Formation pressure data are a crucial guide in an operator’s decision to move a newly drilled well into production. Using pressure data obtained during logging runs, the operator determines fluid contact, fluid properties, reservoir pressures and, ultimately, the production potential of the reservoir. This information subsequently guides reservoir development plans, completion designs, and the type and size of surface facilities.

But in an environment of tight exploration budgets, full well-testing programs can be cost-prohibitive, and operators will instead use wireline instruments to obtain the required data. However, traditional wireline pressure testing methods rely on manual operation and measurements, which can extend testing times and raise the risk of inaccurate data and inconsistent test outcomes.

Any error in pressure measurements can severely impact an operator’s development plans. A wide error bar in the pressure measurements collected at different depths introduces a great deal of uncertainty in determining fluid contacts in a well.

Misjudging a fluid contact by even a few meters can result in the calculated value for the assets being significantly under- or overestimated. If these same erroneous measurements are carried over into subsequent formation tests in other wells, the operator risks making serious mistakes in investment decisions and leaving otherwise recoverable reserves in the ground.

This was the potential risk facing the operator of a major North Sea field development, which required fluid type and contact information for a well drilled into a formation characterized by widely varying permeability and high overbalance pressure in some zones. The operator wanted to conduct formation pressure tests to obtain this information across the different zones, but faced with a tight execution timeline, these tests had to be run efficiently with a minimal number of trips downhole.

Automated solution
Baker Hughes recently developed the FTeX advanced wireline formation pressure testing service designed to deliver reliable and accurate pressure data through a combination of downhole automation and real-time control. The service replaces manual control with an intelligent downhole platform that reduces the possibility of human error by automating pressure measurements and minimizes testing time by real-time optimization of the operating sequence.

The service comprises all the components of a wireline formation tester, including deployment pump, extendable probe, drawdown pump, pressure gauges and packer into a single unit with an outside diameter of 37⁄8 in. A precise drawdown controller commands the motor that directly drives the drawdown pump.

Drawdowns are initialized based on preloaded algorithms that use piston position, speed and pressure for feedback. The controller determines the end of the buildup period based on user-defined stability criteria. The service can operate reliably at ambient temperatures of up to 350 F (177 C), pressures of up to 30,000 psi and in boreholes ranging from 4¾ in. to 16 in. in diameter.

Testing parameters and drawdown control are calculated automatically in the service. It then determines optimal pressure measurements by adapting to the formation response in real time and refining the parameters for subsequent drawdowns, thereby increasing data accuracy and minimizing measurement time. Reservoir engineers can constantly monitor the pressure measurements from the surface and, where necessary, change the progression of each test based on their assessment and expertise, providing a degree of customization to the automation process.

While previous formation pressure testing tools required a standalone run downhole, this automated service can be deployed with the full spectrum of openhole logging tools, including standard petrophysical logs, nuclear magnetic resonance and induced spectroscopy imaging services in the same tool string to save rig time.

Data collected by the automated service, which includes pressure and mobility profiling and fluid contact and density determination, can be obtained in the first logging run. This allows reservoir engineers and petrophysicists to make earlier decisions about how to proceed with their formation evaluation objectives.

Streamlined test sequence
Each testing sequence typically begins by selecting the optimal pretest protocol based on the expected formation response; however, the system also incorporates options for generic test initiation where reservoir properties are less certain. In either case, the automation will optimize the testing sequence once the formation response is measured. Customized pretest protocols can be developed for specific customer needs and specific applications but, in general, the available predefined testing schemes are suitable for all applications.

Once the tool is deployed downhole to the zone of interest, a typical operation would start with selecting a generic testing sequence such as a medium mobility test. This would then initiate a volume-controlled pretest, in which a predefined volume of fluid is withdrawn from the formation. This drawdown is performed and analyzed in such a way that the pressure falls a predetermined value below formation pressure and that any pressure shocks to the formation are minimized.

Downhole software constantly analyzes the pressure response data from this first drawdown test and automatically determines the targeted pressure drop and flow-rate parameters for the next test. The drawdown in the second test is controlled to slowly change the pressure and is controlled automatically until the target pressure is reached, when the drawdown slowly comes to a halt.

The final test evaluates the formation under Darcy flow conditions. Based on the mobility obtained from the previous drawdown, the drawdown rate is increased so that the tool reaches the defined pressure drop quickly. The drawdown rate and pressure drop are then held constant for a period of time—typically 30 seconds or less—to achieve Darcy flow.

Taken together, these three tests allow the testing service to perform a smooth, controlled and efficient pressure measurement in about half the time required for other formation pressure testing tools.

North Sea deployment
A North Sea operator selected the service for its well with the objective of acquiring the pressure measurements through highly variable permeability contrasts. The service had to minimize the error in each pressure measurement to ensure an accurate understanding of the pressure distribution in the field and allow an improved calculation of total hydrocarbons in place.

The service was combined with other openhole wireline services during the logging operation for maximum efficiency. With data from combined logs being used to pick the pressure stations, more than 50 pressure stations were attempted.

Pressures were acquired through formations with mobilities ranging from 0.2 mD/cP to 500 mD/cP, and stable, repeatable pressure measurements were recorded at 0.2 mD with 0.05 psi/min stability criteria within only a few minutes.

The service acquired all data from the well accurately and efficiently, taking on average less than half the time required for previous-generations formation testers in the same environment. Combining it with other openhole logs together with the advances in tool design and automation allowed the operator to avoid an additional trip and saved 20 hours of rig time in the process.

The successes observed in this North Sea well have made the tool the formation pressure tester of choice for the operator. And, thanks to its automated operation and wide specification range, the formation pressure testing service promises similar measurement improvements for both openhole and slimhole logging operations in all formation types.

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