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Source: http://www.doksinet Laboratory Simulation of Vertical Hydrocarbon Microseepage WANG Guojian * CHENG Tongjin FAN Ming REN Chun CHEN Weijun Wuxi Research Institute of Petroleum Geology, Research Institute of Petroleum Exploration and Production,SINOPEC,Wuxi, Jiangsu,214151,China Abstract:Based on a conceptual model of hydrocarbon microseepage,a macro-sized experimental equipment used the matched mixtures of cement and quartz sand as simulant caprock and its overlying strata is first set up to simulate processes of hydrocarbon microseepage and its near-surface expressions. The results of the simulation experiments suggest that simulant caprock and its overlying strata have a certain sealed capability; hydrocarbon microseepage is dominated by the pressure of point gas source, and temperature plays only a subordinate role; on the path of hydrocarbon microseepage, the distribution of hydrocarbon concentration is fan-shaped; differential adsorption of alkanes by the simulant

caprock and its overlying strata results in the occurrence of a chromatographic effect; different migrating patterns within simulant caprock are shown by the ratio of i-butane to n-butane; the concentration of hydrocarbon in the surface soil has good correspondence with the pressure of point gas source. These simulation results are significant to further study of the mechanism of anomalies recovered in surface geochemistry exploration. Key Words: hydrocarbon, microseepage, simulation experiment, equipment,surface geochemistry exploration 1 INTRODUCTION One of the primary impediments to acceptance of geochemical exploration methods in oil industry has been the lack of experiments which could demonstrate the transport of gases from the depth of an oil and/or gas reservoir to the surface without significant dilution and dispersion (Klusman, 1993). In recent years, understanding of the mechanism of hydrocarbon microseepage is thought to result in acceptance of surface geochemistry and

improving its applying effect. So it is necessary to conduct laboratory simulation of vertical hydrocarbon microseepage. The laboratory simulations related to hydrocarbon microseepage is very fewer in spite of many laboratory simulations on primary migration and secondary migration (Zhang,1991;Liu,2008;Li,2008) have been done. Pirson(1977) made an experimental equipment to simulate oxidation-reduction potential effect caused by hydrocarbon microseepage. Antonov(1954,1964,1970) had determined hydrocarbon diffusion coefficients in sedimentary rocks, and conducted experiments of methane and light hydrocarbon diffusing through different lithologies at certain temperatures. Krooss(1985) determined diffusion coefficient of sandstones, siltstones and shales filled with water at temperature from 30 .℃These to 70℃ studies favored to understand the role diffusion play in the process of natural gases passing through caprocks by microseepage. Trost(1994) did experimental simulation to verify

his hypothesis of “non-hydrocarbon carrying gas”. Brown(2000) conducted experiments to study movement of bubbles in a vertical fracture of constant apertures, which can be approximated by spheres moving upwards between parallel plates. A simple pipe equipment made by Zhang Jinlai(1985) had been * Corresponding author. Tel:+8613961871930,E-mail address:kingdomjian@126com 1 Source: http://www.doksinet used to sudy the changes of migration hydrocarbon in overlying medium. In addition, some attempting work have been done by many researchers, such as Zhang Yigang et al.(1991) The above equipments for simulating vertical hydrocarbon microseepage at home and abroad are simple in experimental conditions and devices. Therefore, the simulating results have a big gap with the expected degree on the theory study of vertical hydrocarbon microseepage, Up to now, systemic and large-sized laboratory equipments have not been reported to simulate processes and mechanism of microseepage, mainly

due to the different purposes of experiments, restriction of in situ measurements and equipment material, and complexity of hydrocarbon microseepage. Based on a conceptual model of hydrocarbon microseepage, a macro-sized experimental equipment used the matched mixtures of cement and quartz sand as simulant caprock and its overlying strata is first set up to simulate processes of hydrocarbon microseepage and its near-surface expressions in this paper. It is significant that the results do explain some subsurface and surface observations. 2 METHODOLOGY 2.1 Conceptual model Considering the fact that the complexity of geologic environments can’t be completely simulated, oil and gas accumulation can be looked on as a point source to simplify our study object mainly to the sealed condition and migrating route. The conceptual model of microseepage adopted is shown in Fig. 1 The geological model is only taken into account oil/gas accumulation, porous caprock and overlying strata

