90% of asphaltene particles size surface area

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1 Journal of Engg. Research Year 1 - No.2 - September 2013 pp , 2013 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores MOHAMMAD A.J. ALI *, S.M. KHOLOSY AND A.A AL-HADDAD Kuwait Institute for Scienti c Research, Kuwait. * (Corresponding author) alijumaa@gmail.com ABSTRACT A live oil sample was subjected to a solid detectionsystem (SDS) to measure asphaltene onset point (AOP) at 3850 psi, and asphaltene content of 1.3%. A high-resolution digital camera was used to measure asphaltene particle size distribution. The result showed that asphaltene particles were not uniform in size, but has a normal distribution of m. Asphaltene reversibility to dissolved back into the oil with increasing pressure was only 35% of the original deposition. Two core samples were examined for formation damage due to asphaltene deposition. A Low permeability core showed signi cant permeability reduction exceeding 50% of its baseline permeability, and the higher permeability core showed less permeability decline, even with the same asphaltene precipitation. Keywords: Asphaltene particle size; low permeability; asphaltene content; asphaltene onset point; solid detection system. INTRODUCTION Heavy organic solids, especially asphaltene exist in crude oil, both as soluble and stable colloidal and dispersed conditions surrounded by resins adsorbed to their surface. The stability of the dispersed asphaltene is generally attributed to the citric e ect resins adsorbed on their surface. Any factor, such as changes in pressure, temperature, composition, that disrupts this adsorption equilibrium, can cause asphaltene aggregation, deposition, and precipitation. Asphaltene is soluble in aromatic solvents like xylene and toluene. Asphaltene has high molecular weight, it consists of complex aromatic ring structures containing aromatics, naphthalene, saturates and heavy metals (Mulins, 2008). They give crude oil their color. Heavier black oil will typically have higher asphaltene content. Being a polar molecule, asphaltene adsorbs to formation surfaces, especially clays. Still, asphaltene literature is a bit opaque. Depositionof asphaltene from crude oils, poses serious threats to oil production and transportation. Asphaltene deposition can occur in production systems including tubing, pipelines, and most seriously in formations near

2 60 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad wellbore areas in the vicinity of the pay zone. This could result in severe production in ow impairment, and in extreme situations, to well abandonment. The phenomenon of formation damage is a subject that has attracted much research and investigation over a long period of time, and is well-referenced in the body of available literature. Formationdamage, apart from being the primary cause of overall productivity loss inhydrocarbonreservoirs, is also responsible for rock permeability alterations, rock wettability change, and adverse relative permeability e ects. SOLIDDETECT SYSTEM A high pressure and temperature solid detection laser system (SDS) was used to measure asphaltene onset point. The pressurized sample was transferred to the SDS and measured asphaltene onset point (AOP). The light intensity attenuation was started high at high pressure values and decreased with low pressure. Lowering the sample pressure enhanced asphaltene deposition, which made the oil darker, and hence, less light transmission. AOP graph typically was started with horizontal line at high pressure and thensharply decreased at a certainpressure point. However, Figure 1 shows approximately two straight lines can be withdrawn from the analysis. A line from 6800 psi and another line from 2800 psi, where the two lines intercepted at 3450 psi. This value came on a close agreement with analysis found from the camera image analysis con rming 3450 psi the value of AOP. Fig. 1. Measurement of AOP using SDS DYNAMIC GROWTH OF ASPHALTENE A high resolution colored video camera with high intensity white light power to

