Bridging the gap between wireline formation, drill stem testing
Tests larger section of wellbore than a probe type
David Bowles
Project Engineer
PathFinder Energy Services
To obtain more accurate data in today's complex well environment and to avoid the effects of mud invasion in short term wireline formation testing (WFT), it is often necessary to perform a drill stem test (DST). The time and hardware involved in running a DST can be economically and environmentally prohibi-tive. To bridge the gap between WFT and DST, Pathfinder developed a formation testing tool to add to their fleet of LWD tools.
At the initial design phase of the new tool, several criteria were established:
- Offer a viable alternative to WFT or DST
- Data quality equal to or exceeding current technology
- Should not compromise drilling or operations safety
- Capable of operating in highly deviated complex reservoirs.
The drilling formation tester (DFT) was developed using dual inflatable packers and a custom pump capable of moving 100 cc/sec. A probe-based tool was investigated but rejected as a duplication of existing measurement methods and its inability to move large volumes of fluid in a short period of time.
By using dual packers, the DFT tests a larger section of wellbore than a probe type WFT, making the tool more effective in testing laminated or fractured formations. Even though the DFT moves a larger volume of fluid than the WFT, the fluid velocity is comparatively low, reducing the chances of filter clogging and formation damage.
Because using dual packers can test a larger section of formation, and because the typical probe-based tool may take more than 20-30 tests in a single borehole, the required formation properties can now be obtained with as little as 10-12 tests. When the DFT is conveyed down hole in the drilling assembly, testing may then be conducted in a drill-test-drill mode. Alternatively, the well can be drilled to total depth (TD) and the formation tested on the way out of hole.
The DFT will always reach TD when attached to the drilling assembly. Compared to existing wireline testing, where tools frequently require the assistance of drill pipe to reach TD, the time and cost savings are substantial.
Inflatable packers have been used in formation testing for many years but the DFT is the first to apply this technology to the drilling environment. The initial testing phase of packer survivability proved the concept to be sound after many hours of downhole testing.
System operation
According to the company, the DFT is in "sleep mode" while drilling but capable of transmitting annulus pressure to surface and constantly monitoring the health of its onboard control system. While in this mode, the required test points may be identified using one or more of the LWD sensors (gamma ray, resistivity, density, neutron, or sonic).
The LWD operator instructs the DFT to switch to the test mode once in the correct position, initiating an automated test sequence.
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The flow diverter opens after activating, redirecting mudflow from the drill bit to exit above the packer elements. This allows the packers to be inflated in the static mud environment while still maintaining flow over the upper BHA. Once packers are inflated, the drawdown pump begins to remove the trapped fluid. At each stroke, the pump will move 100 cc of fluid, a typical test removing about 10-20 liters before formation pressure is allowed to build back to the steady state condition.
A pressure gradient plot showed excellent line fit of data, both real time and recorded, indicating high quality data and proving the DFT invaluable for making reservoir decisions.
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Data is transmitted throughout the test to the surface in real time by using the measurement while drilling (MWD) system and stored in high-density memory for further analysis on return to the surface.
The tool may be configured for a single or double drawdown test. The double drawdown has proved effective in removing the need to repeat a formation test. In practice, where a repeat was necessary, the system displayed repeatability to within 0.1 psia.
When the test is complete, the packers retract and the diverter closes, allowing another test to be conducted. Alternatively, the tool may be put back into "sleep mode" and drilling resumed.
System description
The tool is currently available in 6 3/4-in. diameter for use in 8 1/2-in. to 10 3/4-in. holes, and is 37 ft in length. While mechanically complex, the system may be broken down and described in five distinct sections.
- Hydraulic power section: All tool functions are operated using hydraulic fluid power. Hydraulic pressure is generated by a turbine powered by the drilling mud flowing through the tool. The hydraulic fluid operates all internal valves, activation of the diverter, and the cycling of the pressure draw down pump. All the system control valves, hydraulic lines, pressure transducers, and solenoids are housed within this assembly.
- Mudflow diverter: When activated, the diverter re-directs mudflow from the bit to exit above the inflated packers. Maintaining a constant mudflow during the test permits real-time transmission of data and helps maintain optimum hole conditions.
- Control electronics: The electronics section controls the operation and sequencing of the entire tool. The programming sequence is set up by the operator prior to running the tool in hole. This section also contains the non-volatile memory used to store data and process for transmission to surface.
- Dual packer mandrel: This section contains the dual inflatable packers. Inflated with internal pipe pressure and drilling mud to approximately 1,000 psi above annulus pressure. The packers may be inflated/deflated many times and contain a unique deflation system. Once inflated the upper packer will expose the fluid draw down area of the tool. The fluid passage is filtered to prevent ingress of material through the system, and then flushed clean with the fluid removed from the inflated packers at termination of test.
- Lithium battery section: The lower section of the tool is used to house the lithium batteries, which power the electronics. Power consumption is minimal as the mechanical energy required to operate the tool is supplied by drilling mud.
Operational experience
The DFT has been tested extensively in the North Sea, Gulf of Mexico, and North American land operations. To date the tool has operated on 39 separate wells and on many occasions WFT has been canceled as a result of the quality of data produced.
Data was collected from a North Sea well on a semisubmersible rig in seas with wave heights of 15-20 ft. The weather had progressively deteriorated when testing was completed, so the well was sealed and the rig moved off location for seven days. The majority of reservoir decisions were made based on real-time data, to be endorsed by memory data when it was retrieved 10 days later.
The test was initiated by sending a sequence of mud pump signals down hole to the tool. The tool records hydrostatic pressure when pumps are shut down. The initial rise in pressure represents the difference between hydrostatic and hydrodynamic pressure. The rise in pressure after eight min indicates inflation of the packer, a subsequent successful seal, and compression of fluid trapped between the two packers.
In this well, drawdown below formation pressure was quick and build-up to a stable formation pressure after each drawdown was consistent and repeatable. The build-up was monitored for a few minutes before packers were deflated, returning pressure to the almost immediately.
A total of 12 tests were taken in this formation followed by a test conducted to verify system repeatability, which resulted in the confirmation of quality data repeatability to within 0.2 psia. The well itself was highly deviated with an increase of 30 ft from vertical depth over the section tested. A pressure gradient plot shows excellent line fit of data, both real time and recorded, which proved invaluable for making reservoir decisions.
By using dual packers, the DFT tests a larger section of wellbore than a probe type WFT, making the tool far more effective in testing laminated or fractured formations.
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Safety features
The operation of inflatable packers prompted some operational concerns from the drilling operators. However, the design of the DFT incorporates a number of safety features that prevent unwanted packer inflation.
The tool requires two distinct re-programming sequences before it will initiate a test. Once begun, confirmation of the test initiation can be seen in real time on the MWD system and by a physical change in surface system pressure.
Prior to inflating packers, the electronics control monitors internal pressure sensors to confirm system operation and diverter activation.
With further development, the DFT will be positioned for use as a fluid sampling tool. The concept is that a large volume of fluid moved by the reciprocating pump will facilitate storage of clean formation fluids in minutes, compared to hours for probe-based tools. Within the near future, we will see the tool deployed in additional sizes for use in 12 1/4-in. and 6-in. hole sizes.
