Casing while drilling (CWD) is an emerging technology being introduced in different areas around the world This new configuration, where the casing is used as a drillstring, presents new challenges for primary casing cementing operations compared to the conventional cementing operations.
A full understanding of the required changes of the cementing methodology from conventional drillpipe drilling operations can contribute to the success of any CWD campaign. CWD cementing differs from conventional cementing practices because it is impossible to use standard centralizers attached to the casing while drilling because of extended and faster casing rotation.
When more than one bit is required to reach the next casing point, CWD requires full-bore casing access to pull and run bottomhole assemblies (BHA) through the casing. In these instances, conventional floating equipment cannot be used. Wireline logging is normally conducted in cased hole after the cementing job. The cement volumes are calculated with a cement excess factor instead of a caliper log.
This paper describes the methodology developed to successfully cement surface, intermediate, and production casings in more than 125 wells in south Texas where CWD was used. These same techniques can be applied in CWD operations elsewhere.
CWD differs from conventional cementing practices in several ways.
The use of casing attachments, such as centralizers, to provide good pipe standoff. During CWD operations, centralizers are required to be robust enough to drill the entire openhole section while withstanding the pipe rotation when drilling for extended periods of time. This casing hardware must keep its standoff capability while staying in place and in one piece.
The float equipment is different than that used in conventional cementing operations. Where the possibility exists for more than one bit to reach the next casing point, CWD must allow full-bore casing access. To pull and run BHAs with wireline instead of pulling out the complete casing string by single joints, this full-bore access is required. In such cases, the float equipment is installed once the casing reaches the casing setting depth.
When installing the floating equipment with casing on bottom, the float equipment will be exposed to high circulation rates for considerable time while drilling the entire hole section. Damage to the floating valves can be expected.
The cement volume for the surface, intermediate, and production casing jobs is estimated using a cement excess value. In CWD, wireline logging for formation evaluation is often conducted in cased hole after the cementing job, so caliper log information is not available. If caliper hole size or openhole evaluation is required, it may be obtained with normal openhole logging equipment once the casing is pulled out of the previous shoe.
In CWD operations, standard bowspring or welded-body centralizers are not recommended. The casing string will be subjected to longer and faster rotation while drilling the entire openhole section, and standard centralizers are not suitable for these conditions. They may cause severe wear damage and may lose their original placement, decreasing pipe centralization. In addition, these standard centralizers attached to the casing can be lost in the hole, causing additional problems when drilling ahead.
In CWD, there is no option to place any type of centralizers with an OD larger than the gauge hole size. Bow-type centralizers are desirable where washouts are expected because they provide restoring force to centralize the casing in the hole. The bows on this type of centralizer have lower resistance to casing rotation. A good mud system is essential to minimize the hole washouts. If washouts are unavoidable, the reduced pipe standoff should be compensated by enforcing other best cementing practices, such as providing good mud properties, pumping rates, spacer design, etc.1