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Summary This paper presents a case study of borehole instability from four wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging-while-drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure to diagnose the mechanisms of instability and, thus, select the appropriate remedial action. It is observed that instability caused by shear failure of intact rock (borehole breakout) can be suppressed by increasing the MW. However, where pre-existing planes of weakness (such as bedding planes and fractures) dominate the mechanism of instability, mud-weight increases do not necessarily lead to a more stable hole and can, in fact, further destabilize the wellbore. Introduction Despite considerable effort from the drilling, subsurface, and geomechanics communities, many oil wells continue to suffer from wellbore-instability problems during drilling. Although instability is quite common, in the majority of cases a considerable amount of uncertainty exists concerning exactly where, when, and why the instability occurred. Unfortunately, it is almost axiomatic that logs will not be run in an unstable wellbore. Direct measurements of the borehole shape and condition that can be obtained from caliper and image logs are, therefore, rarely acquired in the wellbores from which (from a geomechanics point of view) they would be most valuable. Modeling and cavings analysis alone can leave considerable uncertainty regarding the location and, to some extent, the mechanism of failure. An exception to the axiom can be one in which LWD-image data are acquired. It is still unlikely that LWD-imaging tools would be run in a well in which significant instability was expected. However, LWD is often acquired in wells that turn out to be less stable than anticipated. In these cases, a rare glimpse of the unstable wellbore wall in the early stages of collapse may be captured. This is very useful information that would normally remain the secret of the well. In this paper, a case study from the GOM shelf that includes wellbore instability, LWD imaging, cavings observations, and rock-failure modeling is presented. The authors of this paper were present to provide technical support to the drilling activities of this well soon after initial signs of instability were observed. We were fortunate enough to be able to acquire all the data presented below in a timely manner, such that analysis and recommended remedial action could impact drilling operations. A detailed description and analysis of the data form the bulk of this paper; however, we first discuss mechanisms of wellbore instability within the context of pertinent literature on this subject. Mechanisms of Wellbore Instability We propose that mechanisms of mechanical wellbore instability can be grouped in two main classes:Instability caused by failure of intact rock (i.e., rock that is unbroken and isotropic in strength). Instability because of the failure of rock containing pre-existing planes of weakness (e.g., bedding planes, fractures, and/or cleavage). Rock containing pre-existing weaknesses such as bedding planes or cleavage may be intact in the sense that it is unbroken. For the sake of this discussion, however, intact is defined as above. The majority of quantitative wellbore-stability studies since the 1979 paper by Bradley have modeled the wellbore wall as intact rock subject to the stresses imposed from the far field and the wellbore fluid. This type of failure gives rise to symmetrical break- outs in the wellbore walls. Breakouts can be stabilized by increasing the MW, or they may stabilize after reaching a certain size under favorable combinations of stress and strength. Breakouts are quite often observed in image and multiarm-caliper log data and are clearly a common cause of wellbore instability. Other mechanisms of instability in which pre-existing weaknesses are present do not necessarily stabilize with time or with increased MW. Instability because of such mechanisms is, therefore, rarely calipered or imaged, making the exact location and mechanism of instability uncertain. Consideration of wellbore instability caused by pre-existing weaknesses in oil wells is, for the most part, relatively recent: evidence of these mechanisms came from observations, such as correlations of trouble time with wellbore trajectory, and the existence of pre-existing fracture planes, bedding planes, and/or cleavage in cavings. A particularly insightful documentation of both field and laboratory evidence of this mode of failure from fissile shales in the North Sea is presented by Okland and Cook. From the data presented in the literature, as well as in this paper and in the author's experience, it seems that types of wellbore instability associated with pre-existing weaknesses can be grouped into two classes:Failure because of the existence of "impermeable" pre-existing weaknesses. Failure because of the existence of "preferentially permeable" planes of pre-existing weaknesses. In the case in which the pre-existing weaknesses are not preferentially permeable, an increase in the MW tends to further support the wellbore wall. An example of this type might be where a single set of bedding planes intersected. Where the mud and filtrate preferentially enter pre-existing planes of weakness, increasing the MW does not add support to the wellbore wall and may increase instability. Networks of pre-existing weakness (i.e., where two sets of weakness, such as bedding planes and fractures, intersect) are probably more likely to be permeable than a single plane of weakness (e.g., just bedding planes). In an extreme case, the body of rock may actually be composed of many discrete rock fragments with no cohesion between them, rather like a pile of rubble or the material seen in brittle/semibrittle fault zones. This type of rock mass could be referred to as a rubble zone - the rock having been effectively turned to rubble. These pre-existing weaknesses could be a combination of fractures, cracks, bedding planes, and cleavage planes. Naturally, fissile rock - such as thinly bedded shale - is likely to be particularly susceptible to becoming rubble where it is affected by faulting.
