Frequent Lost Circulation and Borehole Breakout While Drilling? Have You Identified the Real Causes?

Drilling is a core operation in geological exploration, mining, pile foundation construction and other engineering fields. However, projects often encounter the problem of borehole breakout and “losses while drilling” (rapid drilling-fluid loss). This not only leads to large volumes of mud loss and borehole collapse, but also delays schedules, raises costs, and can compromise borehole quality and subsequent construction safety. Based on field practice, this article analyzes the root causes of lost circulation and proposes targeted countermeasures to guide similar projects.

Core causes of lost circulation Lost circulation is rarely caused by a single factor; it typically results from the interaction between formation conditions and operational practice. The causes can be grouped into two major categories:

1. Geological factors — inherent formation constraints The formation itself is the primary predisposing factor for lost circulation. When drilling intersects certain geological units, the risk of mud loss rises sharply:

• Loose sand layers: Large intergranular porosity and weak cementation make it easy for drilling fluid to rapidly seep away through pore spaces. Losses are particularly pronounced in fine sands and silty layers.

• Fractured rock masses: Carbonates, granites and other rocks with developed fractures, joints, or solution cavities contain irregular pathways through which mud can escape quickly, producing the “losses while drilling” phenomenon.

• Other adverse units: Conglomerates, faulted and crushed zones have loose, unstable structures that not only promote fluid loss but also increase the risk of borehole collapse, compounding construction difficulty.

2. Operational factors — controllable construction influences Even in favorable geology, improper operations can trigger lost circulation. Two operational issues are most critical:

• Excessive drilling speed: Pursuing rapid advance by using overly high rotation or penetration rates causes intense disturbance of the borehole wall, destroys its near-field stability, produces cracks or spalling, and allows drilling fluid to escape.

• Inadequate drilling fluid properties: Drilling fluid is the key barrier protecting the borehole. If viscosity is too low it cannot form an effective filter cake; if density is insufficient it cannot balance formation pressure. Either condition prevents effective sealing of pores and fractures and leads to losses.

Targeted countermeasures Addressing lost circulation requires a “prevent first, then remediate” approach with layered and graded treatments that tackle the problem at its source.

1. Optimize drilling-fluid properties to strengthen borehole protection Drilling fluid is the first line of defense against loss. Adjusting the fluid formulation can significantly improve sealing and structural support:

• Increase viscosity and density: Add bentonite, cellulose derivatives, polyacrylamide and similar additives to raise viscosity so the fluid can better adhere to the borehole wall and form a filter cake; moderately increase density to provide greater sealing pressure against formation pores and fractures.

• Targeted modification: For loose sand zones, add fibrous materials such as sawdust or rice husks to improve suspension and plugging ability; for fractured zones, add rapid-setting agents (cement, sodium silicate) so the fluid can quickly gel in fractures and form a seal.

2. Place plugging agents to seal active loss points When losses occur, promptly introduce plugging materials into the bore to directly fill pores and fractures and block fluid pathways:

• Common plugging materials: Clay (bentonite), sawdust, cement, cotton fiber, walnut shells, etc. Choose materials based on loss severity. For minor losses, clay or sawdust may suffice; for moderate losses, a cement-clay mixture uses cement’s hardening to enhance long-term sealing.

• Placement method: Use staged placement and “drill and plug” techniques—slowly feed materials into the loss zone while maintaining mud circulation so the materials are carried to the leak paths, fill voids, and form a stable plug that prevents further loss.

3. Control drilling parameters to reduce borehole disturbance Proper operational practice is essential to prevent lost circulation by minimizing damage to the borehole wall:

• Reduce penetration rate: Lower rotation speed and feed rate, especially when crossing loose sands or fractured strata; adopt a “low rpm, small advance” approach to reduce the impact of the bit on the wall and limit disturbance.

• Allow time for filter-cake formation: Slower drilling gives the fluid time to deposit a uniform, dense filter cake on the borehole wall, further consolidating the wall and sealing micro-pores to reduce loss risk.

4. Refill and re-drill for severe losses When losses are severe and routine plugging fails—or when large-scale borehole collapse occurs—refilling the interval and re-drilling may be required:

• Choice of refill materials: Prefer clay, cement slurry, or a clay–cement blend. Clay is low-cost and effective for sealing; cement slurry hardens to provide high-strength reinforcement of the borehole wall.

• Refill and re-drill procedure: Fill the loss interval and a margin above it with the chosen material until the affected section is filled. Allow the material to cure sufficiently (typically 24–48 hours, adjusted by material and formation conditions) to form a stable seal, then resume drilling. On re-drilling, again optimize the drilling fluid and control penetration to prevent recurrence.

Conclusion Borehole breakout and losses while drilling are common and arise from both inherent formation conditions (loose sands, fractured zones) and controllable factors (excessive drilling speed, poor drilling-fluid performance). To manage the risk effectively, conduct pre-construction formation surveys to anticipate loss-prone zones; during construction, prevent losses by optimizing fluid properties and controlling drilling parameters; and if losses occur, apply a graded response that includes plugging, refilling and, if necessary, re-drilling. Adopting a “prevention-first, remediation-second” mindset and adjusting measures to match formation characteristics and field realities are essential to overcoming lost-circulation problems, ensuring smooth drilling operations, and improving construction quality and efficiency.