Oil exploration has continued to venture into deeper, more adverse environments, and as a result, testing technology has had to redefine the operating envelopes for its tools to meet the challenges presented by the more demanding environments encountered. Not only have high-pressure and high-temperature tools been needed, but tools have also been needed that were more debris-tolerant.
This paper discusses new debris-tolerant tools developed to operate after a fracturing operation has been performed, and ceramic debris remains in the well. Previously, many operations were challenged because of debris, and many times, it was necessary to pull the string out of the hole to clean the well prior to running the DST string again and performing the job.
Debris-tolerant tools now allow the fracturing and acid operations to be performed with a test string, and with the same string, the well can be opened to a well-testing operation without compromising the functioning of the downhole tools. This capability saves time and adds economic value to the job.
Development of debris-tolerant downhole tools also has led to development of new debris-tolerant surface well-testing equipment and subsea tools, all of which are now capable of operating with solids in the well.
The addition of these tools has allowed high-quality well testing to be performed safely. When these tools are used in wells where solids are not expected, and unexpected debris does occur, they can avoid the unexpected rig time previously required to clean the well, further enhancing economic efficiency.
This paper discusses an operation in Brazil where a standard tool was not suitable for performing a testing job; a new debris-tolerant tool was used, and the job was successfully performed. The paper will also discuss the situations that occurred during downhole testing in Brazil that led to the development of the new tools.
In Brazil, the challenge to develop new oil and gas fields has led to the need to investigate deeper reservoirs with higher pressures and temperatures than previously explored. Well testing operations have led to another challenge - the need to enhance the capability of testing tools so that they could offer greater tolerance to debris.
More than any other metocean variable, water depth has the greatest influence on the type of facility selected to drill the well. As the water depth increases, the available options regarding the vessel from which to drill the well decrease. In deep water from greater than 600 m (2000 ft), there are only a few vessels that are capable of mooring to the seabed. The remaining option, to use a dynamically positioned vessel, adds significantly to the costs of the operation, since these vessels are also few in number and command high contract charges. They also consume a considerable quantity of fuel due to the need to remain on station with thrusters that are in constant use throughout the drilling program. (Nardone, 2009).
Some of the difficulties identified in operating downhole testing tools in deeper well conditions have required complete changes to equipment and processes so that they could operate successfully in the new environments. The deepwater conditions have required enhancements to other components used in testing such as heavier mud with solids decantation, tools to maintain integrity when in debris-laden areas during well conditioning operations, and in wellbores where scales of cement and metallic debris or even a mix of different types of debris will be present as well.
As most drillstem-test (DST) tools are operated by applying annulus pressure, debris settles at the area of the moving parts of the tool, preventing the tool from operating properly.