Experiments on Steam Injection into Preformed Steam Chambers of Various Shapes for Maximum Condensate Recovery

Round 1

Reviewer 1 Report

In this manuscript, experiments on steam injection into preformed steam chambers were carried out for an elliptical chamber and a circular one. Results under various mass flow rates were obtained to find the optimal dimensionless mass flow rate. The manuscript is worthy of study. A major revision was required before the next step.

1. Although the test bench has been introduced in previous papers, important parts, as well as the main test steps and conditions, still need to be presented.

2. The experimental results are listed directly without in-depth analysis.

3. The comparision of the results with different steam chambers should be analyzed in detail. 

4. The motivation and innovation need to be further summarized.

 

In this manuscript, experiments on steam injection into preformed steam chambers were carried out for an elliptical chamber and a circular one. Results under various mass flow rates were obtained to find the optimal dimensionless mass flow rate. The manuscript is worthy of study. A major revision was required before the next step.

1. Although the test bench has been introduced in previous papers, important parts, as well as the main test steps and conditions, still need to be presented.

2. The experimental results are listed directly without in-depth analysis.

3. The comparision of the results with different steam chambers should be analyzed in detail. 

4. The motivation and innovation need to be further summarized.

 

Author Response

Reply’s to Reviewer 1

We welcome the points raised by the Reviewer #1 which advise us: “A major revision was required before the next step.

1. Although the test bench has been introduced in the previous papers, important parts, as well as the main steps and conditions, still need be presented.

Reply:  Done, along the introduction and in other parts we added important details related to the experiments and its relation with the actual SAGD process and the theoretical model. Thus, after the second paragraph on page 2 we write:

“We need highlight, based on field trials, that the steam chamber shape evolves slowly in the time [17]. In our theoretical model [10] the (hypothetically instantaneous) knowledge of the shape of chamber it is necessary in the quantitative determination of the optimal injected mass flow rate of steam; this is the fundamental assumption in the proposal for the use of a preformed chamber in the experiments. Since a theoretical point of view, the solution of the steam condensation problem at the chamber edge (of a given form), which must be computed numerically, turns out to be physically admissible only for a particular value of φ_opt, that depends on a set of dimensionless parameters.”

 

Similarly, in the new fifth paragraph on page 2 we clarify how the steam chamber achieves the saturation of steam and the importance of the initial saturation in some heterogeneous reservoirs, where SAGD is inefficient, for instance, due to lean zones. Such a paragraph says:

 

“We will notice that in each of our ideal injection experiments, the chamber is initially dry, i.e., there is not any initial water saturation and, eventually, a steam-saturated stage will be reached. However, the SAGD process is sensitive to the heterogeneity of oil sand formation, as has been detected due to the presence of lean zones during the steam injection [19–21]. The lean zone is a zone in which water saturation is higher than 40%. The lean zone can be found in reservoirs as bottom water, top water, or pockets of water at different elevations and behaves as a thief zone during SAGD production. Field-scale studies show that a well pair is most affected by lean zones above its injector, than the other cases. The high heat capacity of water, which is almost twice that of oil, when combined with high water saturation, leads to high steam demand when steam intercepts lean zones. Due to it, the steam-oil ratio (SOR) increases and decreases oil production. When these inhomogeneities are absent the SAGD technique is generally efficient, with steam-oil ratios in the 2 to 5 range, which means SAGD generally requires converting between two to five barrels of water into steam for the recovery of one barrel of crude oil. The pressure of the lean zone is another parameter that influences SOR and bitumen rate during SAGD operation. Lean bitumen at low pressure (lower than 1000 kPa) may require a large volume of steam to increase the pressure to that of SAGD operation pressure.”

 

We detailed in the paragraph previous to subsection 2.2 that the preformed elliptic chambers have the same size and the circular one is such that it can be inscribed in the ellipses:

 

“As is shown in Fig. 1, the areas of the ellipses are the same A_e = πab = 0.052 m² and the area of the small circumference is A_c = πR² = 0.031 m², which means that the area of the circumference is around 0.6Ae, our criterion for choice the size of the circumference was that it could be inscribed in the ellipse.”

