The Research Infrastructure for borehole stress measurements "Stress trailer"

 

The Stress trailer provides data that support the understanding of ongoing geological processes in the crust and assessment of dimensioning loads on bedrock and geological structures, including information about the orientation of the stress field and its magnitudes is important for the design of boreholes with respect to borehole orientation (dip, hole azimuth) and drilling mud composition.
The Stress trailer uses a wire-line activated straddle-packer equipped with an electrical imager. Imaging data are used to produce a pre-log to select test sections and to identify potential de-coupling zones, and to produce a post-log after completed pressure testing to analyze the orientation of tested fractures. The electrical imager is a major advantage within field testing, especially regarding imaging of hydraulic injection tests. The electric imager was first developed by Cornet and Mosnier (1989). The electrical images and the fixed distance between packers and imager radically reduce the uncertainty in data interpretation. For the Stress trailer, the first (and only) digital version of the electrical imager was developed that in its most recent version (December 2020) includes direct digital communication between tool and surface.
The equipment is mounted in a 40-foot container on a mega-trailer and includes three winches, low- and high-pressure water hydraulics, manifold, water tanks, borehole equipment (straddle-packers, electric imager for three borehole dimensions) and heater for measurements in cold climate. At surface, approximately 45 data streams (electrodes, pressure sensors, orientation sensors, temperatures) from the borehole are integrated with those from surface sensors (pressure sensor, flow-meter, length, speed, cable tension) using Python. These files are then converted to Matlab for data interpretation. The Stress trailer has a maximum depth capacity of 3 km in N, H, and P borehole sizes (76, 96, and 123 mm, respectively), i.e., it has been designed to comply with the coring dimensions of Riksriggen. The main features of the infrastructure are:

  • Load carrying winch with 3,000 m, 15/32”, armored 7 conductor cable
  • Coiled tubing winch with 3,000 m, dimension 1/4”, coiled tubing for straddle packer
  • Coiled tubing winch with 3,000 m, dimension 3/8”, coiled tubing for test section
  • The three winches are frequency controlled and synchronized
  • In-house-developed straddle packer systems for N, H, P borehole dimensions (76, 96, 123 mm, respectively)
  • Digital electric imager, temperature rated to 85ºC, integrated with straddle packer system
  • Digital electric imager can be run as stand-alone unit
  • Systems for low- and high-pressure water hydraulics with fresh and saltwater tank and control panel
  • Pressure capacity of borehole equipment 650 bar/65 MPa, surface equipment 1378 bar/137.8 MPa
  • Pressure sensors for test section and straddle packer, downhole and on surface
  • Flow capacity, up to 6 l/min where the flow is measured with a vortex meter at surface
  • Running-line tensiometer on container roof indicating tension in cable, length, and logging speed
  • Real-time digital data collection and transfer to surface through load-carrying geophysics cable. About 45 signals are sent to surface where they are integrated with surface sensors.
  • Raw data interpretation is done in Matlab according to standard methods recommended by ISRM (International Society of Rock Mechanics; Haimson & Cornet 2003)
  • The 40’ container is insulated and equipped with a heater, enabling testing in freezing conditions. Dimensions: Weight, 41.5 t; Length, 15.7 m; Width, 3.0 m; Height, 4.2 m

Three hydraulic stress testing methods can be applied: hydraulic fracturing, sleeve fracturing and hydraulic testing of pre-existing fractures. The three-dimensional stress tensor and its variation with depth within a continuous rock mass can be determined in a scientific unambiguous way by integrating results from the three test methods. The ISRM standard (Haimson and Cornet 2003) present two types of tests, hydraulic fracturing (HF) and hydraulic testing on pre-existing fractures (HTPF). In HF tests, a new fracture is created. In HTPF tests, the straddle packer is positioned over an existing fracture. The fracture is carefully opened by stepwise and slow increase of pressure. For both tests, the normal stress of the fracture (i.e. pressure-time and flow-time data collection) and its orientation is measured. Data from each test include depth, normal stress, and the dip and dip direction of the fracture. Stress measurements are not large-scale pumping tests. The theoretical stress at the well bore is known, and the aim is to stimulate the new/pre-existing fracture there. The further out from the wellbore, the less is known, e.g. increased risk for stimulating an intersecting fracture and get reduced test quality. Testing does not particularly influence the borehole quality (other than forming new thin tensile fractures in HF test sections, and opening and possible breaking seals of pre-existing fractures).