3 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 61 capture images of the live-oil as it pass through two thinsheets of glass. The camera is equipped with software to detect and analyze asphaltene particle size distribution based on color di erence. Asphaltene particles are usually dark to black solid particles suspended in oil which make them possible to be distinguished from the oil phase. The reservoir pressure is gradually decreased insmall steps (about 200 psi each step) until it reaches the bubble point pressure. The asphaltene size detector assists to monitor the size of asphaltene particles, and to monitor the change of asphaltene concentration. The registered size and concentration are not absolute values, but the values that are captured inthe micro camera lens. The sample cylinder was connected to a high pressure precision pump and kept at a constant pressure of 7000 psi. The pressure volume temperature (PVT) cell and all the lines that connected through the camera lens were all vacuumed and heated at 110 o C for two hours. Then, the oil was injected into the PVT cell, tanks, and lines, and kept under high pressure and temperature for two hours. The oil sample was pressurized to 7000 psi and slowly was decreased to 6800 psi to detect asphaltene deposition. The camera image showed very small concentration of particles. Those particles could be anything, such as asphaltene, sand, wax, or any other impurity solids. The system was not capable of identifying the type of these particles, other than measuring its size. In this case, it was assumed that all solid particles were asphaltene. Figures 2-6 show asphaltene particles at di erent pressures. Clearly, asphaltene concentration increased with decreasing pressure, similar to Hirschberg et. al., (1984). Fig. 2. Asphaltene deposition at 6800 psi. Fig. 3. Asphaltene deposition at 6200 psi. Fig. 4. Asphaltene deposition at 4400 psi.. Fig. 5.Asphaltene deposition at 3200 psi..

4 62 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad Fig. 6. Asphaltene deposition at 2960 psi.. Apparently, asphaltene particles started at about 4800 psi, but became very clear at 4000 psi and air bubble point started at 2960 psi. Table 1 illustrates data measurements for asphaltene particle size distribution and concentration at di erent pressures. It shows that the number of particles was increased from 5 to 276 particles. Table 1. Asphaltene particle growth size distribution Pressures No. of particles 25% of asphaltene in 2 µm2 particles size cell surface e area Pressure Depletion 50% of asphaltene particles size surface area 90% of asphaltene particles size surface area Total surface area Pressure Increment ASPH. No. of particles in 2 µm2 cell psi no. (µm) (µm) (µm) (µm) (%) no Asphaltene particles were relatively small at the start of the test at 7000 psi until 4800 psi when the asphaltene particle numbers started to increase. However, the

5 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 63 increase in asphaltene became more evident at 4000 psi. The deposition of asphaltene particles continued to increase until pressure reached the bubble point pressure, whenbubbles of gas were coming out of the solutionas seeninfigure 6. At that point, the test was terminated, and the sample was discarded. Analysis of crude oil showed 1.3% asphaltene content and therefore, if the highest asphaltene concentration were to be considered as 1.3%, then it would be a valid assumption to consider a pressure at 3200 psi resulting in the total deposited asphaltene. Asphaltene Wt.% shown in Table 1 is calculated from a correlation between deposited asphaltene inside the core samples and pressure as discussed later. Determining the exact AOP using the digital camera was very challenging since particles were shown at di erent pressures, even at high pressure. It would be incorrect to consider the AOP to be just as the rst particles of asphaltene appeared on the camera. One source of error was that asphaltene deposition was not always completely reversible. Therefore, asphaltene may have been deposited inside the cylinder when it was brought to the lab, and was never redissolved back into the oil. Another source of error could be due to impure solids that the software measure as asphaltene. There could be other unknown reasons to mistake for early asphaltene precipitation. A comparison of the two techniques to measure asphaltene onset point has been investigated. Both techniques gave valuable results and information. The digital camera gave actual image of the asphaltene particles and size distribution, and indicated the number of asphaltene particles and the dynamic growth of size and concentration. The disadvantages were in the inability to distinguish between the asphaltene particles and any other solids detected by the camera. The SDS may sound to be a fast technique that could detect AOP automatically by the software where humanerror is not a factor. The disadvantage is the inability to measure the asphaltene particle size. Therefore, it is recommended to use both systems, initially to measure AOP by the SDS, and to identify the exact pressure values. Figure 7 shows the increment of asphaltene particles whendecreasing pressure from 6800 to 3200 psi. It appeared that asphaltene particles were almost constant from 6800 to 5500 psi, and started to increase at 5400 down to 3200 psi. Although asphaltene particle count was increasing during pressure depletion (blue line), when the sample was depressurized to its initial condition, it was observed that asphaltene particle count was high. Table 1 presents asphaltene particle count for tests, pressure reduction and pressure increment. Asphaltene reversibility was earlier studied by Anderson et. al., (1996). Inthis study, it was observed that asphaltene was not completely reversible, and that approximately, only 35% of the deposited asphaltene was dissolved back into the oil at 7000 psi.

