Pressure Measurements Plus Simulation Help Differentiate Between Downhole Events
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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.
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.
Understanding Wellbore Events
It is essential to be able to differentiate between each possible wellbore event because an inappropriate response can worsen the problem and lead to lost circulation or a loss of well control. This study uses pressure-while-drilling data and transient-simulation software to analyze the causes for the ECD change during a flow check or a connection.
A flow check is performed with the objective of ensuring stable well conditions. It usually consists of stopping the current operation, spacing out from the rotary table to allow shut-in of the well in the case of flow. Then, the well is monitored for a given period. Monitored parameters usually include pit or trip-tank volume and downhole-pressure changes. In the case of riserless operations, robotic cameras are used to monitor mud flows at the wellhead and look for indications of gas in the mud.
A connection is defined as adding/removing a length of drillpipe to/from the drillstring. To do so, mud circulation needs to be stopped, which means that every connection is effectively a small-scale flow check.
Pressure While Drilling—ECD Management. Typically, the bottomhole assembly is equipped with one or several downhole pressure-monitoring devices that measure annular pressure. While drilling, the pressure is transmitted continuously to the surface by means of mud-pulse telemetry. During pump-off events such as connections, flow checks, leakoff tests, and formation-integrity tests, the pressure data are buffered in the downhole-tool memory and sent to the surface by means of mud-pulse telemetry once mud circulation is re-established.
From the measured pressure and true vertical depth, one can infer an equivalent density, typically referred to as ECD. It is technically the equivalent mud density when the mud is circulating. When the mud is not circulating, equivalent density is referred to as equivalent static density (ESD). ECD is often used as a general term to encompass both ECD and ESD and is an important parameter that represents the integrated measure of the fluid behavior in the annulus.
If the measured ECD trend deviates from what is expected, corrective or other responsive steps may be taken to try to keep the ECD within the desired range. Measured ECD as a downhole parameter can help prevent costly drilling problems and can aid in positively identifying kicks, inflows, and other events that can lead to unsafe drilling conditions. Some of those events are described in Fig. 1 above.
Fig. 1 shows the typical bottomhole ECD response during a flow check. The blue line represents the pump rate from the stand pipe. The green line shows the mud weight. The orange line shows the ECD at the depth of the annular-pressure-while-drilling sensor.
- In a normal flow check, a square-shaped ECD is expected.
- In a well-ballooning situation, the ECD curve normally would follow an exponential drop off when the pumps are shut down. As pre-existing and drilling-induced fractures close when the annular pressure is lowered below the fracture-propagation pressure, the flow from those fractures compensates the ECD drop, leading to a delayed stabilization of the downhole annular pressure. That pressure will remain above the surface mud weight.
- During a kick, the ECD will show a declining trend and the minimum ECD during the kick flow check is typically lower than the mud weight because of the lightweight influx.
- Finally, in the case of thermal expansion, the ECD curve will take a shape very similar to that of the kick ECD, but the minimum ECD value is not less than the mud weight.
Pit-Volume Change and Differential Flow Rates. As well as monitoring ECD, drilling engineers closely monitor pit gain/loss rates and volumes. Those are different for wellbore ballooning, kick influx, and thermal expansion. Precisely measuring the flow rate in and flow rate out can help identify the reasons for the pit volume change. Wellbore ballooning displays mud loss once pumps are initiated. In the first few minutes, the flow rate out is less than the flow rate in and a continuous seepage loss during circulation would occur. For thermal expansion, there is no obvious flow-rate difference once a circulating temperature profile is established.
A multiphase transient-simulation software was used to carry out this study. For the software, three momentum equations are used, one for each of the continuous liquid phases and one for the combination of gas with liquid droplets. This yields seven conservation equations and one equation of state to be solved. Of the seven conservation equations, three are for mass, three are for momentum, and one is for energy, while the equation of state is for pressure.
Well Description and Preparation. The well is an offshore vertical well in the Gulf of Mexico. Several offset wells in the area encountered shallow flows and hole problems. In addition to the offset data, the planned well trajectory encountered numerous faults extending from the top of salt to the mudline. These factors led to a large amount of pore-pressure uncertainty and drilling risk.
To assess those risks, specific prejob modeling was performed. Specifically, the objective was to establish if a high-pressure kick could be killed dynamically with water-based mud.
Results. An analysis of four events shows that combined drillstring U-tube effect and gas-kick ECD lead to a pump-off pressure profile very similar to that of wellbore ballooning. However, the minimum ECD was lower than the mud weight. Gas influx is assumed to be a small volume because the amount of gas bubbles at the wellhead was relatively minor.
For 14.7-lbm/gal mud, on the basis of a simulation, the total volume of flow was 51 bbl in the first 9 minutes during flow check. A volume of 35 bbl is from flowback because of well ballooning. A volume of 16 bbl was from the drillstring U‑tube effect. As the mud weight increased from 14.7 to 15.0 lbm/gal, wellbore ballooning worsened and the volume of flowback because of wellbore ballooning reached 37 bbl.
The transient-flow simulator, considered with pressure-while-drilling pumps-off annular-pressure data, makes it possible to investigate the reasons for the downhole pressure change and equivalent-mud-density changes in a riserless drilling environment. A lack of data for pit volume, flow rate out, and gas-compositional analysis makes differentiating wellbore ballooning, thermal expansion, and gas influx very difficult. Furthermore, continuous ESD profiles usually are not readily available while drilling and the operator must wait for the tool memory to be retrieved once the bottomhole assembly is available. The availability of the pumps-off annular pressure immediately after a flow check makes this work flow efficient and valuable for troubleshooting downhole drilling events as a post-analysis to determine with more certainty what actually happened as learning for further operations.
The effects of U-tubing must be included to obtain a proper understanding of what is happening downhole. A transient-flow simulator facilitates more-complete understanding of these interactions.
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