Evaluation of Oil Desorption from Shale and Sandstone by Surfactant Solutions with Implication to Oily Drill-cuttings Decontamination

In this paper, batch experiments of shale/sandstone aqueous systems were conducted to evaluate desorption of spiked oil from shale and sandstone using surfactant solutions. The desorption experimental study was designed to determine if selected surfactants in aqueous solutions with varying pH, initial oil on shale/sandstone, surfactant concentrations, surfactant type, solid/liquid ratio, with and without sonication and at different contact time could enhance the desorption of spiked oil (Sarapar147) from pre-spiked shale and sandstone. The experiments tested the influence of the selected variables in batch experiments. The early screening results obtained for sandstone and shale show that sandstone are very much easier to clean than shales. Furthermore, the desorption efficiencies was lower for water compared to surfactant solutions particularly for sandstones.  Nevertheless, ultrasound was able to improve the desorption efficiencies for sandstone washings but not for shale washings. In the other hand, the results of the full factorial experiments showed that the variations of solution pH, mechanical interruption by ultrasound, and prolonged desorption times did not significantly improve the desorption process. These results give strong evidence to the existence of a considerable and irreversibly bound fraction of oil to shale.


I. INTRODUCTION
When drilling for oil and gas, more than 75% of the drilled sections contain shales which are the major source (90%) of wellbore instability problems [1], [2].Borehole instability and related drilling problems while drilling shales is a continuing hurdle in offshore drilling operations.Such problems result in substantial annual expenditure (estimated to cost more than US$1.3 billion per year) for the petroleum industry [1].Technical challenges in offshore drilling have led to the requirement of drilling fluids with drilling properties that exceed those of water-based muds (WBMs).
The effectiveness of oil-based muds (OBMs) has been well documented in offshore drilling applications all over the world [3], [4].The characteristics benefits of oil-based mud drilling fluids were usually cited as to offer excellent shale inhibition and thus maintain wellbore stability, provide high lubricity to mitigate high torque/drag trends in highlydeviated/horizontal and extended reach wells, stability at high temperature, tolerance to chemical/solid contamination, high-temperature/high-pressure fluid loss control, and ease of maintenance [5].
However, the unacceptable environmental impacts of drilling waste (oily-drill cuttings and drilling fluid itself) generated from OBMs use and discharge were recognized, in particular, this applied to the effect of discharging oily cuttings from offshore platforms.Increasingly, stringent regulation and much legislation have been enforced in response to environmental concerns and led particularly to continually tightening the environmental discharge limits of allowable oil on cuttings.Regulatory bodies nearly all around the world specifies that oily cuttings have to be cleaned to a limit of 1% residual oil on cuttings, ROC, (10g oil per kg dry cuttings) following the pioneering ban imposed offshore North Sea countries by 1997 [6].
Current waste treatment technologies are unable to clean oil out of cuttings to such levels, this leads to a discharge prohibition.Contaminated cuttings may, therefore, be taken onshore for treatment and disposal.Other options for disposal of oily cuttings were also evaluated by the industry such as onsite re-injection; which if technically possible and economically feasible is believed to be a best solution to the problem [7].
Unfortunately, onsite re-injection is not possible in many occasions.This favours shipping the contaminated-cuttings to shore for treatment and disposal whenever the facilities are available and local regulations backs it or to other offshore sites where cuttings re-injection operations are functioning.However, in deep and ultradeepwater drilling operations logistics are difficult, boat trips are long, mud volumes are high (due to riser requirements), and deck loads are limited [8].
Furthermore, given the regional differences in current waste management practices, the cost of complying with local environmental regulations varies widely.Taking into account the scarce or absence of on-land treatment facilities and various on-land disposal options in vast majority of less developed countries in one hand and the rising cost of handling drilling waste in more developed countries in the other hand, it seems more likely that the industry will face excessive pressure as to implement cost effective and novel technologies and approaches to meet very tightening regulations.
Much of today's new exploration and production is taking place in less developed countries and remote areas.Companies must develop their own waste management facilities that take into account local laws, climate, geological conditions, and long term effect on local residents.The most cost effective and less depending way of disposal is onsite disposal by overboard discharge of Evaluation of Oil Desorption from Shale and Sandstone by Surfactant Solutions with Implication to Oily Drillcuttings Decontamination Mazen Ahmed Muherei contaminated cuttings.So there is an urgent need to develop cost-effective and energy efficient methods for cleaning the solids before disposal.Drilled cuttings from the solids control separation process typically retain (10-25%) oil by weight [9].Many methods and systems have been developed and millions of dollars spent to find the best way to reduce the amount of residual oil on cuttings (ROC).However, the industry is still striving for a breakthrough that could decrease the final residual oil content to less than 1wt%.The most important issue is the capability of the method to fit the offshore rig limitations (technical feasibility, rig compatibility, environmental, safety, and costs).Several innovative methods are currently under investigation so as to clean oily-cuttings effectively to meet OSPAR/discharge limit.For example use of super critical carbon dioxide (SCCO2) as solvent extractor of adhered oil on cuttings [10], [11], use of new surfactants are also documented [9], [12].Thermal desorption was also implemented with success through the development of hammer mills [13].
Though hammer mills and thermal desorption options was successfully used to desorb mostly all oil from treated cuttings, however, the health and safety and other offshore rig consideration excluded them from offshore usage [14].SCCO2 extraction is recognized as a feasible method for removal of certain solutes from solid matrices such as soils and adsorbents.However, it requires expensive highpressure equipment and is not as familiar a technology as surfactant washing [12].Reference [9] and [12] desorption studies on real drill cuttings reveal that it was not possible for surfactants and chemical treatments to reduce retained oil on cuttings to less than 3wt%.
Despite these achievements, there is still a great interest in further development of established methods, re-evaluation of innovative methods as well as introduction of advantageous alternative approaches.The main focus of this paper is to re-evaluate the possible use of surfactants to decontaminate drill cuttings.It has been found in surfactant cleaning of contaminated soils that surfactant adsorption is significant particularly to shale clay minerals [15]- [20].Surfactant adsorption may lead to significant sequestration and unavailability of surfactants for the washing process.Excessive surfactant concentration may be needed to maintain surfactants at active concentrations which significantly affect the cost of the process.
The objective of the study is to determine the efficiency of surfactant systems in the treatment of shale and sandstone intentionally contaminated with mineral oil (Sarapar147).Batch desorption experiments are designed to determine the effect of surfactant concentration on the removal rate of oil from pre-spiked shale and sandstone and to study how different levels of contamination in shale affect the removal efficiency and finally to evaluate the effect of solid/liquid ratio, pH, equilibration time and ultrasound on removal efficiency.