6 64 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad Fig. 7. Asphaltene particle number measured by the camera. ASPHALTENE DEPOSITION IN CORE SAMPLES A low permeability core from Berea sandstone and a high permeability core from Bentheimer sandstone were used to investigate the e ect of asphaltene deposition on pore blocking and permeability decline, and their basic rock properties are shownintable 2. Table 2. Basic properties of core samples Test Berea Bentheimer Sandstone Sandstone Porosity, % Air Permeability, md Average grain diameter, µm Average pore throat, µm 7 12 Mean hydraulic radius, µm Grain density, g/cm Core Length, cm Core Diameter, cm Pore Volume, cm Two core samples were used inthis study. A high permeability Bentheimer sandstone core of 400 md, and a low permeability Berea sandstone cores of 40 md. The objectives of using two di erent permeability cores were to understand the e ect of asphaltene deposition on permeability and pore throat blockage,

7 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 65 and to study the distance of formation damage away from the wellbore. A similar study concluded that asphaltene deposition causes more damage in low permeability rock thaninhigh permeability rock. Both cores were in one piece 1-ft long each (non composite cores). The cores were weighted dry and then saturated with 2% KCl water for 24 hr at 2000 psi, thenthe weight of the wet cores were measured. The pore volumes for both cores were measured at 90 cc and 50 cc for the Bentheimer and Berea, respectively. The cores were inserted into a rubber Viton sleeve with stainless steel pressure taps. There were 6 pressure taps to measure the di erential pressure at each action of the core sample and at the outlet side of the core, and a backpressure regulator (BPR) that controlled the pressure to allow uid to ow. To study the e ect of asphaltene deposition, live-oil containing 1.3% asphaltene was injected for approximately 2 pore-volumes. But before injecting the live-oil, baseline water permeability was established. The same procedure was followed for both cores separately. Initially, water was injected at low owrate and back pressure of 100 psi. to measure baseline permeability. Water was injected for 2 pore volumes until permeability stabilized at about 400 md for the Bentheimer, and 50 md for the Berea core. After establishing the water permeability, dead oil was injected at 2 cc/min. to sweep water from the core to reach initial water saturation (Swi) of 23% for the Bentheimer core and 18% for the Berea core. During oil injection, the owrate was gradually increased to produced additional and 2 ml of a demulsi er was added in a beaker at the outlet for better oil-water separation and precise measurement. Live oil was injected at a constant pressure of 5000 psi, and the BPR was kept constant at 4995 psi to displace one pore-volume for each core separately. A di erential pressure of 5 psi was kept inside the core to prevent sudden deposition of asphaltene or ashing the gas out of the oil. It required about 90 cc of live-oil to displace the crude oil (dead volume) of lines and one porevolume for the Bentheimer core, and 50 cc for the Berea core. After displacing all the crude oil, thenthe backpressure was gradually reduced from 5000 to 4750 psi and the di erential pressure across the core sample was measured. The experiments for both cores were stopped at BPR of 4750 psi which was above the AOP. Further reductionof BPR to reach the AOP cold not be achieved for two reasons; the di erential pressure would exceed the maximum limit of the pressure transducers, and it would result in extremely high owrate.

