(1. Dagang Oilfield Petroleum Engineering Research Institute Tianjin 300280 2. Qinghai Oilfield Drilling and Production Technology Research Institute Dunhuang 736002)
This paper introduces a testing technique using geophysical methods, that is, potential method to determine the orientation and length of fracturing fractures. It is a scientific research achievement based on the inherent characteristics of oil (coal) layer and through a lot of indoor and outdoor tests and theoretical research. On the basis of briefly expounding the basic principle, measuring methods and measuring instruments of potential testing technology, the field application effects of Jishi 1 Well and Yanchang Oil Mine 8 1 18 Well are emphatically analyzed, which proves the feasibility of potential testing technology and its important role in the exploration and development of oil (coal) gas fields.
Keywords: potential measuring instrument, process crack monitoring
Dynamic testing technology of coal seam fracturing potential method
Zhang Jincheng 1 Wang Aiguo 1 Wang Xiaojian 1 Ding Na2
1. Danggang Oilfield Company, Tianjin 300280; 2. Qinghai Oilfield Company, Dunhuang, China 736002.
Abstract: This paper introduces an applied geophysical method, which is a new technology to test the orientation and length by using the fracturing test potential of coal seam. In order to achieve the purpose of scientific research, based on a large number of physical models and indoor experiments, a large number of field experiments and theoretical studies have been carried out in view of the inherent characteristics of coal seams. The application evaluation shows that compared with other measurement methods, this measurement technology has higher accuracy and does not destroy product ions. After expounding the basic principle, measuring instruments and methods of testing, the testing data of Jishi 1 well and Wushi 5 3 well are emphatically analyzed. The results show that the potential directional testing technology is completely feasible and more meaningful for coal seam fracturing.
Key words: potentiometric analysis; Measuring instruments; Measurement technology; Coal seam fracturing direction
About the author: Zhang Jincheng, 196 1 born, senior engineer; 1990 graduated from the Geophysical Exploration Department of Chengdu Institute of Geology, and graduated from the Geological Exploration College of Jilin University with a master's degree in engineering in 2002; He has published more than ten academic papers in relevant journals, and won the first prize of Dagang Oilfield Group for the research on cross-well monitoring technology by potential method. Engaged in cross-well monitoring technology research for many years. Tel: 0225925803 (13802162056) E-mail: zjc _ 2056 @ sohu.com.
Research background of 1
After the economic evaluation of recoverable reserves of coalbed methane reservoir, in order to exploit coalbed methane economically, a fracture system must be developed and widely distributed in the coal seam (cleavage plane must be connected with the wellbore), so as to accelerate the extraction and depressurization of coalbed methane and promote its desorption and flow to the bottom of the well. As we all know, the main characteristics of coal seam are: developed cleat and low elastic modulus, and it is extremely difficult for hydraulic fracturing to form and support long cracks in coal seam. In view of this, people often regard hydraulic fracturing as an operation process connecting the wellbore with the cleavage system, but it is still composed of zigzag vertical fractures and horizontal fractures parallel to the direction of maximum principal stress, just like ordinary sandstone after being far away from the wellbore.
In view of the inherent characteristics of coal seam (close to inelastic), the research and experimental work of determining the fracture orientation of coalbed methane wells by surface potential method were carried out during the Ninth Five-Year Plan period. In 2000, based on the ground potential method, the research work of "monitoring technology for determining fractures in fractured wells by dynamic method" was carried out, and the DCT50 dynamic image monitoring system was successfully developed, which can realize real-time and visual dynamic monitoring of the whole fracturing process and further expand the application scope of this method. On this basis, the integrated precision instrument system DDPI-EM was developed in 2008, and two related invention patents were applied. The system can provide controllable signals with high measurement accuracy and strong anti-interference ability, and can be loaded with pseudo-random codes, in which the controllable signals are loaded with pseudo-random codes and then emitted to the depths of coalbed methane wells. When the artificial electric field is tested on the ground, the interference background can be eliminated and the deep low resistance anomalies can be clearly distinguished. Up to now, a complete and unique dynamic method has been formed to determine the fracture orientation of coalbed methane fracturing.
