High equivalent circulation density (ECD) is responsible for many wellbore pressure-related problems during drilling. Proper wellbore pressure management is a critical part of normal drilling practices, where static and dynamic fluid pressures are used to contain formation pressures and assure wellbore stability. Excessive fluid pressure while circulating can create problems including reduced operating margins between fracture and pore pressures and, in the extreme, lost circulation. To address these problems, an ECD Reduction Tool (ECD RT) is being developed.
The ECD RT is designed to counter downhole pressure increase due to friction in the annulus by reducing the hydrostatic head. The tool is expected to have a broad range of drilling applications including the narrow pore/fracture pressure margin in the deepwater environment, wellbores prone to instability, pressure depleted reservoirs and extended reach wells. The ECD RT will also potentially reduce the impact of uncertainty when selecting casing setting depth by widening the usable margin between pore pressure and fracture gradient.
Managing downhole pressure is a critical element of most drilling jobs. It becomes paramount under difficult conditions of deepwater and extended reach drilling. The downhole pressure of circulating fluid is the sum of hydrostatic head (a function of mud density and cuttings loading) and frictional loss (a function of mud rheology, mud density, annular geometry, and flow rate). It is converted into pound per gallon to signify effective mud density and referred to as ECD. This paper describes the development of a downhole tool for reducing the ECD of circulating mud. It covers the design and testing of a prototype ECD RT that will be valuable for use in managed pressure drilling applications, both onshore and offshore.
A prototype ECD RT has undergone a series of tests in a flow loop and in experimental wells. The ECD RT provided downhole pressure reduction of 358 psi when run with 11.6 ppg water-based drilling mud at a 550 gpm flow rate. A 25 ft column of concrete was successfully drilled from 13–3/8" casing located in a vertical well using a 12–1/4" drill bit. All returns flowed through the ECD RT without plugging the system. The ECD RT was also tested in a horizontal well to assure compatibility with the mud pulse telemetry systems used in MWD tools. Tests were conducted for both positive pulse and negative pulse signals and in both cases the mud pulse signal passed through the ECD RT without significant attenuation.
Lessons learned during successful technology trials in experimental wells have been incorporated into the current design. The plan is to test the tool in a benign land well environment prior to commercial applications. The tool is expected to be ready for use in an actual drilling operation in the second half of 2004.
The initial focus in developing technologies for ECD reduction was directed towards applications in deepwater, where the issue to overcome is the significant hydrostatic pressure in the riser when it is full of weighted mud1. As the industry has strived to recover hydrocarbons in increasingly challenging areas, it has become apparent that one of the major challenges is to maintain downhole pressures within the narrow window between the pore and fracture pressures. In practice, the window may become even narrower if the minimum required downhole pressure is governed by wellbore stability issues, rather than just pore pressure2,3. Since the size of this operating window dictates the maximum ECD that the well can tolerate, there is clearly a need for reducing the magnitude of ECD4,5. Conventional methods to minimize ECD include:
Reducing frictional losses through the use of low fluid rheologies.
Selection of drill strings and casing strings that provide greater annular clearances.
Application of expandable tubulars to preserve hole size.
Use of drilling liners rather than full casing strings.