Borehole geophysics

Geophysical exploration in a well, or drilling, refers to various geophysical exploration methods that put receiving sensors into a well to collect physical data, so as to obtain useful information about some hidden targets around the well.

Different from the above logging methods, the exploration scope of borehole geophysical exploration is a large space around drilling, between drilling or under drilling. Its specific scope depends on the geophysical methods and techniques used and the detection target state. At present, it is generally tens to hundreds of meters in the horizontal direction of wellbore or vertical direction of bottom hole.

It should be noted that some active source electricity methods and elastic wave methods have the working mode of putting the excitation source in the well and collecting relevant data on the ground, that is, the "well-ground" working mode. According to our classification principle, they should belong to surface geophysical exploration. In fact, some geophysical methods, such as charging method, contact polarization curve method and inverse vertical seismic profile method, which mainly work in well-ground mode, are usually regarded as surface geophysical methods by Chinese geophysicists. There are also some geophysical methods, such as induced polarization method and acoustic perspective method. Many geophysicists in China are used to taking their well-to-ground working methods and their well-to-well and well-to-well working methods as well-to-well geophysical methods. On the other hand, the vertical seismic profiling method, which is listed as the borehole geophysical exploration method, is often regarded as a part of the surface seismic method by seismologists.

Application development

In China, borehole geophysical prospecting is first used for metal mineral exploration. 1958 The single-component magnetic survey conducted by the geological department in Xiuyu lead-zinc mine area of Datong, Liaoning Province should be the beginning of borehole geophysical exploration in China. Since then, with the large-scale development of iron ore prospecting in China, borehole magnetic survey method has been widely used nationwide in the 1960s and 1980s, with hundreds of instruments and thousands of survey boreholes, with outstanding results. Since the mid-1980s, with the vigorous development of coalfield seismic work, vertical seismic profiles and cross-well seismic methods have also been applied in the field of coal exploration. Geophysical prospecting workers in China have also made a preliminary study on the inversion algorithm of cross-hole electromagnetic imaging. Geophysical prospectors in China have also systematically discussed the borehole transient electromagnetic method in the monograph on transient electromagnetic method.

Vertical seismic profiling method is mainly used in oil and gas exploration in China, but also in coal and engineering exploration. It plays an important role in providing elastic mechanical parameters of strata, improving the quality of surface seismic data processing and interpretation, studying the geological profile near wells and predicting the reflection level in front of drill bits. Vertical seismic profiling began to develop abroad in 1970s, and soon attracted the attention of geophysicists in China. From 1983 to 1984, our geological and petroleum departments used imported borehole three-component geophones, which were first tested in Jiangsu, Central Plains and South China Sea, and then quickly popularized and applied in the whole country. 1988 China geophysicists compiled and published relevant monographs [15]. Our factory of petroleum geology department has produced various types of underground three-component geophones, which meet the needs of field work. Geophysical prospectors in China have made many valuable progress and achievements in data processing and inversion interpretation of vertical seismic profiles, including migration processing, wave field separation, directional well data processing, anisotropic medium data processing and interpretation, and joint interpretation of P-wave and S-wave [68 ~ 76]. Geophysical prospectors in China have also made a preliminary attempt to improve the data quality of vertical seismic profiles by using artificial electric fields [77].

The research on cross-well seismic methods abroad began in 1970s, and it was not until 1980s that it developed with the solution of technical problems such as underground seismic source and the rise of geophysical tomography technology. It can provide images of cross-well strata, strata, reservoir characteristics and structures with much higher resolution than ground earthquakes, so it has been paid more and more attention. At the end of 1980s, China began to carry out experimental research on cross-well seismic methods and techniques. In the late 1980s and 1990s, cross-well seismic method made some important achievements in China engineering field, such as foundation exploration of large or high-rise buildings. From 1994 to 1995, it was used to study the oil and gas reservoirs in Jilin, Liaohe and Shengli oilfields, and then it was applied in other oilfields. Various sources are used in the work, such as electric spark source, hammer source, underground special explosion source and underground hydraulic controllable source. Geophysical prospecting workers in China have made some useful experiences or research achievements in data acquisition technology, cross-well observation system design and acquisition system improvement [78 ~ 82]. The so-called "three-dimensional seismic method" has also achieved good results by comprehensively applying cross-well seismic and vertical seismic profiles, inverse vertical seismic profiles and ground earthquakes [83].

The practical application history of cross-well seismic method in China is still short, but as an important part of geophysical tomography technology, China geophysicists have paid full attention to it since the late 1980s, and have done a lot of work in theory, methods and software. He has written and published monographs and published many related expositions, involving the methods and algorithms of ray tomography and wave equation tomography, such as bending ray, shortest path, maximum entropy, cascade, transmission, reflection, refraction, longitudinal wave and shear wave, among which there are many creative progress and research results [84 ~ 108].

4. Gravity method in well

The development of borehole gravimetry mainly depends on the manufacturing technology of borehole gravimeter. Since 1966, when the first practical borehole gravimeter was developed abroad, although some new products have come out, it can only be used for oil and gas drilling because of its large outer diameter. It is mainly used to determine the macro density of the formation around the well, and then obtain the porosity and the development of fractures and caves that are not affected by the invasion of mud filtrate. 199 1 year, China's Ministry of Petroleum introduced the downhole gravimeter of lacoste Company of the United States, and conducted deep well survey in Chongqing, and made a preliminary analysis and study on its effect and limitations [17, 18].

(3) Evaluation and gap

In China, borehole geophysical exploration started earlier, which attracted the attention of geophysicists in different fields and relevant departments, and made active efforts in expanding application and developing technology. The technology and application of borehole electromagnetic wave method and borehole induced polarization method in China are at the advanced level in the world, and the technology and application of borehole three-component magnetic survey were once in the leading position in the world in the 1970s and 1980s. China has also made many advanced achievements in the research on the theory and method of electromagnetic wave, electromagnetic field and elastic wave tomography.

As the main way of geophysical exploration extending to deep and three-dimensional space, the development of borehole geophysical exploration in China is generally not ideal. Especially in the past decade, the gap between China and the international level has increased in some aspects. The development of borehole three-component magnetometer in China has been stagnant for a long time, and it still stays at the low precision level twenty years ago, while there are high-precision products abroad. In-well transient electromagnetic method has become a powerful tool to find large concealed ore bodies with deep depth and good conductivity in old mining areas abroad, and many types of three-component deep well probe high-power transient electromagnetic systems have been produced, but at present, only some shallow single-component work can be carried out in China. The practical borehole electromagnetic wave method in China still stays at the stage of using only amplitude parameters. Geophysical exploration in wells is inseparable from drilling. Due to the reasons of understanding and management, the non-technical difficulty of using geological exploration drilling in metal mines in China has increased in recent years, which has also affected the development of geophysical exploration technology in wells suitable for this field.