A. Surfactants
Polyethylene glycol tert-octylphenyl ether-Triton X-100 (TX100) was supplied by Scharlau Chemie, Spain.It is a non-ionic surfactant (C8H17C6H4(OCH2CH2)xOH) and it has an average of 9.5 ethylene oxide units per molecule with an average molecular weight of 646.37 g/mol.Triton X114 (TX114) has a similar structure as TX100 with 7-8 average ethylene oxide units per molecule and an average molecular weight of 536.72 (EO=7.5).TX114 was purchased from Sigma-Aldrich.TX114 chemical structure is usually written as C8H17C6H4(OCH2CH2)7.5OH or C29H52O8.5.Polyoxyethylene sorbitan monooleate-Tween 80 (TW80) was supplied by Fisher Scientific, Hong Kong and it has an average of 20 ethylene oxide units per molecule with an average molecular weight of 1310g/mol.TW80 chemical structure may be written as C17H36COS6(OCH2CH2)20 or C64H124O26.Sodium Dodecyl Sulphate-SDS is an anionic surfactant with an approximate molecular weight of 288.4 g/mol.SDS was supplied by Merck with a high grade of purity (99%).The chemical structure of SDS can be written as C12H25NaSO4 or CH3(CH2)11OSO3Na.

B. Preparation of Spiked Samples
Samples were disintegrated into small pieces by jaw crusher and then ground using rock pulverizer (Fritsch, Germany).Samples were air dried for 24hrs followed by oven drying at 105ºC for 24hrs.Dried rock samples were sieved to obtain particles less than 2mm and larger than 1mm in all experiments.The method of preparation of spiked shale and sandstone samples was similar to that published in [21]- [23].

