wellbore instability causes and consequences

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Rudarsko-geoloko-naftni zbornik UDC 622.32:622.248 UDK 622.32:622.248 Language/Jezik:English/Engleski

Vol. 19

str. 87 - 98

Zagreb, 2007. Review Pregledni rad


Key words: well, instability, stress

Kljune rijei: buotina, nestabilnost, naprezanje

Abstract Wellbore instability is one of the main problems that engineers meet during drilling. The causes of wellbore instability are often classified into either mechanical (for example, failure of the rock around the hole because of high stresses, low rock strength, or inappropriate drilling practice) or chemical effects which arise from damaging interaction between the rock, generally shale, and the drilling fluid. Often, field instances of instability are a result of a combination of both chemical and mechanical. This problem might cause serious complication in well and in some case can lead to expensive operational problems. The increasing demand for wellbore stability analyses during the planning stage of a field arise from economic considerations and the increasing use of deviated, extended reach and horizontal wells. This paper presents causes, indicators and diagnosing of wellbore instability as well as the wellbore stresses model.

Saetak Nestabilnost kanala buotine jedan je od glavnih problema s kojima se inenjeri susreu tijekom buenja. Nestabilnost kanala moe biti posljedica mehanikih uzroka (naprimjer, lom stijene oko kanala zbog velikih naprezanja, mala vrstoa stijene ili neodgovarajua praksa buenja) ili kemijskih uzroka (tetne reakcije izmeu stijene, obino ejla i isplake). U praksi je nestabilnost kanala buotine esto posljedica zajednikog djelovanja kemijskih i mehanikih mehanizama. Ovaj problem moe izazvati ozbiljne komplikacije u buotini i u nekim sluajevima moe dovesti do skupih operativnih problema. Poveana potreba za analizama nestabilnosti kanala buotine tijekom faze planiranja polja proizlazi iz ekonomskih razmatranja i rastue upotrebe otklonjenih, poveanog dosega i horizontalnih buotina. U radu se prikazuju uzroci i pokazatelji nestabilnosti kanala buotine, dijagnosticiranje problema te naprezanja u stijeni pribuotinske zone.

Introduction Unexpected or unknown behavior of rock is often the cause of drilling problems, resulting in an expensive loss of time, sometimes in a loss of part or even whole borehole. Borehole stability is a continuing problem which results in substantial yearly expenditures by the petroleum industry (Bradley, 1978, Awal et al., 2001). As result, a major concern of the drilling engineers is keeping the borehole wall from falling in or breaking down. Detailed attention is paid to drilling fluid programs, casing programs, and operating procedures in drilling a well to minimize these costly problems. Wellbore instability has become an increasing concern for horizontal and extended reach wells, especially with the move towards completely open hole lateral section,

and in some cases, open hole build-up section through shale cap rocks. More recent drilling innovations such as underbalanced drilling techniques, high pressure jet drilling, re-entry horizontal wells and multiple laterals from a single vertical or horizontal well often give rise to challenging wellbore stability question (Martins et al., 1999; Kristiansen, 2004; Tan et al., 2004). In many cases the section of an optimal strategy to prevent or mitigate the risk of wellbore collapse might compromise one or more of the other elements in the overall well design, e.g., drilling rate of penetration, the risk of differential sticking, hole cleaning ability, or formation damage. For drilling situations it is therefore desirable to apply integrated predictive methods that can, for instance, help to optimize the mud density, chemistry, rheology, the selection of filter cake building additives,


Rud.-geol.-naft. zb., Vol. 19, 2007. B. Pai, N. Gaurina-Meimurec, D. Matanovi: Wellbore instability: Causes and...

and possibly temperature. Sensitivity studies can also help assess if there is any additional risk due to the selected well trajectory and inclination. Wellbore stability predictive models may also be used to design appropriate completions for inflow problems where hole collapse and associated sand production, or even the complete loss of the well, may concerned. For example, in highly permeable and weakly cemented sandstones such predictive tools can be used to decide whether a slotted or perforated liner completion would be preferred over leaving a horizontal well completely open hole (McLellan at all,1994b).Table 1 Causes of wellbore instability Tablica 1. Uzroci nastabilnosti kanala buotine

Causes of wellbore instability Wellbore instability is usually caused by a combination of factors which may be broadly classified as being either controllable or uncontrollable (natural) in origin. These factors are shown in Table 1 (McLellan et al., 1994a, Bowes and Procter , 1997; Chen et al., 1998; Mohiuddin et al., 2001).

