In terms of the casing design, the setting to be used will have a basic structure from the GM-1 field. This includes the 20” conductor casing, a 13 3/8” intermediate casing and a 9 5/8” production casing. These settings used in landmark are casings which are readily available off the shelves and therefore the design will mostly consist of the default information available in the software.
Using the Casing seat software, it will then calculate possible casing shoe depths based on the pore and fracture pressures inputted and this will automatically generate a casing design for each well and the kick tolerance used for the casing setting depths is.
The range for the casing shoe depths differ for each of the producer well and the summary is tabulated below:
Table 7.11: Summary of casing shoe depths
Well name GMP-1 GMP-2 GMP-3 GMP-4 GMP-5 GMP-6 GMP-7 GMP-8 Setting Depth (m MD)
20" Conductor 125.12 130 130.14 130 130 130 125 130.15 13 ⅜" Intermediate 573.81 566.71 770.71 768.59 1270.22 568 1293.99 766.13 9 ⅝" Production 1967.14 2079.83 2150.45 1757.68 2595.59 2508.38 2345.83 2094.31 Target Depth 2399.84 2351.49 2376.32 2202.82 2928.24 2870.49 2665.13 2428.59
During the design phase of the casing seat, the final casing (9 ⅝’) was placed with a setting depth just before the targets to allow the final section (8 ½”) to be an open hole covering the target zones provided. The kick tolerance used for each section is as follow:
Table 7.12: Kick tolerance used in designing the casing shoes
Casing size Kick tolerance (bbls)
20” 10
13 ⅜” 25
9 ⅝” 50
All of the design factors for the safety including the pipe body and the connection have been considered and the values used have been tabulated below:
Table 7.13: Design factors used in the casing designs
Pipe body Connection
Burst 1.100 Burst/Leak 1.100
Axial Axial
Tension 1.300 Tesion 1.300 Compression 1.300 Compression 1.300 Collapse 1.000
Triaxial 1.250
Input required when making the stress checks to be included in the burst and collapse design is the production data which is the packer depth, brine weight and the specific gravity gas gradient. The packer depth is assumed to be just at the 9 ⅝” casing shoe and for the brine selected for the packer fluid depended on the equivalent mudweight of the water from the MDT information obtained from the GM-1 well. The water gradient was 0.4295 psi/ft in which at the average target depths will give a rough pressure of 2100 psi and adding for the 150 psi overbalance for brine will give an EMW of 8.6ppg which will be used for the packer fluid. The specific gravity of gas was obtained from the production technology data and was found to be 0.65.
Other factors were taken into consideration when making the burst or collapse study for the casing design obtained from the design. Listed below are the considerations that were taken into account when making the Burst loads for a well:
• Displacement to gas
• Gas kick profile
• Fracture at shoe with above gas gradient
• Fracture at shoe with 1/3 bottom hole pressure at surface
• Lost returns with water
• Pressure testing
• Cement pressure testing and
• Drilling ahead
For the study for the burst loads in the production casings, the list below shows what was taken into account:
• Pressure test
• Cement pressure test
• Tubing leaks
• Stimulation surface leak and
• Injection down casing
As for the Collapse load calculation, to further make the design much more justifiable, the following factors were taken into consideration:
• Cementing loads
• Fluid evacuation below and above the packer
• Gas migration
A final description of the study will then be automatically generated and to further improve on the design, the casing material and be selected but at the same time the collapse or burst rating of the pipe must not exceed the pressure exerted by the formation in the casing. A sample of how the results appear for the Gelama Merah
Producer-8 is placed in Appendix Figure D.2-1, D.2-2 and D.2-3. All other wells have shown the same trend by using the selected casing material.
For the casing design selection, there is a need to know the conditions of the reservoir and the conditions at the surface as well. The reservoir has traces of carbon dioxide with temperatures reaching 155 degrees Fahrenheit. No traces of Hydrogen sulphate were mentioned and by accordance of the casing steel grade standard and code, the following casings were considered:
Table 7.14: Casing material selection
Casing type Material Selection Connection
20” Conductor J-55 BTC
13 3/8” intermediate L-80 BTC
9 5/8” production L-80 BTC
The reason for using the J-55 steel for the 20” is because the steel grade is one of the most common casing available in the market and although the K-55 have the same minimum yield strength, the deciding factor is the ultimate tensile strength as the K-55 has a higher UTS than the J-K-55 and having a thicker wall making it as a better option to be used as it can for higher temperature wells but with the J-55, it can have better mechanical and thermal fatigue resistance and crack resistance than the K-55 so as to endure the monsoon seasons. There may be need to add further materials to coat the casings to prevent corrosion by seawater.
As for the selection of L-80, the deciding factor on why it is to be used is to protect against corrosion in the reservoir. Even though there are just small traces of carbon dioxide in the well, it should not be taken lightly as there could be a possibility that the concentration may increase throughout the development phase.
The type of connection structure selected will be the buttress thread connection on the basis that it is one of the most typical casing connection types to be used.
7.10.1 Casing Cementation Programme
After placing the casings into the open hole, it is then required to cement the casings into place. The function of the cement which is typically made up from calcareous and argillaceous rocks is to prevent any movement of fluid between the permeable zones and to provide support of the wellbore, preventing any collapse of the formation inside the reservoir while drilling. It will also give support to the casing string being put in place while providing protection against corrosion from the reservoir fluids.
There are different classes of cements available and in terms of selection of which cement to use is heavily dependent on the conditions of the well being drilled.
Classes ranged from A to J and each class depend on how deep the well is being drilled as well as the temperature of the reservoir at its own specific target depths.
One of the basic cements considered for the design is the Class G or the Class H cement. The properties of the cements are effectively similar in terms of both can use accelerators or retarders with moderate to high sulphate resistance by using water as the addition to create the cement. The main difference between the Class G and the Class H is that the Class G cement has a much finer texture as oppose to the Class H with a courser texture.
Looking back into the exploration wells, the type of cement used was the Class G cement was the Class G cement, as there were no problems were encountered during the cementation programme, for the basis of cementing programme calculation and estimation, the same type of cement will be used for the producing wells.
With Class G cement being used, the available information from previous well reports informs that the weight used was at 15.8ppg with a yield of 1.19ft3/sack. This will be used as a basis for the calculating the estimated amount of cement sacks required. As for the mixwater, referencing from the Harriot Watt Drilling engineering notes suggest using a value of 4.96 gallons/sack.
The casing seats for each of the wells have been mentioned in the previous section and therefore the calculations are done manually based on each casing section for each well. Below is the summary for the cementing calculations:
Table 7.15: Cementing summary for all the producing wells 1 to 4
Table 7.16: Cementing summary for all the producing wells 5 to 8
Summing up the total amount of cement (791.53 tonnes) to be used and by using the estimate for the yield of the class G cement, the total number of sacks of Class G cement is estimated to be at 27,971 sacks of cement. Note that this estimate includes a 10% excess used while calculating the volume of cement.