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Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Maintaining a stable wellbore is of primary importance during drilling andproduction of oil and gas wells. The shape and direction of the hole must becontrolled during drilling, and hole collapse and solid particle influx must beprevented during production. Wellbore stability requires a proper balancebetween production. Wellbore stability requires a proper balance between theuncontrollable factors of earth stresses, rock strength, and pore pressure, andthe controllable factors of wellbore fluid pore pressure, and the controllablefactors of wellbore fluid pressure and mud chemical composition. pressure andmud chemical composition. Wellbore instabilities can take several forms (Fig.1). Hole size reduction can occur when plastic rock is squeezed into the hole,and hole enlargement can be caused by caving shales or hard rock spalling. Ifthe wellbore fluid pressure is too high, lost circulation can occur as a resultof unintentional hydraulic fracturing of the formation; if it is too low, thehole may collapse. Excessive production rates can lead to solid particleinflux. Hole instabilities can cause stuck drillpipe as well as casing or linercollapse. These problems can result in sidetracked holes and abandoned wells.Since 1940 considerable effort has been directed toward solving rock mechanicsproblems associated with wellbore instabilities, and much progress has beenmade during the past 10 years toward providing predictive analytical methods.Some of the literature representative of this work is discussed in thisarticle. Emphasis here is on understanding factors that influence wellborestability in open holes, prediction of wellbore failures, and applications ofrock mechanics concepts to control wellbore stability, A brief historicaloverview is followed by discussion of various types of wellbore instabilitiesand descriptions of studies of field wellbore stability problems.
Stresses Around Wellbores
H.M. Westergaard published a paper entitled "Plastic State of Stress Arounda Deep Well" in 1940. This now-classic paper defined the wellbore stabilityproblem as follows.
The analysis that follows is a result of conversations with Dr. KarlTerzaghi who raised this question: What distributions of stress are possible inthe soil around an unlined drill hole for a deep well? What distributions ofstress make it possible for the hole not to collapse but remain stable for sometime, either with no lining or with a thin "stove pipe" lining of smallstructural strength?
Westergaard uses stress functions in cylindrical coordinates to solve theelastic-plastic wellbore problem for zero pressure in the hole and all normalstress components equal to the overburden far from the hole. Hooke's law wasapplied for the elastic region and a Coulomb yield condition* where "thelimiting curve for Mohr's circle is a straight line" was assumed for theplastic region. His conclusions were:
The plastic action makes it possible for the great circumferential pressuresthat are necessary for stability to occur not at the cylindrical surface of thehole but at some distance behind the surface, where they may be combined withsufficiently great radial pressures. The formulas that have been derived serveto explain the circumstances under which the drill hole for a deep well mayremain stable.
Westergaard's elasticity solution agrees with the Lame solution for athick-walled cylinder subjected to the same boundary conditions. Hubbert andWillis (1957) demonstrated how earth stresses can vary from regions of normalfaulting to those with thrust faulting. On the basis of a Coulomb failuremodel, they suggest that the maximum value of the ratio of the maximum to theminimum principal stress in the earth's crust should be about 3:1.
Abstract Coupled poroelastic response of formation around wellbore is expected to differ from the classic linear elastic theory when subject to changes in the state of stress. This may lead to redistribution of stress and pore pressure around the wellbore and consequent time-dependant wellbore deformation. These effects could cause delayed wellbore failure, loss of circulation and even the total loss of the well, especially in the case of underbalanced drilling. This paper presents a fully coupled poroelastic model developed for wellbore stability analysis with particular emphasis on fluid flow induced stress changes around wellbore. By using finite element methods, the model is able to simultaneously compute the geomechanical and hydraulic variables. In analogy with the common approach of wellbore stability analysis through initial and infinite time stresses calculation, the model can evaluate transient stress and pressure profiles to demonstrate the behavior of a wellbore throughout its history. This allows us to widen the safety mud weight window which leads to reduced drilling time and costs. The time of failure can be predicted by using modified Mohr-Coulomb criteria for breakout failure and tensile failure criteria for fracture. Thus, the coupled poroelastic approach can examine the factors influencing the wellbore stability more accurately than linear elastic method. The model was used to analyse wellbore stability in different formation types, stress conditions and drilling situations. Time-dependent shear stress distributions are presented so that they can be compared with shear strength of these rocks at different depths to select appropriate mud weights for underbalanced drilling. Introduction The major advantage of underbalanced drilling technology is that it can minimize formation damage due to drilling fluid invasion, reduce lost circulation risks and increase the rate of penetration. Thus underbalanced drilling provides a viable technique of developing depleted, complex and low quality hydrocarbon reservoirs (Parra et al., 2000). Drilling at an underbalanced condition (with a bottomhole pressure less than the formation pore pressure), however, increases the risk of borehole instability due to shear failure of the rock adjacent to the borehole. The instability of wellbore is primarily a function of how rocks respond to the induced stress concentration around the wellbore during drilling. If the rock is stronger than the induced stresses, the borehole remains stable. Otherwise wellbore instability occurs due to rock failure (shear or tensile). Generally for underbalanced drilling a stable wellbore is that the rock on the wellbore can sustain a drilling density less than the pore fluid density without collapse (Alajmi and Schubert, 2003). As the drilling progresses, the original support of the removed rock is lost and leads to a stress change (concentration) around the wellbore. A primary function of drilling fluid is to provide a support to the wellbore in the form of hydraulic pressure (adjusted through drilling fluid density). In conventional overbalanced drilling, the drilling fluid density is usually high enough to provide this support. However, in the case of underbalanced drilling, the density of drilling fluid is lower then the pore pressure, thus, providing a limited support to the wellbore. From rock mechanics point of view, not all the formations are suitable for underbalanced drilling technique. For a given in-situ reservoir condition, such as in-situ stresses, pore pressure, rock mechanical properties etc., certain mud weight is required to manage a minimum collapse pressure gradient. If the mud weight is lower than the minimum collapse gradient, then underbalanced drilling is not technically feasible. Therefore, determination of minimum collapse pressure is, among other factors, a key to the success of underbalanced drilling. Since reservoir rocks are porous, the interaction of drilling fluid with pore fluid due to pressure difference will cause pore pressure change around the wellbore, which in turn results in time-dependant local stress changes. Hence, the minimum collapse pressure is also time-dependent (Freij-Ayoub, et al., 2003). Recently, the interaction between geomechnics and fluid flow is widely recognized in multiple aspects of petroleum engineering such as borehole stability, hydraulic fracturing, production-induced compaction and subsidence (Gutierrez, et al., 2001; Chen et al., 1995; Advani et al., 1986).
