*Corresponding author. Keywords: Shale, Rock drillability, Anisotropy, Drilling, Acoustic travel time.

Transcription

1 2017 International Conference on Energy, Power and Environmental Engineering (ICEPEE 2017) ISBN: Quantitative Prediction of Rock Drillability in Various Drilling Directions in Shale Formation Hui ZHANG 1, Ming-kun ZHAO 2, Zheng SONG 2, Xia MIAO 1, Shuo WANG 1 and De-li GAO 1 1 College of Petroleum Engineering, China University of Petroleum, Beijing, China; 2 Sinopec Chongqing Fuling Shale Gas Exploration and Development Company, Chongqing, China. *Corresponding author Keywords: Shale, Rock drillability, Anisotropy, Drilling, Acoustic travel time. Abstract. As a typical bedding formation, the rock mechanical properties of shale are strongly anisotropic. Quantitative evaluation of shale formation drillability in various directions is essential for drilling bit selection, drilling parameters optimization and directional well trajectory design. Based on drillability testing device and rock acoustic velocity measurement apparatus, we carried out drillability anisotropy and acoustic characteristic experiments in various drilling directions. The experimental results showed that 1) shale formations have strong drillability anisotropy and acoustic anisotropy characteristics; 2) the drillability at perpendicular to the bedding plane direction is better than that parallel to the bedding plane; 3) the acoustic travel time at perpendicular to the bedding plane direction is larger than that parallel to the bedding plane and the peak value appears at 60 to the bedding plane. Mathematical statistics methods have been applied to established a shale formation drillability forecasting model at various drilling directions, with an average relative error as low as 4.9%. This model provides an important approach to quantitative assess shale formation drillability in various drilling directions using logging data, and has vital importance for the efficient development of shale gas. Introduction Shale gas development has become a hot topic in unconventional reserve development. As a typical bedding formation, the rock mechanical properties of shale are strongly anisotropic. J. B. Cheatham qualitatively points out that drillability will change significantly while drilling in different directions in shale formations [1]. By introducing certain assumptions, Carol A. Tosaya established acoustic anisotropy model for shale formations in Cotton Valley area according to Hooke's law [2]. O. Cazacu established the anisotropic model of shale formations using elastoplastic constitutive equation [3]. C. Jones conducted acoustic velocity and permeability experiments to study the anisotropy of shale formations [4]. J. Schön et al. studied the effect of clay content in shale formation to its acoustic anisotropy [5]. N. Dyaur et al. studied the correlation between X-ray images and acoustic anisotropy of Barnett Shale [6]. Pan Qifeng et al. studied the correlation between the rock acoustic characteristics and drillability anisotropy index [7,8]. Currently, the quantitative predictions of shale formation s drillability at various drilling directions are rare. Based on drillability testing device and rock acoustic velocity measurement apparatus, the drillability anisotropy and acoustic characteristic experiments are carried out at various drilling directions. Mathematical statistics methods have been applied to analyze the correlation between rock drillability index, the acoustic travel time and drilling directions, and thus established a shale formation drillability forecasting model at various drilling directions, with an average relative error as low as 4.9%. This model provides an important approach to quantitatively assess shale formation drillability at various drilling directions using log data, and has vital importance on drilling bit selection, drilling parameters optimization and directional well trajectory design. 192

2 Shale Formation Drillability Experiments Rock Rock Drillability refers to the degree of difficulty to crush a rock; it can be understood as the ability of the rock to resist the broken of a drilling bit under certain conditions [10]. It is well known that rock drillability may vary due to different working conditions. Therefore, we try to fix some of the working conditions, then use "Rock " to quantify the degree of difficulty to crush rocks. According to Chinese Ministry of Petroleum Industry standards, this study uses the following fixed operating conditions: Micro drill diameter: mm; Drilling bit Type: PDC bit. Weight on bit: 500 ± 20 N Rotary speed: 55 ± 1 r / min. Drilling depth: 3mm. Rock is defined as follows: Kd log 2 T (1) Where: K d is Rock, dimensionless; T is the time used to penetrate the rocks. Equation (1) shows that a larger value of Kd indicates it is harder to break the rock. Kd reflects the characteristics of the rock to resist breaking. Experimental Apparatus The drillability experimental apparatus used in this study is shown in Figure 1. The drill bit is a 31.75mm diameter PDC type bit. The contact force at the drill bit can be controlled within an error of less than 20N. The rotary speed of the drill bit can be controlled within an error of less than 1 rpm. Figure 1. Testing device for rock drillability. (Note:1. Rock sample; 2. micro-bit; 3. cutting tray; 4. turbine rod; 5. lever; 6. weight; 7. meter for measuring depth; 8. bar with thread for adjusting lever; 9.worktable; 10.compaction bar with thread.) Procedure The test procedure involves preparing core samples and drilling the core samples. The following procedure was followed in the experiments. 1) As shown in Figure 2, sampling shale rocks at different drilling directions (i.e., when the angles between drilling direction and bedding normal direction were at 0, 15, 30, 45, 60, 75, 90 respectively) 2) Cut core sample to 50 mm long and polish its test surface. 3) Install the core sample in the core holder. 4) Set rotary speed to 55 rpm. 193