Causes of Wellbore Instability Uncontrollable (Natural) Factors Naturally Fractured or Faulted Formations Tectonically Stressed Formations High In-situ Stresses Mobile Formations Unconsolidated Formations Naturally Over-Pressured Shale Collapse Induced Over-Pressured Shale Collapse Controllable Factors Bottom Hole Pressure (Mud Density) Well Inclination and Azimuth Transient Pore Pressures Physico/chemical Rock-Fluid Interaction Drill String Vibrations Erosion Temperature

Uncontrollable factors Naturally fractured or faulted formations A natural fracture system in the rock can often be found near faults. Rock near faults can be broken into large or small pieces. If they are loose they can fall into the wellbore and jam the string in the hole (Nguyen et al., 2007). Even if the pieces are bonded together, impacts from the BHA due to drill string vibrations can cause the formation to fall into the wellbore. This type of sticking is particularly unusual in that stuck pipe can occur while drilling. Figure 1 shows possible problems as result drilling a naturally fractured or faulted system. This mechanism can occur in tectonically active zones, in prognosed fractured limestone, and as the formation is drilled. Drill string vibration have to be minimized to help stabilize these formations (Bowes and Procter, 1997). Hole collapse problems may become quite severe if weak bedding planes intersect a wellbore at unfavorable angles. Such fractures in shales may provide a pathway for mud or fluid invasion that can lead to time-depended strength degradation, softening and ultimately to hole collapse. The relationship between hole size and the fracture spacing will be important in such formations.

Figure 1 Drilling through naturally fractured or faulted formations Slika 1. Buenje kroz prirodno raspucane ili rasjedima ispresjecane formacije

Tectonically Stressed Formations Wellbore instability is caused when highly stressed formations are drilled and if exists a significant difference between the near wellbore stress and the restraining pressure provided by the drilling fluid density. Tectonic

Rud.-geol.-naft. zb., Vol. 19, 2007. B. Pai, N. Gaurina-Meimurec, D. Matanovi: Wellbore instability: Causes and...


stresses build up in areas where rock is being compressed or stretched due movement of the earths crust. The rock in these areas is being buckled by the pressure of the moving tectonic plates. When a hole is drilled in an area of high tectonic stresses the rock around the wellbore will collapse into the wellbore and produce splintery cavings similar to those produced by over-pressured shale (Figure 2). In the tectonic stress case the hydrostatic pressure required to stabilize the wellbore may be much higher than the fracture pressure of the other exposed formations (Bowes and Procter, 1997). This mechanism usually occurs in or near mountainous regions. Planning to case off these formations as quickly as possible and maintaining adequate drilling fluid weight can help to stabilize these formations

the formation squeezing into the wellbore (Bowes and Procter, 1997). This mechanism normally occurs while drilling salt. An appropriate drilling fluid and maintaining sufficient drilling fluid weight are required to help stabilize these formations.

Figure 3 Drilling through mobile formations Slika 3. Buenje kroz pokretljive formacije

Unconsolidated formations An unconsolidated formation falls into the wellbore because it is loosely packed with little or no bonding between particles, pebbles or boulders. The collapse of formations is caused by removing the supporting rock as the well is drilled (Figure 4). It happens in a wellbore when little or no filter cake is present. The un-bonded formation (sand, gravel, etc.) cannot be supported by hydrostatic overbalance as the fluid simply flows into the formations. Sand or gravel then falls into the hole and packs off the drill string. The effect can be a gradual increase in drag over a number of meters, or can be sudden (Bowes and Procter, 1997). This mechanism is normally associated with shallow formation. An adequate filter cake is required to help stabilize these formations.

Figure 2 Drilling through tectonically stressed formations Slika 2. Buenje kroz tektonski napregnute formacije

High in-situ stresses Anomaliously height in-situ stresses, such as may be found in the vicinity of salt domes, near faults, or in the inner limbs of a folds may give rise to wellbore instability. Stress concentrations may also occur in particularly stiff rocks such as quartzose sandstones


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