Hydraulic Fracturing Stress Measurements Research Articles

Comparison and evaluation of overcoring and hydraulic fracturing stress measurements

The stress measurements determined by both the overcoring (OC) and hydraulic fracturing (HF) methods in the Shuichang iron mine and Sanshandao gold mine were compared and evaluated, respectively. The results indicate that the independent OC and HF data in the two mines reveal the same dominant faulting stress regime. The σH orientations derived from the OC and HF methods in the Shuichang iron mine are dominantly oriented in the N81.1°W–N89.4°W and N77.0°E–N88.0°E, respectively, and the σH orientations yielded from the OC and HF techniques in the Sanshandao gold mine are predominantly in the N30°W–N90°W and N55.5°W–N60.4°W, respectively; hence, the σH orientations obtained by the two different methods in the two mines are comparatively similar. In addition, the shapes of the probability density diagrams using an improved Bayesian regression approach of the three principal stresses measured by the OC and HF methods in the same mine are quite similar, and all the obtained Kolmogorov–Smirnov test p-values are larger than the selected significance level of 0.01, indicating that the stress data interpreted by the two methods approximately follow the same distribution law. Thus, the performance of the two techniques and the reliability of the measured data are satisfactory.

Present-day state of tectonic stress and tectonization in coastal gold mine area near Laizhou Gulf, North China

AbstractTo investigate the present-day state of tectonic stress and tectonization in a coastal gold mine area near the Laizhou Gulf, North China, overcoring, anelastic strain recovery, and hydraulic fracturing stress measurement campaigns were performed, and a total of 49 groups of stress data were determined. The results indicate that horizontal tectonic stress plays a predominant role in the present-day stress field, which is a typical tectonic stress field. The stress fields are characterized by σH>σh>σv (σH, σh and σv are the maximum horizontal, minimum horizontal and vertical principal stresses, respectively) and σH>σv>σh, which appear to favor thrust and strike-slip faulting, respectively. The stress level is relatively high and is not conducive to the stability of underground excavations. σH is oriented in the WNW−ESE or approximately E−W directions, which is in accord with that indicated by focal mechanism solutions and regional geological structure analysis. The contemporary stress field is the result of dynamic action and tectonic movement in various geological periods; the contemporary stress field generally inherited the characteristics of the third stage tectonic stress field (Himalayan movement period) while partially retaining the characteristics of the second stage tectonic stress field (Yanshanian period), and finally evolved into the tectonic stress field characterized by nearly E−W extrusion.

Current Tectonic Stress State in an Iron Mine District, North China, Based on Overcoring, Hydraulic Fracturing, and Acoustic Emission Stress Measurements

Abstract Full knowledge of the current tectonic stress state is crucial for assessing open-pit mine slope stability and regional tectonic evolution and geodynamic processes. Overcoring, hydraulic fracturing, and acoustic emission in situ stress measurement techniques were adopted to determine the 3D stress tensor in an iron mine district, North China, and 25 sets of stress data ranging from 56 to 490 m were measured. Accordingly, the current tectonic stress state and its relationship to regional geological tectonics were investigated. The results indicated that the stress condition seemed to favor thrust and strike-slip faulting, and the stress field was particularly controlled by the horizontal tectonic stress. A high horizontal tectonic stress considerably influenced the stability of high and steep slopes in this mine district, which requires great attention. The stress directions derived from different methods were almost similar, indicating a dominant NEE–SWW stress field direction or near-E–W direction, comparable to the direction revealed by focal mechanism solutions and other stress indicators around the mine district. According to geological structure analysis, the present-day stress field in this district generally inherited the third-stage tectonic stress field while partially retaining the characteristics of the second-stage tectonic stress field, which is the result of dynamic action and tectonic movement during different geological periods, and the maximum principal stress direction of the tectonic stress field that affects the modern tectonic activity in this area is the NEE–EW direction.

Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress for Unlined Pressure Tunnels: A Field Experiment Using the Rapid Step-Rate Test

This paper describes an experimental campaign where a simplified test method. The Rapid Step-Rate Test (RSRT) has been investigated in a full-scale field setting. The test, originally developed to assess rock stresses for pressure tunnel considerations, had in an earlier laboratory study demonstrated a promising ability to assess the magnitude of fracture normal stress. The field campaign presented herein was therefore aimed at assessing the field applicability of this test. The field experiments were conducted inside the pressure tunnel of the Løkjelsvatn Hydroelectric Power Plant in Norway, under field conditions identical to those for which the test originally was developed. A total of 29 individual tests were conducted in 7 boreholes, providing 20 test cycles with interpretable test results. The stress estimates made from the test cycles not affected by the near-field stress of the tunnel showed good correlation with the magnitude of minimum principal stress found from a preceding hydraulic fracturing and over-coring stress measurement campaign. The experience from the field campaign suggests that the RSRT can represent an efficient and reliable method to assess the magnitude of minimum principal stress, particularly useful for the final design of unlined pressure tunnels.