3 5) Drill the core sample for a depth of 2.4 mm with a force on rock of 500 N. 6) Record the actual depth of penetration and drilling time. 7) Use equation 1 to calculate the rock drillability index of each sample. Figure 2. Sampling Shale Rocks at Different Drilling Directions. Results and Analysis The testing results of the core samples rock drillability obtained in laboratory are shown in Table 1 and Figure 3. Table 1. Shale at Various Directions. Drilliing Direction ( ) PDC Bit Drilling Time (s) Through the analysis of the above data, the relationship between shale drillability index and drilling direction is obtained, as shown in Figure 3. PDC Drilling Bit Drilling Direction / Figure 3. Shale Versus Drilling Direction Figure 3 shows that: (1) shale samples have strong drillability anisotropy. When drilling in 194

4 different directions, the shale samples have different drillability index. (2) The drillability index at the direction perpendicular to the bedding plane is lower than that parallel to the bedding plane (i.e., the drillability at the direction perpendicular to the bedding plane is better than that parallel to the bedding plane), hence the drillability anisotropy is very strong. Shale Acoustic Velocity Experiments Testing Method With the method of making the ultrasonic pulse penetrating through a rock sample, the acoustic travel time Δt can be measured in laboratory. The ultrasonic testing system used in laboratory is shown in Fig.4, in which the ultrasonic transducers can provide a frequency of 1 MHz and the butter and honey can be used as its coupling media. The pulse generator can generate electric pulses with a strength range of V. The width and iteration frequency of the electric pulse can be adjusted and controlled. During testing, the signal generator makes an electric pulse signal which will trigger the energy exchanger to generate ultrasonic pulses. The ultrasonic pulses (acoustic waves) propagating through the rock sample are incepted by the reception end of energy exchanger. Finally, the propagation time and the signal strength of the ultrasonic pulses (acoustic waves) through the rock sample are logged by a digital memory oscillograph. In order to reduce the errors from the artificial operations, the emission end of the energy exchanger is aimed at its reception end as accurately as possible during testing. Before each test, the ultrasonic testing system should be calibrated using the aluminum rod to ensure the accuracy of the test results. Testing for each point of a rock sample is conducted for three times in the actual testing. The average value of the test data of three times for each point is taken as a final test result for the point of a rock sample. With the test data, the acoustic wave time may be calculated by the following equation: t t l 1000 (2) Where: Δt is the acoustic travel time; l is length of the rock sample, mm; t is propagation time of the acoustic wave, μs. Figure 4. The Ultrasonic Testing System. Results and Analysis Acoustic velocity experiments have been conducted on shale samples from various drilling direction, and the results are shown in Table 2. Table 2. Shale Acoustic Travel Time at Various Drilling Directions. Drilliing Direction ( ) Acoustic Travel Time (μs/m) The relationship between shale acoustic travel time and drilling direction is shown in Figure

