Behind casing wash and cement

Abstract

The invention relates to a method of conducting a perf wash cement (“P/W/C”) abandonment job in an offshore oil or gas well annulus, in particular the washing or cementing operation using a rotating head with nozzles dispensing wash fluid or cement at pressure. A new design of bottom hole assembly is proposed in which the cementing tool has a relatively large diameter in order to optimize pressure whilst the wash tool has a relatively small diameter. The wash process, for a number of reasons, appears to be less sensitive to tool diameter and making the wash tool smaller reduces the overall risk of stuck pipe.

Claims

1. A method of performing a plug and abandon operation in an oil or gas well having a casing, the method including: a. passing through the casing a bottom hole assembly comprising a generally cylindrical wash tool having a plurality of wash fluid nozzles and, axially connected to the wash tool, a generally cylindrical cementing tool having one or more cement nozzles, wherein the outer diameter of the wash tool is less than the outer diameter of the cementing tool, and the outer diameter of the wash tool is about 2.54 cm to 10.16 cm smaller than the drift diameter of the casing and the outer diameter of the cementing tool is about 0.64 cm to 2.54 cm smaller than the drift diameter of the casing; b. delivering wash fluid through apertures in the casing into a region outside the casing; and c. delivering cement through the apertures into the region outside the casing.

2. The method according to claim 1, wherein the outer diameter of the wash tool is selected from about 2.54, 3.81, 5.08, 6.35, 7.62, 8.89 and 10.16 cm smaller than the drift diameter of the casing and the outer diameter of the cementing tool is selected from about 0.64, 1.27, 1.9 and 2.54 cm smaller than the drift diameter of the casing.

3. The method according to claim 1, wherein the length of the cementing tool is selected from about 60, 70, 80, 90, 100, 110, 120, 130, 140 and 150 cm long.

4. The method according to claim 1, wherein position of the cement nozzle nearest the upper end of the cementing tool is selected from about 26, 30, 36, 40, 46, 50, 60, 70, 80, 90, 100, 110, and 120 cm from the upper end.

5. The method according to claim 1, wherein two nozzles of the cementing tool are spaced apart axially by a distance selected from at least 5.08, 7.62, 10.16, 12.70, 15.24, 17.78 and 20.32 cm.

6. The method according to claim 1, wherein some or all of the wash nozzles of the wash tool are angled at angle selected from about 10, 20, 30, 40, 50, 60, 70, and 80 degrees to the axis of the tool.

7. The method according to claim 1, wherein some or all of the wash nozzles are angled downwardly.

8. The method according to claim 1, wherein the cementing tool is connected to the upper end of the wash tool.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

(2) FIGS. 1(a), 1(b), 1(c) and 1(d) are graphic results from CFD analysis showing a comparison between modelled cementing tools with different lengths;

(3) FIGS. 2(a) and 2(b) are graphic results from CFD analysis of modelled cementing tools having axially spaced nozzles;

(4) FIGS. 3(a), 3 (b), and 3(c) are graphic results from CFD analysis showing a comparison between modelled cementing tools with different outer diameters; and

(5) FIGS. 4(a) and 4(b) are graphic results from CFD analysis showing a comparison between modelled wash tools with different outer diameters.

DETAILED DESCRIPTION

(6) Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

(7) In the following examples, the behavior of wash and cement fluid being delivered by cement and wash tools in a casing surrounded by an annulus filled with fluid and debris were modelled. The CFD modelling employed software marketed under the trade name “Fluent” by Ansys Inc. These simulations have since been repeated using a different CFD software package, Star CCM+ marketed by Siemens, and found to give similar results. However, the examples cite here are from the Fluent software. The models were based on a 9%″ (24.4 cm) casing with an 8½″ (20.3 cm) drift diameter. It is believed that the results may be generalized to other casing diameters such as 10¾″ (27.3 cm) or 7¾″ (19.7 cm) casing.

(8) The CFD model was Reynolds Average Navier Stokes (RANS)-based unsteady multiphase Volume of Fluid (VOF) with multiple interacting phases (fluids). It used S.S.T. k-ω turbulence model in the Fluent software. Debris and wash fluids were modeled as non-Newtonian fluids based on Bingham plastic or Herschel-Bulkley models as appropriate. All fluids were considered homogeneous.

