Rocks buried beneath the earth’s surface are subject to a variety of stresses. Determining these stresses is very important in trenchless technology as they will dictate certain factors in the design of the wellbore, including the selection of the drill bit and the constituents of the drilling fluid. Preliminary measurements reveal information about the stress state of the rock that becomes useful as the project advances.
In-situ stresses that are released during trenchless excavation can cause ground control problems such as rockburst, buckling, or spilling. Some factors that affect the magnitude and orientation of in-situ stress include the weight of overlying rock and soil strata, tectonic forces, and residual stress. (Read also: Testing In-Situ Stress: What You Need to Know and Why it Matters.)
Types of Stress Measurement Methods
According to E.R. Leeman,
“It is impossible to measure stress directly since, in fact, it is a fictitious quantity. It is only possible to deduce stresses in a solid body from the results of measurements using some indirect method”.
The in-situ stress measurement techniques require that the rock be disrupted, and the response of the rock to that disturbance is measured and analyzed.
Various techniques have been developed to measure in-situ stress and are broadly divided into 6 groups and their subgroups:
- Surface relief methods.
- Borehole relief methods (overcoring, borehole slotting, etc.).
- Relief of large rock volumes (bored raise, under-excavation technique, etc.).
Strain Recovery Methods
- Anelastic strain recovery (ASR).
- Differential strain curve analysis (DSCA).
Borehole Breakout Methods
- Caliper and dipmeter analysis.
- Borehole televiewer analysis.
- Fault slip data analysis.
- Earthquake focal mechanisms.
- Indirect method (Kaiser effect).
- Residual stress measurement.
Most Common Measurement Methods
The most common and popularly used techniques for stress measurement are the flat jack method, overcoring method, and hydraulic fracturing method. In this article, we will take a look into these three techniques to measure the stress in rocks. (Read also: Carrying Out In-Situ Stress Methods: Hydraulic Fracturing versus Overcoring Methods.)
Flat Jack Method
This method is the oldest practiced method, preceding overcoring and hydraulic fracturing methods. The test is used to measure the acting stresses or mechanical parameters of structures. It's based on the stress release in a small area of a structure by cutting on a plane perpendicular to the surface.
The flat jack test method consists of a test apparatus with two steel plates, circular or square in shape that are welded around the periphery. A feeder tube inserted in the middle of the plates allows the pressurization of the apparatus with oil or water. Two pins are fixed into the walls of the excavation and the distance between them is measured. A slot is cut into the excavation between the pins. In the case of compressive stress, the movement of the pins will take place together.
The prepared flat jack is then placed into this slot. The flat jack is pressurized until the pin or strain gauge readings return to their original position. At least six flat jack tests need to be conducted in six different directions to get a complete picture of the stress state in a particular area.
Overcoring is done in a pilot hole fitted with a strain gauge, beginning at the borehole diameter. It measures the undisturbed stress levels and involves drilling a large diameter borehole (60 – 150 mm). At the end of the large borehole, a small pilot hole (300 to 500 mm in length) is drilled as concentric to the larger hole as possible. The larger hole is again resumed relieving stresses and strains partially or totally within the cylinder of rock.
These changes in strain and dimension are recorded along with the rocks modulus and Poisson's ratio derived from cored samples. The radial displacement is measured and converted to stress magnitudes. Strain gauges can be used to measure the changes and can be mechanical or electrical resistance strain gauges.
The strain gauges are used to measure the in-situ stress based on stress release around the borehole after overcoring. Radial displacement is recorded by the strain gauge and used for estimating stress for various geological conditions. The recovered overcore can be taken for laboratory testing to determine other physical characteristics of the rock.
Hydraulic Fracturing Method
Hydraulic fracturing, also known as fracking, is the process of creating fractures in sub-surface rocks using high-pressure fluid. The rate at which fluid is injected into the formation is high enough to fracture it. Usually, water, sand, and chemicals under high pressure are injected into the bedrock formation through the well.
This method can be used to increase oil and gas flow to the well from formations and is commonly used in low-permeability rocks such as tight sandstone, shale, and some coal beds. It is also used to determine the in-situ stress of rocks and for applications such as tunneling, geothermal energy, groundwater remediation, and water well development.
Hydraulic fractures propagate perpendicular to the least principal stress, which in some formations is the overburden stress, resulting in a hydraulic fracture in the horizontal direction. In very deep reservoirs, the least principle stress will mostly be horizontal, creating a vertical hydraulic fracture.
Fractures always form perpendicular to the minimum in-situ stress and in almost all cases, the vertical stress equals the weight of the overburden per unit area. In some cases, higher sub-surface vertical stresses are created by upward forces greater than the overburden weight. At shallow depth, the minimum stress is the vertical stress, causing fractures in the horizontal direction. (Read also: Fracture Mechanics and Material Safety Testing.)
Proppants such as sand or gritty material that suspend in water-based or other types of drilling fluid are added to keep the fractures open. Fracking is carried out by injecting high-pressure fluid containing about 95% water, 0.5% additives, and 4.5% proppant. When the fracture fluid is injected, the generated fractures expand as the proppant fills them and keeps them open.