C. Desorption Experiments
Desorption of oil from soils/sediments, is a complex kinetic process that includes contributions from the wash system physical properties, time and temperature of wash, and the hydrodynamic forces exerted during the wash process.Accordingly, Desorption tests were conducted by adding sufficient volume of surfactants (20mL-100mL) to contaminated shale/sandstone (1-10g).The total volume and thus solid to liquid ratio was variable.Test tube desorption experiments were designed for treating 1g of contaminated shale and sandstone by 20mL different surfactant solutions (SDS, TX100, TX114, TW80, SDS+TX100, SDS+TX114, SDS+TW80, TX100+TW80, TW80+TX114; mixture solution are at 1:1 volume ratio).Sandstones at one initial contamination level (3wt%) while shales at two different initial contamination levels, i.e., 4wt% and 12wt%).Two surfactant concentrations are investigated namely 0.5wt% and 4wt% using distilled water as control without adjusting pH and using 4wt% ionic strength for anionic (SDS) and anionic+nonionic surfactants mixtures (SDS+TX100, SDS+TX114, SDS+TW80).Treatments were investigated at different conditions and contact times; experiments were run for 1hr at laboratory temperature and compared to experiments conducted for 1hr in an ultrasound bath operating at 47kHz and 240W as well as experiments conducted for 1hr in a water heated bath operating at 75°C.
After performing desorption on shale and sandstone, shale and sandstone were separated from surfactant solutions using Whatman filter paper and rinsed with 50mL distilled water.The separated material was then dried in oven at 40 for 18hrs and finally solvent extracted to determine the final ROC.
Desorption experiments were also run for 1/100 solid/liquid ratio in 250mL volumetric flasks using SDS+TX100 and distilled water as control.1g of shales at both initial loadings of oil (4 and 12wt%) was precisely measured then SDS+TX100 surfactant solution at 4wt% concentrations or water was added.Effect of ultrasound (1hr at 47kHz, 240W) and temperature (1hr at 75°C) were compared to experiments conducted for 1hr in laboratory temperature.
Effect of prolonged contact times at 1day, 2days, 3days and 4days was also investigated.Experiments were also conducted to investigate the effect of pH; pH of shales was adjusted by adding 2wt% NaOH during which the pH have been changed from pH=3 to pH=12.Experiments were similarly conducted at both pH values for 1hr in an ultrasound bath and compared to experiments conducted for 1hr at laboratory temperature.Effect of prolonged contact time at pH=12 was also investigated.

D. Determination of Retained Oil on Cuttings (ROC)
The retained oil on cuttings (ROC) in surfactant treated shale and sandstone cuttings was extracted first by test tube solvent extraction enhanced by 1hr sonication after 18hrs contact time at laboratory temperature and in final tests by most accurate Soxhlet extraction.The amount of extracted oil is estimated by a gravimetric method.The gravimetric method was selected because of its simplicity and availability and suitability to ROC.Taking into account that what matters for offshore regulation for ROC is the total amount of oil retained on cuttings no matter what is the source of oil (organic matter, formation oil, drilling fluid), hence, the retort method is usually used for determination of ROC on offshore [24].In this work oil contribution is expected from different sources such as organic matter, spiked oil, and most importantly from surfactant used for treatment.Hence, gravimetric method will be capable to estimate final ROC regardless of its origin.
Test tube solvent extractions were first optimized.Several experiments were conducted by extracting pre-spiked shale and sandstone.The extractions of these samples were made with different solvents in test tubes.Exactly, 1g of each sample was mixed with 5g Na2SO4, the mixture then was extracted by 20mL of different solvents.Four solvents were used; these are dichloromethane, hexane, acetone and 1:1 mixture of acetone and hexane in test tubes.The solventshale and sandstone mixtures were placed in an ultrasound bath for 1hr or left for 18hrs at room temperature followed by 1hr sonication.The solvent-shale and sandstone mixtures were also left for 48hrs at room temperature.After extraction took place the solvent was decanted through a Whatman filter paper into a pre-weighed 50mL volumetric flasks and the amount of extracted oil was determined gravimetrically.The solvents were allowed to evaporate at room temperature until a stable reading was obtained.The results showed that extraction with 1:1 hexane-acetone mixture for 18hrs followed by 1hr sonication give comparable results to 48hrs contact time.
The test tube solvent extraction enhanced by sonication procedure used in this study involved the following steps: (1) Weighing-put spiked shale and sandstone samples in an extraction test tube with Teflon cap, (2) Drying-mix about 5g of anhydrous sodium sulphate into the shale and sandstone to further dehydrate shale and sandstone, (3) Extraction-add 20mL of solvent, (4) Sonication-sonicate for about 1hr in a water bath, after the extraction tubes was placed on lab-bench overnight (18hrs), ( 5) Separationseparate solvent solution from sediment by filtering through a Whatman filter paper into a pre-weighed 50mL volumetric flasks.( 6) Drying-dry the solvent at room temperature for 48hrs and then weigh the flask every 3hrs until the difference between two consecutive measurements is negligible.The ultrasonic equipment was a Branson 8200 ultrasonic bath which was operated at 47kHz and 240W.