Prediction of hydraulic fracture initiation pressure in a borehole based on a neural network model considering plastic critical distance

Prediction of the hydraulic fracture initiation pressure is essential for in-situ hydraulic fracturing design and stress measurement. The prediction models based on the theory of critical distance have been proofed to fit the abnormal initiation pressure caused by the bore size. However, the critical distance used by the previous models is not physics-based, which may result in errors for prediction. In addition, the laboratory determination of the critical distance neglected the boundary loading effect, causing an underestimating of the critical distance. This work aims to provide an efficient and accurate model for predicting the in-situ hydraulic fracture initiation pressure with consideration of a physic-based plastic critical distance. We first establish a universal analytical solution for stress distribution and plastic zone around a borehole in a cylindrical sample through ideal elastoplastic theory. Using the analytical solution, the plastic critical distance of shale rock is analyzed based on a hydraulic fracturing experiment, and compared with the one estimated by the previous methods. Then, a set of numerical simulation are carried out to build a data set of hydraulic fracture initiation pressure under different stresses, tensile strength, and ratio of plastic critical distance to borehole radius. Finally, the nonlinear relationship between these parameters is mapped by a deep neural network model. According to this study, the following conclusions are obtained: 1) The plastic critical distance increases as the loading stress increases. 2) The established neural network trained by numerical simulation results is plausible, meanwhile, with high precision and high efficiency. 3) Increase in stresses and tensile strength can strengthen the influence of critical plastic distance. 4) Kirsch’s approximate stress solution can bring an error for determination of the critical distance in lab-scale experiments.

Data interpretation method in HF stress measurement for the total stress tensor determination in rock mass

The data interpretation method is validated for the hydraulic fracturing stress measurements for determining simultaneously all components of external stress tensor in rock mass in the vicinity of underground geotechnical structures. The analysis of the alternative patterns of 6 measurement boreholes reveals allowable drilling directions of boreholes to ensure the formulated problem resolvability. The analysis includes the solution of direct problems on finding “the normal stresses in the areas orthogonal to the axes of the boreholes at the known external stresses” and the solution of an inverse problem on finding the external stresses. In the proposed method, the input information may be the data obtained in the in-situ HF stress measurement. The numerical experiments with the synthetic input data determine allowable variations in orientation of measurement holes in directional drilling such that the measuring fault of external stresses is no thigher than 10%.

Determination of stresses in salt rocks byhydraulic fracturing: Case-study

The paper presents the experimental results of stress assessment in salt rock mass around underground excavations in Solikamsk Mines 1, 2 and 3, Uralkali, at the Upper Kama potash deposit. The experiments were a part of the geomechanical research aimed at justification of project designs and engineering solutions made in selection of safe and efficient mining methods, including safety of watertight strata. The experimental works were accomplished using the measurement and computation system Gidrorazryv designed at the Institute of Mining, SB RAS. In the salt rock testing, the system was equipped with new-design downhole survey tool and wireless communication between pressure sensors and portable computer. The sites for measurement stations were selected with regard to current mining situation, distance between the sites and stoping front, absence of undermining or overmining and installability of drilling equipment. A particular attention was paid to a drilling tool capable of geometrically accurate drilling in salt rock mass, in compliance with standards of the hydraulic fracturing stress measurement technique. In the tests, the method of directional hydrofracturing with softening of measurement hole boundary by cutting initiation slots was implemented. All in all, 75 hydraulic fracturing tests were carried in adjacent rock mass in the mines. It is found that stress state in roof rocks of underground excavations is nonuniform: the vertical stress is similar to the calculated value due to weight of overlying rocks while the horizontal stresses exceed the gravitational stress by 2–3 times.

