Fig. 5-1 shows the relationship of some of these casing strings. In addition, the illustration shows some of the problems and drilling hazards that the strings are designed to control.
All wells will not use each type of casing. The conditions to he encountered in each well must be analyzed to determine the types and amount of pipe necessary to drill it. The general functions of all casing strings are as follows:
Drive Pipe or Conductor Casing. The first string ruti or placed in the well is usually the drive pipe, or conductor casing. The normal depths range from 100-300 ft. In soft-rock areas such as southern Louisiana or most offshore environments, the pipe is hammered into the ground with a large diesel hammer. Hard-rock areas require that a large-diameter, shallow hole be drilled before running and cementing the pipe. Conductor casing can be as elaborate as heavy-wall steel pipe or as simple as a few old oil drums tacked together.
A primary purpose of this string of pipe is to provide a fluid conduit from the bit to the surface. Very shallow formations tend to wash out severely and must be protected with pipe. In addition, most shallow formations exhibit some type of lost circulation problem that must be minimized.
An additional function of the pipe is to minimize hole caving problems. Gravel beds and unconsolidated rock will continue to fall into the well if not stabilized with casing. Typically, the operator is required to drill through these zones by pumping viscous muds at high rates.
Structural Casing. Occasionally, drilling conditions will require that an additional string of casing be run between the drive pipe and surface casing. Typical depths range from 600-Î ,000 ft. Purposes for the pipe include solving additional lost circulation or hole caving problems and minimizing kick problems from shallow gas zones.
Intermediate Casing. The primary applications of intermediate casing involve abnormally high formation pressures. Since higher mud weights are required to control these pressures, the shallower weak formations must be protected to prevent lost circulation or stuck pipe. Occasionally, intermediate
Abnormally I high T pressure
Fig. 5-1 Typical casing string relationships
Abnormally I high T pressure
Drilling (and production) liners arc used frequently as a cost-effective method to attain pressure or fracture gradient control without the expense of running a string to the surface. When a liner is used, the upper exposed easing, usually intermediate pipe, must he evaluated with respect to burst and collapse pressures for drilling the open hole below the liner. Remember that a full string of casing can be run to the surface instead of a liner if required, i.e., two intermediate strings.
Production Casing. The production casing is often called the oil string. The pipe may be set at a depth slightly above, midway through, or below the pay zone. The pipe has the following purposes:
Tie-back String. The drilling liner is often used as part of the production casing rather than running an additional full string of pipe from the surface to the producing zone. The liner is tied-back or connected to the surface by running the amount of pipe required to connect to the liner top. This procedure is particularly common when 1) producing hydrocarbons are behind the liner and 2) the deeper section is not commercial.
Casing seat depths are directly affected by geological conditions. In some cases, the prime criterion for selecting casing seats is to cover exposed, severe lost circulation zones. In others, the seat selection may be based on differential sticking problems, perhaps resulting from pressure depletion in a field. In deep wells, however, the primary consideration is usually based on controlling abnormal formation pressures and preventing their exposure to weaker shallow zones. The design criteria of controlling formation pressures generally applies to most drilling areas.
Selecting casing seats for pressure control purposes starts with knowing geological conditions such as formation pressures and fracture gradients. This information is generally available within an acceptable degree of accuracy. Pre-
spud calculations and the actual drilling conditions will determine the exact locations for each casing seat.
The principle used to determine setting depth selection can be adequately described by the adage, "hindsight is 20-20." The initial step is to determine the formation pressures and fracture gradients that will be penetrated in the well. After these have been established, the operator must design a casing program based on the assumption that he already knows the behavior of the well even before it is drilled.
This principle is used extensively for infill drilling where the known conditions dictate the casing program. Using these guidelines, the operator can select the most effective casing program that will meet the necessary pressure requirements and minimize the casing cost.
Setting Depth Selection for Intermediate and Deeper Strings. Setting depth selection should be made for the deepest strings to be run in the well and then successively designed from the bottom string to the surface. Although this procedure may appear at first to be reversed, it avoids several time-consuming iterative procedures. Surface casing design procedures are based on other cri-eria.
The first criteria for selecting deep casing depths is to let mud weights control formation pressures without fracturing shallow formations. This procedure is implemented bottom-to-top. After these depths have been established, differential pressure sticking considerations are made to determine if the casing string will become stuck when running it into the well. These considerations are made from top-to-bottom, the reverse from the first selection criteria.
The initial design step is to establish the projected formation pressures and fracture gradients. In Fig. 5-2a, a 15.6-lb/gal (equivalent) formation pressure exists at the hole bottom. To reach this depth, wellbore pressures greater than 15.6 lb/gai will be necessary and must be taken into account.
The pressures that must be considered include a trip margin of mud weight to control swab pressures, an equivalent mud weight increase due to surge pressures associated with running the easing, and a safety factor. These pressures usually range from 0.2—0.3 lb/gal, respectively, and may vary due to mud viscosity and hole geometry. Therefore, the actual pressures at the bottom of the well include the mud weight required to control the 15.6-lb/gal pore pressure and the 0.6-0.9-lb/gal mud weight increases from the swab, surge, and safety factor considerations. As a result, formations exhibiting fracture gradients less than 16.5 lb/gal or less (15,6 lb/gal + 0.9 lb/gal) must be protected with casing. The depth at which this fracture gradient is encountered is the tentative intermediate pipe setting depth.
The next step is to determine if pipe sticking will occur when running the casing. Pipe sticking generally can occur at the point where the maximum differential pressures are encountered. In most cases, this depth is the deepest normal pressure zone, i.e., at the transition into abnormal pressures.
Fig. 5-2 (a) Projected formation pressures and fracture gradients, (b) Selection of the tentative intermediate setting depth for Example 5. (
Field studies have been used to establish general values for the amount of differential pressure that can be tolerated before sticking occurs:
Normal pressure zones 2,000-2,300 psi
Abnormal pressure zones 3,000-3,300 psi
These values are recommended as reasonable guides. Their accuracy in day-to-day operations depends on the general attention given to mud properties and drillstring configuration.
The tentative intermediate pipe setting depth is the actual setting depth if the differential pressure at the deepest normal zone is less than 2,000-2,300 psi. If the value is greater than this arbitrary limit, the depth is defined as the shallowest liner setting depth required to drill the well. In this case, an additional step is necessary to determine the intermediate pipe depth.
An example problem will be used to illustrate this procedure. The section following the example will illustrate the case in which differentia] pressure considerations cause the additional step to select the intermediate pipe depth.
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