Laboratory studies of hydraulic fractures in tight sands at different applied stresses

In the recent years with the increasing interest in the shale gas and tight sand reservoirs hydraulic fracturing has gained importance. Microseismicity is the most common tool used in hydraulic fracture mapping. The success of a fracturing job depends on the fracture dimensions. Any changes in the insitu stress conditions affect these fracture dimensions. The recorded seismic waveforms are processed and analyzed to locate the source of an event, to extract information about the frequency content and fracture mechanisms. The purpose of our study is to understand the effect of external stress applied on fracture propagation through acoustic emission studies. Focal mechanism analysis indicates that hydraulic fracturing is dominated by shear failure mechanism. Fracture direction seems to be affected by the magnitude of applied stress. Spatial and temporal distribution of hypocenters is observed. Introduction Microseismic event locations provide a measure of the fracture dimensions. Hypocenter location accuracy affects the resolution of these dimensions. In field, monitoring programs include surface or downhole array of sensors and perforation shots are used for calibration. But in our laboratory experiments proper azimuthal coverage of sample is possible with an array of sensors. Orientation of the hydraulic fracture is controlled by in-situ principal stresses. We duplicate this by applying a horizontal stress. Height containment is controlled by treatment parameter, rock moduli and stresses. By changing the magnitude of horizontal stress applied we try to understand the changes in the fracture dimensions and orientation. The fluid injection in the subsurface changes the effective stress thereby creating new fractures or re-opens existing ones. A portion of the energy released during this process is radiated as seismic waves (P-waves and S-waves) (Reyes et al., 2009), and these are detected by an array of sensors located at surface or in an observation well. Shear failure is the dominant mechanism and has been supported using several studies (Ishida et al., 1997; Baria and Green, 1986; Talebi and Boone, 1998; Urbancic et al., 1999). Experimental setup Sixteen B-1025 transducers having a frequency range of 50 KHz to 1.5 MHz are used to record the seismic waveforms. Each sensor is connected to the signal conditioning unit via a preamplifier. The total amplification for waveforms and the trigger are 70 and 66 dB, respectively. All events were recorded at a trigger threshold voltage of 100 mV. The recording on all 16 channels starts simultaneously once the threshold voltage is reached. In this study only the P-wave arrival times have been used in our hypocenter location algorithm. Sample preparation For our study, Lyons sandstone samples are used as the test specimen. Circumferential Velocity Analysis (CVA) was used to determine azimuthal velocity variation in each sample. CVA is a pulse transmission technique where the velocity is measured as a function of azimuth along the circumference of the sample. The velocity deviation across the sandstone samples was less than 4%; therefore this sample was treated as homogeneous and isotropic. A uniaxial horizontal stress is applied to control the fracture direction. Here, two cases having different magnitude of stresses: 4000 psi and 500 psi (see Fig. 1), by keeping rest of the parameters same are studied. Fig 1 Lyons sandstone sample instrumented and stressed while undergoing hydraulic fracture stimulation. Petrophysical and Completion parameters are as reported in Table 1. © 2011 SEG SEG San Antonio 2011 Annual Meeting 1550 2 Table 1 Properties of test samples, completion geometry Lyons sandstone Petrophysical characteristics Porosity, % 10 Permeability 20 μd Mineralogy, wgt% Quartz, 85 Sample and stimulation properties Length, in 4 Diameter, in 4 Borehole depth, in 2.5 Counter-bore depth, in 0.4 Perforation depth, in 1.6 Frac fluid, viscosity, cp Oil , 50 Pumping rate, cc/min 10 In this study, a set of 16 sensors are used. A subarray containing 4 sensors is placed on one edge of the sample and two surface sensors are also used to have proper Zcorrelation. The sensors are distributed to maximize azimuthal coverage. Before pumping any fluid the sensor arrangement is calibrated using a pencil-break source (Hsu and Brekenridge 1981) at 8 surface locations. This process provides calibration for velocity models, transducer polarities and hypocenter locations. The best resolution of hypocenters in the Lyons sandstone is ±3.6 mm (see Fig 2). Fig 2 Plan view of a Lyons sandstone sample showing the pencil breaks (black dots) and the calculated locations (green dots). Error ellipses are indicated by red lines and sensors are indicated by cyan squares. The average absolute error for sandstone samples is ±3.6 mm Hypocenter locations Two different cases are studied: Sample A with a horizontal stress of 4000 psi and Sample B with 500 psi. (a) Sample A (external stress=4000 psi) Fig 3 Pump pressure (Blue), Pumping rate (Red) and Cumulative AE events (Green) plotted as a function of time for Sample A. The fracturing fluid (oil, 50 cp) is injected at a constant rate of 10 cc/min. The breakdown pressure values reported is 3154 psi. 2644 events were recorded in this process. Majority of the AE activity was confined to the pressure buildup period before reaching the breakdown pressure (Fig 2). Also, a secondary activity coinciding with the pump shut off was observed. This sudden burst of events may be due to the closure of asperities created during the fracturing process. A constant velocity model along with the source location algorithm is used to compute the hypocenter locations (Fig 4). Spatial and temporal evolution of events in the direction of stress applied is observed for Sample A.