Well Design and Well Integrity by Runar Nygaard, January 4, 2012, Energy and Environmental Systems Group, University of Calgary
Several recent studies have investigated the integrity of wells around the world. They have identified that out of 316,000 wells analyzed in Alberta—4.6% have leaks. Gas migration occurred in 0.6% of the wells and surface casing vent flow (SCVF) in 3.9% (Watson and Bachu, 2007). In a subset of 20,500 wells, 15% leaked with drilled and abandoned wells making up 0.5% and cased wells 14.5%. … In the Norwegian sector of the North Sea, between 13 and 19% of the production wells experienced leakage, while 37 to 41% of the injectors experienced leakage (Randhol and Carlsen, 2008; NPA, 2008). Further, estimates from the Gulf of Mexico indicate that a significant portion of wells have sustained casing pressure, which is believed to be caused by gas flow through cement matrix (Crow, 2006). In a study of the K-12B gas field in the Dutch sector of the North Sea where CO2 is injected, 5% of tubulars where degraded because of pitting corrosion (Mulders, 2006). The main observation from these studies is that cased wells as more prone to leakage than drilled and abandoned wells, and injection wells are more prone to leakage than producing wells.
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Cementing can be divided into two broad categories, primary and remedial. Primary cementing is used during regular drilling operations to support the casing and stop fluid movement outside the casing (zonal isolation). Cement also protects the casing from corrosion and loads in deeper zones, prevents blow outs and seals off thief and lost circulation zones. … The well construction process only allows one chance to design and install a primary cementing system. … During the drilling phase of a well, the cement sheath must withstand the continuous impact of the drill string, particularly with directional wells. During well completion when the drilling fluid is replaced by a relatively lightweight completion fluid, the negative pressure differential can cause de-bonding at the casing cement and/or cement formation interfaces. The cement sheath must withstand the stresses caused by the perforating operation and resist cracking from the extreme pressure created by the hydraulic fracturing operation.
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It is important to run the casing at a speed that will not fracture the formation. After the casing is in place, common cement failures occur in one of two ways: poor primary cementing or cement failure after setting. Poor primary cementing occurs because a thick mud filter cake lines the hole and prevents good formation bonding. Proper displacement techniques, such as pre-flush, spacers and cement plugs, may not be sufficient because the conventional cement is not the best displacement fluid. Secondly, gas can invade the cement while it sets. During gelling and prior to complete hydration, conventional cement slurry actually loses its ability to transmit hydrostatic pressure to the formation and fluids from the formation migrate freely into the cement. This forms channels that can create future gas leaks. Cement failure after setting occur from mechanical shock from pipe tripping, expansion of the casing and compression of the cement during pressure testing, or expansion and contraction of the pipe due to cycles in injection pressure and temperature.
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ABANDONMENT OF WELLS … For a well that has production casing, the abandonment procedure is more customized. All nonsaline water sources have to be protected and hydraulic isolation must exist between porous zones. This rigorous requirement has been in place since 2003. There are five different options to abandon cased wells using plugs, packers or cement plugs. The three main types are 1) bridge plug set above the perforations with cement over top the plug, 2) squeeze cement in the perforations, and 3) cement plug across perforations. All methods have one common requirement, and that is to have at least 8 m of cement inside the casing that has been pressure tested to 7000 kPa. At the surface, casing strings are cut 1 to 2 m below the surface and a steel plate is welded to prevent access to the casing strings. This is done after the well is tested for gas migration and surface casing vent flow. Squeezing cement into openings in casing as remedial cement is often not successful because of the cement’s high viscosity. Metal alloy that expand (~ 1%) upon solidification has recently been suggested for remediate cementing and cement plugs (Canitron, 2008). The alloy is placed in the wellbore and a heating tool melts it. The alloy flows to fit the openings of the casing and the volume inside the casing. The expansion helps to avoid micro-fissures that cement can experience because of its shrinkage. Alloy is also claimed to not go through a weak transitional phase during solidification like cement does, and it bonds stronger against clean steel than pure Portland cement. Molten alloy has low surface tension and viscosity and is claimed to fill small fissures and perforations efficiently (Figure 13). … Removing the casing in certain areas is another method to mitigate leakage caused by poor bond or de-bonding between casing and cement. If wells are plugged and abandoned permanently, both Gray et al, 2007 and Carlsen and Abdollahi, 2007 (Figure 9) suggest the casing steel be removed before installing the final cement plugs. This will remove the most-likely leakage path along the casing. …
Produced sections with perforations and stimulation through hydraulic fracturing and/or acidizing creates fractures that may have caused increased permeability of the cement sheath. Further bridge plugs with capped cement has shown to be prone to leakage inside the casing.
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