Patent US 5,660,239

An iterative planning and monitoring method for drilling (and completing) difficult boreholes which avoids unnecessary risk or cost. The method provides multiple point value probability estimates of an indicator of drilling problems based upon a range of possible drilling variables, supplanting single point estimates. Expected drilling variables are perturbed within physically feasible bounds, and multiple estimates of the corresponding indicator values are made. The probability of each estimate is used to calculate the likelihood of an indicator of an unwanted condition. Mitigation measures are implemented if the probability of an unwanted condition exceeding a threshold value is unacceptable and the mitigated probability is reassessed. If the perturbed indicator change is not significant, the drilling variable is deleted from further analysis. Critical variables are thus quickly identified, allowing monitoring and selection of mitigation measures which are the most cost effective. Unnecessary mitigation procedures or unwanted drilling risks are avoided by these procedures.