Apache Corp. was investigating new methods to efficiently produce trapped gas in its Stratton Field in South Texas. Recent wells drilled conventionally did not achieve the desired commercial and operational objectives. Casing drilling was selected as a method to achieve these objectives through the operational efficiencies it achieves1 and the ability to minimize lost circulation.
Two options were considered during the rig selection phase of the project. The first option was to utilize a purpose built casing drilling rig and the second option was to convert a conventional rig to drill with casing. The project also dictated that a retrievable casing drilling system2 be used to meet logging requirements. Specialized equipment to drill with casing and to retrieve bottom hole assemblies was required on these rigs. Key components of the analysis included the commercial implications of the rig choice, operational performance, and rig crew experience with casing drilling.
The Stratton Field discovered in the late 1930's3, is a gulf coast field located about 45 miles west of Corpus Christi. To date, approximately 495 wells have been drilled in the Stratton Field on a spacing of 20 to 40 acres.
The previous drilling program conducted in 2000/2001 yielded below average results both commercially and operationally. The below average performance can be attributed to three main areas. The first area of concern was the amount of drilling fluid lost to the well bore during drilling and its effect on well production and well economics. The second was well bore stability through the Anahuac shale section. The third area was a low drilling rate of penetration in shale sections of the wells.
A pilot project utilizing casing drilling was selected as a possible solution. Casing drilling was selected as a method to drill the wells with minimum losses and in turn less risk. The analysis also determined that the wells could be drilled in less time minimizing time dependent well bore stability problems in the Anahuac shale. Casing drilling was also selected based on the analysis that the wells could be drilled more efficiently and with equal or greater rates of penetration than with conventional drilling.
A three well program was conducted to test the casing drilling technology and its ability to lower costs, minimize lost circulation, and lower drilling risks.
Due to certain requirements of the drilling and completion program, a retrievable casing drilling system was selected. This required the use of a rig capable of using a retrievable casing drilling system. The rig selection centered on two options; utilization of a rig purpose built for casing drilling or a conventional rig converted to handle casing drilling with retrievable systems. This paper explores the rig selection process and its effect on the drilling program.
Critical to the analysis were the actual additional rig components required to drill with casing. The additional components included a top drive, casing drive assembly, split crown block, split traveling block, wireline winch, and wireline blowout preventers.