The present study focuses on the description of a methodology that is used to elaborate a complete analysis of a blowout in the field of Mexilhao, which is the largest undeveloped gas accumulation in Brazil. The methodology comprises a hazard and operability analysis (HAZOP) and a quantified risk assessment (QRA), which are necessary processes for identifying and prioritizing potential hazards. In addition, the combination of the HAZOP with the QRA makes easier the introduction of appropriate measures for reducing overall operational risks. Besides those activities, some details of a blowout contingency plan are presented, including bubble plume calculations, relief well planning, and dynamic killing design. Introduction The Mexilhao field is the largest undeveloped gas accumulation in the Brazilian Continental Shelf, in Santos Basin, in water depths ranging from 320 to 500 m. The field is situated approximately 137 km from the coastline (see Fig. 1) and reserve estimates are in the 90 billion Sm3 of gas and 6 million m3 of oil range. The reservoir is a sandstone and the original pressure is 9,774 psi (67.4 MPa) at 4,656-m of vertical depth. The field development plan is based on the construction of seven subsea wells (6 horizontals and 1 vertical) connected to a subsea manifold that will be responsible for exporting the production to a fixed platform located in 170 m of water depth, at about 20 km from the field. The planned start of the production is scheduled for 2008, reaching the production peak of 10 million Sm3 of gas per day in 2009. Some details of a typical horizontal well in the Mexilhao field are presented in Fig. 2. The well is of conventional design utilizing a 30" conductor pipe, which is set at about 75 m bellow the bottom of the sea. The conductor pipe is followed by a 26" hole section and the 20" surface casing is set at 1,250 m. After cementing the 20", the 17–1/2" hole section is drilled and the 13–3/8" casing is set at 2,500 m. The construction of the directional trajectory starts while drilling the 12–1/4" hole section. The kickoff point is placed at 4,100 m and the build-up ratio is at about 30/30 m. The 12–1/4" hole section is drilled down to 4,705 m (MD, TVD 4,573 m), just above the zone of interest. After cementing the 9–5/8" casing, the 8–1/2" pilot section is drilled to facilitate the construction of the final section in accordance with the reservoir needs. The production section is drilled with 8–1/2" bits through the reservoir rock down to 5,584-m (MD, TVD 4,755-m). After reaching TD, a formation test is performed and the well is completed in line with the sketch presented in Fig. 2. As the gas production flow rates of the horizontal wells may be higher than 1 million Sm3/day, the planned drilling and production activities on the Mexi1hiio field meet considerable challenges for safety in drilling and well operations. A comprehensive HAZOP, including risk assessment, is performed and the main findings are presented. Not only this HAZOP study but also some of the blowout contingency conclusions are actively used in the process of well design as an important risk reduction tool. Most of the practical implications of a blowout intervention project are addressed, including intervention strategy, pumping requirements, and mud storage. The paper also presents details of the blowout rate calculations and relief well kill requirements. It is worth to mention that the necessity of using a relief well would involve high pumping rates and challenging requirements.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe present study describes the steps taken to drill the final section of an exploratory well in Santos Basin in the underbalanced (UB) condition, in 1,500-m (4921-ft) of water depth. The first well in the area reached a fractured carbonate reservoir in which massive losses of circulation caused so many operational difficulties that the formation evaluation was not conclusive. As consequence, in the third quarter of 2005, another well will be drilled, reaching the same reservoir in the UB condition. Pre-planning and planning activities, additional required equipment, riser modifications, and new procedures are presented and discussed.
MPD techniques are used nowadays to precisely control the pressure profile throughout the wellbore. They are usually employed in drilling with narrow pressure windows, such as through highly fractured reservoirs, regions near depleted wells, regions with small fracture pressure, among others. Low drilling pressures may result in undesirable influxes from reservoir fluids, whereas high drilling pressures may cause mud loss and may damage the near-well region, reducing the future well productivity. This work details the development of a MPD simulator (hydraulics and control strategy). The hydraulic model is transient and based on the explicit and simultaneous solution of mass and momentum flow equations. This model is different from the usual control purpose hydraulic models since the well geometry is actually discretized, allowing a more precise determination of the pressure profile along the well. Furthermore, this discretization also enables the simulation of the delay between an action at the choke and the corresponding response at the bottom hole, which is fundamental for a realistic evaluation of a control strategy. A nonlinear control strategy was also created in this work. This strategy is based on the aim of a smooth exponential evolution of the choke pressure until it reaches its setpoint. The execution of the hydraulic simulation several times is not required, as in typical model predictive controllers (MPCs). The control strategy is also not so susceptible to control parameters, as in PID classical controllers. An experimental drilling plant was employed for validating the nonlinear control strategy. The nonlinear control algorithm and the classic control scheme were compared. Using real-time measurements, the automated continuous drilling unit performance is analyzed under a scenario of flow disturbance. The fast and precise hydraulic model coupled with an efficient control method makes the present simulator feasible for real-time applications.
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