This paper presents a local constitutive model for modelling the linear and non linear behavior of soft and hard cohesive materials with the discrete element method (DEM). We present the results obtained in the analysis with the DEM of cylindrical samples of cement, concrete and shale rock materials under a uniaxial compressive strength test, different triaxial tests, a uniaxial strain compaction test and Electronic supplementary material The online version of this article (
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTwo challenges facing operators as the energy industry moves into the next century are accessing of new reservoirs that currently cannot be reached economically and maintaining profitable production from older fields. Recent advances in one of the oldest and most fundamental areas of exploration and production, namely tubular technology, will play a key role in meeting these challenges.A method has been developed whereby the diameter of solid tubulars can be expanded downhole. This paper will describe the process and how this significant technological breakthrough provides cost-effective solutions to several tubular problems that have loomed as obstacles to comprehensive reservoir exploitation. In deepwater and subsalt environments such as the Gulf of Mexico, the ability to expand casing and tubing in-situ enables hole-size maintenance and conservation of internal tubular diameter for increased efficiency. Hence, operators are less likely to run out of hole diameter before evaluating all pay zones. Operators can now use smaller holes to drill deeper vertical wells or to extend the reach of deviated wells to access untapped reservoirs. In older fields, existing wellbores can be retrofitted with expanded tubulars for repair purposes or to increase strength and integrity. In the latter case, deeper high-pressure objectives can be supported, and thus, new in-fill wells can possibly be reduced in number or even eliminated.In addition to a description of the process employed to expand solid tubulars, the paper will present applications of expandable tubular technology and results of large-scale testing that has been conducted in support of the applications. Potential commercial applications are also presented.
This paper was prepared for presentation at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
A dynamic stiff string torque and drag (T&D) model is presented that assumes steady state motion of the drillstring as its basis for calculations. Results are compared to previously published T&D models that are based on static equilibrium. The novelty of the new dynamic model is the ability to solve T&D operations of the entire drillstring from bit to top drive in reasonable time using standard engineering computers.The new approach is based on a 3D dynamic model of drillstring and BHA in an elastic borehole. It considers bending stiffness, torsional stiffness, contact forces, and friction with localization of contact points. A numerical method is described that has proven to have excellent convergence. Complete governing equations are provided and the method is described in detail to permit readers to replicate results.The dynamic model is compared to two static stiff string models. Comparisons are also provided for three conventional soft string models including the Lubinski-Paslay-Cernocky bending stress magnification factor. Three field case studies are presented for horizontal wells. One well is short radius with dogleg severity over 50 deg/100 ft and two wells are unconventional shale wells with doglegs up to 15 deg/100 ft. Predictions for surface torque and drag up and down for the new dynamic stiff string model are compared to the static stiff and soft string models. In many situations modeled the top-level results for surface torque and drag up/down are close enough for all six models to be within the uncertainty range associated with the commonly used, lumped-parameter friction factor. However, some major differences in hook load for sliding and slack off operations are observed, which are shown to be caused by differences in location and magnitude of contact force between the drillstring and wellbore. Further, significantly lower surface torque is predicted by the new dynamic stiff string compared to other models for one case history because of lower contact forces in the vertical section of the well. In fact, the key finding of this paper is that major differences are observed for contact forces for the new dynamic stiff string model compared to all five other models, including the two static stiff string models. These differences in contact forces are most significant when the drillstring has helically buckled or when doglegs in the wellbore are high. Contact forces have a large impact on local stress behavior, which is important for predictions of casing and drillpipe wear, drillstring fatigue, and failure points in the drillstring.Although several previous papers have published stiff string models there is no industry standard formulation. The main problem holding back the development of an industry standard stiff model is perhaps the complexity of the numerical algorithm and substantial running time. To address this problem, some previous stiff string models account for bending stiffness of the drillstring but not for radial clearance while others appear to model only portions of the drillstring ...
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