adsorbed with water (wet pores), and Quaternary sediments(or surface soil) as a top layer;Only wet-pores were taken into account on migration path. The expressions of hydrocarbon microseepage in both caprock and its overlying strata and surface soil will be explored in the preliminary laboratory simulation. 2.2 Experimental equipment According to the simplified conceptual model of oil/gas microseepage, a set of experimental equipment(Figure 2) is built to simulate processes of hydrocarbon microseepage and its near-surface expressions. The equipment includes a point gas source, an injecting water system, a temperature controlling system, and a lattice of sampling sites. A mixture of cement, quartz sand, and water are used to model caprock and its overlying strata. Two characteristics of the experiments are as follows. Firstly, the experimental model is of a large size Increasing of the size of experimental model makes the model not only more contiguous to actual conditions, but also

convenient for combining together different kinds of geologic considerations and installing more sampling devices and measuring detectors needed for a more elaborated study. It is also match the trend of oil/gas migration study. The simulant caprock and its overlying strata are built as a cuboid of 50 centimeter long, 50 centimeter wide and 60 centimeter high. The soil profile on the top of model is 15 centimeter thick. This kind of large-scaled microseepage-simulating box-model is the first attempt and has not been found in literature. Secondly, the laboratory conditions are elaborately designed to simulate real underground condition of gas accumulations and microseepage. Gas composition used in the experiments is composed of light hydrocarbons according to the composition of the real wet gas in one gas accumulation in a petroliferous basin 2 Source: http://www.doksinet in China(Zhou Xingxi,2004). Controlling temperature system at the bottom of the model can simulate temperature

field. Injecting water system at the bottom of the model can keep the cuboid wet so as to make the simulant caprock possess a kind of sealed capability. The porosity and permeability of the simulant caprock is in accordance with that of mudstone caprock level 5(You,1991). It is impossible to consider all kinds of caprocks in one experiment, such as evaporate caprocks(Jin,2005;Jin and Cai,2006),bauxite caprocks(Xiao and Chen,1988), and so on. The soil profile is similar to Quaternary sediments. 2.3 Experimental method 2.31 Background hydrocarbon determination Water is injected under 0.3MPa pressure after modeling cuboid had been maintained for 28 days,meanwhile, water is also sprayed on surface of soil profile to keep wet. The background content of hydrocarbon in modeling cuboid and soil profile is determined after injecting water for one month. The background methane and ethane contents in modeling cuboid are shown on table 1, the average values of methane and ethane are 25.4ppm,

244ppm, respectively The sand used in experiment is white, pure quartz sand, so its total organic carbon content (TOC) is very low and will not affect its hydrocarbon content. The TOC in the cement used is 02% and will transfer to a small quantity of hydrocarbon by heat within the cement concrete. The background hydrocarbon is homogeneous approximatively because coefficient of variation is small. Methane, ethane, propane, butane and pentane may be applied as the tracers of microseepage in the laboratory simulation. 2.32 Experimental conditions and sampling method The room temperature is 22℃ during the experiments. The bottom temperature of the model is therefore kept 27℃ at first for 20 days, and then 40℃ for 21 days in order to learn how the temperature influence on the results. The pressure of point gas source is controlled from 0.145MPa to 035MPa so as to find the sealed capability of the simulant caprock Natural gas is injected from the point source after water have been

injected into the modeling cuboid for a month. Each inner sampling site is connected to an exit through a capillary conduit. Five rows of sampling sites are laid with a row space of 10cm from the vertical center profile, and five points in every row with a point space of 12cm (Figure 2). The hydrocarbon microseepage feature of vertical profile through simulant caprock and its overlying strata can be depicted. Eighteen sampling sites are laid in two layers in soil(Figure 2). A gas sample of 80μl is collected by syringes and then injected into a gas chromatograph to analyze its hydrocarbon concentrations during the simulation experiment. 3 RESULTS 3.1 Sealed capability of simulant caprock The relationship between temperature, pressure and concentrations of nearest sampling sites over time is shown as figure 3. When the bottom temperature of the model is 27 ℃ and the pressure of the point gas source 3 Source: http://www.doksinet is tuned up from 0.145MPa to 0276MPa, the

concentrations of nearest sampling sites do not change in two days. The phenomenon indicates that hydrocarbon does not microseep upwards at such a pressure. When pressure of the gas source is tuned up to 03MPa, the concentration of nearest sample space point rises up slightly at first and significantly afterwards, indicating that hydrocarbons begin to microseep upwards through the simulant caprock. The biggest sealed capability of simulant caprock is possible to be from 0.28MPa to 03MPa when the bottom temperature of the model is 27 ℃. 3.2 Temperature and pressure influence on hydrocarbon migration concentrations The gas pressure in the middle vessel is dropped from 0.275MPa to 0264MPa because hydrocarbon microseep through simulant caprock, while the concentrations of hydrocarbon of nearest sampling sites still goes up. The gas source is then shut off for an hour during experiment in order to study its influence on hydrocarbon microseepage when the pressure of gas source changes.

