Shale swelling is a major cause of wellbore instability. It can lead to a variety of problems, such as hole sloughing, pipe drag, bit balling, stuck pipe, or other “gumbo” related issues. Shale swelling is caused by clay (one of the major components of shale) absorbing water, usually from drilling fluid. The proper drilling fluid additives can mitigate shale swelling and prevent the loss of the entire drilling operation.
The OFITE Dynamic Linear Swellmeter measures the interaction between drilling fluids and mineral samples containing clays. This instrument can test up to four (expandable to eight) samples simultaneously and compare the results over time. Each test consists of a cylindrical wafer of compressed bentonite, rev dust, or downhole sample. The wafer is constrained around its circumference and exposed to fluid. As the clay reacts to the fluid, the wafer can only expand linearly. A Linear Variable Displacement Transducer (LVDT) measures the expansion of the water while the software tracks the percent of swelling over time. Each station includes independent heating and stirring.
Many drilling problems can be traced to the interaction between shale rock materials and drilling fluids. Testing the characteristics of the shale is the first step in developing an appropriate drilling fluid system that will be less reactive and cause fewer problems. One of these characteristics is hardness. Shale hardness can be related to the inhibitive properties of the drilling mud. When shale interacts with a fluid, water will often adsorb, the shale will swell, and fine particles will disperse. These can then reduce the compressive strength or cause the shale to spall or fracture, leading to significant wellbore stability problems. In addition, soft and/or sticky cuttings can cause mud rings, sticking problems, and bit balling.
The Bulk Hardness Tester (#150-87) was designed to evaluate the hardness of a shale sample after it has been exposed to a drilling fluid. In the test, the operator uses a torque wrench attached to a threaded piston to extrude the sample through a perforated plate. The torque required for each turn is recorded and evaluated. The change in torque during the test provides valuable information about the characteristics of the sample. Harder samples will require more torque, while software sample will require less.