One of the many benefits of a managed pressure drilling (MPD) system is the reduction in the non productive time associated with kick and loss events. While such an approach has merit, a pressure determination system (PDS)1–6 has been developed to progress MPD from a reactive system to one which anticipates changing formation pore and fracture pressure regimes as the well depth increases. Ultimately the objective of the PDS is to prevent a recordable well control event from occurring over the duration of the drilling process.
The PDS is deployed in conjunction with an MPD Pressure Control Valve (PCV), a rotating or non-rotating annular sealing device, and a flow metering sensor system. The PDS is based on the premise that a small ID PCV, positioned in parallel with a larger ID MPD PCV, oscillates with a programmed open-close cycling speed to generate a pressure pulse in the drilling returns annulus. The programmed PDS PCV thus produces the annular "pulse" with amplitude parameters specified by the operator within the PDS control system that oscillates the annular pressure within a predetermined narrow pressure band while keeping the overall average annular pressure constant. As the cyclic annular pressure changes occur, the models and algorithms within the PDS analyze the relationship between the return flow rate measured by the flow meter sensor and the surface PCV pressure to determine if either pore or fracture pressure margins have been breached. The PDS then readjusts the target bottom-hole pressure (BHP) using the MPD PCV such that the BHP continues to remain within the new drilling window. Please note that at no point is average BHP expected to fall out of drilling margins. Wellbore compressibility of fluids, solids, and gas, wellbore storage effects, and the efficacy of the pulse transmission are key factors to facilitate the analysis3.
Since the PDS PCV is rapidly oscillating its orifice size, a degree of influx or loss is potentially expected to occur in the presence of changing pore or fractures downhole as drilling progresses further. The preset amplitude of the generated pulse either begins to increase beyond the fracture pressure (in the case of an unexpected decrease in the fracture pressure) or decrease below the pore pressure (in the case of an unexpected increase in the pore pressure). The result is that for a brief moment in time drilling fluid is lost to the formation or formation fluid enters the wellbore. What is critical to note is that the resultant loss or gain volumes are negligible and occur instantaneously with the associated peak amplitude of the pressure pulse as it dips below the pore pressure or above the fracture pressure. The flow meter sensor data analyzed by the alogirthms of the PDS detect these miniscule volumetric changes in the annulus and make adjustments before a recordable well control event can occur. Once the average BHP has neared any changes in the geo-margin limit detected and calculated by the pressure pulse analysis of the PDS, the MPD PCV can be manipulated to change the average BHP to continuously remain within the drilling window. Therefore, a recordable well control event is prevented.
The PDS will proactively "ascertain the downhole pressure environment limits" as stated in the IADC definition of MPD. This paper will discuss the engineering concepts, practical implementation, and a preliminary field testing program for the PDS system.