2 test principle and basic formula
Assuming that the stratum is an infinite homogeneous medium, if power is supplied to the stratum with constant current through wires and casings, an artificial electric field will be formed in the stratum. The electric field potential observed at any point M(x, y, z) except the power electrode is as follows:
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
For plane annular measurement, it is only related to well depth h and measuring ring radius r, and the above formula can be rewritten as:
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 (m) from the observation point m to the point source dz.
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.
Comprehensive external electric field potential:
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
It can be seen from Equation (3) that when the observation points m are the same, the generated potential values are different due to the different geometric shapes of the field sources.
In fracturing construction, if the fracturing fluid used is a good conductor of the formation, that is, when the resistivity of the fluid is quite different from that of the formation medium, the logging casing is used to provide high stable current (pseudo-random code modulation) to the formation, and this fracturing fluid can be regarded as a field source in the formation. Because of its existence, the distribution of the original electric field (the ground electric field before fracturing construction) will change, that is, most of the current will be concentrated in the low resistivity body area, which will inevitably cause. In view of this, if several groups of measuring points are annularly arranged around the test fracturing well, and the high-precision potential observation system is used to monitor the change of ground potential in real time during fracturing construction, the purpose of real-time interpretation of fracture extension orientation and other related parameters can be achieved through certain data processing (Figure 1).
Figure 1 Schematic Diagram of Fracture Monitoring
3 measuring instrument system
System overall development scheme (Figure 2): The main design idea of instrument overall design is to adopt the overall system thinking method. Transmitter and receiver are no longer regarded as independent modules, but a unity of mutual work and feedback, which is managed by single-chip computer C805 1F236***. Single chip microcomputer communicates with pc, and finally realizes unified management of computer. The main performance indicators of the final instrument system are as follows:
Maximum output current: 20A
Maximum output voltage: 500V v.
Steady flow accuracy: within 65438 0% (within 20% of load change and within 20% of input change);
Frequency stability: 0.01%;
Input impedance: 80MΩ
Resolution:1μ v;
Accuracy of potential measurement: better than 0.5%;
The dynamic monitoring range is 2V.
Figure 2 Overall development scheme of the system
4 methods and skills of field work
4. 1 Layout of measuring points and lines
The layout of measuring points is centered on Well A, and a multi-ring measuring point is set radially inside (N), middle (COM) and outside (M), and the included angle between the measuring points is 15. The radius of the measuring ring can be measured by theodolite or infrared rangefinder, and the position of the measuring point should be clearly marked to ensure that there is no geometric error in the two measurements. After the measuring points are laid, the measuring network is laid, and the measuring electrodes, measuring wires and power supply wires are buried or laid in advance in the conditional areas, which is an important aspect to ensure the measuring accuracy (Figure 3).
Fig. 3 Layout of measuring points and lines
4.2 Selection of Wells
An artificial electric field is formed around fracturing well A, and another well B is selected around A to form a closed loop with fracturing well A. Generally, the distance between two wells AB should be greater than the depth of fracturing interval of well A, but not too small. Improving the high surface current density between AB is beneficial to improving the resolution of abnormal charging. Generally, the following principles are followed for selection: (1) ab d > Fracture horizon depth H(m), (2) Well B depth HB ≥ Well A fracture horizon depth H(m).
4.3 Reduce the resistivity of fracturing fluid
The greater the difference between the resistivity of fracturing fluid and the resistivity of surrounding rock medium in fractured interval, the more favorable it is for abnormal display. In order to achieve this goal. In fracturing construction, it is necessary to add conductive metal salts into fracturing fluid. Usually, 3% salt can be added to fracturing fluid to meet the requirement of conductivity difference.
4.4 Construction process
The main construction steps are as follows: ① according to the construction design, the measuring points (the included angle is generally 15, and the number of measuring rings depends on the geological task), measuring lines and power supply lines are arranged; (2) Select the parameters of the sending and receiving system (such as code width and code length) and debug them to meet the measurement accuracy required by the design; (3) At the same time as the liquid injection construction, the test work starts until the end of the liquid injection construction.
4.5 data processing
In the actual data processing, we choose the "abnormal method" to process the data. Considering the change of power supply current, it is necessary to normalize the potential difference data measured before and after injecting working fluid. Namely:
Technical progress of coalbed methane in China: 20 1 1 Proceedings of the symposium on coalbed methane.
