Impression Packer Test

[Kerrie Gingrich]

Theimpression packer is designed to make an impression of the sides of a borehole wall on a wax-covered paper. This provides the information needed to determine the location of discontinuities in the strata at different depths. The impression packer consists of a split steel tube of the same diameter as the borehole itself. The two halves of this tube are pressed against the sides of the borehole when a pneumatic packer in the centre of the assembly is inflated. To the outer surface of the split steel tube is attached a thin layer of highly compressible rubber onto which the wax paper is taped.

Together with the spatial orientation of the cracks determined from impression packer tests or imaging bore logs, the measured pressure values necessary to create, widen and keep open the cracks are used to estimate the size and direction of the in-situ stress field.

MeSy has more than 30 years of know-how in the field of hydraulic fracturing for in-situ stress measurement in boreholes up to 5 km depth and in every common borehole diameter. In addition to conventional testing on drill pipe or tubing string, the tests are mainly carried out with the wireline equipment developed by us. Therefore, the test performance is independent of the drilling rig and very cost-efficient.

Our inflatable packers are the tool of choice for geotechnical investigations such as rock stress, perm testing and grouting. Built to be compatible with the most standard drillhole sizes in the industry, we can provide low, medium and high pressure packers at any diameter to suit your needs.

BIP (Borehole Image Processer) is a most-advanced borehole televiewer to conduct continuous scanning and processing of digital images of borehole wall in real-time. Our BIP equipment is capable of logging up to 500m depth in 60mm diameter or larger borehole. In Southeast Asia region, Kiso-Jiban has applied BIP into over a hundred of boreholes and obtained high-resolution images with full-color graphics from which three-dimensional characteristics of the rock mass, such as joint, foliation, bedding and lithology have been assessed for engineering consideration.

The hydraulic fracturing test accompanied with the impression packer test is a common method to measure the magnitude and orientation of in-situ rock stress in a borehole. Kiso-Jiban developed the measurement system and apparatus which are able to conduct the tests up to 300m depth borehole. The hydraulic fracturing test system includes two inflatable rubber packer elements straddled by an interval spacer. The packer elements are used to seal off a segment of the borehole so as to enable its leak-free hydraulic pressurization. The impression packer comprises of a compass-like (gyroscopic) device and a packer element covered with a thin rubber membrane for taking an imprint of the hydraulic induced fractures. In Southeast Asia region, Kiso-Jiban has carried out the tests in several boreholes and the results have been published in technical papers.

P-S velocity logging is to determine the in-situ velocities of the P (Primary; Compressional) and S (Secondary; Shear) waves propagate through the soil or rock at the immediate vicinity of the borehole. Kiso-Jiban employs the suspension P-S velocity logging system which is designed to measure velocities with two receivers and one wave source mounted in a probe and can be applied to deep boreholes. Kiso-Jiban has numerous proven track records of performing the suspension P-S velocity logging in soft soil to hard rock up to 250m depth boreholes in Southeast Asia region.

Why does this matter? Businesses that rely on underground operations, like the petroleum and geothermal industries, must have accurate estimates of in-situ stress (which is the existing pressure underground before a hole is drilled). This affects drilling, surveying, and fluid injection (hydraulic fracturing, water flooding, CO2 sequestration) as well as phenomena such as fault re-activation and induced seismicity.

Embedding HD-FOS sensing fiber in a helical pattern allows for precise measurement of radial and axial strain as the hydraulic pressure inside the sleeve is increased. The continuous measurement of strain around the circumference and down the depth of the hole provides new types of data that have previously been unavailable, such as fracture initiation and evolution in real-time. The methodology to measure in-situ stress is shown in Figure 2.

After executing a complete measurement at a single location, the pressure in the sensor sleeve is reduced. The fractures will close, and the sleeve is free to be positioned at the next location. During this process, all hydraulic fluid remains contained within the sensor. Because no fluid is lost and the outer diameter of the sensor sleeve can be as small as 66 mm, it is anticipated that all the equipment necessary to perform a measurement will fit on a utility truck (hydraulic pump, a spool of sensor/cable, reservoir of fluid, instrument electronics, and computer).

A shallow HQ borehole was drilled at a local limestone quarry to demonstrate the fracture-sensing capability of the CHISL prototype (Figure 3). The tensile strength of the rock was determined to be 3,100 psi using Brazilian disc tests of core samples.

An impression packer was used before and after CHISL operation to verify the fracture state pre- and post-test. In the measurement described here, a pre-existing horizontal fracture was the only impression recorded before the test.

As the CHISL assembly was inflated, this same horizontal fracture was visible in the strain map of the borehole wall, as seen in Figure 4. The x-axis of the plot is along the depth of the hole, while the y-axis is the polar angle around the circumference of the cylindrical CHISL sensor. The color map indicates the strain level observed at every location within the embedded fiber optic sensor.

Further pressurizing the CHISL sensor, bi-wing fractures appeared below and above the horizontal fracture, but at different angles and at different pressure applied to the borehole wall, as seen in Figure 5.

The color map in this plot represents the derivative of strain with respect to pressure increase and is helpful in identifying rapidly growing locations of strain as pressure is ramped. Below the horizontal partition the fractures occurred at 90 and 270 at a lower pressure of 2,500 psi, while the formation above the horizontal partition fractured at 180 and 330 at a pressure of 3,700 psi. This demonstrates the power of the distributed sensing concept. Within the 1 m sensing region there is sufficient spatial resolution to detect individual fractures opening, their length, and changes in stress orientation along the depth of the hole, and even measure two distinct formations simultaneously.

The borehole was further pressurized with the CHISL sensor until a double fracture occurred at 4,900 psi, seen in Figure 6. The new fracture at 270 is roughly 90 between the bi-wing fracture that originated at the lower pressure. It is also observed that the fracture at 330 decreases while a new longer fracture at 0 opens. According to theory, this new fracture is related to the maximum in-situ stress in the rock formation.

This experiment proves that the CHISL sensor has the potential to create, measure, and visualize both fractures corresponding to minimum and maximum horizontal stress. When deployed, this technique would provide critical measurements with unmatched consistency and depth resolution, allowing safer and more efficient well operations.

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The ability to control induced seismicity in energy technologies such as geothermal heat and shale gas is an important factor in improving the safety and reducing the seismic hazard of reservoirs. As fracture propagation can be unavoidable during energy extraction, we propose a new approach that optimises the radiated seismicity and hydraulic energy during fluid injection by using cyclic- and pulse-pumping schemes. We use data from laboratory-, mine-, and field-scale injection experiments performed in granitic rock and observe that both the seismic energy and the permeability-enhancement process strongly depend on the injection style and rock type. Replacing constant-flow-rate schemes with cyclic pulse injections with variable flow rates (1) lowers the breakdown pressure, (2) modifies the magnitude-frequency distribution of seismic events, and (3) has a fundamental impact on the resulting fracture pattern. The concept of fatigue hydraulic fracturing serves as a possible explanation for such rock behaviour by making use of depressurisation phases to relax crack-tip stresses. During hydraulic fatigue, a significant portion of the hydraulic energy is converted into rock damage and fracturing. This finding may have significant implications for managing the economic and physical risks posed to communities affected by fluid-injection-induced seismicity.

Hydraulic-fracturing intervals from tests HF1, HF2, and HF3 are located in vr granodiorite (AG) in the deeper part of the hydraulic-testing borehole, HF4 and HF5 intervals are located in fine-grained diorite-gabbro (fgDG), and the HF6 interval is situated in fine-grained granite (fgG) at a distance of 5 m from the tunnel wall (Fig. 1c).

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