The correlation between low tectonic stress and the Appalachian Basin Quiet Zone

In order to assess the nature of the Appalachian Basin Quiet Zone (ABQZ), a region of low seismic activity, a minimum horizontal stress profile from sonic logs was calibrated using hydraulic fracture stress measurements in a well from McKean County, PA. The calibrated minimum horizontal stress profile that best fits with the hydraulic fracture stress data was established using an extended Eaton's model that treats rock as a transverse isotropic material with a vertical symmetry axis (TIV). To achieve a best fit, several assumptions are introduced: (1) a constant tectonically-induced elastic strain of εHmax = 3.3 × 10−4, (2) an anisotropic velocity model ANNIE approximation for shales, and (3) a bimodal pore pressure distribution with abnormally high pore pressure in the Marcellus gas shale. Other calibration methods assume an isotropic medium, a constant tectonic strain gradient corresponding to zero stress at ground surface, or constraints from sidewall core data to estimate stiffness constants. By testing four calibration methods, we find that stress profiles are affected by multiple factors, including the ratio of maximum tectonically-induced elastic strain over minimum tectonic strain, pore pressure in the Marcellus, Biot's coefficients, and elastic properties. Sensitivity analysis was conducted to analyze the influence of these parameters. Our stress profile is most influenced by pore pressure and tectonically-induced elastic strain while other factors had much smaller effects. Tectonically-included strain in the ABQZ is insufficient to achieve the critical stress necessary to induce frictional slip on favorably oriented faults in the sedimentary cover which presumably speaks to the lack of earthquakes in the basement of the ABQZ. However, induced earthquakes from deep disposal wells suggest that basement at the edge of the ABQZ may be decoupled from the sedimentary cover, in which case the cover has relaxed through a time-dependent deformation mechanism such as pressure solution creep.

Hydraulic fracturing stress measurement in underground salt rock mines at Upper Kama Deposit

The paper reports the experimental results on hydraulic fracturing in-situ stress measurements in potash mines of Uralkali. The selected HF procedure, as well as locations and designs of measuring points are substantiated. From the evidence of 78 HF stress measurement tests at eight measuring points, it has been found that the in-situ stress field is nonequicomponent, with the vertical stresses having value close to the estimates obtained with respect to the overlying rock weight while the horizontal stresses exceed the gravity stresses by 2–3 times.

Hydraulic Fracturing Stress Measurements and Their Implication for the Design of the First Concrete Lined Pressure Shaft in Sri Lanka

Hydraulic Fracturing Stress Measurements and Their Implication for the Design of the First Concrete Lined Pressure Shaft in Sri Lanka

Heterogeneous in situ stress magnitudes due to the presence of weak natural discontinuities in granitic rocks

Two field examples of hydraulic fracturing stress measurements are reported, in which the determined stress magnitudes exhibit severe variations with depth. The stress measurements were conducted in vertical boreholes drilled in granites in two different locations in South Korea. Several isolated intervals of intact rocks in the boreholes were vertically fractured by injecting water. The magnitudes of the minimum horizontal principal compressive stress (Shmin) were determined from shut-in pressures. The magnitudes of the maximum horizontal principal compressive stress (SHmax) were estimated based on the Kirsch equation using tensile strengths determined from hollow cylinder tests and Brazilian tests, in which pressurization-rate effects on tensile strength were taken into account. The stress states in both locations are in reverse-faulting stress regimes. The magnitudes of SHmax are generally within a stress range defined by frictional limits of favorably oriented fractures having frictional coefficients of 0.6 and 1.0. However, SHmax magnitudes do not increase linearly with depth, but rather scatter quite severely. It is noted that near the depths where the measured stresses are relatively low, natural discontinuities with wide apertures containing weak filling material exist, whereas near the depths of high stress, such wide discontinuities are scarce. Wide aperture discontinuities are predominantly oriented such that their slip tendency is high under the given stress conditions, meaning that if excessive shear stress is exerted, the weak discontinuities would slip to release the excessive stress. Such local processes would restrict SHmax magnitudes within values that can only be sustained by the shear strengths of the discontinuities, leading to severe variations of SHmax with depth. This result suggests that stress magnitudes are controlled quite locally by the frictional property of natural discontinuities, and that the stress state in granitic rock might be inherently heterogeneous because of the heterogeneous distribution of natural discontinuities having various frictional properties.

