Wireline formation testing in high-pressure/high-temperature (HP/HT) wells has long been a challenge due to both the hostile operating environment and the general nature of formation testing. Acquiring multiple pressure measurements and extracting fluid samples can be a long process, and in hot, deep and small boreholes that are typical of HP/HT wells, this can be difficult. To enable logging tools to operate in high temperatures, vacuum flasks are used to protect their electronics, but these can be bulky and offer only finite protection. To operate successfully at high pressures, the design of every housing and seal must be carefully analyzed to ensure mechanical integrity at extreme pressures. The ideal wireline formation tester (WFT) for HP/HT wells would have a small tool diameter and be specially designed for hostile environments. This article describes and demonstrates the design features of a new HP/HT WFT tool in a case study from the Gulf of Thailand.

Generally, environments above 350° F (176° C) are considered high-temperature and pose difficulties for the precision electronics found in logging tools. To operate successfully in HP/HT wells, the tool electronics must be protected from the intense wellbore heat. To keep this heat away from the electronics, a vacuum flask surrounds the instrument. Flasks have been used for many years in the industry and have often been used with existing logging technologies to extend their operating temperature range. The drawback with vacuum flasks is that the inherent heat dissipation from the electronics causes "self-heating" inside the flask. Since the flask prevents dissipated heat from escaping, the electronics will overheat and eventually fail. There are two effective approaches to solving the problem of logging hot wells: One is to design electronics that can operate at a high temperature, and the second is to design very efficient electronics that dissipate the minimum amount of heat. A new WFT tool uses both these methods in its design for operating in high-temperature wells.

The electronic systems in conventional logging tools have generally been limited to 350° F and, to log wells beyond this temperature range, a flask would be installed around the instrument. The specially designed electronics of the new tool can operate continuously at 375° F (190° C), which is achieved by careful electronic component selection and extensive thermal management. To reduce heat dissipation, the electronics use new low-voltage (3.3v) technology that significantly cuts the integrated circuit power consumption. There is a low heat-dissipating "sleep mode" that turns off unused circuitry during periods of inactivity, such as when moving between pressure test intervals. A highly efficient DC-DC power converter and the ability to dump excess power outside the flask further reduce internal heat dissipation. Placing all the electronic assemblies on a beryllium-copper chassis improves thermal management within the flask. All these measures give an extended operating time above 375° F to allow for extensive formation testing programs.

There are other WFT challenges in the HP/HT environment, one of which is to ensure reliable pad seals during the multiple sets of a typical test program. Extensive lab tests and field trials have demonstrated that specially reinforced rubber material and a careful mechanical design, which allows for rubber extrusion at high temperature, has been successful. The standard pad currently deployed with the tool can be expected to make in excess of 75 sets at temperatures over 350° F in synthetic drilling fluids without a seal failure. To minimize the chances of downtime from solids plugging the sample probe, the tool has a hydraulically controlled wiper that only opens the fluid entry point (Figure 1) after the pad is securely deployed to the borehole wall. Elastomer seals which are typically used in WFT tools to isolate the tool flowline and sample chambers from the hydrostatic column have proven to be unreliable at temperatures over 350° F. The presence of gas while sampling exacerbates the problems with elastomers at these temperatures. To overcome these difficulties and to ensure proper sample integrity, the tool uses specially designed metal/metal shear seals in place of elastomers. These shear seals have been proven to operate reliably during testing at 400° F (204° C) and at a differential pressure of 20,000 psi. More than 40 samples have been collected in the Gulf of Thailand, of which most were gas samples. The tool can be configured with two one-gallon H2S-resistant sample chambers with either an air or water cushion.

Besides the specialized electronics, the tool has other design features specific for hostile conditions. One of the most important is that the maximum tool diameter is 31⁄8 in. This reduced size not only allows logging in well bores as small as 4 in. in diameter, but it also reduces the contact area between the tool and the borehole wall. Since wireline formation testers spend time stationary in the well bore, it is important to minimize the chances of differential sticking, particularly in areas where depletion is common. After 2 years and more than 180 successful jobs in the Gulf of Thailand, there has not been a single stuck logging tool that required fishing. Moreover, many of these jobs were testing infill development wells that often encounter depleted intervals. The small diameter of the tool, along with other design features such as built-in standoffs, allow successful wireline conveyance in extremely difficult well geometries. In fact, experience in the Gulf of Thailand has shown that cable tensions are typically lower than resistivity, density-neutron triple combo logs.

When operating wireline tools in difficult well conditions, a surface-controlled releasable cablehead eliminates mechanical weak points and allows pulling to the maximum allowed tension of the wireline cable. In the event of "sticky" wellbore conditions, being able to pull to the maximum safe wireline tension - instead of being limited to the safe margin of a mechanical weak point - can make all the difference in avoiding a lengthy fishing operation. Run in conjunction with the new WFT tool, a 23⁄4-in. wireline cablehead was designed for high-temperature operations and has been run with every job in the Gulf of Thailand. In more than 180 jobs performed to date, the average bottomhole temperature has been in excess of 350° F, with the hottest well reaching a maximum temperature of 398° F (203° C).

There are big advantages to working efficiently in hostile environments, and getting in, taking the measurements quickly and getting out are primary objectives. The tool has been designed to get accurate pressure data rapidly. The quartz pressure gauge, rated to 400° F, is housed in such a way as to expose it directly to the wellbore fluids (Figure 1), which ensures rapid temperature stabilization. Pad deployment takes about 30 seconds, and drawdown is completed in less than 10 seconds. A complete test sequence, as shown in Figure 2, takes appropriately 5 minutes from stopping the winch to moving off the point. During 2004 in the Gulf of Thailand, more than 3,000 pressure measurements were made that took on average less than 6 minutes each to complete the pretest sequence, from pad set to pad retract.

There are many applications for reservoir pressure data; petrophysicists often request multiple pressure tests within an apparent continuous reservoir interval to evaluate the type of fluid it may contain. When an operator is plotting pressure data versus true vertical depth and applying regression analysis, the slope of the best fit line can represent formation fluid density at that depth. The density contrast between gas, oil and water is generally sufficient to identify the major fluid types of a reservoir.

Having additional fluid information is helpful to the petrophysicist during log interpretation, particularly where there is a low resistivity contrast between gas-, oil- or water-bearing formations, or when mineralogy distorts conventional log analysis.

Pressure data is used in other ways, too; geologists look for inter-well correlations and try to infer reservoir connectivity. Reservoir and well service engineers use the pressure data to help design completion strategies as well as input to reservoir models and simulations. Whatever the application, reliable pressure data and sampling capacity are just as important in hostile environments as they are in more benign conditions, but they have been tough to acquire until now.