A gas extraction well casing design
A gas extraction well casing design relies on the specifications for casing and cementing designs on the specific location of well. According to the report by West Virginia Department of Environmental Protection (1), any drilling activities should be conducted in consideration of potential environmental impacts. The key factors to be considered include fresh water locations and the depth of coal and oil deposits. The project of design is a well with a total depth of 18,500 ft and MD of 7420 ft in the region comprising of lower Marcellus shale. The project was conducted following the geological prognosis for West Virginia region, since it is to be located in Morgantown, WV.
According to the standards provided by the West Virginia of Environmental protection (1), any well casing must be designed to support at least 20% more than the anticipated maximum fracturing pressure drop in the well. For the well under consideration, the objective of the operations would be to fracture gas at 1.12 Psi/ ft. The production line has to be sunk up to 7420 ft from the ground level. For this, the anticipated pressure in the well would be 1.12 psi/ft * 7420 ft = 8310.4 psi. The design pressure for the production line should be 20% more than the anticipated well pressure which is equal to 9972.48 psi.
The most suitable steel casing for this pressure is S-95 with an outer diameter of 7 inches and a normal weight of 32. The production line drift will thus be 6.000 while the internal diameter is 6.094 inches. The collapse pressure is 10400 psi. The cement is recommended to be at least 1.5 inches all round. However, the intermediate casing should have a drift larger than the production bit line. This means that the most reasonable intermediate casing would be the S-95 with a normal weight of 71, drift of 10.430 and an outer diameter of 11.75. The total width of cement for the first casing will thus be 1.715, given by the difference between the outer diameter of the production line casing and the drift of the intermediate casing.
A similar approach is applied to determine the drift of the surface casing to be 14.822 and the outer diameter to be 16 inches. The difference between the drift of the surface casing and the intermediate line gives the cement for the intermediate as 1.536 inches. The surface casing will thus be Q-125 with a normal weight of 84 and an inner diameter of 14.868 inches. The last casing (the conductor) must also have a drift bigger than the size of the surface casing. As such, the most suitable size based on the expected structural strength will be H- 40, with a normal weight of 94 and a drift of 18.936. The outer diameter will be 20 and the inner diameter will be 19.124 inches. The cementing around the surface and the conductor casings will both be 1.5 inches thick.
To determine the depth of each casing, the reported depth of different strata as provided in the geological prognosis was used. The depths of the casings were calculated to be at least 20% more than the depth of the recommended strata location. For instance, the longest casing will be the production casing, going to a total depth of 7420 ft from the ground surface. The total length of the casing will however be equal to the total length of the well which is 18,500 ft. This means that after forming the rat hole at the bottom of the production line, the lateral length of the production casing will be 10,080 ft. The intermediate casing goes as deep as the end of the earth stratum where the concentration of salty water is high as well as the concentrations of oil and gas. The intermediate casing should go to the end of the oil and gas strata according to the geological prognosis provided for the project (PNGE310 3).
From the geological prognosis, the oil and gas strata end at approximately 3407 ft below ground level. However, well design directions as given by the West Virginia Department of Environmental Protection (3) indicates that the depth of the casing should be at depths that enhance well stabilization, zone separation and safety of all staff. Based on the prognosis, the most commendable depth to sink the casing would be 3750 ft. For the surface casing, the overall depth to the end of the strata containing salty water and coal should be considered to determine the depth of the casing. As such, the prognosis shows that the salty water and coal regions go up to 758 ft hence the surface casing may go up to 910 ft below the ground surface. The desirable depth for the conductor casing should be below the total fresh water depth. The geological prognosis indicates that the fresh water depth is 80 ft below ground surface. This gives a tentative depth of about 100 ft for the conductor casing. The table below gives a summary of the key features of the different casings.
|Casing||Model||Drift (inches)||Outer Diameter (inches)||Cement thickness (inches)||Drilling diameter||Depth of sinking (ft)|
Cementing the different casings is design depending on the depth of the casing and the casing type. West Virginia Department of Environmental Protection (2) provides guidelines into how to design the cement around the drilling well casings. The objective of cementing in this case is to prevent the flow of fresh water, oil and gas into the annulus of the casings. At the same time, the cement also prevents movement of gas from the casing annulus to the surrounding environment. These bars help in preventing environmental degradation. The key area of concern in designing the cementing structure is to ensure that the right slurry blends are used for each of the sections of casing required. The PGNE 310 manual provides guidelines into the right slurry blends to use for specific cementing sections. Each of the sections will be cemented using the displacement method to obtain the required structural strength.
The volumes of slurry to be used were calculated using the cylindrical volume formula. The following diagram shows the aerial view of the independent casing and cementing appearances.
di – Outer diameter of specific casing
Di – outer diameter of specific casing + cement
From the diagram above, the area of cement is equivalent to the area of the annular space between the outer diameter of the specific casing and the outer diameter of the casing + cement. The formula for calculating the area of an annular space is thus used i.e.
A = ᴫ (R2 – r2); The volume of the slurry used should be equal to the volume of the space to be cemented which is the area multiplied by the total depth of sinking.
For instance, for the conductor casing, Di = 23; di = 20. This gives R = 11.5 and r = 10 inches. The respective radii are thus R = 0.955 ft and r = 0.830 ft. For a total depth of 100 ft, the volume of slurry will be 70.067ft3. A similar approach is used for calculating the slurry volumes for all the other casings. The surface cement will be 520.378 ft3; the intermediate cement will be 1649.55 cubic feet while the production cement will be 5923 cubic feet for the vertical and lateral drills. The total slurry volume will be 8162.997 cubic feet.
To determine the total yield in sacks of cement to be used, the specific yield in cubic feet per sack of cement is used. The total slurry volume divided by the specific yield should give the total yield for each section. In the production casing for instance, the calculated total yield is given by 5923 ft3/ 1.260 = 4700.79 sacks of cement. In the same manner, the data provided in the PGNE 310 manual (2) used for the other casings. The total calculated yield for the intermediate cement will be 1186.73 sacks; the surface cementing will consume 434.74 sacks of cement while the conductor will consume 58.54 sacks. The total cement for the drilling line will thus be 6381 sacks.
The water consumption per sack of cement is given by the mix ration in gallons/ sack of cement. For the different sections, different blends are recommended. The production line has a recommended blend of 5.77 gal/ sack of cement. The total water consumption for the section will thus be the recommended mix multiplied by the number of cement sacks giving 27123.56 gallons of water for the production casing cementing. The other sections will consume 7986.69, 2281.08 and 307.16 gals for the intermediate, surface and conductor cementing respectively. The total water consumed will be 37698.469 gallons. The total setting time to be allowed for each of the sections will be 8 hours as per the directions of the West Virginia Department of Environmental Protection (3). In 72 hours, the cement for each of the sections will be tested to determine the compressive strengths which should be at least 200 psi. The table below provides a summary of the cement designs for various casings. The total yield in the table is given to the next whole number of sacks that can be purchased.
|Casing||Slurry Volume (ft3)||Total yield (sacks)||Total water consumption (gallons)||Settling time (hrs)|
The total water consumption per section remains the same since not all of the cement is consumed during the construction process. The total duration between the start of construction and readiness for use will depend on a number of factors including the available man power, productivity per person, duration of work per day and the process challenges. From the data provided, the drilling well can be constructed effectively as long as the project plans are plausible in terms of time and resources.
West Virginia Department of Environmental Protection. Casing and cementing standards and best management practices. Office of Oil and Gas.
PNGE 310 Manual. Drilling Engineering Project #2 – Summer 2017.