Casing Shoe Depth Determination: General Points

Defining the casing setting depths has to take several different fac- tors into account. In a directional well, casing points and directional planning are intimately entwined and may take several iterations to achieve a good overall design.

The first step is to decide which formations would give a compe- tent shoe (i.e., one that will hold a reasonable wellbore pressure assum- ing that the cement job is good). Refer to the geological information, lithology column, pore pressure and frac gradient prognosis, hole sec- tion summaries of the offset wells, and any other available data. You are normally looking for competent shales or unfractured limestones that are impermeable and have a reasonable fracture gradient.

Of particular significance is the existence of a pressure transition zone. If casing is set just above a transition zone then pore pressure in the next section will increase shortly after drilling out and the kick tol- erance will reduce as mud density is increased. It may reduce enough to prevent drilling the following section to the planned depth. On the other hand, if a transition zone is penetrated too deeply a kick may result in the worst circumstances since there will be a lot of open hole under the previous casing shoe. (See Fig. 1-7)

Ideally, casing should be set deeply enough in the transition zone to give a sufficient kick tolerance for the next hole section, while main- taining enough kick tolerance in the current hole section.

Where a drop in pore pressure is expected, casing could be set just above it to give the best shoe strength prior to drilling in to the weak- er zone (see Fig. 1-7)

p

p

o

f

v

v

₊









−

−

=

1

[ ]

The next step is to note which formations may give problems relat- ing to wellbore stability, losses, mobile formations, differential stick- ing, etc. Examination of these may flag where separation of problem areas is needed or where a hazard would be created by having two sets of problems in the same section (e.g., a weak “loss” zone in the same section as a hydrocarbon bearing zone). It might also show where deal- ing with the problems would cause serious incompatibility in the required solutions (e.g., different/conflicting mud properties required). Offset well data as presented in “Hole section summaries” in Section 1.1.2 will be helpful here.

Fig. 1-7 Pressure-Depth Graph Illustrating a Transition Zone

If there are several target or reservoir zones to penetrate, it may be necessary to drill through these in one hole section for efficient comple- tion. In this case designing the casing points to set a casing shortly above the top reservoir zone would minimize open-hole time in the reservoirs and would offer the best chance to get through in one attempt.

With the possible casing points and problem areas in mind for a directional well, draw up a rough directional plan. Formations giving

1.4.5 Well Design

Fracture Gradient

Pore Pressure Gradient

Normal Pressure Gradient Overburden Gradient

wellbore stability problems should be avoided for the buildup section since hole enlargement will make directional control more difficult. Builds should be finished some distance before setting the next casing. Note those formations that should not be used for the buildup part of the well.

It is good practice to run casing to cover the kickoff section of a directional well. This consolidates the well and reduces the danger of keyseating in the dogleg section. However, the casing point should not be inside or just at the end of the kickoff section or else a keyseat could be worn in the casing shoe, which may be impossible to get free from if you get stuck in it.

In a horizontal well it is common to build to horizontal in the reservoir, run the production casing, and then drill ahead horizontally for a slotted liner or prepacked screen completion. This protects the buildup section and (assuming a good cement job) isolates production fluids from the formations above.

Now decide where suitable formations will fit in with the potential problems and the tentative directional plan. This will give the preferred casing setting points.

Starting at the bottom section, define the required hole size for each section. Once the preferred casing setting points and hole sizes are established, kick tolerances should be checked to ensure that these can be safely reached. A good target to set casing is in a transition zone giving you the shoe strength to drill ahead. Starting at the surface, cal- culate kick tolerances for each hole section so that, based on the assumed mud densities required and the likely fracture gradients, each section can be drilled to the preferred casing setting points with acceptable kick tolerance limits. If this is not the case, the casing shoe depths should be revised and it may be that an extra casing string is needed. (See Appendix 1 for calculating kick tolerances.)

Once the casing points allow a safe well, isolate problem areas, and with acceptable kick tolerances, revisit the directional plan. Adjust kickoff points and build rates as necessary to fit in with the casing points.

Having finalized the casing and directional program, note the TVD and measured depth (MD) of each shoe and the maximum anticipated pore pressure in the subsequent hole section.

We looked previously at summarizing offset data by using hole section summaries. Even in exploration areas these should be done with any offset wells that may be relevant even if they are quite far

[ ]

away—because they may give useful data. Now get together the lithol- ogy and pore pressure/fracture gradient prognoses for your well, with the hole section summaries, well proposal, and site survey (including shallow gas indicators, if any).