Wireline formation pressure testing benefits extended to low-mobility environments

Precise pressure data are critical during all phases of oilfield operations. Reservoir pressure data are critical in determining fluid contact, fluid properties and, ultimately, the production potential of the well. Formation testing in ultra-low-mobility formations has plagued the industry since the first probe was extended against rock, and formation pressure measurements were attempted.

Fig. 1. The Baker Hughes FTeX advanced formation pressure testing service uses closed-loop downhole intelligence and real-time control to extend wireline formation pressure testing benefits to low-mobility environments.

Every reservoir contains sections of rock where mobility is lower than expected. Understanding the pressure and subsequent pressure gradients through these low-mobility sections can be critical to determining the ultimate production potential of the well, and designing the optimum well design, placement, and completion and production scheme for maximum ultimate recovery. However, traditional wireline-deployed pressure testing services require significant manual operation and measurements, which are prone to longer testing times, and raise the risk of inaccurate or incomplete data and inconsistent test outcomes.

A new, advanced formation pressure testing service from Baker Hughes uses downhole automation and closed-loop, real-time control of an intelligent packer and pump configuration to deliver critical formation data—including pressure profiles, fluid contact and mobility information—reliably, accurately and efficiently, even in ultra-low-mobility environments, Fig. 1.

Previous formation pressure testing tools required a stand-alone run downhole. By contrast, the automated service can be deployed with the full spectrum of openhole logging tools—including standard petrophysical logs, nuclear magnetic resonance (NMR) and induced spectroscopy imaging services—as early as the first logging run, to significantly reduce overall logging time. The result is an earlier and better understanding of the reserves in place, so operators can quickly identify the production potential of a reservoir and plan accordingly.

TESTING CHALLENGES

In single-probe formation tests, an elastomeric probe is extended into the formation, and a small volume of fluid is withdrawn. The pressure changes during the drawdown and build-up periods are recorded and analyzed to estimate the formation pressure, mobility and, occasionally, fluid compressibility.

A number of techniques to analyze pressure transient data of formation testers have been adapted from well testing analysis. Typically, these techniques are associated with spherical and cylindrical flow regimes. These methods use only build-up pressure data. In a tight formation, where pressure builds up slowly, a long testing period is not desirable because of potential tool sticking problems. Although late time data are critical for both cylindrical and spherical flow analyses, the data are difficult to acquire. In low-mobility formations, operators would have to wait a substantial amount of time to get past the storage portion of the build-up, before they could even confirm that they are actively communicating with the formation. The best way to confirm formation connectivity is to allow a flowing period, where the flowing pressure and the rate are held constant for a period of time.

ELIMINATING SUPERCHARGING

In any well, the near-wellbore formation remains under the influence of the drilling mud until it has time to stabilize with in-situ reservoir conditions. The lower the mobility of the near-wellbore rock, the longer it will take to stabilize with the rest of the formation. In low-mobility formations, active mud circulation can limit filter cake growth and lead to drilling-fluid-filtrate leak-off. This, in turn, can elevate near-wellbore formation pressure beyond the formation’s ability to dissipate it, resulting in a phenomenon known as supercharging. Additionally, if the formation is tested shortly after it has first been drilled, there may be insufficient time for elevated pressures to relax.

With traditional formation testers, the ability to measure the true formation pressure is limited by the necessity of removing enough fluid to eliminate the supercharging, without driving the sandface pressure so far below formation pressure that build-up times are prolonged. Prolonged build-up times result in tests that are not stabilized and not representative of the formation pressure. Conversely, if not enough fluid is removed, test time is not always extensive, but the data are supercharged and not representative of the formation. Multiple repeat tests can be performed, but the process is very time-consuming, because each time fluid is withdrawn, it is necessary to wait for the pressure to build up again. There is a tendency to give up before the tests are representative.

One method of addressing the supercharging challenge has been to use straddle- or radial-type inflatable packers with extremely large flow areas. However, multiple runs may be necessary, to acquire the large number of pressure tests required to map the entire pressure profile of the formation. This method also increases the risk of stuck tools and fishing.