The gas pressure is then elevated to 0295MPa, and the concentrations of hydrocarbons in nearest sampling sites begin to drop and keep to drop within 5 days even after tuning the gas pressure to 0.3MPa This reflects the fact that hydrocarbon cannot penetrate into simulant caprock when the pressure of gas source disappears abruptly and reincreases. The hydrocarbon concentrations of nearest sampling sites increase immediately as the pressure reaches 0.35MPa ,and goes up gradually when the pressure range is from 035MPa to 0297MPa The bottom temperature of the model has changed to 40℃ and the pressure of gas source to 0.26MPa in order to learn the influence of temperature and pressure on hydrocarbon migration As a result of such a temperature change, the hydrocarbon concentrations go down with the drop of gas source pressure. The hydrocarbon concentrations of nearest sampling sites increase gradually after the pressure changed to 0.3MPa It is therefore clear that hydrocarbon microseepage

is dominated by the pressure of point gas source, and temperature plays only a subordinate role through laboratory simulation of the model. 3.3 Vertical profile characteristics of hydrocarbon microseepage All the hydrocarbon concentrations of sampling sites are determined after hydrocarbon microseepage found through the simulant caprock. The vertical concentration profiles of methane and ethane in simulant caprock on seventh day are shown in figure 4. The high concentration can be first found in the nearest sampling site (sample 3-1) after methane microseep from the gas source. The distribution of hydrocarbon concentrations in the overlying medium is not substantially different from the background values, though they do increase slightly. As time goes on, the distribution of methane concentration presents a fan shape, but keeps a dispersion trace from middle to top. The variation trend of ethane content is almost the same as methane in simulant caprock except that its velocity is

slower than that of methane and its change lags behind methane. As a matter of fact, hydrocarbons migrate from point source into the surrounding space 4 Source: http://www.doksinet driven by pressure and temperature. So, when pressure and temperature are low, hydrocarbons can only migrate to a limited distance, or the site directly above the gas source, e.g the point 3-1.When the processes of infiltration and diffusion go on, more hydrocarbons are accumulated above the gas source to form a fan shape. The peak value of hydrocarbon microseepage is corresponding with the gas source, namely, the anomaly of hydrocarbon concentrations is apical above the gas source. 3.4 One-dimensional vertical microseepage of hydrocarbons The characteristics of methane concentration on five vertical detecting lines show a positive gradient from top to bottom (Figure 6). Profiles of totally free hydrocarbons of passive seeps (or microseepages), including a headspace plus cuttings of low molecular weight

hydrocarbons(C 1 to C 5+ ), often display increases with depth(Abrams,1993; Figure 5). Some results have been shown in the offshore Gulf of Mexico(Anderson et al.1983, Figure 6) in deep gravity cores near active seeps(or macroseepage). Note that the magnitude of concentrations and changes are very different in the active and passive seep zones. Our laboratory simulation demonstrates the characteristic of passive seeps (or microseepage) on detecting lines 2, 3 and 4. The trace of vertical hydrocarbon microseepage is often used to search the origin of surface anomaly based on the characteristics of light hydrocarbons on vertical borehole profiles. Tang(2005) reported that the concentrations of headspace gas on vertical borehole profiles display increases of hydrocarbon concentrations with depth until approaching gas reservoir, but this is not the case for dry holes. Klusman (1993) reported the same phenomenon The five vertical detecting lines in simulant caprock in the experiment are

just like five boreholes in a geologic condition. Line 3 above point gas source in the experiment is similar to oil and gas well, lines 2 and 4 to the wells at contact of oil and water, while lines 1 and 5 to the dry wells. The methane concentration increased gradually with depth on lines 2,3 and 4 indicates the existence of vertical microseepage, but lines 1 and 5 show no such a rule. The above conclusion drawn from the laboratary simulation is consistent with the characteristics of microseepage found in real geological situations. 4 DISCUSSION 4.1 Mechanism of hydrocarbon migration Two mechanisms, diffusion and infiltration, are proven to exist in the hydrocarbon migration in our laboratory simulation in which the pressure of gas source and temperature are strictly controlled to certain values at the bottom of the model. Different migration patterns can be studied with the ratio of i-butane to n-butane on the five vertical detecting lines. It is generally accepted that infiltration

