Well Construction in Complex Formations: Techniques for Dealing with Sand, Gravel, and Fractured Rock Layers

Well Construction in Complex Formations: Techniques for Dealing with Sand, Gravel, and Fractured Rock Layers

Drilling wells in complex formations, such as quicksand, thick gravel, and fractured rock layers, presents a dual challenge to both technology and equipment. Different drilling techniques and specialized equipment configurations are required depending on the characteristics of each formation. The core objectives are effective borehole formation, prevention of collapse, and ensuring safety.

Dealing with Sand Layers (especially Quicksand): These formations are extremely loose and easily flow when exposed to water, making boreholes prone to collapse or narrowing. The key technique is "fast drilling speed and strong borehole wall protection." Rotary drilling rigs with scraper bits are typically used for rapid drilling, while high-viscosity, high-quality drilling mud must be used. This mud quickly forms a thick, dense "mud cake" on the borehole wall, firmly fixing the sand grains like glue. In areas with severe quicksand, "casing drilling" is often necessary, where a robust, thin-walled casing is simultaneously driven into the borehole while drilling, directly isolating the quicksand through physical means. This is the most reliable solution.

For pebble and gravel layers: The strata consist of loose, porous rocks and pebbles of varying sizes, making it easy for the drill bit to get stuck or deviate, and causing significant mud loss. The core techniques are "leakage prevention, jamming prevention, and deviation prevention." Percussion drilling rigs are preferred, using heavy drill bits to repeatedly impact and break up the pebbles; alternatively, reverse circulation drilling rigs can be used, which act like a large suction pipe to powerfully extract the broken rock fragments, resulting in high efficiency and thorough borehole cleaning. Thick, fibrous, and sheet-like mud must be used to seal formation fissures and prevent mud loss. Similarly, casing drilling is the safest method for traversing thick pebble layers.

For fractured rock strata and fractured layers: The rock mass itself is hard, but it is cut into pieces by numerous fissures, making it easy for rock from the borehole wall to fall and jam the drill bit. The key technique is "stability first, maintaining borehole wall integrity." Rotary drilling rigs equipped with roller cone or diamond drill bits are recommended, employing a parameter combination of "light pressure, slow rotation, and high pump volume" for smooth drilling and minimizing vibration and disturbance to the fractured borehole walls. The drilling mud must possess excellent "cementing and wall-protecting" properties, capable of penetrating and bonding even micro-cracks. In extremely fractured sections, it may be necessary to inject specialized cement grout or chemical grout for reinforcement before continuing drilling.

The general principle for equipment configuration is "one main rig, multiple backup rigs." Typically, a single main drilling rig (such as a multi-functional hydraulic drilling rig) is used as the foundation, but based on prior geological surveys, specialized drill bits, drilling tools, and treatment materials (such as plugging agents and casing) must be prepared for the anticipated complex strata. Flexible combinations and extensive field experience are often decisive factors for successfully traversing complex strata.