Evaluation of the Kinetic and Thermodynamic Behavior of Tracers for Their Applicability in SWCTT

Reviewer #1: Comments

Response

It is recommended to add references to geophysical logs and cores in LINES 30-31

References to geophysical logs and cores were added in the text of this sentence

In line 61-62, it says that SWCTT is also widely used in polymer flooding, surfactant flooding and ASP flooding, whether these chemicals will affect the tracer? I think the tracers used will also vary. What are the main uses of ester-tracers described in this paper?

When determining the residual oil saturation after each of the above types of flooding, esters (formates, acetates, propionate) are used as partitioning tracers, which is their main application area in SWCTT. Commercial surfactants and polymers were tested in our laboratory and they showed no effect on the tracers under study. For other types of flooding, we can rely on literature.

The decimal point of some data in Tables 1 and 2 should be unified instead of ", ", if 7,7 should be changed to 7.7

In Tables 1 and 2 "," have been changed to "."

In formula (9), “or” is the subscript, 0,5 is changed to 0.5, 1,5 is changed to 1.5

Formula 9 has changed its subscript from "," to "."

Reviewer #3: Comments

Response

What is the novelty in this manuscript? Please further explain.

The main novelty of the study is to determine the boundary conditions for the application of typical tracers (esters) used in SWCTT technology, taking into account their kinetic and thermodynamic behavior under reservoir conditions of low and high temperatures and brine salinity.

Please explain the relationship between eq.(3) and eq.(4). Because they are K values. In eq.(3), the water-oil phase are involved, but in eq.(4), only the parameters in water phase is used.

The solubility preference of primary tracer (ester) is represented by the oil/water partitioning coefficient, K-value, where for partition coefficient the general formula is K=C_o/C_w, Co and C_w - concentrations at equilibrium of the tracer in oil and water phases. To calculate the K-value we use in our studies it is convenient to use formula 4, determining the K-value through the concentration of the tracer in the aqueous phase, considering the mass loss of the ether during hydrolysis and transition into the oil phase, as already suggested in paper “Bursaux, R.; Peltier, S.; Nguyen, M.; Romero, C.; Danielle Morel, D. 2016. Single Well Tracer Test Results in a High Temper-ature, High Salinity Offshore Carbonate Reservoir for Chemical EOR Pilot Evaluation. SPE Improved Oil Recovery Con-ference, Tulsa, Oklahoma, USA. https://doi.org/10.2118/179579-MS”

Please correct comma to points in the table 1

In Table 1 "," have been changed to "."

Please make the proposed algorithm clearer in line 69.

In our manuscript, the term "algorithm" has been replaced by the term "method", which is more logically suited to this process by the method of selecting the working interval of the primary tracer in SWCTT

Reviewer #4: Comments

Response

Terminology should be unified. There are numerous terms used for the same situation. Examples include “shut-in period” & “shut-down period” & “soak time”, “1/2 hydrolysis” & “half-life”.

Yes, we agree that all the terms in the article in the latest version should be reduced to one form. This has been carried out.

More explanation related to the research background could be helpful. Especially related to the range of temperature. An explanation of the typical temperature range for a reservoir of EOR would help understand the range of applied temperature during the experiments.

The use of SWCTT is possible in fields with different reservoir temperatures, which is one of the main factors in choosing a primary tracer. Considering in the manuscript the influence of the ester structure, we describe the change in the rate of hydrolysis of the tracer at different reservoir temperatures and accordingly the possibility of its use in the selected reservoir. In the introduction, the part on temperature range for a reservoir of EOR was supplemented.

Additional information about the experiment settings seems to be needed. It seemed that the experiment was introduced but not explained. What were the experimental conditions and what do they each represent within the revoir environment? What kind of procedure was taken to conduct the experiment? Is this experiment setting typical for this kind of study? This experiment seems to be the key of this research, but there seems to be no emphasis and explanation about it.

The K-value measurements for low and medium temperatures without the application of pressure in our previous studies were carried out in sealed vials. Since in this work we wanted to achieve measurements in more realistic reservoir conditions, an experimental setup was prepared, which was adapted for K-value studies in harsh conditions. It is analogous to the high pressure and high temperature cells, which made our tests successful. If you need to add more details, we will try to add. Experiments to determine ester K-value in a two-phase system were carried out at the same pressure of 7 MPa (pressure effect on hydrolysis rate is low, as mentioned in paper “Tang, J.S., Harker, B., 1990. Mass Balance Method To Determine Residual Oil Saturation From Single Well Tracer Test Data. J. Can. Pet. Technol. 29 (02), 115-124. https://doi.org/10.2118/90-02-08”) and different temperatures already mentioned in the Results section.

