Planning phase

Well path design

The well trajectory affects the total depth that has to be drilled, and will therefore affect ECD’s at depth (the ECD’s increases with increasing depth) [1]. It also affects the required casing design (high wellbore angles may require an additional casing string for stability purposes, but it is mainly determined by the relationship between the drilling window (further explained in section 4.3) and target depth). According to Sheppard et al. [70] the well path should ideally be as short and simple as possible and within the limits of other important parameters like T&D, casing wear, buckling, hole cleaning and wellbore/borehole stability [16].

Hole size optimization

It is not given that the standard hole sizes (20” for the 17” liner, 16” for the 13⅝” liner, 13½“

for the 10¾” liner and 9½“ for the 7⅝“ liner) is the optimum choice regarding management of ECD [1], [28].

According to [1] increasing the hole size from 8½” to 9⅞” increases the annular area by 53%

for 5” drillpipe, and 108% for 7” casing. This shows that ECD’s can be drastically decreased by upsizing from a 8½” hole to a 9⅞” hole or from a 12¼” to a 15” hole [1].

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Mims and Krepp [1] states that wellbores with small clearances (between the drillstring and the wellbore) may produce high ECD values. This is particularly a problem on long hole sections or shallow hole sections [1].

Casing plan

It is possible to adjust the casing plan in order to reduce the ECD’s when the deeper hole sections are being drilled [1], [9]. The ECD generally increases with depth (is a function of the TVD) and may thus have critical values in the deepest hole sections:

a. Run the intermediate casing as a liner: Running the intermediate casing as a liner may contribute to a reduction in ECD’s (since the long string of casing is exchanged to a shorter one that starts from the last set casing). This will ensure a larger OD in parts of the annulus, seen in Figure 44.

Figure 44: Running the intermediate casing as a liner vs. running it as a casing.

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b. Use alternative casing connections and centralizers: The connections and/or centralizer type of the casing may affect the downhole pressure while running or circulating the casing [1]. The first step to minimize ECD’s is to therefore to reduce the number of centralizers to the minimum (required for cementing objectives) [1].

c. Use different sizes of casing: Using smaller casing sizes may have certain advantages (e.g. run a 6⅝” liner in a 8½” hole rather than running a 7” liner); it will reduce the ECD while both running and cementing the liner according to [1].

d. Casing flotation and ECD: The ECD’s created while running casing is often ignored. This can be a problem when running long strings of floating casing. The collapse pressure is usually acceptable in a static situation, but if the ECD’s increase to a certain limit it may result in casing collapse.

Drilling fluids

According to Payne et al. [18] the drilling fluid must:

1. Provide a stable wellbore for drilling long open hole intervals at high angles;

2. Maximize lubricity to reduce toque and drag;

3. Develop proper rheology for effective cuttings transport;

4. Minimize the potential for problems such as differential sticking and lost circulation;

5. Minimize formation damage of production intervals;

6. Limit environmental exposure through the fluid system design and the well-site waste minimization program.

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The selection and design of drilling fluids is very important in order to achieve a robust ECD management [1].

a. Rheology: According to K&M [1] the fluid used in an ERW should always be as thin as possible (within the hole cleaning limits), and thinned even more right before running and cementing the casing. If the fluid has good shear thinning capabilities it will further improve the ECD conditions downhole.

b. Gel strengths: The gel strengths should be flat and easily broken down when the fluid is sheared [1] (by either pipe rotation or by breaking circulation) in order to minimize ECD effects.

c. Sweeps: Sweeps are pretty effective in vertical wells, but causes more harm than good in high-angle wells. They have problems in dealing with the cuttings, and struggles with the transportation out of the wellbore. Avoid using sweeps as far as possible to avoid spikes in the annular pressure and ECD fluctuations.

Drillstring design

The drillstring design is an important factor regarding ECD management, especially in shallow ERW, due to the following [1]:

(a) Low formation integrity

(b) Large OD pipe is used to avoid and reduce buckling – reduced annular clearance may result in increased ECD

A common solution to overcome buckling tendencies in (shallow) ERW is to use HWDP or larger OD pipe to increase the stiffness and weight of the drillstring (which again may lead to increased ECD’s) [1]. The drillstring design must therefore be viewed in perspective with ECD constraints in order to obtain a safe and effective drilling operation.

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Designing the ERD well for maximum junk slot area reduces the risk of tripping problems and for swabbing in a kick when the drillstring is being pulled through cuttings beds [1].

Pressure while drilling (PWD) technology

PWD can be an important tool in order to fully understand what is going on downhole with that particular well’s circumstances. PWD technology will provide the operations and

engineering personnel with applicable information in order to improve planning and practices regarding ECD reduction [1].