By Katie Mazerov, contributing editor
Managed pressure drilling (MPD) has made production possible in wells with narrow pressure margins, but well control in these environments is still primarily a manual process dependent on many variables that can increase the risk of a blowout, an expert said at the 2013 IADC/SPE Managed Pressure Drilling & Underbalanced Operations Conference & Exhibition, 17-18 April in San Antonio. “Today, well control, the technique used to prevent the influx of formation fluids into the wellbore by estimating pore and fracture pressure, is a manual process carried out by the driller using limited instrumentation,” said John-Morten Godhavn, principal researcher, Automatic Control Systems, for Statoil. Making the case for the use of an automatic well control system, Mr Godhavn presented test results indicating the method improves real-time early kick detection through continuous monitoring, detects minor influxes that a driller cannot see and eliminates human misinterpretation.
Manual operations, which depend on the driller’s awareness, understanding of well hydraulics, and ability to monitor and interpret trends based on his experience and certification from well control courses, can result in kicks being detected too late, Mr Godhavn warned. Kicks occur when pressure in the wellbore is too low due to low mud density, insufficient filling when tripping, tripping too fast, lost circulation and higher pore pressure than expected.
To avoid such conditions, drillers must continually monitor indicators such as a rapid increase in the rate of penetration, increased return rates and pit levels and reduced standpipe pressure. In MPD operations, the driller must also monitor the choke pressure and position and use flow meters to measure the density and rate of the return mud.
“In today’s challenging environments, the motivation for automatic well control, which can significantly aid the driller in managing these scenarios, centers first and foremost on the safety and health of workers and avoidance of environment-damaging spills,” he emphasized. “Reliable well control also gives industry the license to drill in places such as the Gulf of Mexico and improves economics by reducing downtime and reaching total depth.”
In collaboration with the International Research Institute of Stavanger (IRIS), Statoil tested algorithms on a hydraulic simulator and successfully applied kick detection to field data for a horizontal flow loop with a gas injection. “The goal of the tests was to improve well control by automation for both MPD and conventional drilling,” Mr Godhavn said. The system was designed to aid the drilling crew in all aspects of well control, including early kick detection, automatic pressure control, including automatic choking to maintain constant bottomhole pressures, during circulation and well shut-in.
The system also was designed to minimize false alarms and includes a mechanism for detecting choke plugging and washout, bit nozzle plugging and washout, pump failures such as instability and power loss and failure of choke and stand pipe pressure sensors. It also allows for semi-automatic shut-in. “This function should not be fully automatic, allowing the driller to make the decision to shut in the well, including manual closing of the blowout preventer,” Mr Godhavn said.
Test results indicated the automatic system can enhance well control. In the MPD flow loop test, for example, it took less than a minute for the automatic system to detect the influx and overall less than two minutes for the driller to confirm the alarm, stop the pump and shut in the well. Pumping resumed six minutes later at a slow circulation rate, with gas reaching the choke in less than three minutes. Gas was out of the system in 30 minutes.