied sample. The mixture D with the activated carbon as adsorbent gave the best result, then that with the sodium silicate (mixture C) (Coz et al., 2009). These results are confirmed by other research results that have used the activated carbon as adsorbent for hazardous wastes treatment (Arafat et al., 1999) and in another for decomposition and remediation of hazardous organic materials(Zhang and Bishop, 2002)(Rho et al., 2001).
The results recorded in Table 6 showed that the sand improves the retention of hydrocarbons in both combinations. Because it strengthens the matrix formed by the cement which in turn gives the treated waste a better solidification. Moreover, the sand effect is more pronounced with sodium silicate (Mixture C) than that with activated carbon (Mixture D). For the same amount of sand, a decrease of 1.25 % of hydrocarbons rate is obtained with the sodium silicate, whereas, only a decrease of 0.62% of hydrocarbons with the activated carbon is found. From the obtained results, sodium silicate (Mixture C), is selected to be treated with more stabilizers (optimal amounts) i.e., orgnophilic clay and activated carbon, as follows:
Cuttings + optimum of cement + optimum of sand + optimum of sodium silicate + optimum of orgnophilic clay = Mixture F.
cuttings + optimum of cement + optimum of sand + optimum of sodium silicate + optimum of activated carbon = Mixture G.
Table 7 summarizes the results of these new combinations. From these results, the combination of an adsorbent with an encapsulant improves the results obtained in the samples stabilized by adsorption. Nevertheless, this decrease is not very important compared with the results obtained in the samples without combinations of stabilizers.
3.5. Centrifugation Effect and Acceptance Test For Landfilling
The optimised results using (Mixture G, 5.102%) still insufficient for the landfilling, since they possess values that are above the tolerated value which is 5% (Abbas, 2011). Thus, for the solidification/stabilization process for drill cuttings highly contaminated by hydrocarbons, to be effective, the cuttings must be subjected to the pre-treatment (centrifugation, dilution) operation to confirm the regulations set by SONATRACH(Khodja, 2008),(Abbas, 2011). The results of the sample before (Mixture G) and after centrifugation operation (Mixture ) are presented in Table 8. From this table, it is found that the hydrocarbon content of our centrifuged sample dropped from 5.102 to 1.999% which meets the standards set by SONATRACH (mass percentage of hydrocarbon less than or equal to 5%) (Abbas, 2011).
So, it can be declared that the sample of mixture at this rate, 1.999% of hydrocarbon content, had successfully passed the acceptance test for landfilling (in what concern the amount of hydrocarbons).
3.5.1 Matrix Permeability
After the analysis, the permeability of the treated sample is 59.71 mD. The treatment by solidification/stabilization gives a good result, because the cement fills the void that exists between the particles (contaminated by hydrocarbon) of the sample before treatment and solidifies it. Therefore, it makes the fluid passage within the matrix difficult. The solidification/stabilization effectiveness was confirmed by Ogechi et al. (S et al., 2010).
3.5.2. Matrix Solidification Test: Resistance to Free Compression (RFC)
Fig 3.a shows the Compressive strength of the treated sample during curing time. It can be noticed that curing of the sample increases with time, as: 84.24, 139.18, 186.39, 211.88 and 246.04 (kg/cm2) after 3, 7, 21, 28 and 96 days, respectively. The Strength test is used to provide a comparison between stabilized and unstabilized drill waste cuttings (Malviya and Chaudhary, 2006). Hence, it can be deduced that the proposed treatment criteria using the solidification/stabilisation method improves the mechanical properties of the drilling waste. The EPA (United States Environmental Protection Agency) Guide considered that 0.35 MPa is the value to have a satisfactory compressive strength (“Guide to disposal of chemically stabilized and solidified wastes,” 1982). This value (0.35 Mpa) has been proposed as a minimum value for materials to be landfilled. Accordingly, this confirms that our treated cuttings are very acceptable for landfilling. Compressive strength was tested at different time by Malviya and Chaudhary (Malviya and Chaudhary, 2004). They reported that compressive strength increases as the curing time increases (Malviya and Chaudhary, 2006). However, many other factors affect the effectiveness of solidification/stabilization such as cuttings nature and its proprieties, waste/cement ratio, type of cement, curing days etc (Malviya and Chaudhary, 2004)(Malviya and Chaudhary, 2006). .
3.5.3. Stabilization Tests of the Treated Waste
3.5.3.1. Atomic absorption spectrophotometry for the retention of heavy metals
Table 9 presents the results of Atomic Absorption Spectrophotometry Analysis (AAS) of the waste sample before and after treatment, compared to the maximum tolerated values for the landfilling. From the results recorded in Table 9, it can be seen that the solidification/stabilization treatment is very effective for heavy metals elimination (S et al., 2010). Moreover, the results of the analysis of our sample after treatment pass the testing standards (below the maximum tolerated values) only traces of Zn remained. This confirms that our treated sample strongly passes the leaching test.
3.6. Acceptance Test of the Treated Waste for Recovery
Fig.3.b represents the variation of the resistance to free compression as a function of the cement concentration after 7 curing days. According to Fig. 3.b, the resistance to free compression (RFC) of our treated sample increases with increasing cement content which surpasses the allowed minimum value (0.35 MPa), (“Guide to disposal of chemically stabilized and solidified wastes,” 1982), by many times. It is deduced from this test that the pre-treated sample was very brittle, so, the production of concrete from the treated waste requires the addition of considerable quantities of cement. The cement offers a sturdy structure to the treated sample and bind well the harmful heavy metals which prevents their movement in a monolithic mass (Masrullita et al., 2018).
3.7. Shape and Morphology of the Treated Drill Cuttings
To discuss the effects of the solidification/stabilization process on the treated drill cuttings, it is important to investigate the surface morphology before landfilling. Figs. 4 a,b, and c show the images of the treated drill cuttings at different times after mixing: 7, 28, and 96 days respectively, obtained using the SEM technic. The SEM structure indicates that the samples consist of granules of clay materials, with silicon as the predominant component. They also show high carbon (HC), calcium hydroxide, C-SH, and some aggregates of grain-like. Therefore, it can be concluded that the time after mixing has no effect on the sample morphology but rather had an effect on hardness and durability of the sample. Furthermore, the shapes and surfaces of the three obtained samples, indicate that the sample C is more solid and stable compared to the other samples. This later result was confirmed by the results of compressive strength which is proportional to the curing time because the porosity has an impact on the strength (Wang et al., 2019). The tests have also revealed that the final stabilized outcome of Portland cement is a concrete of low permeability sturdy compact matrix related to liquid substances (hydrocarbons) (Clark and Perry, 1985;Poon et al.,1985; Zhao et al.,1999; Lake and Menzies, 2007), which offer an appropriate pore arrangement for storing the substances within it (Young,1992). These facts clarify the reason of the very low amounts of heavy metals for the sample after S/S treatment found in Table 9.