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Acquiring bottomhole pressure and temperature data
The acquisition of bottomhole pressure and temperature data can be planned and executed in a cost-effective manner with a minimum disruption to normal operating routines. In many cases, early on-site interpretation is useful in guiding decisions about continuing the acquisition program. This article discusses options for obtaining pressure and temperature data.
Several questions should be answered at the design stage:
- What are the objectives of measurement: static pressure, reservoir dynamics, fluid characterization, vertical pressure and temperature profile, well flow characterization, or other?
- Is the environment openhole or cased hole? An exploration or development well?
- Is there a need for real-time surface readout (SRO) measurements or can the data be recorded downhole and reviewed later?
- What metrology is needed for the measurements (e.g., maximum temperature and pressure, measurement resolution and accuracy)? 
- How will the gauges be conveyed to the bottomhole measuring points?
- Is there a need to perform continuous or repeated measurements over months or years?
- What economics apply, and do they imply a possible tradeoff with the quality or quantity of the measurements?
Surface readout vs. downhole recording
Measurements can be transmitted to the surface, usually via an electric cable, or recorded in downhole memory powered by batteries.
SRO has the obvious advantage of providing data in real time. Real-time readouts are especially beneficial for transient measurements that require time for the pressure to stabilize and radial flow to develop. Because stabilization times depend on reservoir and fluid properties and because the determination of these parameters is often the purpose of pressure measurements in well tests, predicting the duration of stabilization periods is often difficult. SRO is preferred in these cases.
Some applications, usually those in the lower economic tier, can be conducted without the need for SRO. The benefits include lower operating costs and a fixed operations schedule. The drawback is the difficulty of guaranteeing the quality of the acquired data, including the potential for significant operating losses if the bottomhole recording equipment malfunctions. For these reasons, downhole recording (DHR) should be chosen only when the measurement target does not necessarily depend on stabilization times or when stabilization times are already known (e.g., to measure the average reservoir pressure in a reservoir of known mobility).
Many industry tools provide both SRO and DHR. The measuring section of these tools is common to both options. In the SRO option, the sensor electronics are coupled to a telemetry system for uphole transmission, and the cable supplies downhole power. In the DHR option, downhole batteries supply power, and the data are stored in memory boards for future readout or downloading to suitable computer systems.
Surface shut-in vs. downhole shut-in
The practice of downhole shut-in during a buildup test as opposed to surface shut-in is discussed in Pressure transient testing. The advantages of downhole shut-in include:
- Control of the wellbore volume (afterflow)
- Reduction of the duration of buildup tests
- Choice on the recording mode (SRO or DHR)
Downhole shut-in can be performed during conventional drillstem tests (DST) or during tests performed on production wells. In a DST, the downhole shut-in valve is usually the main test valve. Shut-in is performed at the end of a flow period by actuating the test valve. Traditionally buildup pressures are recorded in DHR mode. Another procedure is to use the DST valve as the main shut-in valve while the DataLatch electrical wireline downhole recorder/transmitter is used to observe pressures in real time during the buildup. DataLatch technology is described later on in this page.
In producing wells, downhole shut-in is performed by setting a valve assembly in the tubing before performing the test. The tubing must have been previously equipped with locator nipples so that the valve can be anchored at the appropriate depth. The valve is run on either a slickline carrying a DHR pressure gauge or an electric line equipped with an SRO recording gauge. The shut-in valve is actuated by a sequence of pulls and releases on the slickline or cable. Commonly operated shut-in valves can perform in the range of up to 12 open-close cycles, after which the valve assembly is released from the setting nipples by an appropriate pull on the line. Other versions of shut-in valves can be operated by a clock, small explosive squib, or battery.
Bottomhole conveyance of gauges
Pressure and temperature gauges can be conveyed to the downhole environment by a number of methods, possibly in tandem with other measurements. Several possibilities are discussed here.
Drillstem test string
This topic is discussed below.
Electric line operations provide surface readout and can be conducted any time during the life of the well. In openhole, wireline pressure testing offers a unique opportunity to efficiently collect distributed pressure data on the entire stratigraphic sequence penetrated by the well. In cased holes, pressure and temperature measurements are taken repeatedly along with other production logging measurements to monitor well performance and diagnose flow and completion problems. In addition, pressure buildups and other transient tests are frequently conducted in producing wells, using production logging tools or with "hanging" gauges, to check for variations in the productivity index and for skin development and to monitor multilayer producing systems. In the exploration environment, however, electric lines are not typically used because of the risks associated with having cable in the well while flowing. These risks include difficulty closing safety valves or subsea trees with cable in the borehole and sticking of the cable because of sanding from unconsolidated formations or hydrate formation in subsea wells. The DataLatch electrical wireline downhole recorder/transmitter, introduced in the late 1980s, has largely mitigated this risk. The DataLatch system is briefly described under Drillstem Testing below.
