A case study of mud weight design with finite element method for subsalt wells
6.7 A SOLUTION WITH 1D TOOL DRILLWORKS AND ITS COMPARISON WITH 3D SOLUTION
The results of SFG and FG values obtained with 1D software are shown in Fig. 6.31 for comparison purposes. The dashed, dark blue curve is the SFG solution obtained using the 1D method, and the solid blue curve is the FG solution obtained with the 1D method.
The input data used for the 1D solution includes the following:
• Effective stress ratio: the value of k0 = 0.8 is used for the entire well section.
• Tectonic factor: the value of tf= 0.5 is used for the entire well section.
• Overburden gradient is given by the same density used in 3D model and is equivalent to the vertical stress of the 3D model at the TVD of the salt exit; the strength parameters are the same as those used by the 3D model.
• The pore pressure shown in Figure 6.7 is adopted in the calculation.
A comparison between the 1D solution with its corresponding 3D MWW solutions shows that a significant difference exists between the two in the TVD interval from 5,000 to 8,000 m.
The two sets of MWW results are almost the same beyond the previously described TVD.
The maximum difference between the two sets of SFG values is approximately 1,300 Pa/m, Figure 6.27. An example of mud-weight logging data used in practice.
SHEN.indb 115
SHEN.indb 115 9/8/2011 8:38:28 PM9/8/2011 8:38:28 PM
which occurs at the TVD of approximately 6,130 m. Accounting for the depth value, the mud weight pressure difference obtained by the 3D and 1D methods are significant, and the discrepancy between the value of the effective stress ratio used in the 1D analysis and that implicitly used in the 3D analysis is believed to be the major reason for this phenomena.
Figure 6.29. Effective stress ratio distribution of σx /σz.
Figure 6.30. Effective stress ratio distribution of σy/σz. Figure 6.28. Geometry of the salt body in the model.
SHEN.indb 116
SHEN.indb 116 9/8/2011 8:38:28 PM9/8/2011 8:38:28 PM
5500
6000
6500
7000
7500
8000
15000 16000 17000 18000 19000 20000 21000
Pressure gradient /Pa/m
TVD depth /m
SFG 3D
FG 3D
FG 1D SFG 1D
Figure 6.31. Comparison between 1D and 3D MWW solutions.
6.8 CONCLUSIONS
Using FEM, a 3D numerical solution of the MWW was obtained for a subsalt wellbore sec-tion. Based on the numerical results presented in this chapter, the following conclusions can be obtained:
• There is a region of the subsalt formation where the reverse faulting stress pattern may be formed. It is most likely to be true, particularly when an anticline structure exists at the salt base.
• As the wellbore trajectory penetrates this region, the FG and SFG values at this region may appear as abnormally high values. Consequently, the MWW obtained for this well-bore trajectory shown in the results is shifted/greater than the normal values as compared to the 1D solution obtained with 1D software.
• The numerical results indicate that the effective stress ratio within the salt base formation changes with TVD and with the horizontal position. For an accurate MWW solution for subsalt wellbore sections, the 3D FEM tool should be used in the design of the MWW.
NOMENCLATURE
A = Model parameter in creep model n = Model parameter in creep model m = Model parameter in creep model k0 = Effective stress ratio
Sh = Minimum horizontal stress component, Pa SH = Maximum horizontal stress component, Pa tf = Tectonic factor
σx = Effective stress component in x-direction σy = Effective stress component in y-direction σz = Effective stress component in z-direction ρ = Density, kg/m3
CS = Cohesive strength, Pa FA = Internal friction angle, o FEM = Finite Element method FG = Fracture gradient, ppg
SHEN.indb 117
SHEN.indb 117 9/8/2011 8:38:29 PM9/8/2011 8:38:29 PM
MWW = Mud weight window OBG = Overburden gradient, ppg pp = Pore pressure gradient, ppg SFG = Shear failure gradient, ppg TVD = True vertical depth, m
UCS = Unconfined compressive strength, Pa
REFERENCES
Cullen, P.C., Taylor, J.M.R., Thomas, W.C., Whitehead, P., Brudy, M. and vander Zee, W.: Technologies to identify salt-related deep-water drilling hazards. Paper OTC 20854 presented at the 2010 Offshore Technology Conference, Houston, TX, USA, 3–6 May, 2010.
Dassault Systems: Abaqus Analysis User’s Manual. Vol. 3: Materials, Version 6.8, Vélizy-Villacoublay, France: Dassault Systems, 19.3.1-17 – 19.3.2-14, 2008.
Dusseault, M.B., Maury, V., Sanfilippo, F. and Santarelli, F.J: Drilling through salt: constitutive behav-ior and drilling strategies. Paper ARMA/NARMS 04–608 Gulf Rock 2004 presented at the 6th North America Rock Mechanics Symposium, Houston, TX, 5–9 June, 2004.
Fredrich, J.T., Coblentz, D., Fossum, A.F. and Thorne, B.J.: Stress perturbation adjacent to salt bodies in the deepwater Gulf of México. Paper SPE 84554 presented at the Annual Technical Conference and Exhibition, Denver, CO, USA, 5–8 October, 2003.
Power, D., Ivan, C.D. and Brooks, S.W.: The top 10 lost circulation concerns in deepwater drilling. Paper SPE 81133 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Port of Spain, Trinidad, West Indies, 27–30 April, 2003.
Shen, X.P.: DEA-161 Joint Industry Project to Develop an Improved Methodology for Wellbore Stability Prediction: Deepwater Gulf of Mexico Viosca Knoll 989 Field Area. Halliburton Consulting, Hou-ston, TX, USA, 18 August, 2009.
Shen, X.P., Diaz, A. and Sheehy, T.: A case study on mud-weight design with finite-element method for subsalt sells. Paper 1120101214087 accepted by and to be presented at the 2011 ICCES, Nanjing, China, 18–21 April, 2011.
Tsai, F.C., O’Rouke, J.E. and Silva, W.: Basement rock faulting as a primary mechanism for initiat-ing major salt deformation features. Paper 87–0621 presented at the 28th US Symposium on Rock Mechanics, Tucson, AZ, USA, 29 June-1 July, 1987.
Whitson, C.D. and McFadyen, M.K.: Lesson learned in the planning and drilling of deep, subsalt wells in the deepwater Gulf of Mexico. Paper SPE 71363 presented at the SPE Annual Technical Confer-ence and Exhibition, New Orleans, LA, USA, 30 September-3 October, 2001.
Willson, S.M. and, Fredrich, J.T.: Geomechanics considerations for through- and near-salt well design.
Paper SPE 95621 presented at the 2005 SPE Annual Technical Conference and Exhibition, Dallas, TX, 9–12 October, 2005.
SHEN.indb 118
SHEN.indb 118 9/8/2011 8:38:29 PM9/8/2011 8:38:29 PM
119