- J.B. Cheatham Jr. (Rice U)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- June 1984
- Document Type
- Journal Paper
- 889 - 896
- 1984. Society of Petroleum Engineers
- 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 4.3.1 Hydrates, 2.2.2 Perforating, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.1.2 Faults and Fracture Characterisation, 1.10 Drilling Equipment, 1.2.3 Rock properties, 1.6 Drilling Operations, 1.1.6 Hole Openers & Under-reamers, 4.1.5 Processing Equipment, 1.11 Drilling Fluids and Materials, 2 Well Completion
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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.
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