III. RESULTS
It is noteworthy that it is the shale formations which require the use of OBMs or SBMs for better drilling operations.Hence it was hypothesized that use of surfactants may not be suitable for cleaning shale cuttings and may in one way or another add to the final ROC in the same manner as the original organic matter of the shale.In this work, the capability of surfactant systems to decontaminate pre-spiked shale and sandstone was evaluated.The amount of oil retained after the washing process was determined directly through solvent extraction and the weight of the extracted oil was determined gravimetrically and expressed as retained oil on cuttings "ROC".No discrimination was made on the origin of oil, whether, is it of surfactant origin or from pre-spiked oil or oil originally present in cuttings (organic matter).All are considered hydrocarbons and may pose toxicity and hence harm the marine biota.In more than one occasion, surfactants are considered even more toxic to marine biota than contaminants; a lot of quality references can be found on the subject.Experiments were run first with test tube extraction and followed by the most accurate Soxhlet extraction.Several parameters were investigated such as contact time, solid/liquid ratio, surfactant type, surfactant concentration, use of ultrasound.

A. Screening Desorption Experiments
Surfactant desorption experiments were first evaluated with test tube surfactant desorption and the ROC was determined by test tube solvent extractions followed by gravimetric method.The intention was to figure out approximately the effects of different variables on desorption of oil from spiked shale and sandstone for 1hr of contact time.The variables investigated include the surfactant type (single and binary mixtures), adsorbent type (shale and sandstone), initial spiked oil for shale (4wt% and 12wt%), effect of temperature (28°C and 75°C) and finally the effect of mechanical mixing (47kHz-240W ultrasound).

B. Desorption Experiments for Spiked Sandstones
Surfactant desorption experiments were run for sandstone with initial spiked oil of 3wt% using 1:20 solid/liquid ratio in test tubes.The oil remained in sandstone was determined by solvent extraction using 1:1 mixture of acetone and hexane left to equilibrate for 18hrs at room temperature followed by 1hr exposure to ultrasound.The solvent/Sarpar147 extract was removed and dried in preweighed vials at room temperature to determine the ROC gravimetrically.
As seen in Fig. 1, the result indicates that all the surfactant solutions and water were fairly successful at achieving contaminant removal.Although surfactant solutions at 0.5wt% give better results than water at different conditions, it was not possible to differentiate the effect of different surfactant on desorption of oil.Furthermore, it was apparent that desorption experiments done in an ultrasound bath desorbs more oil from contaminated sandstone for both water and surfactant solutions.It is also prevalent that surfactant solutions under ultrasound and high temperatures desorb slightly more oil form sandstone than do the water.Again it was not possible to differentiate between different types of surfactants used whether they are in single or binary mixture state.Surfactants, extracted approximately 72 and 75% of the total oil previously present in sandstones when used at low (28°C) and high (75°C) temperatures, respectively.In the other hand water as a control was able to remove approximately 65% of the oil.Incorporating ultrasound energy with surfactant washings further improved desorption efficiencies.Roughly, 87% of the oil could be removed by surfactants under the ultrasound energy.Similarly, water was able to remove 83% of oil under the effect of ultrasound.