mechanism plays a main role if the ratio of i-butane to n-butane increases gradually in the migration route, but diffusion mechanism if the ratio decreases gradually (Yang et al.,1995;Li et al2002) Our experiments show that the distribution of hydrocarbon concentration reaches a steady state after a reasonably long time. Therefore, the mechanism of vertical hydrocarbon migration is predominantly infiltrative on lines 2,3 and 4, especially above the gas source(Figure 9). The rule on lines 1 and 5 is less obvious than that of lines 2,3 and 4 4.2 Characteristics of hydrocarbon in soil 4.21 Hydrocarbons on horizontal detecting Lines in the soil layer 5 Source: http://www.doksinet Hydrocarbon concentration in the soil layer rises significantly as the pressure of the source reached 0.3Mpa and this indicates hydrocarbon from gas source has seeped into soil As time goes on, the hydrocarbon concentration drops to a certain quantity and keeps stable owing to the fact that hydrocarbons seep

into the soil, dissipate and is adsorbed by it until saturation is reached. Hydrocarbon concentrations change with time too, the reason of which may be the variation of the pressure of the gas source and room temperature or the humidity of the soil and microorganism. But hydrocarbon concentrations keep steady state in a certain stage. 4.22 Relationship between hydrocarbon concentrations in soil and pressure of the gas source Different injection pressures are applied in the experiment and changes of methane concentration in the soil with time are observed(Figure 10). Hydrocarbon concentration in the soil layer rises slightly when the pressure of the gas source is from 0.145MPa to 0276Mpa, due to minor background hydrocarbon of modeling caprock migrate to soil. Hydrocarbon concentration in the soil layer raises abruptly and reaches its highest value as the gas source pressure is 0.3Mpa and this indicates hydrocarbon from gas source has seeped into soil, and finally drop before long.

Accordingly, the concentration of methane changes with the pressure of gas source. Such a rule is obvious though the magnitude is not very significant and demonstrates the fact that the pressure of point gas source contribute to the hydrocarbon concentration in the soil, a phenomenon coincident with the knowledge that the hydrocarbon concentrations in soil gases and fluxes from the soil surface should change in response to reservoir production. Horvitz(1985) had demonstrated such an anomaly observed over the Hastings field in Texas,USA (Fig. 11) The field was discovered in 1938 and the depth to the productive zone is approximately 1830m. The survey shown in the Figure 11 is of ethane and heavier(C 2+ ) hydrocarbons desorbed from soil samples by weak acid. The soil samples were collected at a depth of 2.4-37m and the survey was run in 1946 The productive area is outlined by the dashed line and the field exhibited a halo-type hydrocarbon anomaly at the surface. A similar survey was run

in the same region in 1968, and the anomaly previously revealed had nearly completely disappeared(Fig. 12) Whether this is due to drawdown of pressure is not known, but the effect had clearly reached the surface in a time period greater than 8 years but less than 30 years(Klusman,1993). 4.3 Discussion on type of caprocks The stimulant caprock used in this experiment belongs to mudstone caprock level 5(You,1991) by determined parameters of micropores. Vertical hydrocarbon microseepage is easily occurred under such experimental conditions and its characteristic rules is revealed. Hydrocarbon microseepage and its surface geochemical anomalies are related to sealed capability of the caprock, depth and age of oil/gas pool under actual geologic condition. Many studies show the phenomena of hydrocarbons microseepage are prevalent in petroliferous basins, not only in the area where the caprocks are not good or the depth of oil ang gas pool is shallow, but also in the area where the caprocks

are thick salt or gypsum, the depth of oil and gas pool is deep. Microseepage hydrocarbon in soil have been detected through thick evaporitic sequence in Egypt(Sherif El-Bishlawy et al.,2001) Hydrocarbon inclusions distributing in star-shape have been found in thick salt caprock in depth 2300m,Zhongyuan oil field,Henan province,China(Xu et al.,2001), this phenomenon also suggests hydrocarbon can seep through the best caprock,salt sequence,which is thought not to be penetrated. The depth of oil and gas reservoir is more 5000m 6 Source: http://www.doksinet in Yakela condensed gas field,Tarim basin,Xinjiang,China. It has been vertified that hydrocarbon can microseep to surface by detecting cuttings from borehole and soil in surface(Hou and Su,2001). With further studies in future, different types of caprocks will possibly be considered,such as compacted mudstones,evaporates, bauxites;the horizontal plane area and vertical height of simulant caprocks will be augmented; Simulation

