Comparison of Wellbore Hydraulics Models to Maximize Control of BHP and Minimize Risk of Formation Damage

Abstract

Underbalanced drilling is considered a technique for improving productivity, since the target zone in the well is drilled with bottom-hole drilling pressure lower than pore pressure in the target zone. These differences in pressure may minimize the risk of formation damage if the bottom hole circulating pressure is maintained lower than reservoir pressure at all times. For this reason, it is very important to have good knowledge of pore pressures and bottom hole circulating pressures and their variations while drilling. The ability to accurately predict bottom hole pressure is critical for both designing an underbalanced drilling operation as well as for predicting the effect of variations during the actual drilling operation. Commercial models are available to calculate wellbore hydraulics for fluids used in underbalanced operations. Modeling is employed for designing and monitoring underbalanced drilling operations and for modifying operational parameters in response to changing conditions. It is also very important to realize that, in case the bottom hole pressure signal from the downhole tool is lost, it is necessary to rely on the model being used. If calibration has already been made and predictions agree with actual measurements there is more confidence in the model and the prediction that the system is underbalanced. Otherwise underbalanced conditions may not be guaranteed, and formation damage could occur. Field data from oil and gas wells are used in this paper for comparing the prediction of bottom hole pressure using different wellbore hydraulic simulators against the actual pressure while drilling underbalanced. Results show which simulator and correlations give better predictions based upon characteristics of the wells and fluids type. The source of differences in results is examined and recommendations are made. Introduction Preventing or reducing formation damage in the near wellbore can enhance production, reduce stimulation needs, and delay the onset of gas or water break-through. When drilling conventionally overbalanced, one of the primary mechanisms of damage in the near wellbore is solids and filtrate invasion in the formation pore spaces. The damage can be due to mechanical or chemical factors. The main mechanical mechanism is drilling solids/fines blockage of pore throats. Another major factor is the change in the relative permeability's due to a change in the residual saturation profile due to filtrate invasion. Chemical causes include permeability reduction due to chemical interaction between the mud filtrate and pore fluids. Such interaction can result in the formation of scales, sludge, or emulsions which block fluid flow. Interaction can also take place with any shales or clay present, causing them to swell and reduce permeability. Care must also be taken when drilling underbalanced to avoid periods of overbalance. It has been shown that going overbalanced when drilling underbalanced can be even more damaging than if conventional drilling fluids had been used. Essentially no mud cake is created while drilling underbalanced. Hence, no "barrier" is created between the productive interval and the wellbore; and invasion of the filtrate from a fluid used in underbalanced drilling can travel deeply into the target reservoir.1 Wellbore hydraulics modeling is key when designing the underbalanced drilling operation, when monitoring during the operation, and when modifying operational parameters in response to changing conditions. During the underbalanced drilling operation, the bottom hole pressure (BHP) can be estimated using hydraulics simulators to verify whether an operational change will allow the system to remain underbalanced. Reliable simulation is key when avoidance of formation damage is one of the objectives in drilling underbalanced. Prior to making a change in the operational parameters the new, intended change can be simulated to determine the effect on the pressures. It is important for the engineer to know the degree of accuracy of the predictions to gauge the impact of recommended changes and to calibrate the simulator results with actual downhole pressure measurements.