The stress state of the Beichuan-Jiangyou segment of the Longmenshan fault before and after the Wenchuan M S 8.0 Earthquake

After the Wenchuan M S 8.0 Earthquake, which occurred on May 12, 2008, in Sichuan Province, China, we conducted a series of hydraulic fracturing stress measurements in three 200 m deep boreholes (ZK01, ZK02, and ZK03) drilled in Beichuan and Jiangyou regions near the northeastern segment of Longmenshan fault belt in 2009. These measurements revealed the near-surface stress field in the fault region one year after the Wenchuan M S 8.0 Earthquake. However, the lack of the stress measurements before the earthquake in the same region makes it difficult to understand variations of the in situ stress field (near-surface) by comparative analysis. In order to determine the unknown horizontal principal stresses before the earthquake in Beichuan and Jiangyou regions, the following research method was tentatively applied. Firstly, we calculate the static co-seismic stress field by linear elastic finite element numerical simulation with ANSYS, based on the co-seismic static displacement generated by the Wenchuan M S 8.0 Earthquake along the central Longmenshan fault plane in Beichuan and Jiangyou. Secondly, combining hydraulic fracturing measurements (after the earthquake) with the co-seismic stress (simulation), the magnitudes and orientations of horizontal principal stresses before the earthquake were calculated. Finally, the variation of the in situ stress (near-surface) in Beichuan and Jinagyou, both before and after the Wenchuan M S 8.0 Earthquake, were obtained by comparative analysis. To do this the magnitude of S Hmax was decreased on average by 13.01 and 6.54 MPa after the earthquake in ZK02 and ZK03, respectively and the magnitude of S Hmin was decreased by 2.54 and 5.29 MPa in ZK02 and ZK03, respectively. Following the earthquake, the average direction of S Hmax rotated anticlockwise by 42.5°.

A Seismic Evidence for the Reliability of the 3-D Stress Measurement by Hydraulic Fracturing

Determining the in-situ stress of wall rock is important for the underground cavern project. The three-dimensional (3-D) stress measurement, which is obtained by hydraulic fracturing stress measurement via measuring the stresses in boreholes with three different orientations, has been applied for the designation of cavern project. However, there are few examples that can demonstrate the reliability of 3-D stress measurement method. In this study, we showed one example of the 3-D stress results measured by hydraulic fracturing in Western China. The measured results, especially the dip angles of the maximum principle stress are consistent with those determined by focal mechanism of nearby earthquakes. This consistence indicates the reliability of the 3-D stress results by hydraulic fracturing and an expected application for regional geostress study.

수압파쇄시험 해석을 위한 중공원통 인장시험과 압열인장시험 화강암 인장강도 비교

We conducted hollow cylinder tensile strength tests and Brazilian tests in Seokmo granite to measure tensile strength necessary for estimating the magnitude of the maximum horizontal principal stress in hydraulic fracturing stress measurements. Two different pressurization rates were used in hollow cylinder tests. Tensile strengths were determined to be higher at higher pressurization rate, which suggests that tensile strength should be measurement at the same rate used in actual in situ hydraulic fracturing tests. Considering the effect of pressurization rate and specimen size on tensile strength, the hollow cylinder tests and Brazilian tests yield similar results each other. This demonstrates that Brazilian tests can be utilized to produce representative tensile strengths for interpretation of hydraulic fracturing test results.

The Application of Hydraulic Fracturing Stress Measurement in Hydroelectric Projects of China

Compared to the other methods of stress measurement, classical hydraulic fracturing (HF) technique is a relatively convenient and economic method to determine stress and interpret the test results. However the HF method can only obtain the local dimensional (2D) stress of the plane perpendicular to the axis of test holes. In this paper, two HF measurement campaigns applied in two hydroelectric projects in China have been presented to describe how to obtain the three dimensional (3D) stress from three different direction shallow boreholes in the same section by HF, and how to carry out the back analysis of a large scale area stress field based on the test results.

In-situ stress measurements and stress distribution characteristics in underground coal mines in China

The theory, equipment and method of hydraulic fracturing stress measurement used in underground coal mine roadways in China are presented. A quality assurance system and a quality ranking scheme were developed for hydraulic fracturing stress measurements under specific conditions encountered in Chinese underground coal mines. During the last 8 years, a total of 204 hydraulic fracturing stress measurement campaigns have been carried out in 49 coal mines within 13 Chinese coal districts. The test sites which are located in sub-horizontal, inclined and steeply dipping coal measures range in depth from 69.2 m to 1 283 m. Each stress data record was assigned a quality between A and E according to a proposed data quality ranking scheme. A total of 97 stress data records of A and B quality were used to investigate the distribution characteristics of in-situ stresses within the coal districts. Generally, in-situ stresses increase with depth. In shallow coal mines, the rate of increase in horizontal stresses with depth is greater than the rate of increase of vertical stress. With increasing depth the rate of increase in horizontal stresses decreases. A considerable scatter in the in-situ stress test data may be due to distinct differences in both the strength and deformation moduli of strata located in varying geological environments and different coal districts. In addition, large folds and faults often result in a change in orientation of the maximum horizontal stress, and in distinct changes in stress values. Three types of stress field distribution have been noted with σ H > σ h > σ v found mainly in relatively shallow coal mines (< 400 m), the σ H > σ v > σ h type in moderately deep coal mines (400–600 m), and the σ v > σ H > σ h type predominantly in moderately deep to deep coal mines. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.5 and 2.0 in the coal districts.

