Formation testing tools
1. Importance of Formation Testing
In petroleum exploration, drilling is the most direct exploration method. By using formation testing tools to test drilling, cementing, completion and well-testing operations, operators can determine the reserves of an oilfield, understand reservoir characteristics, and establish a reasonable development plan. The process from drilling to well testing is generally time-consuming and costly. Running casing and cementing are especially expensive, and cost much more than drilling alone. In many exploratory wells, when well-test results do not reach commercial production value, many wells are abandoned, which wastes substantial capital and delays the exploration schedule. To change this situation, countries around the world have carried out extensive research on formation testing tools, and these tools have already been applied in actual exploration work. In recent years, they have also been gradually promoted and used in China.
Formation testing tools can be used for open-hole testing and cased-hole testing on land and offshore. Cased-hole testing is not the main application and is relatively simple, so it is not described here. Open-hole testing plays a central role in exploration and has the following advantages:
1. Testing can be carried out promptly, avoiding long-term soaking of the oil zone by drilling mud, which may contaminate or plug the reservoir, or avoiding plugging of a low-pressure oil zone caused by cementing. This makes it easier to identify low-pressure oil zones and obtain more accurate data.
2. Testing is fast. When oil or gas shows are encountered during drilling, testing can be performed immediately. Under normal conditions, the required data can usually be obtained in one to two days. Drilling can then continue, or after the whole well is drilled, the oil and gas show intervals can be tested in stages to obtain data for each interval. This is much faster than running casing and then conducting a well test.
3. A large amount of casing, cement and unnecessary working time can be saved. After drilling is completed, oil and gas intervals determined by electric logging are sometimes not very accurate. In some cases, a water zone is mistaken for an oil zone and the well is completed. If a formation tester is used, the error in electric logging can be corrected and exploration cost can be greatly reduced.
The disadvantages of formation testing tools are relatively high manufacturing cost and the possibility of sticking during open-hole testing. These shortcomings can be continuously improved through extensive field practice.
2. Structure and Working Principle of Formation Testing Tools
Formation testing tools mainly consist of surface control equipment and downhole tools. The downhole tool assembly is run into the well on drill pipe. The drill pipe serves both as the connecting member and as the fluid-flow passage, linking the downhole tool with the surface control equipment. The functions of the surface control equipment are: 1. to actuate the downhole tool. Through controlled rotation, lifting or lowering of the drill pipe, the test valve in the downhole tool is opened or closed, allowing formation liquid or gas to enter the drill pipe and flow to the surface. 2. to serve as a temporary oil or gas production wellhead assembly. 3. to allow necessary tools to be dropped into the drill pipe. The surface equipment mainly includes
a Well Test Flowhead, Drill Floor Manifold, indicating head, dropping device and Hose Loop.
The main functions of the downhole tool assembly are:
1. To anchor the testing tool, especially when testing is carried out in the upper part of the wellbore.
2. To isolate the producing interval, separating the target zone from other formations so that gas and liquid in the target zone enter the testing tool.
3. To control the flow direction of drilling mud and formation fluid, so that the testing tool is accurately positioned and tested until it is completely pulled out to the surface.
4. To measure and record the flowing pressure, formation pressure and temperature of the oil zone, and to recover a certain amount of formation-fluid sample under downhole conditions.
5. To prevent the testing tool from sticking, and to provide fishing or releasing tools for freeing a stuck assembly.
The downhole assembly mainly consists of an anchor pipe, open-hole packer, reverse-circulation valve, multi-flow tester, open-hole bypass valve, safety seal, pressure and temperature recorders, hydraulic jar, safety joint and related components.
The working process of the complete formation testing tool is as follows. After drilling is completed, the testing tool is run into the well. When it reaches the predetermined depth, the surface equipment is connected and anchoring is performed while the packer is set. After the formation is isolated, conventional well-testing operations such as well washing and induced flow are carried out, and the required reservoir parameters, such as production rate, are measured. The downhole instruments begin operating at the same time. After the operation is completed, drilling mud is circulated again; the packer is unset and the anchor is released; then the downhole tools are pulled out. If resistance is encountered or the tool becomes stuck and cannot be pulled out, jars or other releasing tools can be used to free it. After the drilling string and tools are completely recovered to the surface, the testing job is completed.
3. Performance Specifications of Formation Testers
In recent years, some formation testers have been imported from abroad, mainly products from Johnston and Lynes in the United States. The main performance of these two types of formation tester is basically similar, but there are certain differences in operating method and in parts of the structure. The following introduces the main performance specifications of the Johnston formation tester:
I. Surface Equipment
1. Working pressure: 5,000, 10,000 and 15,000 psi
(350), (700) and (1050) kg/cm2
2. Test pressure: 10,000, 15,000 and 20,000 psi
(700), (1050) and (1400) kg/cm2
3. Working medium: no hydrogen sulfide (H2S)
4. Minimum bore: 1-1/2 in. (flexible flowline)
5. Working load: 350,000 lb (158 tons)
II. Downhole Tools and Instruments
1. Applicable range: open hole or cased hole
2. Applicable borehole sizes: 5 in. to 6-1/4 in.
5-3/4 in. to 7-3/4 in.
7-1/2 in. to 10-3/4 in.
9-5/8 in. to 12-1/4 in.
3. Pressure Recorder Specifications
Maximum pressure
1,600, 2,800, 4,700, 6,400, 9,000, 14,000 and 20,000 psi
Clock running time: 24, 48, 96 and 192 hours
4. Thermometer Specifications
60 to 220 F (15.55 to 104.44 C)
100 to 300 F (37.78 to 148.89 C)
200 to 400 F (93.33 to 204.44 C)