C. Desorption Experiments for Spiked Shales
Similar to sandstone, the surfactant desorption experiments were run for shale with initial spiked oil.The results, however, are quite different as the amount of oil desorbed from shale was significantly less than in the case of sandstone (Fig. 2).The amount of retained oil on shale cuttings (ROC) spans between 2.3 and 3.5wt% with an average of approximately 3wt%.Most importantly, all different surfactants and conditions of temperature and ultrasound do not show any improvements in the amount of oil desorbed during the 1hr contact time.However, the variations in altitude of columns could simply attributed to experimental artifacts rather than any true effect.Furthermore, the ROC determined by test tube extractions may underestimate the actual values of ROC particularly for shales.
Experiments were also run for 12wt % oil contaminated shales (Fig. 3).Similarly, it is quite clear that altering the conditions and type of surfactant were ineffective to improve desorption of oil from shale.The amount of ROC spans between 6 and 8wt%.
Overall insignificant desorption of oil from shale were observed at all conditions since the maximum amount that could be desorbed never exceed the 50wt%.Increasing the contact time from 1day to 4days could not improve the desorption efficiencies.All desorption experiments seem to give similar results at different contact times.All shale samples retain a value of 9wt% ROC (25wt% has been desorbed) after washing with distilled water.Increasing solid/liquid ratio from 1/20 to 1/100 while using water as the washing medium, give no improvements.However, when 0.5wt% surfactant solutions were used minor improvements could be detected as the amount of oil desorbed from shale increased to approximately 33wt%.Similar to water increasing solid/liquid ratio from 1/20 to 1/100 as well as increasing contact time from 1day to 4days give no improvements.Further increase in surfactant concentration from 0.5 to 4wt% at 1/20 solid/liquid ratio did not improve desorption amounts of oil from shales even when contact times have been increased from 1day to 4days.Increasing pH or ion hydrogen concentration from 3 to 12 for 4wt% surfactant solutions do not improve the amount of oil desorbed, however it was evident that the humic acid on shale has been released as the colour of surfactant solutions has turned to yellow.Humic acids are the major extractable component of soil humic substances, they are not soluble in water under conditions (pH<2) but are soluble at higher pH values.
The fluvic acids are the fraction of humic substances that are soluble in water under all pH conditions.Reference [25] found that humic acid did not contribute to pesticide desorption hysteresis in humic acid-clay.Their study has demonstrated the intercalation of the pesticide within the clay mineral and has suggested that humic acids are restricted to the external surfaces of clay tactoids.Accordingly, their result has indicated that clay mineral fractions in soils, including those with organic coatings, may play an important role in the retention of certain pesticides.
The final attempt was to increase the solid/liquid ratio of the 4wt% surfactant solutions from 1/20 to 1/100 while maintaining the pH values at 3. This effort fruitful as the ROC decreased to 6wt% and the amount of oil desorbed increased to 50wt%.Further increase in contact time was not fruitful.
The next round of experiments were restricted to 1hr washing time while used to investigate the effect of surfactant concentration (0 and 4wt%), pH value (3 and 12), solid/liquid ratio (1/20 and 1/100) and ultrasound mixing effect (Fig. 5).

Fig. 5. Effect of pH on desorption of 12wt% shale
As seen in Fig. 5, adding ultrasound energy to water at 1/100 solid/liquid ratio could not improve desorption efficiencies.Similarly, increasing solid/liquid ratio from 1/20 to 1/100 for 4wt% surfactant solutions do not show any improvements mean while surfactant solutions at 4wt% give slightly better desorption than water (ROC decreased from 9 to 8wt%).Adding ultrasound energy to 1/20 solid/liquid ratio of 4wt% surfactant solution barley give any difference.However, on the other hand, adding ultrasound energy to 1/100 solid/liquid ratio of 4wt% surfactant solution give profound improvements as the ROC has been reduced from 8 to 6wt%.The minimum ROC that can be obtained, using all different condition, while cleaning 12wt% contaminated shale was barley 6wt%.

