Advanced techniques determine casing points while drilling

A new procedure based on detection / analysis of "wellbore breathing" lets the operator drill more footage before setting casing, while preventing mud loss and well-control problems

*D. Weisinger, **W. Bell, *B. Baker, Vastar Resources; D. Power and C. Hight, Halliburton Energy Services-Baroid; and T. Pruitt, Sperry-Sun Drilling Services

n deepwater drilling operations, the drive to set casing points as deep as possible is often compromised by the tight operating window between pore pressure and fracture gradient. As a result, the number of casing strings required is typically greater for deepwater wells than for shallow-water or onshore wells with the same drilled depth. A new procedure developed for early kick detection and quantification of wellbore breathing allows the operator to maximize the interval depth while avoiding lost circulation and preventing the well from flowing.¹ In this article, a review of this procedure is presented, together with examples of field data generated using the technique.

Loss-Gain Phenomenon

Wellbore breathing and / or ballooning is usually identified during connections when mud flowback is observed. When the pumps are switched on, pressure applied to the formation can open fractures, which then take fluid. Shutting the pumps off relieves the downhole pressure, allowing the fractures to close, subsequently displacing the same volume of fluid back into the wellbore. This phenomenon, wellbore breathing or ballooning, can often cause serious confusion, resulting in a breathing well being incorrectly diagnosed as a flowing well. An inappropriate response can, in this instance, serve to worsen the problem and lead to lost circulation. When drilling with oil- or synthetic-based drilling fluids, compressibility can also lead to further confusion.

The fluid-flowback phenomenon can be driven by the following: surface equipment drainage, fluid compressibility, thermal expansion, wellbore breathing, or an actual fluid influx or kick. Understanding the contribution of each of these factors to observed flowback on connections is critical for successful drilling. In deepwater drilling, thermal expansion and contraction can be a large contributor to flowback, although, typically, drilling fluid should contract when static in a deepwater well due to the large volume of fluid in the riser.

Compressibility is often overemphasized when considering causes of fluid flowback. Drilling fluid compressibility is definitely an issue when considering surface density vs. downhole density, but the difference in circulating density and static density is often less significant. One of the largest contributing factors is drainage of surface equipment.

The combined effects of thermal expansion / contraction, compressibility and surface equipment drainage should be evaluated while drilling the cement shoe at the top of an interval. This process generates a reference curve, whereby the flowback volume as a function of time is recorded. This curve can be used as a guide to determine the onset of breathing, or the onset of a kick when drilling the openhole section. This method has been proven to be a rapid and accurate method for early kick detection. Examples of these two scenarios are shown here.

Example 1: Onset Of Breathing

In this example, the onset of breathing resulted in a significant increase in fluid flowback. At the top of the interval, immediately after drilling through the shoe, the stable flowback profile showed the well to be returning about 80 bbl of fluid to the active pit system when the pumps were shut down within a 6-min. period, Fig. 1. On previous wells drilled with the same rig, similar flowback volumes were recorded for a stable well (typically 70 to 80 bbl).

Fig. 1. Connection flowback volume illustrating volumes associated with breathing wellbore.

When drilling ahead in the openhole section – after several instances of packing off – the stable flowback profile increased to over 140 bbl. The time for the flow to decrease to zero was also extended to over 20 min. This extended flowback time can greatly increase the cost of drilling if, on each connection, the operator chooses to wait until zero flow is observed before continuing to drill ahead. At this time in the drilling operation, it was also noted that seepage losses were occurring. On each connection, the volume returned on pump shutdown equaled the volume lost on pump start up – a classic indication of breathing for such large volumes.

In order to reduce the seepage losses, sized calcium carbonate was added to the fluid system. The calcium carbonate eliminated the seepage losses and simultaneously reduced the volume returned on connections. The calcium carbonate particles appear to have acted as bridging agents and thus minimized, though not fully eliminated, the degree of fracture opening when the pumps are on. After adding calcium carbonate, flowback volume was reduced to 112 bbl.

Example 2: Onset Of Kick

In the next example, the flowback monitoring technique is used to determine the onset of a well control scenario. Two flowback curves are shown for the top of the interval in Fig. 2. The flowback profiles are essentially identical and repeatable for the first two curves shown. The steady-state volume is about 100 bbl, with just over 8 min. being required for the flow to go to zero.

Fig. 2. Connection flowback data illustrating onset of well control.

The third curve shows a significantly higher flowback volume, which does not decrease over time to zero. One of the key distinguishing features of a flowback profile for a flowing well is the point of inflection indicated on Fig. 2. At this point, the slope of the curve actually increases, whereas, for a stable well, the slope should be continually decreasing. Increasing mud weight by 0.3 ppg killed the well.