The FTeX advanced formation pressure testing service uses a closed-loop, intelligent system design, downhole automation and real-time control to eliminate supercharging, and drive new formation testing efficiencies. The closed-loop system analyzes the pressure at 100 times/sec, making micro-adjustments to the flowrate to optimize the pressure drop, while making sure to remove enough fluid. This allows the flowrate to go as low as 0.001cc/sec. The system also is designed to minimize tool storage effect—that is, the amount of fluid inside the tool that needs to be compressed and decompressed. This feature minimizes required system volume. Additionally, optimizing the pads’ flow area through intensive modeling enables sufficient exposure to the sandface to permit flow, even at extremely low mobilities. The service can be combined with other wireline logging tools to allow true evaluation of tight clastics or carbonates, from gas to water.

AUTOMATING THE PROCESS

The service integrates all the components of a wireline formation tester, including deployment pump, extendable probe, drawdown pump, pressure gauges and packer, into a single, 3.875-in. OD unit. A precise drawdown controller commands the motor that directly drives the drawdown pump. Drawdowns are initialized, based on pre-loaded algorithms that use piston position, speed and pressure for feedback. The controller determines the end of the build-up period, based on pre-loaded, user-defined stability criteria. The service can operate reliably at ambient temperatures of up to 350°F and pressures up to 30,000 psi, in boreholes ranging from 4.75 in. to 16 in. in diameter.

Pressure measurements are automated to eliminate the need for human operator control from the surface and, with it, the possibility of human error. Test parameters and controls for drawdowns are set automatically, requiring minimal effort to run a test. By adapting the formation response from the first drawdown, and defining a behavior for subsequent drawdowns in real time, the service determines optimal pressure measurements, while reducing logging time. The system’s precise pressure gauge ensures highly accurate measurements, while the electrical architecture minimizes the risk of system failure.

Optimizing the operation sequence not only expedites testing, but also reduces the amount of time that the tool is in contact with the formation. This, in turn, lessens the risk of differential sticking and the potential for costly fishing operations.

Throughout the test, the service continually analyzes formation response at a rate of 100 times/sec. Additionally, the pressure measurements are sent to the surface, so reservoir engineers can constantly monitor pressure measurements from the surface. The system’s intelligence also adds a degree of customization to the automated process. Because it can adjust its behavior based on the real-time feedback it is receiving, the reservoir engineers can change the progression of each test when necessary, based on their assessment and expertise. Finally, because the process is so swift, each pressure test can be repeated to verify that a true, repeatable formation pressure is reached.

TEST SEQUENCE

A typical testing sequence, using the automated formation pressure testing service, is as follows:

1. Pre-test protocol selection is 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, automation optimizes the testing sequence, once the formation response is measured. Customized pre-test protocols can be developed for specific operator needs and specific applications. In general, however, the available predefined testing schemes are suitable for all applications.
2. Once the tool is deployed downhole to the zone of interest, a typical operation begins with the selection of a generic testing sequence, such as a medium-mobility test. A volume-controlled pre-test is initiated, 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 to a predetermined value below formation pressure, while removing a set volume, so any pressure shocks to the formation are minimized.
3. Downhole software constantly analyzes the pressure response data from this first drawdown test, and automatically determines the targeted pressure drop and flowrate parameters for the next test. The drawdown in the second test is controlled automatically, to slowly change the pressure until the target pressure is reached, then the drawdown slowly stops.
4. The final test evaluates the formation under Darcy flow conditions. Based on the mobility obtained from the previous drawdown, the drawdown rate is increased, such 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 4 sec or less—to achieve Darcy flow.

Fig. 2. An operator of a major North Sea field used the advanced formation pressure testing service to successfully map the pressure profile through a low-porosity, very-low-mobility section.

Taken together, these three tests acquire a smooth, controlled and efficient pressure measurement in approximately half the time required for other formation pressure testing tools.

