Wellbore Stability While Drilling
Jumat, 26 Januari 2018
Edit
Encrypting your link and protect the link from viruses, malware, thief, etc! Made your link safe to visit.
 |
| Wellbore stability while drilling |
New challenges additionally emerged since the increasing use of horizontal wells, drilling in naturally broken media, in terribly deep formations, and in troublesome geologic conditions, wherever wellbore stability is of major concern (Willson and Willis 1986). for instance, a 8,715 m deep well was trained in crystalline rock in Federal Republic of Germany and a few styles of wellbore instabilities (breakouts, washout, undergauged sections) were discovered (Hoffers et al. 1994). Some wellbore instabilities related to advanced earth science conditions, wherever the strain regime was controlled by active faults, area unit reportable within the Cusiana field (Colombia), the Pedernales field (Venezuela), the Alberta Basin (Canada), the Tarim Basin (China), sure areas of the Norwegian Sea, and offshore state (Willson et al. 1999, Plumb et al. 1998, Wiprut and Zoback 1998, Ramos et al. 1998).
When a borehole is trained in an exceedingly naturally broken formation, overly high mud density permits the lubricating substance to penetrate into fractures, mobilizing the rock blocks, and aggravating ovalization (Charlez 1999). once this happens, the broken blocks are not any longer subject to the mud overbalance pressure, and also the destabilized blocks will cave into the well bore as a results of scouring the formation once tripping (Willson et al. 1999). once a borehole crosses a fault, drilling fluid might invade the separation plane. with the exception of mud losses, penetration of the fluid reduces the traditional stress and induces a displacement on the crack planes that shears the well, as shown in Fig. 7.1. the implications will quickly become dramatic and will cause partial or maybe total loss of a well. 2 case histories in Aquitaine, France were delineated that resulted within the loss of the wells and also the want for the drilling of 2 new wells, cost accounting within the vary of US$30 million (Maury and Zurdo 1996).
Wellbore instability may end up in lost circulation wherever tensile stresses have occurred thanks to high drilling fluid pressure (Fig. 7.2a); breakouts and hole closure just in case of compressive and shear failures (Fig. 7.2b). throughout drilling stage associate open hole is supported by drilling fluid pressure to stay wellbore from collapse. If the mud weight is less than the shear failure stress or collapse stress, the shear failure and compressive failure occur within the wellbore within the minimum far-field stress (Sh) direction, inflicting hole collapse or gaolbreak. If the mud weight exceeds the rock enduringness, the tensile fracture is elicited within the most far-field stress (SH) direction. Consequently, this might cause lubricating substance losses or lost circulation.
Figure 7.3 shows a typical wellbore instability thanks to gaolbreak and drilling elicited tensile fracture. For a circular gap with giant diameter the hole/tunnel gaolbreak contains a similar behavior as little boreholes. Figure 7.4 presents hole gaolbreak in an exceedingly circular tunnel with a radius of R = one.75 m within the Underground work of North American country (Martino and Chandler 2004). The absolutely developed notch (breakout) within the roof was caused by stress distribution thanks to excavation. The notch is stable, attributable to its naturally shaped form, that develops a confining pressure at the notch tip. The notch can stay stable unless disturbed by dynamic conditions, like enlarged temperature, little stress changes caused by near excavations.
In severe cases the borehole instability will cause loss of the open hole section. The borehole stability downside will be thought-about by separating the potential rock failure mechanisms into the subsequent four classes (Roegiers1990):
- failures associated with pre-existing or drilling-induced formation damage;
- failures caused by the elicited stress, pore pressure concentrations, and temperature redistributions;
- failures attributed to deliberate or unintentional further stresses; and,
- failures associated with shock-wave loading.
Borehole instabilities area unit the most explanation for drilling difficulties, leading to a rich loss of your time, typically in an exceedingly loss of a part of or maybe whole boreholes. Wellbore instabilities build work terribly troublesome to perform and to interpret (Maury and Sauzay 1987). a nasty condition of the borehole wall alters by artificial means the annulus zone reminiscent of the depth of investigation of most of the work tools.
The shape of the borehole will be powerfully changed giving associate elongated hole in one direction, diameter reduction within the alternative direction and additionally nearly circular cavings in places. within the Cusiana field in South American nation, even supposing some measures to forestall borehole instability were taken, in depth breakouts in fissile and naturally broken shales – of up to 44" in 121/4" hole – occurred (Willson et al. 1999). close to 100% of the well prices within the Cusiana field were spent dealing with unhealthy holes, primarily owing to abnormally high tectonic stresses elicited by a lively thrust-faulting surroundings (Addis et al. 1993, Last and McLean 1995). additionally to the price related to borehole instability whereas drilling, borehole stability additionally contains a substantial impact on reservoir productions (Bradley 1979).
There area unit many stages within the lifetime of a well, i.e., drilling, completion, stimulation, flow tests, production, and depletion. Borehole instabilities will be encountered altogether these stages (Ramos et al. 1996). within the drilling stage, the most issues area unit to see the mud composition and density (mud weight) which is able to maintain the integrity of the well, while not the loss of drilling fluids. throughout the completion and stimulation stage, the reservoir should be connected to the well via perforations.
This operation may fail if the rock adjacent to the cemented casing is non-brittle. before full production, downhole tests embrace open-hole work, fluid sampling, build-up, drawdown, injection, and deliverability tests. it's attainable to induce wellbore failure and casing collapse throughout these processes (Peng et al. 2007). As hydrocarbons area unit depleted, the drained region compacts that may induce solids production, casing harm, surface subsidence and wellbore failure. of these stages within the lifetime of a well, integrated borehole stability analyses area unit vital to make sure the reservoir economical production and minimize the expensive issues elicited by the wellbore instabilities.
Link will be apear in 15 seconds.
Well done! you have successfully gained access to Decrypted Link.