Consequences of Formation Breakdown During Well Control: A Study of Underground Crossflow While Drilling an HPHT Well
Authors: Element D.J. , van der Vossen H. , Diamond S. , Hamilton T.A.P. , James J. 
Published: December 2007

D.J. Element, SPE, AEA Technology plc, H. van der Vossen, S. Diamond, Shell U.K. Exploration & Production (Operator in the UK Sector of the North Sea for Shell & Esso), T.A.P. Hamilton, SPE, Offshore Safety Division, UK Health & Safety Executive, J. James, Anadrill

Introduction
Formation breakdown during well control operations can lead to underground flow between the kicking formation and the loss zone. Such crossflow will complicate well control operations, making it difficult to interpret the behaviour of the well from pressure and flow measurements at the surface. Increased costs and threats to safety can result from this uncertainty. This paper presents results from a project to study the behaviour of an HPHT well kick using a gas kick simulator to reproduce surface pressure measurements observed during a period of underground crossflow The project required some simulator development and included evaluation of the sensitivity of the results of the simulation to assumptions about unknowns - e.g. the depth of the loss zone.
Analysis of such underground crossflow scenarios has not previously been discussed in detail in the open literature, in spite of the fact that a number of studies of kicks and well control have been reported in recent years.
The selected case study concerns a gas condensate kick in a Central North Sea HPHT well. The kick was taken in the 12 1/4 in. hole section at a depth of 13,858 ft, while drilling the Hod formation in the Chalk Group. Unexpectedly high formation pressures meant that the well was underbalanced by some 1,500 psi at the time. The tight nature of the kick zone limited influx rates even at this high underbalance. During the shut-in period which followed kick detection, surface pressures increased to over 1,000 psi, causing the formation to fracture. Following the fracture, influx and losses continued for a considerable period.
This study has made it possible to understand the variations observed in surface pressures during the extended shut-in period following the formation breakdown.
The analysis of the underground crossflow case study has given physical insight into complex downhole phenomena. On the basis of the study comments are made concerning the choice of actions to take following formation breakdown during well control.
Further simulations have been employed to model the theoretical behaviour of other, shallower wells, comparing the results for oil-based and water-based muds.

Field Example
The case history discussed in detail in this paper concerns a near-vertical HPHT well. The 13 3/8 in. shoe was placed at 11,261 ft. While drilling 12 1/4 in. hole in the Hod formation (Chalk Group) at a depth of 13,858 ft the well took a kick, which was later determined to be condensate. 15.7 ppg pseudo- oil-based mud was being used. The stopping and starting of pumps for a directional survey and the slow rate of influx meant that the kick was not detected until about an hour after the influx began. By the time of detection, some 20 bbl of condensate had entered the well, although at the time of the incident reported pit levels were interpreted as indicating a 10 bbl gain, followed by a further 3 1/2 bbl gain during the flow check. (For another example of a survey causing a delay in kick detection, see Frigaard)
The well was closed in, but initially the surface drillpipe and annulus pressures (Pdp and Pann) remained steady at approximately 80 and 35 psi respectively. After some twenty minutes of shut-in both Pdp and Pann began to build-up at a noticeable rate. When the pressures began to rise (prior to the start of the fracture), Pdp exceeded Pann by some 180 psi. No logical explanation has been found for the delay in pressure rise and the observation that Pdp exceeded Pann. P. 145^