Stress concentration around the wellbore can create breakouts, fractures, or failures. Understanding the stresses on rocks around wellbores is important to well design. For a vertical well drilled in a homogeneous and isotropic elastic rock in which one principal stress (the overburden stress, Sv) is parallel to the wellbore axis, the effective hoop stress, σθθ, at the wall of a cylindrical wellbore is given by Eq. 1. Here, θ is measured from the azimuth of the maximum horizontal stress, SHmax SHmin is the minimum horizontal stress; Pp is the pore pressure; ΔP is the difference between the wellbore pressure (mud weight) and the pore pressure, and σΔT is the thermal stress induced by cooling of the wellbore by ΔT. At the point of minimum compression around the wellbore (i.e., at θ 0, parallel to SHmax), Eq. 1 reduces to The equations for σθθ; and σzz are illustrated in Figure 1 for a strike-slip/normal faulting stress regime (SHmax Sv SHmin) at a depth of 5 km, where the pore pressure is hydrostatic and both ΔP and σΔT are assumed to be zero for simplicity.
Manriquez, Alberto Lopez (PEMEX Drilling Engineering Management/Department of Geosystems and Petroleum Engineering, The University of Texas) | Podio, Augusto L. (Department of Geosystems and Petroleum Engineering, The University of Texas) | Sepehrnoori, Kamy (Department of Geosystems and Petroleum Engineering, The University of Texas)
Abstract Wellbore completion and stability in multilateral junctions to date presents one of the most challenging operations to field engineers. Given the complex geometry of the multilateral junction, comprehensive mechanical stability and well completion have not yet been fully investigated. In this paper, the newly-derived analytical solution for the stress in and around the junction in a non-hydrostatic state of in-situ stress for an open-hole completion is used to assess the overall stability of the multilateral junction. This study models the main wellbore and the lateral wellbore as two circular holes in an infinite plane, with various radii and variable separation distance, thus simulating the multilateral junction extension. The system is subjected to the in-situ minimum and maximum principal stresses, with variable wellbore pressures and mudweights. The theoretical solution is used to reproduce the stress distribution in and around a multilateral junction, of an equivalent experimental program conducted and published in paper SPE 78212. This paper shows that the relative orientation of the junction to the in-situ stress field and the radii's ratio of the main wellbore to lateral wellbore both have a significant impact on well completion, stability, and stress distribution in and around the multilateral junction. Introduction Multilateral wells can not only improve accessibility and recovery but also cut drilling costs. Their effectiveness has been validated in many oil fields throughout the world. However, problems emerge more frequently with the increase in drilling of multilaterals. To date, the mechanical stability of the multilateral junction still presents one of the most challenging problems in the industry. A multilateral well consists of a main or a mother wellbore with one or more deviating wellbore branches. These laterals or secondary wellbores are drilled to increase the well productivity by increasing the drainage area. They are also used to produce oil from different isolated formations. Due to geomechanical effects, the second or even the main wellbore can be lost during drilling, completion, or production, leading to enormous losses and delays in production schedule. Numerical analyses of stability of multilateral junctions, as well as the stress distribution in and around the junction, have been widely conducted. On the other hand, the analytical solution only exists for the special case of hydrostatic in-situ stress, equal wellbore size, and same mud pressure in each well. Experimental analysis is also very limited. This paper uses the newly-derived analytical solution to assess the general model of the multilateral junction, and attempt to reproduce the results of an experimental test series on multilateral junction. The main wellbore and the lateral wellbore are modeled as two circular holes in an infinite plane, with various radii and variable separation distance, thus simulating the junction extension. The system is subjected to the in-situ minimum and maximum principal stresses, with variable wellbore pressures. This general model supplies insights into the importance of each parameter and the complexity of their interplay.