Understanding pore pressure

Gregory Hale

November 1, 2015

Gregory Hale discusses how managed pressure drilling helps to eliminate the guess work.

Automated managed pressure drilling is the future of conventional drilling. Photos from Weatherford.

Realizing the potential benefits automation brings, numerous drilling giants are integrating managed pressure drilling (MPD) into drilling rigs, which not only saves time and money, but also enhances the safety capabilities of their rigs.

“The drilling industry is behind other industries, but the technology is shifting towards automation,” said Sayamik Ameen, MPD engineer at Weatherford.

Using MPD enables drilling with a better understanding of pore pressure (PP), which is one of the most critical parameters in well planning along with geomechanical analyses of wellbore stability.

Several empirical methods aim to estimate the pore pressure using seismic and logging-while-drilling data, and drilling parameters. Experts have studied the uncertainty in pore pressure estimates using these methods because of noise and fluctuations in the received data as well as the under-compaction effect in over-pressured formations. It is common practice to calibrate these methods with data from the repeat formation tester (RFT) or drill stem test (DST). MPD, on the other hand, enables precise measurement of the PP with dynamic pore pressure testing (DPPT), which reduces the nonproductive time inherent in RFT and DST without compromising safety.

Old vs new

“During conventional drilling, we predict pore pressure values using the available common correlations in the industry,” Ameen said. “When a driller experiences a kick, he controls it. With MPD however, the automated control system provides early kick and loss detection, hence eliminating the uncertainty in providing pore pressure values along the wellbore.”

“Conventionally, we couldn’t just reduce the pump rate to decrease bottomhole pressure because of the risk that an influx would emerge when measuring pore pressure while drilling. In comparison, MPD provides data and enables the control needed to precisely reduce pressure on the surface and measure pore pressure at specific depths.

Automated managed pressure drilling software platform.

MPD in real time

As mentioned, the whole idea behind MPD is to enable real-time management of the annular pressure profile and accurate monitoring of the return flow through a closed-loop circulation system.

The annular pressure profile is managed by manipulating the surface backpressure (SBP) using automated hydraulic chokes, operated by a programmable logic controller, according to Ameen’s paper entitled, “Dynamic Calibration of the Empirical Pore Pressure Estimation Methods Using MPD Data,” presented at OTC this year. The return flow, temperature, and mud density are measured using a Coriolis flowmeter, and the standpipe pressure (SPP) and SBP are measured using precision digital sensors.

MPD enables performing repetitive dynamic formation integrity tests (FIT) and pore pressure tests (PPT) without the need to stop circulation. Drillers perform dynamic FIT and PPT to safely confirm the upper and lower boundaries, respectively, of the downhole pressure envelope without causing downtime.

In a dynamic FIT on an intact formation, the integrity of the entire open hole is confirmed by increasing the surface pressure until a predetermined set point while the return flow is continuously monitored, the paper describes. If there is a loss, the pressure unloads and the downhole pressure at the location where the micro-loss occurred provides the direct and precise measurement of the leak off pressure. In a dynamic PPT, the surface pressure is incrementally reduced from an initial set point until a micro-influx is observed.

Increase in accuracy

In tests, maintaining the full circulation rate enables the pressure while drilling (PWD) tool to continue reading the downhole pressure, which increases the accuracy of the pore pressure and the effectiveness of fracture pressure measurements, the paper said. Continuous measurement of the downhole pressure using PWD minimizes the uncertainties inherent in the bottomhole pressure calculations, which are caused by cutting loading and pipe rotation.

Besides direct measurement of the pore and fracture pressures through dynamic FIT and PPT, monitoring and correlation of the surface parameters provide qualitative indication of the pore pressure. Correlation of the bottoms up gas after the connections as well as monitoring of the rate of penetration and cutting analysis provide complimentary data for pore pressure interpretation.

Eliminating unplanned downtime

“The unwanted downtime that comes with conventional methodologies is not the only setback. Using the old school method requires you to increase your mud density to increase your bottomhole pressure,” Ameen said. “With MPD you don’t have to increase your mud density, rather, you have one density and instead manipulate the surface backpressure as needed. That way, it is under your control.”

Taking advantage of actionable data gathered through MPD can ensure a more cost-effective and safe operation.

“In this industry downturn when there are limited operations, minor mistakes could cost millions of dollars and may come with an abundance of harsh scrutiny,” Ameen said. “Having more certainty and obtaining more precise downhole data ensures that you conduct your job in a safer and more systematic manner. More precision means fewer mistakes.”


Gregory Hale
is the editor and founder of Industrial Safety and Security Source (ISSSource.com) and is the contributing Automation Editor at Offshore Engineer.

 

References:

Ameen Rostami, S., Gumus, F., Kinik, K., Kirchoff, K.“Dynamic Calibration of the Empirical Pore Pressure Estimation Methods Using MPD Data,” OTC-25953-MS, Offshore Technology Conference, May 2015.