Where: US is the standard apparent pure anomaly (MV/A); UQMN and UHMN are potential difference data (mV) measured before and after injection of working fluid; IQ and IH are the supply currents (a) before and after the injection of the working fluid.
After data processing, the apparent anomaly curve and ring diagram are given. In the apparent anomaly map, the abscissa represents the azimuth angle of the measuring point, and the ordinate represents the apparent anomaly value; In the annular chart of apparent anomalies, the points are recorded, and the azimuth of the test points is marked outside the annular chart. True north (n) is 0, rotate clockwise, 90 is due east (e), 180 is due south (s), and 270 is due west (w).
5 Field application example
5. 1 well test
Jishi 1 well is a CBM exploration and evaluation well deployed by the CBM Project Management Department of Jixian County, Daning County, Shanxi Province. Its geographical location is 200 meters west of Pitiaogou Village, Puxian County, Shanxi Province, and its structural location is in Guyi anticline in Rao fold belt in the east of Ordos Basin. In order to determine the extension direction of fracturing cracks in Jishi 1 well, the CBM project management department entrusted Dagang Oilfield Drilling and Production Institute to test the direction of fracturing cracks in 8# coal in this well. As can be seen from Figure 4 to Figure 6, the apparent anomaly curve of Us has changed for nearly two periods within the range of 360, and the minimum value corresponds toNo. 16 (N45°E) and No.,so it is considered that the fractures formed by fracturing construction are symmetrical fractures with different lengths. According to the inversion calculation, the crack length in the direction of 16 (N45°E) is 89m, and that in the direction of No.4 (S45°W) is 66m (Figure 6).
5.2 Well Wushi 5-3 Test
Fig. 4 apparent anomaly curve of No.8 coal in Jishi oilfield 1 well at 80 100.
Fig. 5 Apparent pure anomaly curve of Jishi 1 Well 8 coal 100 120m.
See Figure 7 for the well location of Well Cluster Wushi 5. The fracturing fracture test was carried out in Wushi 53 well in this oilfield. The construction data of exploration wells in the block where Well Cluster Wushi 5 is located show that the extension pressure gradient in this block changes greatly, and some wells have high extension pressure gradient, especially the central well Wushi 5, with the extension pressure gradient as high as 0.044MPa/m, even as high as 0.05MPa/m in the pre-fluid stage, which reflects the heterogeneity of regional coal seams. On the other hand, the overall evaluation is: ultra-low porosity and ultra-low permeability, and the upper and lower interlayers of the target layer have certain stress shielding effect; The extension pressure gradient changes greatly, some wells have high extension pressure gradient, and many cracks in coal seam are highly developed, which makes fracture extension difficult.
Fig. 6 Test results of 8# coal in Jichang Oilfield 1 Well
Fig. 7 Location Map of Well Group Wushi 5
See fig. 8 ~ fig. 10 for the apparent anomaly curve obtained from field test data after data processing. ① The apparent anomaly curve has changed for nearly two cycles within the range of 360, and it is considered that symmetrical two-wing unequal-length fractures were formed during fracturing construction, and the azimuth of the fracture center was 30 and 210, of which 60 was a long fracture (Figures 8 and 9); ② Through simulation calculation, the crack length in 30 direction is 79.96m, and that in 2 10 direction is 60.97m (see figure10).
Fig. 8 Apparent Pure Abnormal Curve of Well Wushi 5-3
6 conclusion
It is of practical significance in production and scientific research to study and determine the orientation of hydraulic fracturing fractures in oil (coal) layers by geophysical methods, and the research results also open up a new field for potential methods. Based on the basic theory of charging method, combined with the actual situation, the mathematical model is analyzed reasonably, and the physical simulation test is carried out systematically. For example, according to a set of on-site working methods and techniques and the developed dynamic observation system, it can be successfully used to determine the dominant orientation of fracturing fractures with a buried depth of less than 3000m, and predict the fracture length accordingly, which is not only of great guiding significance for studying the fracturing technology effect and formulating a reasonable and economic development plan, but also has certain reference value for solving other similar engineering problems and has broad application prospects.
Fig. 9 Annular diagram of apparent pure anomaly in Well Wushi 5-3.
Figure 10 Isogram of Fracture Length of Well Wushi 5-3
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