Management Of High Pressure Water Flows While Drilling A Large Gas Reservoir
Authors: Witt Joseph William , Gahr Jason Z. , Aly Maher A. , Super Dan W. 
Published: February 2016

An extensive drilling program is ongoing for the development of Qatar's North Field. This massive offshore carbonate field contains natural gas produced from the thick Khuff reservoir at approximately 10,000 ft true vertical depth (TVD). Capable of flow of over 200 MMScf/D each, these complex wells pose considerable challenges to both drilling and completion operations. One of the most significant challenges is over-pressured water flows which are encountered randomly within the primarily shale- and clay-stone section above the production zone. The major risks associated with these high-pressure flows include the masking of gas kicks within the massive sour gas reservoir below it; reduction of hydrostatic head during underbalanced operations and trips; added complexity to any simultaneous lost returns events; obtaining the required open-hole logs; and adequately cementing the production liner. During the ongoing program, the development of an advanced engineering and operational response to these persistent water flows has dramatically reduced their time and cost impacts without compromising safety or well objectives.

Acquiring quality data to determine the characteristics of the "water flow reservoir" is the first step in effectively managing each well control event. Shut-in data is collected first to verify the influx is due to a water flow interval. If it is verified the influx is due to a water flow, inflow testing is done to assess the deliverability characteristics of the water-flow reservoir. Empirical equations derived from inflow test data are input into a model to calculate how the pressure exerted by the fluid column at the top of the gas reservoir will decrease over time given a specified mud density. The model assumes no wellbore circulation, as its primary purpose it to predict how long the fluid column pressure will remain overbalanced to the gas reservoir pressure during trips or other periods when the well is not circulated. Bottomhole pressure (BHP) vs. Time modeling is also used to determine what mud density will be required to be able to drill, log, and case the production-hole interval. If it is too high, the flow zone is then depleted until subsequent inflow data and modeling indicate the well can be finished with a resonable mud weight (MW). Weighting up to kill the zone or depleting the zone to a zero flow state is not practical.

This paper discusses data collection, modeling, unique well-control risk-management practices, and operational procedures developed for effective wellbore construction in the presence of a persistent water flow.

Introduction

Fig. 1 shows the location of the Qatar North Field.1 Fig. 2 shows the field stratigraphy and a typical optimized big bore (OBB) wellbore configuration.2 The majority of the wells are similar in lithology, hole size, casing configuration, and casing setting points. They differ primarily in inclination and throw. Approximately 10 percent of the wells are vertical, with the remainder varying from 55 to 660 inclination to approx-imately 10,000 ft TVD.3 To date, over one million ft of hole has been drilled by the operator in this field. Previously published work addresses the drilling and completion of these complex wells.1

The operator's Drilling and Completion Department has experienced 12 production-hole water flow events during its ongoing Qatar North Field development drilling program. Eight of these were low-flow-rate (&lt; 2 bbl/hr) events, which were more operational nuisances than barriers to operations. Four of the events had initial flow rates from 15 to 60 bbl/hr and posed significant challenges to continuing operations.

The water flow zones are encountered in the Sudair formation, which is above the Khuff gas producing horizons. The Sudair primarily consists of shale and claystone with interbedded stringers of dolomite and anhydrite. Internal geological studies have been unable to predict where the flows are likely to occur; however, they have been able to correlate the flows to a 5- to 10-ft thick dolomite stringer. The permeability of this stringer varies over the extent of the field. The high flow-rate stringers have pore pressures of up to 16 ppg equivalent mud weight (EMW) and produce brine water. See Table 1 for brine characteristics.