IV. DISCUSSION
Early screening experiments showed a positive effect of

ROC, wt% Time, days
Water-1/20 solid:liquid ratio, pH=3 Water-1/100 solid:liquid ratio, pH=3 0.5wt% SDS-TX100, 1/20 solid:liquid ratio, pH=3 0.5wt% SDS-TX100, 1/100 solid:liquid ratio, pH=3 4wt% SDS-TX100, 1/20 solid:liquid ratio, pH=3 4wt% SDS-TX100, 1/20 solid:liquid ratio, pH=12 4wt% SDS-TX100, 1/100 solid:liquid ratio, pH=3 1hr water @ 28 C and 1/100 solid/liquid ratio 1hr water @ 47KHz, 240W ultrasound and 1/100 solid/liquid ratio 1hr 4wt% SDS-TX100 @ 28 C and 1/20 solid/liquid ratio 1hr 4wt% SDS-TX100 @ 28 C and 1/100 solid/liquid ratio 1hr 4wt% SDS-TX100 @ 47KHz, 240W ultrasound and 1/20 solid/liquid ratio ultrasound on desorption of oil from shale/sandstone.The positive effect of ultrasound on desorption can be attributed to mechanical mixing and/or disintegration/erosion of shale/sandstone matrix caused by bursting bubbles.Whether the effect of energy released on the collapse of the cavitational bubbles upon exposure to ultrasound waves was one of surface blasting or whether the energy released simply produced very high-velocity particle-to-particle collisions, the net effect of ultrasound is towards allowing better interaction of surfactant/water solutions with shale/sandstone and adhered contaminant.This may be by increasing the net surface area available for surfactant to work on by disintegrating sandstone into fine powders as was evident in this work.Maximizing the erosion of contaminant from the surface of sandstone by the solution in contact was also possible if the micro-jet blasting was the dominant mechanism of the blasting bubbles.Generally, the efficiency of the desorption process depends on solute-solvent interactions combined with the ability of the solvent to desorb the solutes from the matrix.Researchers have also indicated that the solvent plays a very important part as a competitor for the active sites on the matrix.If the matrix has a higher attraction for the solvent molecules than for the solute molecules, then they are preferentially attracted to the matrix and in so doing, the solute molecules are desorbed from the matrix.
From the early screening results, it can be concluded that the initial oil on substrate contributed to the amount of oil desorbed.The ROC from 12wt%-shale was approximately 6wt% while ROC from 4wt%-shale was approximately 4wt%.This finding contradicts with the result in [9].As they found that the residual oil content after washing recontaminated Marathon cuttings was the same at about 3wt% for the initial oil content of 20wt% and 10wt%.
As the initial spiked oil concentration increased, the adsorbed amounts of oil onto shale will increase which on the other hand may enable more oil to desorb during desorption experiments.It is clear that the mass of oil remaining on shales is dependent on the initial mass available for adsorption (4wt% versus 12wt%).The mass of oil remaining on shale surfaces can be characterized as slowly desorbing or reside in an inaccessible compartment.On the contrast, the reversible fraction desorbs easily within a short period of time.Generally, it is believed that the compartment containing the slowly desorbing oil fraction is of a finite size and once this compartment is filled to its capacity, the additional mass that adsorbs on shale surfaces are more prone for desorption [26].It is possible to notice that the capacity of the inaccessible compartment is about 3wt% for the re-contaminated Marathon cuttings reported in [9].Accordingly, the two initial ROC levels (20wt% and 10wt%) of these cuttings are both in excess of the inaccessible compartment capacity, hence the extra oil are adsorbed reversibly and both of which give a similar desorption efficiency (final ROC of treated cuttings=3wt%).However, the capacity of this compartment in the shale samples of this study seems to fall between the 12wt% and 4wt%.Therefore, it was possible to desorb some of the extra oil from the 12wt% shales because it contains oil in excess of the inaccessible compartment capacity while it was not possible to desorb oil from the 4wt% shales because it contains oil that is equal or less than the inaccessible compartment capacity (i.e., there is no oil available for desorption).
Before running the experiments, two different possible scenarios were suggested.The first scenario suggests that shales adsorb oils and surfactants reversibly.This was further subdivided into two possible scenarios: (i) surfactants are capable to remove entirely the oil adsorbed to shales without being adsorbed to these surfaces; (ii) surfactants are capable to remove the majority of the adsorbed oil, while surfactants themselves adsorb or do not adsorb to these surfaces.The second possible scenario suggests that shale adsorb oil and surfactants irreversibly.To identify which scenario is more relevant to the study experimental result, a sample of the extract was sent for GC-MS and the result confirmed that the solute in the extract was due mostly to Sarapar147.This means that the main reason for this hysteresis is the irreversible adsorption of oil to shale samples.