In figure 3, is it possible for the K-value to be different at the same temperature? The authors plotted the 1/2 hydrolysis according to the temperature and K-values, but the K-value is the function of temperature. Also, what is the exact meaning of gray shade? The message of figure and the explanation for the figure is not sufficiently written in the text.

We understand your concerns at this point. This graph shows how the value of 1/2 hydrolysis of the ester in the two-phase system could change with increasing K-value. At different Sor these values will be different, we have given here at Sor = 0.5. This graph is more theoretical, i.e. it illustrates the effect of the K-value on the hydrolysis time, and what value of 1/2 hydrolysis time we will get at the same chosen temperature, but with different K-value (depending on the type, the K-value may vary).

To indicate the average soak period, in which ½ hydrolysis of the ester falls, we have highlighted this area on the graph with a blue shade.

References are need for some explanations. What are the references for equations 3 and 4 (units are also needed)? What research proposed that the optimal time for the soak period is usually 2-15 days (line 158)?

Reference to general equation 3 added, equation 4 is our representation of the K-value calculation for the ester. The soak period (2-15 days) that we have outlined is an average of the data that has already been given in the literature, for which several references are given in the text.

Figure 4 seems to be quite similar to the figure used in ref29. If this is referred to, adding the reference to it should be considered. Also, the message that this figure is intended to convey doesn’t seem to be explained that well. In the description, it’s talking about the equilibration of ester between the oil and water phase. However, the circulation sign is misleading in this sense, since it gives the impression of mixing rather than equilibration.

Yes that's right, it's talking about the equilibration of the ester. The sign of circulation at this point explains that since the distribution coefficient must be a constant value, then to maintain the same K-value, the ester balances between two phases (water and oil) to replace the lost ester during hydrolysis. Unfortunately, there may be a mistake, in paper we find no reference to this graph.

Figure 5 could be drawn in a more effective way other than the current method. The 3D graph seems to be unnecessary since for each graph there are only two variables. it the four graphs were plotted in one, then the 3D graph would have been meaningful. However, since this is not the case, there seems to be no reason for this kind of graph style. Also, this type of graph is actually misleading due to the fact that the shadings at the bottom of the columns do not convey any messages. This can confuse readers to think there is something behind this shade of colour, so it would be recommended to use a 2D line graph or something else.

We agree that 3D graphs are more convenient to compare on a single graph from temperature range data.

The graphs in Figure 5 have been reformatted in 2D style, we think they look more digestible in this version.

Reviewer #5: Comments

Response

I find it hard to distinguish background information in the literature from the new work done in this study. I suggest reorganizing the manuscript and move all introductive text into “Introduction” or “Background” or “Methods” sections and leave only new results in the “Results and Discussion” section. It seems like only Table 3 and Figure 5 are the new experimental results.

Are Figures 2 and 3 produced from this study? Are they used to guide experimental design? If so, please state that (in Table 3 or in methods) and put the figures in Methods, to improve the clarity of the paper.

Line 77 mentioned different electrolytes were used (sodium chloride, calcium chloride, magnesium chloride), however the effect of different electrolytes on the experimental results was not mentioned.

The manuscript has been amended to make it clearer, some of the general information has been consolidated in the "Introduction" section. The part of the research that was carried out experimentally is placed in the "Results and Discussion" section. In the same section, the analytically calculated data, which are given in Figures 2 and 3 and respectively form the logical chain in this paper, also shows the idea of the presented method of determining the working conditions of tracer use in SWCTT, which is the main concept of the manuscript.

In this work, we did not investigate the effect of electrolytes on the behavior of tracers under the given reservoir conditions. We simply prepared a reservoir water model (a selected field in Western Siberia) using these salts.

Line 50: Consider adding “(see section xx)” to let readers know that K will be defined later in the paper.

When editing the last manuscript, we added "(see experimental section)" as a calculated definition of K-value.

Line 62: Please include the full name for ASP.