experiments will be conducted under different thermobaric conditions;and microseepage hydrocarbon in surface soil can be detected and its surface distributing rules may be revealed. 5 CONCLUSION It is the first to take cement and quartz sand as simulant caprock and its overlying strata to make the macro-sized model for microseepage simulation, and simulation condition is similar to oil and gas accumulation subsurface. Injecting water system at bottom of the model can keep the simulant caprock wet and has membrane sealed capability. Hydrocarbon microseepage is dominated by the pressure of gas source, and temperature plays only a subordinate role. Some of predecessors’ research results are verified by our laboratory simulation. There are obvious rules governing the vertical hydrocarbon microseepage: the vertical profile of hydrocarbon microseepage demonstrates that the peak value of hydrocarbon microseepage is corresponding to point gas source in the space; differential adsorption of

alkanes by the simulant caprock and its overlying strata results in the occurrence of a chromatographic effect; and the mechanism of hydrocarbon microseepage above gas source may be infiltration-oriented. Hydrocarbon concentrations in surface soils change with the gas source pressure. This result is coincident with the knowledge that the concentration in soil gases and fluxes from the soil surface could change in response to reservoir production and shows that hydrocarbon microseepage is a dynamic process. The results of the laboratory simulation of hydrocarbon microseepage is believed to be significant to make further studies on the mechanism of surface geochemical anomalies. ACKNOWLEDGMENTS The authors express their gratitude to the helpful comments of Prof. ZHANG Yigang, XUE Weiping, XIA Xianghua, BAO Zhengyu, Dr.LI Zhiming; Thanks are also due to MrLI Ji-peng ’s analyzing work in this experiment. Support by the Wuxi Petroleum Geoglogy Laboratory of SINOPEC, China, is also

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J.Wrigley,Holger Stolpmann,2001,Soil gas survey detects microseepage through thick evaporitic sequence in Egypt. Oil&Gas Journal,May 7:36-41 ［26］Hou Weiguo,Su Jiangyu, 2001,The evidence and characteristics of vertical micro-migration of upper pool’s hydrocarbons in northern Tarim basin.Xinjiang Petroleum Geology,22(6):465-468(in Chinese with English abstract). 附文中图名、图注、数据表： Figure 1 Simplified conceptual model of vertical hydrocarbon microseepage Figure 2 Sketch map of experimental equipment Figure 3 Relationship among the pressure of the gas source, bottom temperature,and methane concentrations of the nearest sampling sites in the model. Figure 4 Vertical concentration profiles of methane and ethane in the simulant caprock after injecting hydrocarbon gas for seventh days Vertical concentration profiles of methane and ethane in simulant caprock after Figure 5 injecting hydrocarbon gas for twenty-sixth days Figure 6 Vertical variation of

methane concentrations on five vertical detecting lines at the end of the experiment. Concentration of Hydrocarbons(C 1 to C 5+ ) versus depth. Bering Sea Figure 7 Shelf,offshore Alaska, showing passive seepage(From Abrams,1993) Figure 8 Concentration of hydrocarbons(C 1 to C 5+ ) versus depth. Offshore Gulf of Mexico, showing active seepage.(From Anderson et al,1983) Figure 9 One-dimensional vertical variation of i-butane to n-butane on five detecting lines in modeling caprock at the end of the experiment. Figure 10 the relationship between pressure of point gas source and average methane concentration in soil versus time Figure 11 Weak-acid-extracted ethane+ desorbed in soil samples over the Hastings oil 9 Source: http://www.doksinet field,Brazoria County,Texas,USA in a survey conducted in 1946 (contour intervals at 30,50,and 100ppm)( from Horvitz,1969). Figure 12 Weak-acid-extracted ethane+ desorbed in soil samples over the Hastings oil field,Brazoria County,Texas,USA in a

survey conducted in 1968.Note the change in concentration from that of the 1946 survey(Figure 11)(contour intervals at 30,50,and 100ppm)(from Horvitz,1969) Table1 Characteristics of background hydrocarbon contents in modeling cuboid(ppm，n=25） sample Hydrocarbon Max. Min. Ave. Sd. Cv. Background Methane 45.2 6.48 25.41 10.52 0.41 Ethane 5.39 0.42 2.44 1.30 0.53 contents in modeling cuboid 按要求提供 第一作者王国建的身份证号码为：210222197211134755 10