 

In Section 3, as a first paragraph, we added a comment on the well constraints in our experiments and in actual SAGD:

 

“The wells constraints in experiments were that, along the injection pipe, steam is injected radially into the porous medium at pressure pI and in the recovery pipe, the condensates, and eventually the steam, outflow. In terms of the pressure, the lower end of the chamber is close to the production pipe, and during injection the pressure in the chamber decreases to about the pressure in the production pipe, which is the ambient pressure (in an actual reservoir such a pressure is not very different from that of the reservoir [24]). In this stage, the spatial pressure variations in the steam chamber are small compared with the hydrostatic pressure variation in a distance of the order of the height of the steam chamber. However, the small spatial pressure variations determine the flow of steam in the chamber and the distribution of condensation flux at its boundary.”

 

2. The experimental results are listed without in-depth analysis.

Reply:  We believe that our main experimental results have in-depth analysis because additional to the determination of the existence of the optimal injections, which we show that depend on the steam chamber shapes, as has been predicted through our formal fluid mechanics model [10], we measured experimentally the transient condensate fronts reaching the chamber edges, and to understand it we alluded to other theoretical study on radial hot water injection [28], to fit different power laws of time to data. In these last cases, we also discuss how such fronts interact capillarly with the solid boundary due to the water wetting.  We commented that if water does not wet the steam chamber edge, an emulsion can be produced. The stability of this film, close to the high-viscosity oil region, could depend on the effective gravity and the local curvature of each chamber shape. This last is an open problem. We added this paragraph at the end of Section 4:

 

“We also believe that the local curvature of each steam chamber used in experiments may influence the downward flow intensity, through the effective gravity. Consequently, more careful studies correlating gravity and both types of liquid films will be performed in order to complement our experiments in preformed steam chambers.”

 

3. The comparison of the results with different steam chambers should be analyzed in detail.

Reply:  Done. The pertinent arguments are similar to those given in the previous question.

 

4. The motivation and innovation need to be further summarized.

Reply:  Done. In the improved Introduction and in the improved Conclusions we mentioned that our motivation for this study is that the knowledge of the instantaneous shape of the steam chamber, of the parameters of the reservoir and of the involved fluids, may help to improve the crude oil production in homogeneous reservoirs.

As innovations we express that inhomogeneities (lean zones) in the reservoir and technical procedures as subcool may be imposed in the experimental set-up in order to observe, and measure, the impact of such situations on the performance of the condensate recovery in our steam chambers. Close to the end of the Conclusions we added the following phrase:

“We need comment that in actual field operations subcool is one of the keys for stable SAGD production due to forming a certain height of steam-liquid level to prevent steam breakthrough and improve the utilization efficiency of heat. However, the location of steam-liquid level can’t be monitored in field operation, and can only be approximately adjusted by monitoring and maintaining reasonable injection-production temperature difference [36,37]. Finally, in our current experiments we did not find these situations, but in future studies we could establish them in order to measure in a controlled manner their effects.”

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper studied the optimal mass flow rates in steam-assisted gravity drainage (SAGD) process. In order to gain physical insight into the main mechanisms of heat and mass transfer within the steam chamber, the authors have created three shapes of preformed steam chambers with cross-sections of an ellipse with a horizontal semi-major axis, an ellipse with a vertical semi-major axis, and a circumference.The results indicated that the shape of the chamber edge has a considerable influence on the efficiency of the steam chambers. 

Overall the paper is well structured throughout. The content is well researched in the literature and clearly demonstrates the current state of research and shortcomings in the field. The results are clearly presented and the conclusions are reasonably derived.

There are some problems, which should be solved before it is considered for publication. Fig.1(a) and Fig.1(a) actually have the same area, but their sizes appear significantly different in the image. I do not understand why the cross-sectional areas of the three different preformed steam chambers in Fig.1 are not the same.Moreover, I believe a brief introduction to the optimal mass flow rate per unit length, , for steam injection should be provided.

Overall, it is good.

Author Response

Reply’s to Reviewer 2

We wish to thank the reviewer for his/her thoughtful reading of the paper and the comments and criticism in his/her report.

