An Assessment of the Kick Tolerance Calculation, Its Uncertainty, and Sensitivity

Summary Much of the contemporary literature on kick tolerance relates to the practical aspects of the calculation and the effects of the underlying assumptions and necessary simplifications. It tends to a formulaic approach involving unconstrained uncertainties about pore pressure, kick intensity, drilling fluid density safety factors which tend to overlook the inherent complexities of the subsurface environment, and the reality of model limitations. The literature on kick tolerance highlighting the strengths, weaknesses, and limitations of the classic calculation is reviewed before describing a first-principles approach to determining the consequences which casing seat strength limitations and strength variations in the overall openhole section impose on safe well designs as a result of uncertainties in the input variables and basic model assumptions. The classic kick tolerance calculation, together with its associated assumptions, yields only a single value for a given set of inputs. The uncertainties and sensitivities are those introduced by the variables and the choice of operational procedures. The Monte Carlo method is used to explore the effect of uncertainties in the input parameters and associated assumptions. The analysis is based on the driller’s method for kick circulation, with a single-bubble insoluble gas with optional allowances for gas gravity and thermal regimes. To examine the various factors influencing kick tolerance, the problem is framed graphically in terms of an allowable influx volume, the dependent variable, against bottomhole pressure (BHP), the independent variable, on which pore pressure, its uncertainty, possible variations in shoe strength, strength in the openhole section, drilling fluid density with associated equivalent circulating density (ECD) and swab effects can be examined. Application of the new approach demands significant prior work to determine the uncertainties inherent in the problem. Some of these uncertainties, such as drilling fluid density variations, swabbing, and ECD, must be evaluated by the engineer as part of the normal well design. Other factors, such as shoe strength, pore pressure variability, and formation depth error bars must be developed as part of a multidisciplinary effort within the planning team. Often ignored due to the spatial uncertainties, natural fractures intersecting the borehole could be inherently unstable and prone to hydraulic communication between borehole and formation. Synthesizing some of these considerations into a simple calculation framework allows the effect of these parameters either individually or collectively to be quantified and provides a view of the potential geological uncertainties inherent in a well design. The approach offers a starting point for thinking about the nature of geological uncertainties and carrying them into drilling programs.