The present study aims to propose a methodology to treat probabilistically the prediction of well bottom pressures during drilling, circulation and tripping operations. The pressure generated by the fluid is the primary safety barrier and its failure can immediately initiate gains and losses events in the well, with a direct impact on the safety of the process. Two models were considered in the study: a two-phase flow model commonly used to predict hole cleaning and downhole pressures while drilling and a pressure propagation single flow transient model (which considers fluid compressibility and gelation) to account for peaks in tripping and pump restarts. A Monte Carlo method coupled with a latin hypercube strategy was implemented to propagate the uncertainties in the input variables to the resulting pressure. The probabilistic approach aims to consider the uncertainties in the input variables (weight, rheology, penetration rate, drillstring velocity and acceleration, pump flow, particle diameter, well trajectory, etc.). Monte Carlo Simulation was conducted to generate the pressure distribution curves in each of the operations: drilling, circulation, tripping and pump restarts. Probabilistic pressures are fit into probabilistic operational windows allowing the quantification of the risk of losing the barrier. The paper also details the critical aspects associated with typical pre-salt well drilling project conditions in offshore Brazil where narrow operational widows are a reality. The analysis directly impacts the choice of drilling strategy (conventional or MPD) and, consequently, rig selection. Innovative strategy to enable risk assessment strategies in the analysis of safety barriers. Pilot to a major integrated approach which will include other barriers such as cement, rock and equipment.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDifferential pipe sticking is still today a challenging problem in horizontal well drilling. The main focus of this paper is the analysis of the dynamics of substitution of a viscous fluid (the drilling fluid) by a less viscous one (the spot fluid) in a fully eccentric horizontal annulus. Initially, an experimental study aiming the determination of the flow rates required for the substitution was performed on a pilot scale flow loop using flow visualization techniques. The tests reproduced the drilling of 8 ½ in horizontal wells. An extra test was performed to reproduce the 12 ¼ in phase. Based on results obtained from a 2D simulator predicting the critical shear stresses in horizontal eccentric annular flows, a mechanism for the efficient displacement of spot fluids is proposed.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper deals with the adaptation of synthetic drilling fluid for use in milling operations in offshore wells. Main topics covered include: a discussion on the fundamentals of solids transport highlighting the critical aspects of transporting iron particles in offshore wells; a computer simulation task, based on solids transport mechanistic models, to define minimum requirements for fluid flow rates and rheological properties; an extensive experimental work comparing the carrying capacity of water and synthetic based fluids and; the process of field implementation concentrating on well succeeded experiences in the offshore fields operated by PETROBRAS.
The reduction of the number of sections in the construction of an offshore well is a major driver for cost reduction. In order to achieve this goal, the sections must be longer, resulting in additional challenges. A critical aspect is related to the second section (no fluid return to surface) reaching high inclinations and longer extensions. Such sections are drilled with fluids with no carrying capacity, reaching 1500 m and inclinations as high as 65 degrees, resulting in expressive cuttings accumulation in the wellbore annulus. The scope of this article is to present a novel hydraulics design procedure which accounts for the effect of hole cleaning actions during well construction which reposition hydraulic parameters into acceptable criteria. This paper details novel design procedures which include the following steps: Defining cuttings bed height deposited along the wellbore annulus using traditional steady state cuttings transport models. This information is the initial condition for the design of hole cleaning pills pumping. Estimation of the extension of the cuttings bed through a material balance. Estimation of drag efforts for drillstring movement in the presence of a cuttings bed and the maximum well length which can be handled by the rig without carrying extra hole cleaning procedures. Calculation of the solids removal due to the pumping of hole cleaning pills based on a transient solid-liquid model. Optimization of the frequency, volumes and pump rates for the cleaning pills. This way, the proposed methodology allows the creation of a hydraulics design package which incorporates the optimization of hole cleaning actions during the drilling of a deviated well with fluids which do not present cuttings carrying capacity. The procedure was successfully implemented in 4 offshore wells in Campos basin, offshore Brazil, which were built in only 3 sections including a horizontal section. The present methodology is a milestone in hydraulics design for offshore wells, enabling the construction of horizontal wells in 3 phases resulting in a huge well construction time reduction.
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