5 Acoustic Travel Time (μs/m) Drilliing Direction ( ) Figure 5. Shale Acoustic Travel Time versus Drilling Direction. It can be seen from Figure 5 that shale samples have strong drillability anisotropy. Acoustic travel time at perpendicular to the bedding plane direction is larger than that parallel to the bedding plane direction, the peak value appears at 60 to the bedding plane direction. Shale Formation Drillability Forecasting Model at Various Drilling Directions The Establishment of the Model. A correlation between shale drillability index, acoustic travel time and drilling direction angle can be obtained through nonlinear regression analysis of the experimental data in Table 1 and 2: K d ( ) t p (3) Where: Kd(φ) is shale drillability index under different drilling directions, dimensionless; φ is angle between sample axis and bedding surface normal direction, ; Δtpφ is acoustic travel time under different drilling directions, μs/m. A correlation between acoustic travel time at different drilling directions, acoustic travel time in the direction of perpendicular to the bedding plane and drilling directions can be obtained through nonlinear regression analysis of the experimental data in Table 2: t p t p 0 (4) Where: Δtp0 is acoustic travel time in the direction of perpendicular to the bedding plane,μs/m; By substitute equation (4) into equation (3) the following formula can be obtained: K d ( ) t p (5) Equation (5) is the quantitative forecasting model of shale formation drillability at different drilling directions. This model can be used to predict shale drillability at various drilling directions using acoustic travel time at perpendicular to the bedding plane direction. This study provides an important basis for using logging data to assess shale formation drillability at different drilling directions. Validity Test of the Model. Laboratory experiments data are used to test the accuracy of the prediction model, the results are shown in Table

6 Drilliing Direction ( ) Table 3. Validation Results. (Experiment data) (Model Prediction) Relative Error (%) Table 3 shows that: when the drilling direction is 0, the relative error of the forecasting model is pretty large and reached 17.9%, when at other drilling directions the relative errors are small. The overall average relative error is 4.9%; thus, the forecasting model established in this paper has high accuracy. Conclusions Based on drillability testing device and rock acoustic velocity measurement apparatus, the drillability anisotropy and acoustic characteristic experiments are carried out at various drilling directions, the main conclusions are as follows: 1) Shale formations have strong drillability anisotropy. The drillability at perpendicular to the bedding plane direction is better than that parallel to the bedding plane; 2) Shale formations have clear acoustic anisotropy characteristics. The acoustic travel time at perpendicular to the bedding plane direction is larger than that parallel to the bedding plane and the peak value appears at 60 to the bedding plane direction. 3) Mathematical statistics methods have been applied to establish a shale formation drillability forecasting model at various drilling directions, and the validity test shows the average relative error is 4.9%. This model provides an important approach to quantitative assess shale formation drillability at various drilling directions using logging data, and has vital importance on drilling bit selection, drilling parameters optimization and directional well trajectory design. Acknowledgement The authors gratefully acknowledge the Natural Science Foundation of China(Grant No ,Grant No ), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No ), National Oil and Gas Major Project (Grant No. 2017ZX05009),the State Key Laboratory of Petroleum Resource and Engineering. References [1] A Theoretical Model for Directional Drilling Tendency of a Drill Bit in Anisotropic Rock. J. B. Cheatham, Jr. and C. Y. Ho.1981:1~12. [2] Acoustical Anisotropy of Cotton Valley Shale. Carol A.Tosaya.SPE ~13. [3] Elastic/Viscoplasctic Constitutive Equation for Anisotropic Shale. Cazacu, J.F. Shao & J.P. Henry. Rock Mechanics, Aubertin Hassani & Mitri. 1996:1683~1690. [4] An Experimental Study of Elastic Wave Propagation Anisotropy and Permeability Anisotropy in an Illitic Shale. C. Jones. SPE/lSRM 47369:307~313. [5] Elastic Wave Anisotropy and Shale Distribution. Schön, J. Georgi, D. and Tang, X. SPWLA 46th Annual Logging Symposium.2005:

7 [6] Velocity Anisotropy and X-Ray Imaging of Barnett Shale. N. Dyaur, G. Kullmann, A. Ortiz, V. Pena and E. Chesnokov. SEG Las Vegas 2008 Annual Meeting. 554~558. [7] Model To Evaluate Anisotropy Of Rock Drillability. Pan Qifeng and Gao Deli. Natur. Gas Ind. v. 25, no. 10, pp ,10/25/2005. [8] Experimental Study on Evaluation of Formation Drillability Anisotropy by Acoustic Wave Velocity. Pan Qifeng and Gao Deli. Chinese Journal of Rock Mechanics and Engineering. 2006,25(1): [9] Study of Determining the Rock Drillability with Sonic Interval Transit Time. Zou Deyong, Chen Yonghong. Oil Drilling & Production Technology (6): [10] Drilling Engineering Theory and Technology. Chen Tinggen, Guan ZhiChuan, China University of Petroleum Press. 198