Slickline pressure and temperature surveys are performed with hanging gauges in situations that do not require SRO. Slickline operations are more cost-effective than electric line operations; however, the data quality usually does not match that of SRO data. Depth control is one of the critical factors affecting data accuracy. On the other hand, surface pressure control is easier in slickline operations because of their smaller diameter, typically in the range of 0.1 in. in diameter. A promising development is the "slick conductor line," a thin, hollow-core, 100% steel cable laid around an electric conductor to provide limited SRO capabilities.
A popular alternative to drillpipe, coiled tubing is used to convey downhole gauges and other equipment in deviated holes when gravity is insufficient to pull the tools to the bottom of the well. Insufficient gravity occurs where well deviation exceeds values in the range of 60 to 70°, depending on tool weight and length, friction coefficients, pipe roughness, and the presence and type of completion components. In horizontal wells, the coiled tubing may not reach the toe of the completion because of a helical lockup of the coil inside the completion. Coiled tubing may be equipped with an internal electric cable running the length of the coil to support SRO operations.
Tractors are an emerging technology that complements the use of coiled tubing in difficult, deviated completions. Tractors are self-powered and operated by electric line. They can negotiate bends, crawl up or down, and overcome the limitations of coiled tubing in long horizontal wells. Their main limitation is the large amount of cable power required for operation.
Wireless transmission is a technique that has been in use since the late 1980s. It attempts to provide the advantages of SRO without using an electric line. The downhole tool, a sub that is part of a DST string, features a pressure gauge, battery pack, telemetry, recorder board, and antenna. The antenna sends the signals collected from the pressure recorder at a frequency suitable for transmission through the formation strata. At the surface, the signals are picked up by an array of suitably deployed stake antennae. This technique is limited to land operations and depths of approximately 8,000 ft.
A DST string is a complex array of downhole hardware used for the temporary completion of a well. DSTs provide a safe and efficient method to control flow while gathering essential reservoir data in the exploration, appraisal, and development phases of a reservoir or to perform preconditioning or treatment services before permanent well completion. Fig. 1 shows a typical DST string with its essential components. In exploration well testing in particular, the DST string usually includes tubing-conveyed perforating (TCP) guns, which are shot underbalanced (i.e., wellbore pressure is less than reservoir pressure) at the initial well completion.
DST strings include gauge carriers, which are collars that normally contain up to four pressure gauges bundled together, affording redundancy in long tests in which one or more gauges are likely to fail. These gauges perform only DHR measurements.
Most modern DST strings are fullbore strings, which means they have a flush opening running completely through the string of tools. The opening enables running pressure gauges and other slim tools (typically 1 11/16 in.) in SRO mode.
The DataLatch system is a DST string component that combines the advantages of DHR (typically during flow periods) with the advantages of SRO (typically during shut-in periods). Pressure data are recorded in the tool’s memory boards. In suitable conditions, a latched inductive coupling (LINC) tool is run with an electric cable and latched into the DataLatch mandrel. This combination is used to read out the memories, reprogram the gauge acquisition schedule if necessary, and monitor the test in real time. The DataLatch system is unique for bottomhole pressure measurement applications. It allows simultaneous and continuous acquisition of three different measurements during the course of a DST test:
- Rathole or reservoir pressure
- Cushion or tubing pressure
- Annulus pressure
Openhole wireline pressure testing
Wireline pressure testing is conducted using tools lowered on an electric cable or coiled tubing in deviated wells. The tools consist of function-specific modules selected for a specific operation. Fig. 2 shows the modular arrangement of a modern wireline pressure tester. In a complete configuration, the tools may include a single-probe module for basic pressure testing and sampling, a dual-probe module for permeability applications, a flow-control module for flexible schedule testing, a fluid analyzer module for optical fluid properties monitoring in real time, multisample module for representative fluid sampling at reservoir pressure-volume-temperature (PVT) conditions, sample modules for large-volume fluid sampling, a pumpout module that can recirculate mud filtrate and other unrepresentative fluids out of the tool flow system before representative sampling, and a dual-packer module for very large area sampling, interference testing, and DST emulation.