(9) A 12 feet long perforated section of casing was modelled. Typical CFD mesh count ranged from 7˜8 million cells. The computational timestep was in the range of 1 ms to 3 ms, adjusted for optimum numerical stability and tool rotational speed. The motion of BHA was simulated via a moving-deforming-layering mesh approach including interface. All perforations in the casing were assumed to be circular with no burr. A mass boundary flow condition was applied at the inlet and a pressure boundary condition at the outlet.

Example 1

(10) The BHA currently in use by the applicant has an 8 inch (20.3 cm) outer diameter for the cement and wash tools. The cementing tool has an overall length of 50.5 cm (19.9 inches), the two nozzles are diametrically opposed and at the same position axially, and the distance between the nozzles and the upper (proximal) end of the tool is 16.8 cm (6.6 inches). In this Example, three other geometries for the cement tool are analysed using CFD analysis: in each case the length of the tool proximally of the nozzles is increased without changing the other dimensions. The increase in length is 30 cm, 60 cm and 120 cm. FIGS. 1(a) to 1(d) show the four cases FIG. 1(a) the standard case (tool as in use today), FIG. 1(b) a 30 cm extension, FIG. 1(c) a 60 cm extension and FIG. 1(d) a 120 cm extension.

(11) Looking at FIG. 1, it can be seen that each diagram includes several plots with the distance travelled by the tool (up) on the x axis and the fraction of fluid displaced (in the outer annulus) on the y axis. The value on the y-axis is presented as a fraction of 1, so that for example 0.2 represents 20%, 0.9 represents 90%, etc. The different plots are of different volumes going down the annulus with the uppermost volume being the first plot. It is simplest to compare the first of plot for each of the four diagrams. As can be seen in FIG. 1(a), the first annulus volume reaches a displacement fraction of 0.9 (90%) when the tool has travelled just over 1 foot (just over 30 cm) and the maximum displacement is about 0.95 (95%). Turning now to FIG. 1(b), it can be seen that a displacement fraction of 0.9 is reached when the tool has travelled only about 0.7 of a foot (about 21 cm) and the maximum displacement proportion is about 0.97 or 0.98 (97-98%). This clearly demonstrates that there is a benefit to using a longer cementing tool in terms of increasing the proportion of the original annulus content which is displaced by cement (and by implication the likely quality of the cement job).

(12) Turning now to FIGS. 1(c) and 1(d), these show, respectively, extensions of the cementing tool proximally by 60 cm and 120 cm. Looking at both FIGS. 1(c) and 1(d), there is hardly any discernable difference from FIG. 1(b). A reasonable conclusion is that increasing the cementing tool length by 30 cm provides a distinct benefit, which is also provided by increasing the length more; however, increasing the length beyond 30 cm may not provide an incremental benefit. It would be reasonable to conclude that any proximal extension of the cementing tool from its current length may provide some benefit, however.

Example 2

(13) In this example, CFD analysis was performed on the model of FIG. 1(b) in Example 1, i.e., a cement tool with a 30 cm extension, but with one of the two nozzles moved axially upwardly/proximally by 8″ (20 cm). The result is shown in FIG. 2(a) and can be compared to FIG. 1(b). Comparing the first plotted line, it can be seen that that the maximum displacement fraction for the tool with spaced or offset nozzles reaches a value of 0.99 (99%) or higher. This compares favorably with the maximum displacement fraction of 0.97-0.98 (97-98%) achieved by the cement tool without offset nozzles.

(14) CFD analysis was also performed using the model of FIG. 1(c) in Example 1, i.e., a cement tool with a 60 cm extension, but again with one of the two nozzles moved axially upwardly/proximally by 8″ (20.3 cm). The result is shown in FIG. 2(b) and can be compared to FIG. 1(c). A similar benefit is achieved in terms of maximum displacement fraction although, as with Example 1, there appears to be little difference between the 30 cm and 60 cm extended tools.

