Pressure Measurements Plus Simulation Help Differentiate Between Downhole Events
Authors: Wilson Adam 
Published: December 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IADC/SPE 178835, “Differentiate Drilling-Fluid Thermal Expansion, Wellbore Ballooning, and Real Kick During Flow Check With an Innovative Combination of Transient Simulation and Pumps-Off Annular Pressure While Drilling,” by Zhaoguang Yuan, Dan Morrell, SPE, Aldrick Gracia Mayans, SPE, and Yahya H. Adariani, Schlumberger, and Matthew Bogan, Noble Energy, prepared for the 2016 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 1–3 March. The paper has not been peer reviewed.

Drilling-fluid thermal expansion, wellbore ballooning, and formation kick are similar in terms of surface observations such as pit volume gain. Each of these events, however, is solved in different ways. Treating wellbore ballooning the same way as a kick likely will result in losing the current borehole after days or weeks of unsuccessful operations. In this study, pressure-while-drilling technologies are combined with software simulations to differentiate drilling-fluid thermal expansion, wellbore ballooning, and formation influx during riserless drilling operations.

Wellbore Events

Thermal Expansion. Because mud density is dependent on temperature and fluid compressibility, volume gains or losses because of thermal effects may be substantial, especially in high-pressure/high-temperature and deepwater wells. Thermal expansion typically results in small volume changes and low flow rates because it takes time for the mud to heat up after circulation stops. Depending on the downhole conditions, however, muds can heat up sufficiently to produce significant flowback for a short period of time.

Formation-Fluid Influx. If the mud-weight hydrostatic pressure is insufficient to contain formation influxes, when the pumps are shut down, the loss of the frictional pressure created during pumping can allow formation fluid to flow into the wellbore, assuming the formation fluid has sufficient mobility. This is described as a kick, or formation-fluid influx. It is verified by performing a flow check and observing mud returns at surface over time to determine a trend in pit gain. A steady increase or accelerating trend will be interpreted as a kick, although, in many cases, the well will be shut in before a clear trend can be established.

U-Tube Effect. In riserless drilling, two different fluid densities exist—in the annulus (mud and seawater) and in the drillstring (mud). Because fluids flow from a higher-pressure area to a lower-pressure area, a U-tube effect will occur once the pump stops. This will show as flow at the wellhead with a high flow rate initially, declining as the U-tube effect equalizes. The volume contribution from the U-tube effect is relatively simple to quantify, so, in the cases where surface volumes are measured, this effect will be somewhat easier to distinguish.

Wellbore Ballooning. Changes in equivalent circulating density (ECD) and hydrostatic pressure can result in wellbore ballooning, where the formation takes drilling fluid when pumping and the injected fluid then flows back into the well when the pumps are shut down.