When the well is flowing, formation fluid of lower density is entering the well, which acts to reduce hydrostatic pressure on the formation. The reduced hydrostatic pressure subsequently results in an increase in flow out of the well – without appropriate response, the consequences are obvious. A real-time application that captures and interprets the type of data presented in Fig. 2 provides an excellent tool for early kick detection, and a process by which the drilling crew can better understand what is occurring downhole. The program, Connection Flow Monitor or CFM (Trademark of Halliburton Energy Services.), allows the user to preset alarm levels that are activated once a certain return volume is exceeded. Any tool that can allow the user to listen and respond appropriately to what the well is indicating is a highly valuable asset.

Requirements For Monitoring Flowback

For the Connection Flow Monitoring system to be fully effective, a number of specific procedures must be followed. The volume of fluid returned to the pit system on pump shutdown is a function of the flowrate immediately prior to pump shutdown. Typically, the volume returned will increase with increasing flowrate. As such, it is imperative that the reference curve – a flowback profile generated while drilling through the shoe – be generated using a flowrate equivalent to that planned for drilling the openhole section. In addition, since the volume returned is dependent on drainage of surface equipment, should configuration of the surface equipment change during drilling, then a new reference curve should be generated. Without a valid reference curve or flowback profile, interpretation of any abnormal flowback behavior becomes more complicated.

It is worth noting that the IADC Deepwater Well Control Guidelines state specifically that a record should be made of any flow with the pumps off, including the volume of mud and length of time it flows.² Though this technique is not an all-encompassing, foolproof method for detecting kicks, when used diligently, it can greatly improve the ability of a drilling team to rapidly detect and respond to either kicks or breathing.

Well Construction Tool

In addition to determining if a well is flowing or breathing, the flowback monitoring procedure allows the operator to pinpoint location of the next feasible casing point in real time, without stepping out beyond the limits of the formation. The onset of breathing can be used as an indicator of imminent lost circulation, thus prompting the drilling crew to set casing. Theoretically, breathing onset, unless naturally prevalent, occurs because the fracture-initiation stress has been exceeded.

If the need arises to increase mud weight – such as in well control – the fracture-propagation stress may be exceeded, leading to formation breakdown and massive fluid losses. If the onset of breathing is detected, reducing mud weight could help alleviate the breathing and provide further operating space to continue drilling ahead. If the mud weight cannot be reduced due to pore pressure restrictions, then casing must be set. Using the flowback-monitoring procedure provides an ideal mechanism for determining drilling limits, avoiding lost circulation and minimizing the impact of kicks.

Summary

The examples outlined in Figs. 1 and 2 clearly demonstrate the potential advantages of a real-time, flowback-monitoring and recording system. The procedure described has been used successfully on many deepwater wells, to deliver improvements in rig safety, as well as significant cost savings. These cost savings can be realized through: reduced waiting time on connections, rapid response to well control, reduced seepage losses and reduced whole-mud losses through lost circulation.

Literature Cited

1. Power, D. et al., "Well characterization procedures for early kick detection and wellbore breathing," ETCE/OMAE Conference, New Orleans, Louisiana, February 14Ð17, 2000.
2. International Association of Drilling Contractors, IADC Deepwater Well Control Guidelines, Houston, Texas, 1998.

The authors

	David Power, applications specialist, Halliburton Energy Services-Baroid Drilling Fluids, is a member of Baroid’s High Impact Technology Team specializing in fluid hydraulics and well characterization procedures. He holds BS and PhD degrees in chemical engineering from the University of Melbourne.
	Tommy J. Pruitt, sr. ADT engineer, Sperry Sun Drilling Services, has been with the company since 1981. His present efforts focus on deepwater drilling operations in the Gulf of Mexico, currently with the VRI deepwater division.
	Robert V. Baker is a well design and installation engineer for Vastar Resources, Inc., in Houston. Before joining Vastar in 1994, he served as drilling engineer and drilling supervisor for several major oil companies. He earned his bachelor’s degree in civil engineering from California State Polytechnic University in Pomona, Calif., in 1979.
	Carl Hight is Baroid’s in-house deepwater tech for the Vastar Ocean Victory rig. He has been with Baroid for 26 years, working in both domestic and international operations. He hods a BS degree from Louisiana State University.
	Don Weisinger is deepwater drilling team leader for Vastar Resources, Inc., in Houston, a position he has held for the past 2-1/2 years.