Bottom Hole Pressure Management in a Highly Permeable and Narrow Margin MPD Operation

Application of Managed pressure drilling (MPD) technology with other techniques to maintain constant Bottom Hole pressure (BHP) has been found to enhance drilling operations in applications where the margin between the pore pressure and fracture gradient is narrow and the reservoir permeability is high. Classic examples of such applications are deep water drilling, high pressure and high temperature (HPHT) regime and depleted reservoir environments. In the Niger Delta, HPHT reservoirs can be found in well depths up to 17000 ftss with a drilling window range of 0.4 to1.6ppg. Typical reservoir characteristics are formation permeability of 124 – 204mD and reservoir mobility of 112 – 1000mD/cp. Generally in this type of environment and essentially where there are high uncertainties in the reservoir pressures and formation characteristics, significant process safety incidents have been found to occur during pumps off events as a result of variations in BHP outside the allowable limits of pore pressure (lower limit) and fracture gradient (upper limit). The risks of exceeding the allowable limits are the possibility of taking significant influx volume if BHP falls below the pore pressure and loss of well bore integrity if the BHP exceeds the fracture pressure. Consequences of any of these events are high nonproductive time (NPT), well cost escalation and inability to achieve well objectives. This paper illustrates how in the recent HPHT exploration campaign carried out in Niger Delta, managing BHP was identified as a critical success factor. Hydrocarbon reserves of the exploratory objectives were successfully and safely unlocked by using MPD to maintain BHP within the allowable limits. The paper also illustrates how MPD application was enhanced by the use of high resolution pressure while drilling (PWD) technology.