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 At stages as the well is deepened casing will be run and cemented into the hole. Also at total depth after drilling has been finished a final casing string will be used if the well is to be tested and/or completed. Casing is metal pipe of progressively reduced diameter, cemented into a borehole at intervals. The casing point is the depth at which the base of each casing string is set.
At stages as the well is deepened casing will be run and cemented into the hole. Also at total depth after drilling has been finished a final casing string will be used if the well is to be tested and/or completed. Casing is metal pipe of progressively reduced diameter, cemented into a borehole at intervals. The casing point is the depth at which the base of each casing string is set. Costs for casing and cementing depend on the total depth of the well and whether the final section is to be cased but a rough estimate is around 8% to 15% of total well costs of which maybe half is for the steel in a fully cased well. The casing and cementing programme in appraisal and development wells tends to be greater as a percentage of total well costs because the lower part of the hole will usually be cased. Based on the expected geology, the casing points are estimated and detailed in the drilling programme but actual depths are chosen by reference to geology and hole conditions. There are many purposes to casing and cementing related to the need to mechanically and hydraulically isolate a well for its entire life. They include securing weak formations that may collapse into the well bore and trap the drill string; sealing porous intervals to prevent lost circulation; covering bulging shale intervals that can stick the drill string; isolating high-pressure zones to allow safe drilling; and allowing safe entry and retrieval of equipment. Casing is first run from the surface to the casing point - smaller sizes within larger sizes. The last string to be set is usually called liner, which is hung from the lowest part of the previous string from a liner hanger. When closed-ended casing is run into a hole there are surge pressures arising from the displaced mud that can cause formation damage, particularly in narrow deep holes where there is minimal clearance between the casing and the hole. Running the casing open-ended can reduce surge but allows cuttings and debris into the casing which can cause blockages without filter systems at the shoe. On most wells there are three casing groups. For a typical offshore well these may be:
- conductor (surface) casing with large diameters set at shallow depths (commonly 30" casing in 36" hole at around 200 to 300m);
- Intermediate casing set over specific geological intervals (18%", 13%" and 9%" casing in 24", 17W' and 12'1." hole at around 500, 2,000 and 3,500m;
- Liner set at total depth (7" liner in 8Y>" hole at TO). Liner may be slotted where it crosses a production interval.
Narrower casing sizes are also sometimes necessary in deep wells, in deep water wells and in wells with a series of problematic formations.
Fixing casing
Casing and liner both require a sheath of cement, which must endure for long periods and must be able to withstand all the rigours of drilling and, if required, completion and production. Thus, after casing has been nun into the hole, fluids (called pre-flushes and spacers) are pumped in to flush out the drilling mud. Mud must first be removed for the cement to bond to the wellbore and to the exterior casing surface. Depending on the difficulty of removing the mud, more than one fluid may be used and, in addition, various mechanical devices called scratchers and wipers may be fixed to the outside of the casing, which is reciprocated up and down and rotated. The scratchers and wipers break up the wall cake and gelled drilling mud so the circulating spacers and cement can sweep it from the hole establishing better cement contact and promoting adhesion, Behind the fluids a carefully gauged volume of cement is pumped downhole through the casing and back up the annulus between the casing and wellbore. A plug followed by fluid is pumped behind the cement to force it out of the wellbore as far as the bottom of the casing string and top of the annulus at the surface. The plug hits a stopper at the shoe so that only a short, cemented section will set at the base of the casing. This will be drilled out if drilling is to recommence. The cement is designed to develop rapid early strength to allow continued drilling or completion as soon as possible. Cement must be able to isolate productive zones from the surface and from one another. It also has to protect against external corrosive fluids, physically support the casing and withstand pressure and temperature cycles. There may be a compromise involved in choice of mix, for example, high compressive strengths may be more prone to erosion by high temperatures. Consequently cement mixing is a specialist task, especially in highly deviated wells, where long laterals require longer setting times and hole cleaning is more difficult. Flexible cements that adapt to dynamic well bore conditions have also been developed.
The cement bond at the base of the casing with the borehole is termed the casing shoe, It is the most fragile part of the hole and an area critical for successful completion of a well. The shoe is best located in a hard non-porous formation with high mechanical strength (usually salt, anhydrite, hard shales or un-fractured limestones) although this is not always possible. After drilling out the plug and re-drilling into un-cemented rock, aleak-off test (LOT) is conducted just beneath the shoe, During a leak-off test the mud is pumped to a pressure that causes it to be physically injected into the formation and this indicates the maximum mud weight that may be used in the next open hole section. Cased-hole logs may also be run to determine the quality of the cement Bond.
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