Fracture mechanics is the study of crack propagation in solid materials. On a drilling station, fracture mechanics are used to helping determine borehole stability and potential fracture points within the wellbore. The use of these quantifying techniques is necessary to bore through rock and underlying strata successfully.
These five reasons help explain the usefulness of employing fracture mechanics at the drilling site.
Basic Rock Mechanics
When drilling into a new site, the underlying rock mechanic is essential to ensure proper hole integrity. By using Poisson’s ratio, which says that “the ratio of lateral expansion to longitudinal contraction for a rock under a uniaxial stress condition,” allows workers to mathematically determine the mechanical properties of rock in the drilling area. (Read on in "Signs Your Borehole Is Losing Integrity.")
Theories concerning the stress tolerance of the underlying rock, take the stiffness of the substrate. A stiffer rock surface results in narrower fractures, while a softer material results in more extensive fractures. Surface stiffness calculations include porosity, lithology and other factors. Drilling fluid type is also a factor in stiffness calculation.
It's well known by experienced drillers that underground formations experience stress. This stress is a combination of nonhomogeneous, compressive and anisotropic forces. However, these forces are not equal and vary based on the direction of the force. Determining this stress helps drillers to anticipate fractures possible during drilling.
When tapping a new reservoir, analysts perform tests to determine the in-situ stress in an area.
Drillers use fracture mechanics to sense the orientation of fractures within the well better. Using different injection tests, workers assess the internal minimum principal stress.
Not every well needs each of these tests. New sites may undergo in-situ stress tests before putting the reservoir into use. More commonly, mini fracture and step-down tests are standard before treating a fracture.
In-Situ Stress Tests
To perform an in-situ stress test, workers begin by injecting a few barrels of fluid into the new well. The injection rate is slow at the rate of tens of gallons per minute. The objective is for the fluid to create a small fracture within the well. Once this objective is reached, workers stop pumping fluid and record the pressure within the well. Analysis continues until the fracture closes giving observers a fracture-close pressure, also known as the minimum in-situ stress.
In the event the pressure within the well exceeds the minimum in-situ stress, the fracture reopens. Workers perform multiple stress tests to ensure the data is adequate and repeatable for future use.
Mini Fracture Tests
After completing an in-situ stress test, operators often run a mini fracture test to confirm the data from the other experiments. The mini fracture test also works to estimate the fluid-loss properties of the fluid.
When running a mini fracture test, operators inject several hundred barrels of fluid into the well at a fracturing rate. The purpose of this test is to create a fracture at the same height as one built during fracture treatment. After producing a mini-split, workers shut down the pumps and monitor pressure decline. Like the in-situ test, this data helps to estimate the fracture-closure pressure. This test also measures the total fluid leak-off coefficient.
Before performing a mini fracture test, the step-down analysis is complete. The test uses linear fluids pumped in at fracturing rates. Workers start by injecting fluid into the well for 10 to 15-minutes. The flow rate is reduced in steps to zero. At each step, the flow is held constant for 1-minute to measure the pressure. The objective is to measure the near-wellbore pressure drop as a function of injection rate.
Fracture mechanics helps to determine wellbore strength and stability. The fracturing of a borehole occurs when the drilling-fluid pressure exceeds the in-situ pressure of the hole. This instability can lead to collapse or loss of drill circulation. (See "How to Avoid Tunnel Collapse When Boring Big.")
A detailed analysis occurs to determine the wellbore stability. Using fracture mechanics of in-situ stress and linear elasticity, along with mud properties and pore-fluid pressure, analysts can determine the stability of the borehole. However, many variables play a part in wall strength and certainty is not guaranteed.
Fracture mechanics on a drilling station is not just about the borehole and rock integrity. It also has a part in helping to predict the possibility of fatigue crack growth on drilling pipes during their operation. A surface crack on the rotary bit may seem inconsequential. However, over time, the repeated bending moment produces a fissure in the drill pipe surface. Fatigue cracks represent more than 30 percent of drill pipe damage.
By employing fracture mechanics at every stage of the bore, many of the possible issues are mitigated. While many factors may be difficult to quantify, it is essential to have a technician trained in this area of mechanics.