8 66 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad Although the total pressure drop across the cores was relatively low, yet it damaged the end section of the cores. Approximately, the average permeability for the full 1 ft. length of both cores were not a ected, but only few centimeters toward the exit face of the cores where asphaltene precipitated the most. The permeability of the Bentheimer core reduced to 280 md, but the rst ve sections were undamaged and constant at 400 md. Similarly, the low permeability Berea core showed permeability reductionto 23 md at the exit section of the core, but the remaining ve sections remained constant at 50 md. This change in permeability could be attributed to pore plugging by asphaltene occulation and deposition. It is interesting to observe permeability damage evenat pressures above the AOP. Although it may appear that the total permeability was not a ected, but it is signi cantly reduced at the outlet face of the core sample where di erential pressure reduction is high. At the end of the live-oil injection, water was injected for several porevolumes until no more oil was produced yielding residual oil saturation (Sor) of 35% and 25% for Bentheimer and Berea cores, respectively. Next, the BPR was reduced gradually from 4750 to 50 psi, and heptane was injected for 2 porevolumes to sweep the water and the remaining residual oil trapped in the pores, but keep precipitate asphaltene in place. POST MORTEM TECHNIQUE Asphaltene deposition was con rmed after the termination of the test by visually examining the color of the core. The image of the core sample in Figure 8 clearly shows asphaltene deposition at the core surface. Fig. 8. Asphaltene deposition inside Berea core sample. The depositionwas highest at the core outlet with some traces along the core length. This image was taken at the end of the experiment and after ushing with water and heptane

9 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 67 A technique where after the termination of the core ooding tests, the core samples were cut into equal lengths of smaller core plugs, and the weight of the wet core plugs were measured. The small segments were then dried in an oven for 6 hours and the weights were taken dry but containing asphaltene. The core samples were then cleaned with toluene to remove all hydrocarbon by using a Soxhlet extractor. The cores were next kept in an oven for 6 hours to dry, then weighed. The weight of the dry plugs was subtracted from the wet plug to give the weight of the asphaltene as shown in Figure 9. Fig. 9. Asphaltene deposition pro le in low and high permeability cores The plot shows two distinguished trends, the rst trend was that asphaltene seemed to be deposited along the entire core length, but in reality the deposition took a place during ushing with Heptane and not during the depressurising test. This was con rmed by the constant di erential pressure for rst ve segments of the cores. The second trend shows a high deposition at the end of the core and near the outlet face. The reduction in permeability was 30% for the Bentheimer and 50% for the Berea core due to asphaltene deposition. DATA INTERPRETATION Permeability decline due to asphaltene deposition was examined in a low permeability Berea core and a high permeability Bentheimer core. The deposition of asphaltene took a place mainly at the outlet of the core. The deposition pro les were normalized so that only the deposited asphaltene due to pressure reduction was considered as shown in Figures 10 and 11.

10 68 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad Fig. 10. Permeability decline for the Bentheimer core after 1.1 pore-volume. Fig. 11. Permeability decline for the Berea core after 1.1 pore-volume. It appears that asphaltene deposition was increasing with higher pore volumes of the injected oil. Figure 10 shows that normalized permeability decreased to 70% of the Bentheimer permeability baseline, which made 30% reduction due to asphaltene blocking the pore channels. Figure 10 shows that normalized permeability for the Berea core decreased to approximately 50% of its baseline permeability, which made 50% reduction due to asphaltene blocking the pore channels. Figure 12 is a comparison of the normalized permeability for the two cores for the damaged segment only.

11 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 69 Fig. 12. Permeability decline for the damaged segment of the core. It seems that the reductionrate inpermeability is faster for the Berea low permeability core thanthe Bentheimer high permeability core. However, it appears that eventually the damage will be similar for both cores after more pore-volume injections. Looking back at the average size of asphaltene particles of 120 m (0.12 mm), thenparticles that where smaller thanthe pore throat should pass through the porous rock. However, following the 1/3 rd to 1/7 th rule, thenparticles that where larger than1/3 rd of the pore throats would cause external deposition at the core face. Particles that where smaller than1/7 th of the pore throat would pass through the core causing no damage. Particles that where in between would be deposited causing permeability damage. The analysis showed that for Bentheimer, the 1/3 rd was 4 m and 1/7 th was 1.7 m, and for Berea the 1/3 rd was 1 m and 1/7 th was 2.3 m, were both smaller than the asphaltene particles; and therefore, all asphaltene particles were deposited inside the core. It appeared that the core samples behaved as a lter media. In order to quantify the weight of deposited asphaltene and correlate it with pressure reduction, it was required to correlate between the weights of deposited asphaltene at each segment of the core along with the nal pressure reading at the end of the test. The weight fraction of asphaltene (oâ ) was the ratio of the deposited asphaltene by the weight of the core segment. This correlation was used together with the AOP to generate a correlation as shown in Figure 13, and this correlationwas used to estimate asphaltene depositionintable 1.

