7.2: Well Design

The overriding well design considerations are:

• well costs;
• tubing and completion longevity;
• well simplicity and reliability;
• ability to achieve all (or most) of the objectives of the well.

As in all industries, the oil and gas industry is focused on keeping expenses as low as possible. This is true for well designs which are planned to be as low cost as possible while retaining the ability to keep the well safe for people and the environment. One important aspect of low costs is the longevity, or life span, of the well and completion. While changing tubulars and well completions is a common practice in the oil and gas industry; multiple, frequent changes goes against the concept of low costs.

Two aspects of the well design that lead to a longer life span are the durability of the equipment and the flexibility of the well to handle changing reservoir conditions. As we have seen in earlier lessons, these changing reservoir conditions include lower pressures caused by fluid production, lower production rates caused by the lower pressures, and changing Gas-Oil Ratios (GOR) and Watercuts ($f_w$) over the life of the reservoir. Flexibility in well design includes the ability of the original design to handle all of the anticipated reservoir changes or, if this is not possible, designing the original completion to be able to be modified to handle these changes at the lowest possible costs.

Finally, the well must be able achieve the objectives of the well. There are many objectives other than oil or gas production that can be designed into the well. These include:

• sand control for reservoirs made up of unconsolidated reservoir rock or rock susceptible to Fines Movement (movement of fine-grained materials within the rock);
• zonal isolation for reservoirs with multiple zones (to allow each individual zone to be shut in, if required);
• well stimulation for reservoirs with low permeability or susceptible to well damage; and
• artificial lift for low rate wells.