Patent ID: 7660672
Filing Date: 2010-02-09
Classification: E21B

Abstract:
1. A method of minimizing pore pressure increase near the wellbore wall within argillaceous formations through which a borehole has been drilled, the method comprising: extracting and interpreting relevant drilling data related to wellbore instability mechanisms; delineating, using a processor of a computer, the type, extent and time-dependency of the wellbore instability mechanisms; determining, using the processor, the impact on drilling mud designs on the time-dependent wellbore instability and hole enlargement; determining, using the processor, pore pressure change near the wellbore wall after maximum exposure duration due to a mud pressure penetration mechanism and a chemical potential mechanism, wherein the mud pressure penetration mechanism induces an osmotic outflow to be greater than an osmotic inflow, and wherein the chemical potential mechanism induces a difference in salt concentration related to the osmotic outflow from one of the argillaceous formations; determining, using the processor, at least one field-based criterion relationship between net mud weight reduction percentage ratio and hole enlargement; determining, using the processor, a maximum allowable percentage ratio(s) of net mud weight reduction and either breakout mud weight used for the adopted maximum hole enlargement that the wellbore may experience during drilling; determining, using the processor, drilling mud salinity and salt type to satisfy the maximum allowable percentage ratio(s); monitoring, using the processor, a wellbore condition comprising the drilling mud salinity, the salt type, and the maximum allowable percentage ratio(s) while drilling in the borehole; and adjusting, using the processor, a mud weight based on a change in the wellbore condition to minimize the pore pressure increase and to prevent the one of the argillaceous formations from deteriorating.