(China United Coalbed Methane Co., Ltd. Beijing 1000 1 1)

Abstract: downhole microseismic monitoring technology and potential monitoring technology are used to monitor fracture shape in real time during fracturing. The results show that underground microseismic monitoring can effectively explain the orientation, height, length and symmetry of fractures and their extension with time. Potential testing technology is suitable for large-scale fracturing, especially for large-scale hydraulic fracturing in shallow wells. The monitoring results of the two technologies on the same well show that fracture monitoring can effectively reflect the horizontal trend of fracturing fractures and help to understand the distribution of formation stress in this area, but the vertical expansion can only reflect the frequency of in-phase axis, and can not effectively analyze and monitor the height and width of fractures.

Keywords: microseismic potential method for monitoring coalbed methane by fracturing fractures

Application of fracture monitoring technology in fractured wells of coalbed methane reservoir

Zhang Jian

(China United Coalbed Methane Co., Ltd., Beijing 1000 1 1, China)

Abstract: Underground microseismic monitoring technology and potential monitoring technology are used to display the real-time geometry of fractures. It shows that downhole microseismic monitoring technology can explain the orientation, height, length, symmetry and extension of fractures. Potential monitoring technology is suitable for large-scale fracturing, especially for shallow wells. The monitoring results of the two methods on the same well show that this method can effectively reflect the orientation of horizontal fractures and is conducive to understanding the stress distribution. However, the frequency of rupture can only be characterized in the vertical direction. The height and width of cracks cannot be effectively analyzed.

Keywords: fracturing; Crack monitoring; Coalbed methane; Microseism; potentiometric titration

Fund Project: National Science and Technology Major Project 42 "Technical Research and Equipment Development of CBM Development in Deep Coal Seam" (201kloc-0/zx05042).

About the author: Zhang Jian, born in 198 1, Ph.D., graduated from China Youshi University (Beijing) with a doctorate in 2009; Mainly engaged in coalbed methane development and modern completion engineering research. Address: (1000 1 1) No.88, Anwai Street, Dongcheng District, Beijing. E-mail: Zhang Jian @ chinacbm. com .

1 Introduction

At present, China's coalbed methane development mainly adopts fracturing to improve oil recovery, and the optimal design of fracturing parameters is very important for improving fracturing scheme and increasing single well productivity. The previous fracturing scheme was mainly shallow and experienced. With the increase of coal seam depth, it is necessary to establish a fracturing parameter combination suitable for deeper coal seam. Using downhole microseismic monitoring technology and potential monitoring technology, the construction fracture morphology under the existing fracturing scheme is monitored in real time, which provides technical support for further improving coalbed methane fracturing technology.

2 test principles

2. 1 downhole microseismic testing principle

The downhole microseismic testing method is to monitor the vertical fracturing operation of adjacent wells, monitor the microseismic events generated during fracturing by using the downhole three-component seismic imaging system, and interpret the collected downhole three-component microseismic data to obtain the spatial distribution (azimuth and length) of fractures formed by fracturing [1 2].

2. 1. 1 origin of microseisms

Due to the influence of pressure, microseisms originated in a certain area around hydraulic fractures. Microseismic events in this area include: microseismic events induced by stress change at crack tip, microseismic events induced by fluid filtration and microseismic events induced by weak strata.

2. 1.2 Determination of the distance between microseismic points

Due to the change of stress state, the stratum produces shear slip, which induces compression wave (P wave) and shear wave (S wave). P wave travels faster than S wave. With the increase of propagation distance, the time difference of first break wave increases. The three-component detector is used to receive shear waves and compression waves that can distinguish different components, so as to determine the occurrence distance of microseismic points.

2. 1.3 Determination of microseismic orientation

The amplitude crossplot method is adopted, that is, the amplitude crossplot of the first wave of P wave is established to determine the direction of microseismic source, and the propagation direction of compression wave is consistent with the vibration direction. By tracking the vibration of particles in a period, the propagation direction α can be determined, as shown in figure 1. The field test system includes data recording system, SeisNet workstation and quality control system, which is used to save and analyze data, as shown in Figure 2.

Figure 1 Schematic diagram of microseismic orientation determination

Figure 2 Schematic diagram of test system

2.2 Principle of potential test

Potential monitoring technology is based on the basic theory of conductivity exploration, and parameters such as fracture orientation, length and shape can be obtained by monitoring the changes of ground electric field caused by fracturing fluid injection into the target layer [3,4].

Assuming that the stratum is an infinite homogeneous medium, the electric field potential observed at any m point outside the power supply electrode is:

Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.

Where: ρ is the formation apparent resistivity, ω·m；; I is the power supply current intensity, a; H is the depth of the test target layer, m; R is the distance between the observation point m and the point source, m.

When the field source is arbitrary, a bin ds should be drawn at the field source to calculate the external electric field potential. If the current density at ds is j, the current flowing from ds is jds, and the potential dUM generated at observation point m can be written as:

Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.

The comprehensive external electric field potential is:

Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.

The instrument system used in the field test consists of four parts: measuring system (theodolite), power supply system (ZT7000 generator), sending system and receiving system (HGQ-5/ 10kW/Js-03 transceiver system), as shown in Figure 3.

Fig. 3 schematic diagram of test device

3 Field application and evaluation

Five wells in the construction area of Qinshui basin in Shanxi Province were monitored by potential method, and three wells were monitored by underground microseisms. Monitoring by potential method shows that a group of unequal-length fractures are formed during fracturing, and their wings are basically symmetrical or slightly inclined. As shown in Figure 4, the anisotropy of formation permeability and the complexity of tectonic stress are the main factors causing this phenomenon. The monitoring results of nitrogen injection experimental well show that nitrogen and other gases in coal seam still exist in molecular form because of their inactive chemical properties, so the conductivity of coal reservoir is basically unchanged, and it is difficult to monitor its distribution in coal reservoir by potential method.

The monitoring results of underground microseisms show that the faults extend in two directions and are asymmetrical, and most of the monitored microseismic events are located in the strata above the coal seam, with a wide range of microseismic events, as shown in Figure 5.

4 conclusion

(1) The monitoring of underground microseisms has effectively explained the orientation, length and symmetry of cracks and their extension with time.

(2) Potential testing technology is suitable for large-scale fracturing, especially for large-scale hydraulic fracturing in shallow wells.

(3) The monitoring results of the two technologies on the same well show that fracture monitoring can effectively reflect the horizontal trend of fracturing fractures and help to understand the distribution of formation stress in this area, but vertical expansion can only reflect the frequency of events, and can not effectively analyze and monitor the height and width of fractures.

Fig. 4 Horizontal Projection of Monitoring Cracks by Potential Method

Fig. 5 microseismic monitoring fracturing fracture profile

refer to

Xia, Wang, and so on. Study on fracture and stress distribution characteristics of coal and rock based on high-precision microseismic monitoring [J]. Journal of Coal Science, 36 (2): 239 ~ 243.

[2] Xu Jianping. 20 1 1. Research and application of crack monitoring method [J]. Science and engineering,11(1): 2575 ~ 2577, 25865438+.

Wang xiangzeng. 2006. Application of well-ground potential method in fracture monitoring of coalbed methane wells [J]. Coal Engineering, 5: 36 ~ 37.

[4] Guo Jianchun, Li Yongming, et al. 2009. Research and application of potential fracturing testing technology [J]. Petroleum Geology and Engineering, 23 (3): 127 ~ 129.