- North America > United States > Louisiana (0.34)
- Europe > United Kingdom > North Sea (0.24)
- Europe > Norway > North Sea (0.24)
- (2 more...)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.77)
- Geology > Geological Subdiscipline > Geomechanics (0.69)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Borehole Seismic Surveying (0.35)
Abstract Recent years have seen many scientific and technical progress in the field of well bore stability understanding. The main causes of instabilities have been identified, models developed and practical remedies proposed and often tried successfully in the field. Yet despite this scientific effort, drilling extra costs associated with wellbore instabilities remain very high. This in turn demonstrates that until now, scientific and technical progress have somehow failed to reach their final technico-economical objectives. This paper shows that the main reason for such a state of affairs is the complexity of wellbore stability analysis which cannot be performed for every single well on a worldwide basis. Having identified this, Agip undertook a vast effort of field data back analysis in order to identify from historical records the best way to optimize its drilling planning, practice and post-evaluation procedures. The paper presents part of this effort and in particular the back analysis of four clusters drilled in Southern Italy -i.e. 38 wells in total. It then draws conclusions in terms of procedure optimization. Finally, it presents how the approach is used during the appraisal phase of a field in order to speed up the learning curve and hence significantly cut the development drilling costs.
- Europe (0.68)
- North America > United States (0.47)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock (0.71)
- Geology > Mineral > Silicate > Phyllosilicate (0.47)
Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering. Introduction Maintaining a stable wellbore is of primary importance during drilling and production of oil and gas wells. The shape and direction of the hole must becontrolled during drilling, and hole collapse and solid particle influx must be prevented during production. Wellbore stability requires a proper balance between production. Wellbore stability requires a proper balance between the uncontrollable factors of earth stresses, rock strength, and pore pressure, andthe controllable factors of wellbore fluid pore pressure, and the controllable factors 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. If the wellbore fluid pressure is too high, lost circulation can occur as a resultof unintentional hydraulic fracturing of the formation; if it is too low, the hole may collapse. Excessive production rates can lead to solid particlein flux. Hole instabilities can cause stuck drill pipe as well as casing or liner collapse. These problems can result in side tracked holes and abandoned wells. Since 1940 considerable effort has been directed toward solving rock mechanics problems 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 wellbore stability in open holes, prediction of wellbore failures, and applications of rock mechanics concepts to control wellbore stability, A brief historical overview is followed by discussion of various types of wellbore instabilities and 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 stability problem 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 of stress 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 small structural strength? Westergaard uses stress functions in cylindrical coordinates to solve the elastic-plastic wellbore problem for zero pressure in the hole and all normal stress components equal to the overburden far from the hole. Hooke's law was applied for the elastic region and a Coulomb yield condition* where "the limiting curve for Mohr's circle is a straight line" was assumed for the plastic region. His conclusions were: The plastic action makes it possible for the great circumferential pressures that are necessary for stability to occur not at the cylindrical surface of the hole but at some distance behind the surface, where they may be combined with sufficiently great radial pressures. The formulas that have been derived serveto explain the circumstances under which the drill hole for a deep well may remain stable. Westergaard's elasticity solution agrees with the Lame solution for a thick-walled cylinder subjected to the same boundary conditions. Hubbert and Willis (1957) demonstrated how earth stresses can vary from regions of normal faulting to those with thrust faulting. On the basis of a Coulomb failure model, they suggest that the maximum value of the ratio of the maximum to the minimum principal stress in the earth's crust should be about 3:1. JPT P. 889
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Fault > Dip-Slip Fault (0.54)
Tremendous developments in the analysis of wellbore stability and associated problems have taken place in the last decade. However, the estimated worldwide cost of unscheduled drilling events (attributed to instability sources) has not decreased. Conservative yearly estimates are U.S. $600 million to $1 billion. This apparent constant estimate is not an indication of industry aloofness, but a reflection of the extent to which the limits of drilling technology is constantly being pushed. Viewed in this way, it is evident that analysis of wellbore stability has tremendous effects on the bottom line and will continue to do so more and more as we push to understand and develop tools for estimating closely the parameters required for more accurate predictions. The needs of the industry continue to grow, and wellbore-stability specialists are updating their thinking constantly to cope with the demands and reduce overall drilling cost.