12 70 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad Fig. 13. Estimating asphaltene content at di erent pressure points. Finally, to establish a simple model for formation damage factor (û) due to asphaltene deposition, the relationship obtained from oâ, û,and pressure decline could be estimated as shown in Fig. 14. The constants û 1 and û 2 canonly be obtained experimentally, which could be functions of pore throat, uid viscosity, owrate, etc, and are very challenging to determine the function of each of these variables. The values û 1 and û 2 increase with low permeability samples, but how exactly they are related to predicting permeability decline, remains to be unclear. k k i ˆ 1 : 2 :2 1 1 The relationship in Figure 14 shows that the rate of permeability decline is much faster for low permeability core thanhigh permeability evenfor the same amount of deposited asphaltene. This is logical since high permeability usually means large pore space and pore throats, and giving enough space for asphaltene particle to move with the uids and not trap in the pores. However, low permeability cores with small pore volume and pore throats disable the free movement asphaltene particles and cause them to block the pore channels.

13 Laboratory investigation of dynamic growth of asphaltene deposition and formation damage on sandstone cores 71 Fig. 14. Formationdamage factor for low and high permeability cores. Formationdamage due to asphaltene depositionproved to be very complex to model, especially because it is a function of pressure, pore throat, particle size, asphaltene content, asphaltene particle size distribution, and many other important parameters. Generally however, asphaltene deposition occurs more rapidly in the well tubing and near wellbore radius where pressure drop is highest withinfew feet from the producing well. Similarly, ina laboratory core ood test, asphaltene deposition is more likely to occur at the low pressure zone. However, it is very challenging to mimic pressure pro le from the eld and downscale it to a core size. A major challenge is to keep a high di erential pressure and low ow rate simultaneously CONCLUSIONS Asphaltene deposition poses serious problems to the oil industry. Asphaltene deposition is very challenging to understand and forecast. This study was focused on the investigation of dynamic growth of asphaltene and its impact on permeability decline.. SDS is a useful tool to give very good measurement of asphaltene onset point (AOP), but it does not give size distribution or concentration.. A high resolutioncamera is a useful tool to measure asphaltene particle size distribution, but it does not give asphaltene concentration or AOP pressure.. Asphaltene particle size increased by particle coalesces and aggregations. Asphaltene particle size was seen to be not uniform in size, but reported a normal distribution with an average diameter of m.

14 72 Mohammad A.J. Ali, S.M. Kholosy and A.A Al-Haddad. Asphaltene particles were not completely reversible and did not dissolve back into the oil after restoration. Approximately, 35% of asphaltene was dissolved back into the solution.. Permeability impairment is likely to occur in the vicinity of asphaltic oil. The degree of impairment would depend on many factors, asphaltene content and permeability being the most important parameters.. Asphaltene particle size is usually one order of magnitude larger than pore throat.. Low permeability cores could cause more permeability decline than higher permeability cores for the same amount of deposited asphaltene. Higher permeability cores with large pore throat could allow for asphaltic particles to ow through the porous media. REFERENCES Anderson, S.I. & E.H. Stenby Thermodynamics of asphaltene precipitation and dissolution investigation of temperature and solvent e ects. Fuel Science Technology Int. Journal, 14 (1-2); Hirschberg, A., B.A. Schipper, & J.G, Meijer In uence of temperature and pressure on asphaltene occulation. SPE Production Journal, 1, Mulins, O.C Review of the molecular structure and aggregation of asphaltene and petroleiumic. SPE Journal. 13:

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