Pushing the Limit of Drilling with Casing
Authors: Scott Randy , Brown Pat , Zo Tan Ming , Lin Low Siew 
Published: October 2008

Drilling with Casing (DwC?) technology utilizes the casing string as a drill string so that casing is landed on bottom during the drilling process rather than later in a separate installation process. Many areas of the world have used the practice to drill-in the final tubing string and cement in place with the drill bit still attached. Modern drilling with casing is not limited to only the final string of the well. In fact most modern DwC jobs involve drilling the surface hole and intermediate hole sections.

There are two basic technologies used for Drilling with Casing1. The first technology employs a bottom hole assembly comprised of a positive displacement motor (PDM), drill bit and hole opener. The assembly is latched to the first joint of casing. As the assembly drills ahead, the casing is lowered into the hole, either in a static or rotated mode. Upon reaching total depth (TD), the latch-on bottom hole assembly is recovered with a special retrieval tool. A valve system is run and installed before cementing commences.

The second technology utilizes only casing to transmit rotary torque and weight to the drill bit. A drillable drill bit and valve assembly is made up and run with the first joint of casing. The casing string is rotated during drilling, typically with a spear assembly that provides rotation to the casing. Upon reaching TD, the casing can be cemented immediately, with no need for an additional trip.

The Apache China Zhao Dong DwC project employed the latter technology to provide simple and cost effective methods for surface casing installation. This technology has proven to be capable of saving up to 79% rig time3.

Zhao Dong Program Overview

Zhao Dong field is located 55 km south of the Tanggu supply base in Bohai Bay, PRC (Figure 1). The drilling program is comprised of 25 wells in the initial development. Surface casing depths range from 270 m to 300 m. The water depth is 7 m and rig elevation is 32.3 m above mean sea level (AMSL).

The first objective of the DwC project was to start up a newly modified drilling rig in which all of the crews were Chinese with very little English language skills and no experience working for a western company. These crews were ready and willing to learn. DwC system provided a very simple and repetitive operation so that the crews could get on the learning curve quickly. Training the crews to the western methods and blending cultures was greatly enhanced, as the results demonstrated.

New startup rigs typically experience some degree of downtime and are more prone to safety related incidents in the early stages as the machinery is initially functioned and the personnel become familiar with the equipment. DwC operations greatly simplified the drilling processes and reduced dependence on the rig's ability to function. There was much less handling of heavy equipment and rigging-up and -down than required in conventional drilling, so that many aspects of safety were improved. In general, repetitive operations tend to provide a safer work environment.

The primary objective of the DwC project was to safely and efficiently install 13.3/8" surface casing strings in one large 22-well batch setting operation. Apache worked with the contractor, Weatherford, to plan and design many new ideas and techniques for the installation of surface casing.