The Application of Hydraulic Fracturing in Storage Projects of Liquefied Petroleum Gas

It is very important to study the pristine stress field in Civil, Mining, Petroleum engineering as well as in Geology, Geophysics, and Seismology. There are various methods of determination of in-situ stress in rock mass. However, hydraulic fracturing techniques is the most convenient method to determine and interpret the test results. Based on an hydraulic fracturing stress measurement campaign at an underground liquefied petroleum gas storage project which locates in ZhuHai, China, this paper briefly describes the various uses of stress measurement, details of hydraulic fracturing test system, test procedure adopted and the concept of hydraulic fracturing in arriving at the in-situ stresses of the rock mass.

Study on in-situ stress measurement around coastal marginal land in Fujian

The in-situ hydraulic fracturing stress measurements have been carried out around the coastal marginal land in Fujian Province. And the characteristics of magnitude, direction and distribution of tectonic stress have been obtained. Based on the observed stress data, the characteristics and activities of fault zones are analyzed and studied in the paper according to the Coulomb friction criteria. (1) The maximum horizontal principal compressive stress is in the NW-WNW direction from the north to the south along the coastline verge, which is parallel to the strike of the NW-trending fault zone, consistent with the direction of principal compressive stress obtained from geological structure and across-fault deformation data, and different from that reflected by focal mechanism solution by about 20°. (2) The horizontal principal stress increases with depth, the relation among three stresses is S H>S v>S h or S H≈S v>S h, and the stress state is liable to normal fault and strike-slip fault activities. (3) According to Coulomb friction criteria and taking the friction strength μ as 0.6–1.0 for analysis, the stress state reaching or exceeding the threshold for normal-fault frictional sliding near the fault implies that the current tectonic activity in the measuring area is mainly normal faulting. (4) The force source of current tectonic stress field comes mainly from the westward and northwestward horizontal extrusions from the Pacific and Philippine Plates respectively to the Eurasian Plate.

水圧破砕の引張強度に関する破壊の確率論による検討

A tensile strength deduced from a hydraulic fracturing stress measurement is different from tensile strengths obtained by a Brazilian test and by a bending test. In this paper, the authors discussed the differences based on a probability theory of fracture considering stress distribution in the specimen. At the first, they conducted the three tests in specimens of Kurokami-jima granite, and suppose that variation of their tensile strengths follows Weibull distribution. At the second, from ratios of tensile strengths among the hydraulic fracturing, the Brazilian test and the bending test, they found that Weibull's coefficient of uniformity, m, equals to 10.1. At the third, they elucidated that the obtained coefficient of uniformity is good for a significance level in the Kolmogorov-Smirnov test.Through the discussion, it is concluded that the differences of the tensile strengths among the hydraulic fracturing, the Brazilian test and the bending test can be explained to some extent by the probability theory of fracture.

Shallow hydraulic fracturing measurements in Korea support tectonic and seismic indicators of regional stress

Abstract We have conducted five hydraulic fracturing stress measurement campaigns in Korea, involving 13 test holes ranging in depth from 30 to 250 m, at locations from North Seoul to the southern coast of the peninsula. The measurements reveal consistent crustal stress magnitudes and directions that suggest persistence throughout western and southern Korea. The maximum horizontal stress σ H is oriented between ENE–WSW and E–W, in accord with plate movement and deformation, and with directions indicated by both focal mechanism solutions from earthquakes inland and offshore as well as borehole breakouts in mainland China close to its eastern coast. With respect to magnitudes, the vertical stress is the overall minimum stress at all tested locations, suggesting a thrust faulting regime within the relatively shallow depths reached by our tests. Typically, such a stress regime becomes one favoring strike-slip at greater depths, as is also indicated by the focal mechanism solutions around Korea.