Casing drilling and production costs!
Casing drilling is an alternative option to conventional drilling and uses standard oilfield casing instead of drill-string. This technology involves drilling and casing a well simultaneously. In casing drilling process, downhole tool recovery or replacement can take minutes instead of hours under conventional methods.
The process employs wireline retrievable tools and a drill-lock assembly, permitting bit and bottom hole assembly, BHA changes, coring, electrical logging and even directional or horizontal drilling.
Once the casing point is reached, the casing is cemented in place without tripping pipes. It has been carried out successfully in South Texas (2001, Wyoming (2003) and Brunei (2003). An original paper, now abridged, by Nedilaka Gaurina-Medimurec of the faculty of Mining Geology and Petroleum Engineering of the University of Zahgreb, Croatia, is here served.
Casing drilling is an effective method of reducing the overall drilling costs by reducing drilling time and drill string problems encountered during conventional drilling process. In addition to the productive drilling time lost to tripping, unscheduled events during tripping can make the drilling process even more inefficient, which can lead to losing the well.
Moreover, the savings from reducing hole problems may be more significant. There are many situations where problems such as lost circulation, well control incidents and borehole stability problems are directly attributed to tripping the drill-string and other situations where these problems prevent the drill-string from being tripped.
Since the casing drive system, CDS process provides a continuous ability to circulate the well, it is inherently safer than leaving the well static without a means of circulating in while a conventional drill-string is tripped. Reduced pipe tripping with the CDS should also reduce surge and swab pressure fluctuations.
There are two basic methods of drilling with casing. A latched retrievable BHA inside the casing that incorporates a motor to drive a conventional bit and under-reamer or rotating the string at surface system is incorporating an internal CDS and a drillable “cement in place” drilling BHA.
Casing drilling system has been designed primarily for multi-well offshore platforms, multi-well operations on land, deep-water operations and for situations requiring operators to drill through and place casing across problem formations quickly.
Drilling rigs used for casing drilling can be specially developed for this technology by modifying conventional rigs. One of the most important things on the rig is casing drive system, CDS which provides safe, non-threaded connection between top drive and casing string. Casing drive system is run hydraulically and it transmits torque and mud fluid to the casing string. There are two types of CDS; internal for greater casing radius and external, for smaller casing radius. It is controlled automatically from the driller’s cabin with Programmable Logic Control, Plc.
The BHA is attached to a drill lock that fits into a full bore landing sub on the bottom of the casing in such a way that it can be retrieved with a wireline unit without any need to trip pipes out of well.
The wireline retrievable drill lock assembly is at the heart of the casing drilling system. The BHA generally consists of a pilot bit and under-reamer, but may include other tools needed to perform almost any operation that can be conducted with a conventional drill string.
The pilot bit and under-reamer passes through the drill-casing and drill a hole that provides adequate clearance for the drill-casing and subsequent cementing. Conventional directional tools and logging while drilling, LWD tools can be suspended below the drill-casing shoe for directional drilling.
A conventional core barrel can be run for casing. Designing a well for casing drilling is similar in many ways to conventional well except that the casing is subjected to additional stress while casing-drilling, so buckling and hydraulics deserves special attention.
A significant difference between drilling with a conventional drill-string and casing drilling is that drill collars are not used to provide weight on bit. The lower portion of the drill-casing will support only a limited compressive load before it buckles.
Buckling occurs when the compressive load and casing/hole geometry create a sufficient bending moment so that the casing becomes unstable. Buckling causes two effects. First, the lateral contact forces between the drill-casing and borehole wall can cause wear on the casing and will increase the torque that is required to rotate the casing.
Secondly, the buckling causes the casing to assume a curved geometry within the borehole that increases the stress in the pipe and may increase the tendency toward lateral vibrations. In straight holes, the compressive load that causes buckling is determined by the stiffness of the pipe, the lateral force of gravity (pipe weight and hole inclinations) and distance from the borehole wall (radial clearance).
In conclusion, casing drilling has proven benefits for certain classes of wells, particularly, those with low deviations. Therefore, low torque is generated during the casing drilling process. The casing system may eliminate costs, reduce hole problems associated with tripping and with its state of development, it is well suited for drilling softer formations with casing sizes of 7” or larger. In these situations, the penetration rate can easily match conventional rates while reduced tripping and drill-string handling can be used advantageously. In these times of low priced crude oil, casing drilling may just be another way of reducing production cost by concerned companies.
. Kayode Adeoye is an energy expert in Lagos.
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