Abstract: For a wellbore drilled in a low or subnormal pressure reservoirs, a static loss rate of drilling fluid is monitored within a limit of a drilling rate. In reaching the limit, the annulus is closed off to returns using a rotating control device, or the annulus may remain open to the atmosphere at surface. Operations may not be able to keep the annulus filled with a mud cap so pressurized mud cap drilling cannot be sustained. Instead, an initial fluid level of the mud cap is defined in the annulus. Drilling the wellbore with the mud cap then involves: pumping a sacrificial fluid through the drillstring without returns to surface through the annulus, and monitoring the initial fluid level in the annulus to detect a change. Monitoring uses downhole instrumentation to measure pressure, temperature, and gas level of the mud cap. In response to the detected change, the drilling can be further controlled, including stopping the drilling, turning off pumps, and possibly bullheading the well.

Abstract: For a wellbore drilled in a low or subnormal pressure reservoirs, a static loss rate of drilling fluid is monitored within a limit of a drilling rate. In reaching the limit, the annulus is closed off to returns using a rotating control device, or the annulus may remain open to the atmosphere at surface. Operations may not be able to keep the annulus filled with a mud cap so pressurized mud cap drilling cannot be sustained. Instead, an initial fluid level of the mud cap is defined in the annulus. Drilling the wellbore with the mud cap then involves: pumping a sacrificial fluid through the drillstring without returns to surface through the annulus, and monitoring the initial fluid level in the annulus to detect a change. Monitoring uses downhole instrumentation to measure pressure, temperature, and gas level of the mud cap. In response to the detected change, the drilling can be further controlled, including stopping the drilling, turning off pumps, and possibly bullheading the well.