The paper studied the optimal mass flow rates in steam-assisted gravity drainage (SAGD) process. In order to gain physical insight into the main mechanisms of heat and mass transfer within the steam chamber, the authors have created three shapes of preformed steam chambers with cross-sections of an ellipse with horizontal semi-mayor axis, an ellipse with vertical semi-major axis and a circumference. The results indicated that the shape of the chamber edge has a considerable influence on the efficiency of the steam chamber.

Overall the paper is well structured throughout. The content is well researched in the literature and clearly demonstrates the current state of research and shortcomings in the field. The results are clearly presented and the conclusions are reasonably derived.

There are some problems, which should be solved before it, is considered for publication. Fig. 1(a) and Fig. 1(a) actually have the same area, but their sizes appear significatively different in the image.

 I do not understand why the cross-sectional areas of the tree different preformed steam chambers in Fig. 1 are not the same.

Answer. - We detailed in the paragraph previous to Subsection 2.2, in the manuscript, that the preformed elliptic chambers have the same size and the circular one is such that it can be inscribed in the ellipses:

 

“As is shown in Fig. 1, the areas of the ellipses are the same A_e = πab = 0.052 m² and the area of the small circumference is A_c = πR² = 0.031 m², which means that the area of the circumference is around 0.6Ae, our criterion for choice the size of the circumference was that it could be inscribed in the ellipse.”

 

We also changed Fig. 1(c).

 

Moreover, I believe a brief introduction to the optimal flow rate per unit length, φ_opt, for steam injection should be provided.

Answer. - Yes, after the second paragraph on page 2 we write:

“We need highlight, based on field trials, that the steam chamber shape evolves slowly in the time [17]. In our theoretical model [10] the (hypothetically instantaneous) knowledge of the shape of chamber it is necessary in the quantitative determination of the optimal injected mass flow rate of steam; this is the fundamental assumption in the proposal for the use of a preformed chamber in the experiments. Since a theoretical point of view, the solution of the steam condensation problem at the chamber edge (of a given form), which must be computed numerically, turns out to be physically admissible only for a particular value of φ_opt, that depends on a set of dimensionless parameters.”

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper is focused on experiments on steam injection into preformed steam chambers of various shapes for maximum condensate recovery. The following revisions are needed before next round of review:

(1) The nature of discussing steam injection is the steam injectivity. However the steam injectivity is affected by many parameters including permeability, porosity, initial water saturation... Usually the initial water saturation has a bigger impact, and I suggest the authors to summarize the effects of initial water saturation on steam injectivity in the Introduction part based on following references:

Effects of lean zones on steam-assisted gravity drainage performance

Numerical study of the effects of lean zones on SAGD performance in periodically heterogeneous media

(2) What is the well constraint in the experiment? How is the subcool like for the producer. It is very important production parameter for SAGD operation.

(3) I do not think it is the steam chamber that determines steam injectivity. In stead, it is the steam injectivity in horizontal/vertical direction that determines the steam chamber shape. I want to see more discussions on the mutual impacts between steam injection and steam chamber shape.

Author Response

Reply’s to the Reviewer 3

We wish to thank the reviewer for his/her thoughtful reading of the paper and the comments and criticism in his/her report. Point-by-point we response to the Reviewer.

The paper is focused on experiments on steam injection into preformed steam chambers of various shapes for maximum condensate recovery. The following revisions are needed before next round of review:

(1) The nature of discussing steam injection is the steam injectivity.

However the steam injectivity is affected by many parameters

including permeability, porosity, initial water saturation… Usually

the initial water saturation has a bigger impact, and I suggest the

authors to summarize the effects of initial water saturation on

steam injectivity in the Introduction part based on following

references:

 

Effects of lean zones on steam-assisted gravity drainage

performance.