Many laboratory results and field studies demonstrate, however, that a significant fraction of sediment-bound organic contaminants do not desorb linearly and reversibly, and are difficult to remove by extraction with surfactants or co-solvents.This fraction usually desorbs very slowly and may persist much longer than would be expected by the reversibly adsorbed contaminants and may increase with the "age" of the contaminant in sediments or soils.Adjectives such as resistant, recalcitrant, rate-limiting, irreversibility, and even non equilibrium are used to describe desorption of this particular fraction.Various explanations have been proposed to interpret this abnormal desorption.The desorption-resistance has been ascribed to the heterogeneity of organic matter present in sediments.In addition to the amorphous natural organic matter, condensed phase carbon, similar names as hard carbon, soot, black carbon, glassy polymer, coal derived particle etc., has been widely found in field-contaminated sediments and been postulated to be responsible for the desorption resistance of contaminants in sediments [27]- [33].
Reference [34] studied the adsorption/desorption of gasoline hydrocarbons on three clays (montmorillonite, illite and kaolinite) using the batch equilibrium method.The organic matter in the clays was removed before experimentation.61% of initial adsorbed gasoline could only be desorbed when the adsorption time was 24hrs.Similarly, ref. [35] studied desorption of contaminated Hudson River sediments and reported that a significant fraction of the adsorbed mass (45%) resist desorption.They attributed the cause of slow release to be the slow desorption from a condensed phase of soil organic matter.Researchers found that a maximum irreversible concentration was observed for certain contaminant/sediment systems [26], [30], [31], [36].This concentration may reach 30-50% of the adsorbed contaminant resides in the irreversible adsorbed compartment and the rest resides in the labile and reversible compartment.They, proposed "rearrangement" of the organic matter around some adsorbate molecules during adsorption that places them in a high-affinity local environment characterized by a far higher partition coefficient than the bulk of the adsorbate.
Clay minerals can be grouped into two broad categories: swelling and non-swelling clays.Swelling clays such as montmorillonite, smectite, and vermiculite (2:1 clay) undergo interlayer expansion upon wetting, generating internal surface areas as high as 800m2/g.The nonexpandable clay minerals (i.e., kaolinite) have a very small internal surface area (10-20m2/g).Both clay categories can bind organic contaminants.For instance, ref. [37] found that organic contaminants would reversibly adsorb to kaolinite and irreversibly bind to the iron oxides associated with swelling clays (smectites).These observations are consistent with the presently proposed irreversible adsorption."How clean is clean" is a common issue in all innovative remediation technologies.In considering the applicability of surfactants to real drill cuttings decontamination, the results of this work showed that surfactant-enhanced remediation of contaminated drill cuttings at offshore sites is a difficult task particularly for shale contaminated with drilling fluids.It was recognized that the adsorbed surfactant amounts to shale are significant particularly for single nonionic surfactants which make it difficult for the technology to comply with the economical and regularity issues [20].Furthermore, shales may adsorb oil irreversibly which may render the decontamination of shale or shaly drill-cuttings extremely difficult.
The efforts to optimize and further improve the surfactant technology to suit contaminated drill cuttings are more challenging and allow predicting that the technology will remain under investigation for several decades.Thermal treatment of contaminated drill cuttings in the other hand is well established and efforts must be directed toward improving the technology such as using indirect microwaves radiations in heating the samples rather than direct heat treatment which may pose fire hazard to offshore drilling and/or oil storage installations.

V. CONCLUSION
Results of this study allow concluding the following:  For sandstone decontamination, surfactant solutions at 0.5wt% give better results (72-75%) than water (65%) at high (75°C) and low (28°C) temperature conditions.Moreover, it was apparent that desorption experiments done in an ultrasound bath desorbs more oil from contaminated sandstone for both water (83%) and surfactant solutions (87%). Using each surfactant system, there were no differences in the amount of oil desorbed from shale and sandstone.Similarly, different pH levels of the flushing solution (SDS-TX100) do not make any difference in terms of the wt% of oil desorbed from shale. Using each substrate, sandstone was very much easier to clean than shale.Desorptions are better for sandstone with surfactant solutions and in an ultrasonic bath (47kHz, 240W). Using shale, the percentage of oil desorbed from 12wt% shale was significant while those desorbed from 4wt% shale was negligible. Oil adsorbed irreversibly to shale.The resistant adsorbed fraction was approximately 75wt% of the amount initially adsorbed by spiking.The resistant fraction could not be reduced with water but could be reduced to 50wt% using surfactants (4wt%) at higher solid/liquid ratio (1/100) for one day.Applying ultrasound assist to reduce the necessary contact time to as low as 1hr.