Karst Annular Seal: Lost Returns and a Down-Gradient Spring
A 10-inch borehole is drilled to 220 ft through 20 ft of clay and then karst limestone. A 6-inch PVC casing is set to 180 ft, leaving a 2-inch annulus. A void is encountered at 100 ft; drilling-fluid returns are lost, and foam is used to total depth. The contractor grouts the annulus with a 15%-solids bentonite-cement mix pumped at 10 gpm. The contractor pumped 1.4 times the calculated annulus volume. Grout returns at the surface after 45 minutes. After 24 hours, the grout level in the annulus has dropped 12 ft. A spring 300 ft down-gradient is the client’s water supply. After development, turbidity is 25 NTU and the spring shows a slight increase in turbidity. The state requires a verified annular seal and a 24-hour set before drilling out.
1. Interpret the surface return, the 12-ft drop, and the spring turbidity. Is the annular seal adequate? Explain.
Read the worked answer
Surface return after 45 minutes only proves the grout reached the top of the annulus; it does not prove the annulus is full. The 12-ft drop after 24 hours is a red flag: grout either flowed into the 100-ft void, bridged, or was lost to a fracture. Because the void is behind the casing, a single-stage tremie pour can leave a void or a low-density grout column. The spring turbidity could be from drilling disturbance, grout travel, or development; it is not proof of a bad seal, but it means the seal is not verified. I would not accept the seal based on surface return alone. Check the grout volume, temperature/conductivity logs, and downhole camera, and calculate whether the volume matches the annulus plus the void.
2. Design a remediation and verification plan that protects the spring. Include grout type, placement, and testing.
Read the worked answer
Isolate the loss zone rather than simply re-grouting the whole annulus. Use a packer or staged grouting in the annulus, tremie from the bottom, and a lower-density, thixotropic bentonite-cement or neat cement with accelerator. Pump slowly, monitor pressure and spring turbidity, and stop if pressure rises abnormally or the spring muddies. If the void at 100 ft is taking grout, stage the grout in lifts and wait between stages. Verify with a downhole camera, grout-level measurement after 24 and 48 hours, volume reconciliation, and a pressure test if the state allows. Sample the spring before, during, and after, and have a contingency plan to stop work if turbidity increases sharply.
3. What documentation and notifications would you complete, and what would you tell the client about the spring?
Read the worked answer
Document the borehole log, casing depth, annulus volume, actual grout volume, mix design, pump pressure and rate, return time, 24-hour level drop, development method, turbidity readings, and spring observations. Notify the state regulator of the lost-circulation zone and the seal verification issue before drilling out or completing. Tell the client plainly that the spring is down-gradient of a karst void, that the seal is not yet verified, and that turbidity in the spring must be monitored. Do not tell the client the well is sealed until the verification data support it. Recommend baseline spring testing and a written monitoring plan; if the spring is their sole supply, provide an alternative water plan during remediation.