Example 3

(15) In this example, modelled cementing tools with different outer diameters were analysed using CFD analysis. All the modelled cementing tools were extended proximally by 30 cm and had nozzles offset by 20.3 cm. The aim of the analysis was to determine, by reducing the outer diameter of the tool, at what diameter performance fell significantly. The reference for this Example is FIG. 2(a) which is the result for a 30 cm extended tool with 20.3 cm offset nozzles, where the tool outer diameter was modelled at 8.0 inches (20.3 cm). FIG. 3(a) shows results for CFD analysis using the same model but with an outer diameter of 7% inches (20.0 cm). It can be seen from examining the first plot in the diagram that the performance is affected somewhat. The tool moves though approximately 0.8 feet (24 cm) before achieving a displacement fraction of 0.9 (90%) and the maximum displacement fraction achieved is around 0.98 (98%).

(16) FIG. 3(b) shows the results for a model which is the same in all respects except that the outer diameter is reduced to 7¾ inches (19.7 cm). Looking at the first plot, 0.9 (90%) displacement fraction is not achieved until the tool has moved by 1 foot (30 cm) and the maximum displacement fraction is reduced slightly.

(17) FIG. 3(c) shows the results for a model in which the outer diameter is reduced to 7¼ inches (18.4 cm). Looking at the first plot, it can be seen that performance has fallen off significantly, with 0.9 (90%) displacement fraction not being achieved until the tool has moved about 2 feet (60 cm) and the maximum displacement fraction being about 0.92 (92%). The inventors believe, based on practical experience, that these results show that an actual tool with these dimensions may provide inadequate displacement of cement.

(18) The results for the 7% inch (20.0 cm) and 7¾ (19.7 cm) outer diameter tools are believed to be acceptable, so that the cut off between acceptable and unacceptable performance appears to lie somewhere between 7¾ (19.7 cm) and 7¼ inches (18.4 cm).

Example 4

(19) In this example, a wash tool was modelled using CFD. The wash tool was modelled with a total of 10 nozzles, 2 of which were inclined upwardly at 45 degrees, 4 downwardly at 45 degrees and (between them) 4 nozzles perpendicular to the tool axis. The purpose of this work was primarily to compare the performance of a wash tool with this design and having an 8.0 inch (20.3 cm) outer diameter and a similar wash having a much smaller outer diameter of 5.5 inches (14.0 cm). The results are shown in FIGS. 4(a) and 4(b).

(20) Referring firstly to FIG. 4(a), the volume on the y axis represents the fraction of remaining original fluid in the annulus (represented as a fraction of 1), so that for example a value of 0 indicates that the wash fluid has displaced all of the original contents of the annulus. The wash tool is modelled travelling over an axial distance of about 2 feet (60 cm) in a downward/distal direction, and this is represented on the x axis. The individual plots show the modelled displacement in each of a series of 1 foot (30 cm) long sections of the outer annulus, with the first in the list being the uppermost or most proximal section which the wash tool passes first in its downward travel.

(21) As can be seen from the diagram, the first plot shows, naturally, the uppermost/most proximal section of annulus being displaced to wash fluid more quickly than the others. After 2 feet (60 cm) of travel, there is only about 0.02 (2%) of the original fluid remaining in the annulus. The lower sections of annulus are progressively less efficiently washed, though it should be noted that in a real situation the wash tool will travel further than 2 feet (60 cm) so that these sections will in fact receive more washing.

(22) FIG. 4(b) shows the modelled small (5.5″, 14.0 cm) tool. It is immediately apparent that the washing effect of this tool is at least equivalent, and in some respects somewhat better, than that of the modelled 8″ (20.3 cm) tool. The first section is washed more slowly, but by the 2 foot (60 cm) point a similar displacement is achieved to that achieved by the 8″ (20.3 cm) tool. After 2 feet (60 cm) the lowest section is also displaced to about the same extent as by the 8″ (60 cm) tool, but it appears to achieve this level of displacement more quickly.

(23) When viewed in the light of the results from the cement tool modelling, where reduction of the tool diameter to 7¼ inches (18.4 cm) had a marked negative effect on performance, these results were very notable, and bear out the inventors understanding as set out in, for example, in the Brief Summary of the Disclosure.

REFERENCES

(24) All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. Incorporated references are listed again here for convenience: Ferg, T., et al “Novel Techniques to More Effective Plug and Abandonment Cementing Techniques”, Society of Petroleum Engineers Artic and Extreme Environments Conference, Moscow, 18-20 Oct. 2011 (SPE #148640). US2020/040707A1 (ConocoPhillips)