Wireline pressure testing encompasses several applications described in Reservoir pressure data interpretation. Theses applications include:
- Measuring static reservoir pressure
- Collecting representative fluid samples
- Determining anisotropic permeability
- Identifying reservoir permeability barriers
- Determining reservoir fluid gradients and densities
- Determining rock stress components.
As in conventional well testing, a wireline sampling operation must be carefully designed beforehand to achieve the desired objectives. Key parameters that require special attention include:
- Test type
- Fit of the probe type to the reservoir’s mechanical and hydraulic deliverability characteristics
- Volumes to be withdrawn
- Type of pressure gauges for the expected reservoir permeability
- Test sequence
- Number of pressure points to be taken
- Interpretation objectives
The actual downhole tool configuration and the test sequence directly reflect the design study that precedes the operation.
See Production logging for uses, benefits, and interpretation techniques of production logging.
Measurements while perforating
Pressure and temperature measurements may be performed concurrently with shaped-charge perforating. This technique, called measurement while perforating (MWP), includes the following applications:
- Before perforating, MWP verifies the completion fluid density, directly measures the wellbore pressure, and adjusts the perforating underbalance
- During perforating, MWP positively detects the detonation of the perforating gun
- After perforating, MWP observes the pressure responses and interprets them as a transient test, and it monitors the fluids produced by the perforation
The MWP system is especially adapted to low-flow-rate or short-duration tests such as:
- Impulse measurement while perforating tests
- Closed-chamber tests
- Slug tests
- Flow tests
MWP must be performed in SRO mode; otherwise, the technique offers no benefit. An SRO tool such as the DataLatch recorder/transmitter or an electric line tool must be used. The production well environment is preferred for MWP, with the perforating guns conveyed on an electric line with the MWP sub above the guns. A typical MWP sub for pressure and temperature measurements incorporates a gamma ray detector and casing collar locator for depth control and suitable shock absorbers to mechanically decouple the guns from the measurement system. An optional bottom electric adapter can fire the guns electrically, which is often the procedure in non-TCP applications.
Because MWP also measures the wellbore fluid temperature, it is used for the same applications as production logging.
Permanent pressure measurement installations
Permanent monitoring systems are placed downhole with the completion string near the depth of the reservoir to be monitored. They are connected to the surface by a cable that runs the full length of the completion string and exits the wellbore through suitable connectors crossing any subsurface safety systems and the wellhead. Advanced telemetry allows querying these sensors at any time throughout the life of the reservoir. Most systems in operation today record bottomhole pressure and temperature.
Permanent systems are engineered specifically for monitoring applications and have a life expectancy of several years. The digital electronics within the gauges are designed for extended exposure to high temperature without required maintenance. The metrology characteristics emphasize long-term stability rather than fast dynamic response. Quartz crystals as well as sapphire-based sensors can be used.
The cables for permanent installations are designed to withstand:
- Exposure to highly corrosive fluids
They must also be mechanically rugged to prevent damage during installation. Usually single conductor cables are used.
The connections are similarly designed for durability. They include bottomhole connectors (power and pressure) to the permanent gauge mandrel and uphole connectors that cross through the wellhead. The complexity of the surface installation varies, depending on whether the wellhead is located at the surface—as on a land well or wellhead exposed above the sea on a platform—or subsea. For subsea wells, the acquisition system is typically through existing data-gathering systems through umbilicals. On platforms, several permanent gauges may be connected to an autonomous surface unit that records the measurements and communicates with shore facilities through standard or advanced (satellite) transmission links.
Fig. 3 shows a continuous data stream from an 80-day recording in which the pressure measurement was used to optimize production. The surface production rate was repeatedly adjusted to yield an acceptable bottomhole flowing pressure.
- Veneruso, A.F., Ehlig-Economides, C., and Petitjean, L. 1991. Pressure Gauge Specification Considerations in Practical Well Testing. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 6-9 October 1991. SPE-22752-MS. http://dx.doi.org/10.2118/22752-MS
- Colley, N. et al. 1992. The MDT Tool: A Wireline Testing Breakthrough. Schlumberger Oilfield Review (April): 58.
- Davies, J., van Dillewijn, J., Herve, X. et al. 1997. Spinners Run While Perforating. Presented at the Offshore Europe, Aberdeen, United Kingdom, 9-12 September 1997. SPE-38549-MS. http://dx.doi.org/10.2118/38549-MS
- Eck, J. et al. 1999. Downhole Monitoring: The Story So Far. Schlumberger Oilfield Review (Winter): 20.
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