Uncertainty Analysis of Borehole Stability Problems
- Nobuo Morita (Waseda University)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 22-25 October, Dallas, Texas
- Publication Date
- Document Type
- Conference Paper
- 1995. Society of Petroleum Engineers
- 3.3.2 Borehole Imaging and Wellbore Seismic, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 1.6.10 Running and Setting Casing, 2.4.3 Sand/Solids Control, 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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An uncertainty analysis is conducted for borehole stability problems based on a statistical error analysis method. The uncertainty analysis shows that the parameters impacting on the safe mud weight to stabilize borehole are rock strength followed by shale swelling, in-situ stress, pore pressure, well inclination, and so on. Several procedures to improve borehole stability analysis are proposed.
Many parameters affect borehole stability. These parameters include three principal stresses, pore pressure, mud weight, well azimuth and inclination, rock deformation properties (anisotropy, non-linearity), rock strength, shale swelling, borehole temperature, and borehole size. The problem of applying a borehole stability model is that none of these parameters can be accurately measured. In this paper, an uncertainty analysis is conducted for borehole stability problems based on a statistical error analysis method. The uncertainty analysis provides the following information: (1) which parameters impact borehole stability and what magnitude of impact is expected on the safe mud weight; (2) the range of accuracy required for these input parameters; (3) the range of borehole stability model accuracy required for a given magnitude of the input error; and (4) how to improve the accuracy of borehole stability analysis. These analyses are conducted based on the data acquired during drilling boreholes in a North Sea reservoir.
The uncertainty analysis shows that the parameters impacting on the safe mud weight to stabilize borehole are rock strength followed by shale swelling, in-situ stress, pore pressure, well inclination, and so on. The impact of borehole stability model error on the safe mud weight calculation is relatively small comparing with the error magnitude induced by these input parameter errors if a non-linear borehole stability model with post failure is used. However, if a linear elastic model is used, the safe mud weight error predicted by the model becomes larger than the error induced by these input parameters. Hence, if a linear borehole stability model is used, the model must be corrected by a back analysis method (such as the stress cloud method) or the thick wall borehole collapse tests (a laboratory test using cores from a reservoir) before applying it to design a safe mud weight in a specific reservoir. The data. calibration using existing wells significantly improves the model prediction accuracy.
The technical contributions of the uncertainty analysis are: l. The conventional model prediction gives one safe mud weight for specific input data while the uncertainty model prediction provide a confidence level to each calculated safe mud weight range. This paper clarifies how to apply this procedure.
2. The uncertainty analysis clarifies the input parameter accuracy required for routine borehole stability analysis.
3. The uncertainty analysis clarifies the accuracy of borehole stability models required for routine borehole stability analysis comparing with the error induced by input parameter errors.
|File Size||763 KB||Number of Pages||10|