Managed pressure drilling episode 3
2- ECD Reduction Tool
A better understanding of equivalent circulating density (ECD) is necessary to understand how this tool can help manage the pressure profile window. A high ECD can cause problems in complex wells, including reducing the operating margin between the pore pressure and fracture pressure. A high ECD in this case could result in exceeding the fracture pressure of a formation. With a high ECD, a common problem is lost circulation.
ECD is a function of mud density, mud rheology, cuttings loading, annular geometry and flow rate, where:
- Drilling-fluid density is required for pressure control and wellbore stability.
- Viscosity and flow rate are needed for hole cleaning and barite-sag mitigation.
- Gel strengths are required to suspend drill cuttings.
The goal of ECD management is to find balance between these parameters to successfully drill a well.
Reducing ECD in a well can result in many benefits. These benefits can include:
- Reducing the number of casing strings.
- Improving hole cleaning by using higher flow rates.
- Being able to remain in the pressure window for complex wells.
- Reducing lost circulation and differential sticking.
- Reducing formation damage.
Current techniques that are used to reduce ECD include:
- Using low fluid rheologies to reduce frictional losses.
- Using drillstrings and casing strings that provide greater annular clearance.
- Using expandable tubulars to increase hole size.
- Use of drilling liners in place of casing strings.
- Reducing flow rates to decrease frictional losses.
- Reducing penetration rates to reduce the amount of cuttings in the annulus.
These techniques can solve ECD problems but can result in higher drilling costs. The higher drilling costs could make some wells uneconomical to drill. Dual gradient drilling and riserless drilling also reduce ECD but can have higher capital expenditures than an ECD reduction tool. So, The ECD reduction tool is seen as a low-cost alternative to other methods of ECD reduction. The ECD reduction tool is designed to reduce the bottomhole pressure increase caused by friction in the annulus by providing a pressure boost up annulus.
The tool has three basic parts. The top section of the tool has a turbine motor that is powered by the circulating fluid. The middle section consists of a mixed flow pump that is partly axial and partly centrifugal. This section pumps the fluid up the annulus. The bottom section consists of the bearing and seals. Two nonrotating packer-cup seals in the lower section of the tool provide the seal between the tool and the casing. This causes all the return fluid to flow through the pump.
The ECD tool has some features that will enable the tool to be used in both onshore and offshore operations. The initial design of the tool enables it to be run in 9-5/8-in. to 13-5/8-in. tubing. Drill cuttings up to 5/16-in. can flow through the tool. A grinding mechanism in the bottom section breaks up larger drill cuttings, preventing the pump from being plugged.
Fig. 8 shows the pressure boost seen in the well as a function of the flow rate. The tool started up at a flow rate of 250 gpm and as the flow rate increased, the pressure boost increased as a quadratic function.
Fig. 10 shows the change in down hole pressure that is seen while the tool is running. At the 550 gpm flow rate, the down hole pressure is reduced by about 250 psi.
Important Notes on the Tool:
- Wireline tools can be run through the tool after retrieving a flow diverter that is located in the turbine motor. The tool has a clearance of 1.812-in. inside the pump once the diverter is removed. The mechanical strength of the tool is comparable to that of new 5.-in., 19.5 lb/foot S-135 drillpipe. It is designed to have a maximum pressure boost of 450 psi in the annulus with a flow rate of 550 gpm.
- A test run with Measurement While Drilling (MWD) tools showed that the tool would not interfere with communications from the other tools. The tool was found to be able to work with MWD tools and allowed signals to be passed through the tool, allowing correct measurement of the well inclination. This enables the tool to be used in horizontal wells.
- A potential disadvantage of the ECD reduction tool is the surge and swab effects that could occur during tripping. Surge refers to the downhole pressure increase due to the downward movement of the drill string in the well. Swab refers to a decrease in downhole pressure when the drill string is being pulled out of the hole.
- When using the ECD reduction tool, it is important to consider the depth of the zone of interest. As you can see in Fig. 7, the pressure is only reduced below the location of the tool on the drillstring. This means that if the tool travels below the zone of interest, then the tool will have no effect on the pressure at the zone of interest.
The ECD reduction tool can be used in onshore and offshore environments to help prevent problems associated with drilling wells that have narrow pressure windows. It can help alleviate high ECD that could result in formation damage and mud loss. It may be useful as a low cost alternative to other ECD-reduction techniques. Further testing, though, is necessary to determine if this tool can be used in smaller drillstrings and to further study the effects of the tool on the surge and swab while tripping pipe.