The full name for the ASP was specified in manuscript.

Line 70, 244: The term “interval” appeared at several locations of the manuscript and seem to have different meanings. It should be clarified or re-worded. Should it mean the time period for shut in? or the delay time between primary and secondary tracer? Or the range of optimal K values (line 287)?

In the manuscript, we have reviewed the use of term in necessary and unnecessary places to provide a more detailed definition in specific cases.

Line 76: What type of oil? What’s the API gravity? Please provide oil characterization data,

otherwise it is hard for readers to interpret the results without knowing what oil was used.

The study used oil with a density of 825 kg∙m^-3; we added this information to the manuscript

Line 92: Is sufficient mixing between tracer and brine needed? If so, how was it ensured? Is sufficient mixing between the oil phase and the aqueous phase needed? If so, how was it ensured?

The solubility of primary tracers in water is shown in Table 1. We performed tests with solutions of esters in brine with a concentration of no more than 2%, dissolving them without significant effort. Prolonged mixing of the water-oil phase leads to a faster distribution of the tracer, which can be compensated by a longer exposure in the cylinder. The solutions were mixed immediately after preparation and kept under reservoir conditions for 24 hours, which is sufficient for distribution under our experimental conditions.

Line 96: Please specify how long time was needed for equilibration.

In this part of the manuscript, the time for equilibration of the ester is given.

Line 102 (Figure 1): What is meant by “T = Tres”?

“Tres” refers to the reservoir temperature, noted in the text.

Line 114: It’s vague what “amount” means. Please rephrase.

Here the term refers to the amount of a substance, the unit of measure of which is "mol"

Line 140: Please provide a reference for this statement (A paper? Past experience? etc.).

Other articles on tracer SWCTT studies have not previously emphasized the value of logK_ow.

Line 148: Please spell out “bp”.

Designation “bp” changed to full name in Table 1.

Line 244: What interval? (see comment above)

In this line we have already indicated the K-value interval (from 6 to 50), these data were given in the manuscript.

Table 3: why were different tracer compounds tested under different temperature ranges? It seems to have something to do with Figs 2 and 3 (see general comment above). It should be clarified in the table caption or methods.

The working ranges of the studied primary tracers such as ethyl formate, ethyl acetate and ethyl propionate are different, which is largely due to their rate of hydrolysis at different reservoir temperatures. Figure 2 shows the influence of the structure on the kinetic behavior of the tracer at different temperatures, Figure 3 reflects the contribution of the tracer extraction capacity on the hydrolysis time. This is described in the text of the manuscript. If we require further explanation, we will give it in our paper.

Figure 3: “K = 1-6” for EtOF is vague. Please write the specific K value next to each line for EtOF.

A specific K-value has been added to the graph for EtOF.

Figure 5: I find the 3D layout unhelpful for reading the K values and unnecessary. Each subplot

merely shows K-value versus Sor at a certain temperature. A simple 2D plot is sufficient to plot

the data.

We agree that 3D graphs are more convenient to compare on a single graph from temperature range data.

The graphs in Figure 5 have been reformatted in 2D style, we think they look more digestible in this version.

Figure 5: Ethyl formate was tested at T = 25, 40, 45°C. Why does Figs 5a shows T = 35 °C? What

about T = 25 and 40°C? And what about T = 110 °C for Ehtyl Acetate and T = 60 & 90°C for Ethyl Propionate?

Your question is understandable.

We have presented these graphs as a few examples to illustrate our method of determining operating conditions of use and limitations of the tracers. Therefore we have only chosen 2 examples each with two tracers and some temperatures, in order not to overload the article. The graph for ethyl formate (35 C) is also logical as we have data in the temperature and salinity range for this tracer.

Another issue is that to make such a graph for ethyl propionate was problematic due to the fact that in the calculations there is the rate constant of hydrolysis in water and the activation energy (there is not enough information for these in the literature and we have not defined it yet, but we have plans to define it in the future)

Line 238: “Deans coefficient” is not defined. Where is this coefficient in the equation? How is it relevant to the text afterwards?

The Deans coefficients are the values 0.5 and 1.5, which are given in equation 9 and represent the optimal range of the delay coefficient of the secondary tracer β.  Based on these values, it is explained that you can calculate a possible range of K-values depending on the residual oil saturation, taking into account these coefficients.