 

Numerical study of the effects of lean zones on SAGD

performance in periodically heterogeneous media

 

Answer. - We agree with the referee suggestion, attending his/her suggestion we added in the new fifth paragraph, on page 2 that:

 

“We will notice that in each of our ideal injection experiments, the chamber is initially dry, i.e., there is not any initial water saturation and, eventually, a steam-saturated stage will be reached. However, the SAGD process is sensitive to the heterogeneity of oil sand formation, as has been detected due to the presence of lean zones during the steam injection [19–21]. The lean zone is a zone in which water saturation is higher than 40%. The lean zone can be found in reservoirs as bottom water, top water, or pockets of water at different elevations and behaves as a thief zone during SAGD production. Field-scale studies show that a well pair is most affected by lean zones above its injector, than the other cases. The high heat capacity of water, which is almost twice that of oil, when combined with high water saturation, leads to high steam demand when steam intercepts lean zones. Due to it, the steam-oil ratio (SOR) increases and decreases oil production. When these inhomogeneities are absent the SAGD technique is generally efficient, with steam-oil ratios in the 2 to 5 range, which means SAGD generally requires converting between two to five barrels of water into steam for the recovery of one barrel of crude oil. The pressure of the lean zone is another parameter that influences SOR and bitumen rate during SAGD operation. Lean bitumen at low pressure (lower than 1000 kPa) may require a large volume of steam to increase the pressure to that of SAGD operation pressure.”

 

References suggested and other were included as Refs. [19–21].

 

On page 10, just after Fig. 9 we change a little the original paragraph to include the following text:

“Through Figs. 10(c) and 10(d) we notice that, at the bottom, a water accumulation (liquid pool) appears as a consequence of the recovery orifice (black dots in said figures) being located 0.03 m above the lower edge of the chamber. It is apparent that similar cases could occur in actual SAGD processes, as the lean zone discussed in the Introduction, and, therefore, the experimental visualization of this case becomes of interest [30].”

 

 

(2) What is the well constraint in the experiment? How is the

subcool like for the producer? It a very important production

parameter for SAGD operation.

 

Answer. – On page 6, just at the start of Section 3 we discuss the well constraint in the experiment:

 

“The wells constraints in experiments were that, along the injection pipe, steam is

injected radially into the porous medium at pressure pI and in the recovery pipe, the

condensates, and eventually the steam, outflow. In terms of the pressure, the lower

end of the chamber is close to the production pipe, and during injection the pressure

in the chamber decreases to about the pressure in the production pipe, which is the

ambient pressure (in an actual reservoir such a pressure is not very different from that

of the reservoir [24]). In this stage, the spatial pressure variations in the steam chamber

are small compared with the hydrostatic pressure variation in a distance of the order

of the height of the steam chamber. However, the small spatial pressure variations

determine the flow of steam in the chamber and the distribution of condensation flux at

its boundary.”

 

 

For the subcool we argued that:

 

“We need comment that in actual field operations subcool is one of the keys for stable SAGD production due to forming a certain height of steam-liquid level to prevent steam breakthrough and improve the utilization efficiency of heat. However, the location of steam-liquid level can’t be monitored in field operation, and can only be approximately adjusted by monitoring and maintaining reasonable injection-production temperature difference [36,37]. Finally, in our current experiments we did not find these situations, but in future studies we could establish them in order to measure in a controlled manner their effects.”

 

 

(3) I do not think it is the chamber that determines steam

injectivity. In stead, it is the steam injectivity in horizontal/vertical

direction that determines the steam chamber shape. I want to see

 more discussion on the mutual impacts between steam injection

and steam chamber shape.

 

Answer. - We agree with this observation. In our manuscript the main motivation for the change of the preformed shapes, was just the prediction of the existence of an optimal mass flow rate as a function of the parameters of the reservoir and of the involved fluids, including the produced fluids. We believe that pictures and plots in Figs. 4-13 we give a lot of evidence of the mutual effects between the shapes and the corresponding fluid flows. In order to detail a little about the interaction of the condensed fluids and the chamber edges, we added the following phase at the end of Conclusions:

 

“We also believe that the local curvature of each steam chamber used in experiments may influence the downward flow intensity, through the effective gravity. Consequently, more careful studies correlating gravity and both types of liquid films will be performed in order to complement our experiments in preformed steam chambers.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

There are no additional comments.

There are no additional comments.

Reviewer 3 Report

Good to go now.