Case Study of Enhancing Wellbore Strengthening While Drilling Subnormal and Overpressured Formation Types
Authors: Breuer John , Mammadov Elvin , Al-Hashmy Wamidh , Sadat Ali Yousefi 
Published: August 2016

The drilling industry is an expensive part of the oil and gas sector, especially when drilling through a combination of low pressure and high pressure formations in exploration wells. When these zones are experienced while drilling, maintaining the BHP inside the drilling window is critical to ensure drilling fluid is not lost or formation fluids are not gained. Conventional solutions to help mitigate drilling through the troublesome formations include isolating thief zones, pumping LCM and cementing. These remedies could increase the overall project cost and add delays. One common problem associated with these solutions is how do you verify that the problem is corrected before drilling continues?

From having analyzed a case study from the Duvernay wells in Western Canada, it demonstrates that Managed Pressure Drilling (MPD) was applied with lighter drilling fluids to help adjust the bottom-hole pressure (BHP) as desired before the problematic formations. Through the Winterburn formation, constant losses were recorded and LCM was squeezed by applying the required surface-back-pressure (SBP). A formation limit test for the Winterburn formation was recorded and the bottom-hole equivalent circulating density (BH ECD) at 1495 kg/m3, showed 283 liters losses. Due to continued losses into Winterburn Formation, 1.5 m3 of 1100 kg/m3 LCM pill was mixed, spotted into the annular and then squeezed on top of the formation.

The LCM squeezing operation was started by applying 11,500 kPa static SBP which increased the BH ECD to 1700 kg/m3. After the LCM squeeze operation the well was reamed, and an extra 6 meters was drilled before performing a new formation integrity test (FIT). The second FIT was performed at the bottom of the formation and BH ECD had increased up to 1575 kg/m3 by applying 6,800 kPa SBP and the healing lost circulation zones were continued while drilling unconventionally through the MPD system. In the Beaverhill Lake formation, overpressured zones were encountered but drilling continued and dealt with both abnormal formation pressures. Lost circulation occurred in Winterburn formation with low pore pressure. The constant mud losses in this formation indicated that this problem was resulted from formation permeability, porosity and fractures that can be resolved by squeezing LCM. MPD brought value to the project by performing FIT in each formation, by monitoring and controlling precise LCM and cement squeeze operations. It also provided a solution for both types of abnormal formation problems as drilling continued and maintaining BHP inside the drilling window, increasing the overall safety of the project by detecting micro influxes and controlling them safely.

According to the pressure profile window, this paper illustrates how MPD successfully drilled through an upper formation of low pore pressure, with lost circulation problem, and lower formation with abnormal higher pore pressure without setting a casing between them. It also discusses the effect an MPD-LCM squeeze has on the fracture gradient of a formation and how the drilling window can be increased and manipulated to the operator's advantage.