Experimental Investigation on Wellbore Strengthening Based on a Hydraulic Fracturing Apparatus

Lost circulation is a serious problem which always exists in the petroleum industry. Wellbore strengthening by lost circulation materials (LCMs) is a commonly applied method for mitigating lost circulation. This paper presents a hydraulic fracturing apparatus to investigate the effect of material type, concentration, and particle size distribution (PSD) of LCMs on wellbore strengthening behavior. In addition, the characteristics of pressure curves in the fracturing process are analyzed in detail. The results showed that the fracture pressure of the artificial core can be increased by LCMs, and there exists an optimum concentration of LCMs for the maximum wellbore strengthening effect. The LCMs with wide PSD can significantly increase the fracture pressure. However, some LCMs cannot increase or even decrease the fracture pressure; this is resulting from the LCMs with relatively single PSD that makes the quality of mud cake worse. The representative pressure curve in the fracturing process by drilling fluids with LCMs was divided into five parts: the initial cake formation stage, elastic plastic deformation stage, crack stability development stage, crack instability development stage, and unstable plugging stage. The actual fracturing curves were divided into four typical types due to missing of some stages compared with the representative pressure curve. In order to strengthen the wellbore in effective, good LCMs should be chosen to improve the maximum pressure in the elastic plastic deformation stage, extend the stable time of pressure bearing in the crack stability development stage, and control the crack instability development stage.

Leak-Before-Break Assessment of a Cracked Reactor Vessel Nozzle Using 3D Crack Meshes

A postulated surface crack near a reactor pressure vessel nozzle is evaluated using finite element analysis (FEA) to compute the fatigue crack growth rate, evaluate crack stability, and examine the possibility of a leak-before-break (LBB) condition. For a pressurized vessel with cyclic loading, determining if the crack may have a LBB condition is desirable to allow for the possibility of leak detection leading to corrective action before catastrophic failure. A fatigue crack growth analysis is used to determine how the surface crack dimensions develop before re-categorizing the surface crack as a through thickness crack and evaluating its stability for LBB. To evaluate if a particular crack is unstable and may cause a structural failure, the Failure Assessment Diagram (FAD) method provides an evaluation using two ratios: brittle fracture and plastic collapse. The FAD method is described in the engineering best practice standard API 579-1/ASME FFS-1. The FAD curve and assessment ratios can be obtained from crack front J-integral values, which are computed using 3D crack meshes and elastic and elastic-plastic FEA. Computing custom crack solutions is beneficial when structural component geometries do not have an available stress intensity or reference stress solution.

Validation of Axial Crack Stability Assessment Methods in xLPR Version 2.0 Code

As part of the xLPR (Extremely Low Probability of Rupture) project, validation efforts were conducted to assess the predictive capability of axial surface crack (Ax-SC) and axial through-wall crack (Ax-TWC) stability modules employed for making the crack stability assessment. For the Ax-SC, the plastic collapse solution/criteria from the Ductile Fracture Handbook (DFH) was selected and employed after making detailed comparisons between the solutions obtained from the DFH and API-579/ASME FFS-1 solution methods. Experiments from past tests conducted for the Atomic Energy Commission (AEC) and by MPA-Stuttgart were used to validate the performance of the implemented axial SC stability module. Similarly, for the axial TWC stability assessment, a Limit Load solution, and a numerical analysis procedure for an elastic-plastic fracture mechanics (EPFM) based method employing a J-integral fracture mechanics parameter approach using a GE/EPRI type J-estimation equation, and tabulated plastic influence functions based on pipe dimension and Ramberg-Osgood material parameters, were used. Experimental results from past pipe tests conducted for the AEC, others conducted for the PRCI, and by Tokyo Gas involving larger diameter pipes were used to validate the performance of the implemented axial TWC stability module. In this paper, the selection of stability criteria/solutions, their performance and implications for the xLPR 2.0 code are discussed. The impact of using different flow stresses (function of yield and ultimate stresses), and stress magnification factor (Folias bulging factor) expressions, on the predictions is highlighted. Further, a conservative basis for the assessment of axial crack stability in welds is proposed. In closing, additional empirical corrections that can be incorporated without overly biasing the deterministic solutions/criteria that feed into a probabilistic analysis are highlighted.