MEASURING PRESSURE THROUGH TIGHT CARBONATES

Measuring formation pressure through tight carbonates has posed a significant challenge to the upstream oil and gas industry, because the formation has extremely low-permeability properties, and it is typically assumed that tight carbonates cannot be pressure-tested accurately. An operator of a major North Sea field was able to overcome that challenge and acquire accurate formation pressure, to successfully map the pressure profile through a low-porosity, very-low-mobility carbonate section (Fig. 2), using the FTeX advanced wireline formation pressure testing service with a custom-made, elongated packer and a patented formation rate analysis (FRA) technique.

The formation pressure information was key to the operator, and the timeline for execution was critical. The tool had to not only acquire formation pressures, but also verify them by achieving stable, repeat pressure measurements. The system had to minimize the error in each pressure measurement, to ensure an accurate understanding of the pressure distribution in the field and, consequently, improve the production strategy of the field. To achieve the desired results, storage effects in the low-mobility environment had to be mitigated, and mobility had to be determined accurately.

Pressure points were selected from a previous imaging run, and tool storage confusion was mitigated by validating formation response on the pressure points with real-time pressure transient analysis (PTA). PTA is a technique that analyzes the change in build-up pressure, as the well goes from the flowing pressure to final formation pressure. This analysis contains valuable information about the reservoir properties. Using PTA allowed the team to determine whether the build-up profile of the formation pressure test was coming from the compression of the fluid inside the tool, or because of the formation response. Additionally, because the service induces a flowing pressure in the formation, the team could be absolutely certain that the flow was coming from the formation, not from decompressing the fluid inside the tool.

Fig. 3. The advanced formation pressure testing service measured the formation’s pressure response and rate during the job.

A patented FRA technique was used to calculate mobility. The technique uses a multi-variate, linear regression algorithm with a multi-scale scatter search optimization algorithm to estimate the formation parameters more accurately and efficiently, particularly in low-mobility formations. Unlike the drawdown mobility method, which uses a single data point of pressure versus formation rate, the patented FRA technique uses all the pressure and formation rate data points in the test to account for the storage effects, and calculates the correct formation flowrate. This is critically important in mobility estimation in low-permeability zones. The FRA technique is based on the material balance for the tool’s flowline volume. It uses the modified Darcy’s law with a geometric factor, and calculates the formation rate instead of using the piston-withdrawal rate for the analysis. By correcting the piston rate for the tool-storage effect, the actual flowrate from the formation is calculated. This allows for a significantly more accurate formation mobility to be calculated.

Fig. 4. Close-up of highlighted area in Fig. 3.

To expedite the operation, the operator and service company collaborated to custom-design a packer with three times the normal surface area, to optimize the flow area for low-mobility formation pressure tests. Optimizing the packer’s flow area led to a significant increase in both the quantity and quality of the acquired formation pressure measurements. A rapid, joint engineering and design project produced a packer with three times the flow of a standard probe.

Valid, repeat pressures were acquired from 16 depth stations, with 12 tests achieving a build-up stability of less than 1 psi/min., with the lowest being 0.01 psi/min.

A stable pressure measurement was recorded, at mobility as low as 0.003 mD/cP with 0.9 psi/min. stability and repeat tests within 30 min., Figs. 3 and 4. The mobility data for the operation ranged from 0.003 to 0.4 mD/cP. The electrical pump of the pressure testing service was able to achieve drawdown rates as low as 0.002 cc/sec, which was critical to success at such low mobility.

Using the custom elongated packer reduced the test time 50%, compared to a standard probe deployed in a previous well at the same mobility. The service acquired valid, repeatable pressure measurements and successfully mapped the pressure profile through the tight carbonate section.

Better understanding of the pressures in extremely low-mobility formations has always been important to field development. Until now, however, quality data have been difficult or unreliable to acquire. As a result, operators have faced a conflict of having to choose between quality of data or quantity of data. Advanced, wireline-based formation pressure testing service, such as the one described in this article, allows operators to both achieve sufficient pressure tests, as well as acquire the quality formation pressures required to truly understand what the formation behavior will be when they put the well or field into production.