RIASSUNTO
Abstract
Developments in extended reach drilling and completion technologies allow to economically access a number of scattered small hydrocarbon pockets and will open up further opportunities for maximizing recovery from these fields. Effective use of these developments requires us to better understand the transient multiphase flow behaviour.
Undulation is associated to horizontal wells with some degrees of deviation from the horizontal. The inclination angle could be a result of a lack of sufficient drilling control or could be designed on purpose, for instance, fish-hook wells, snake wells and undulating wells. A complicated and undulating trajectory may initiate severe slugging at the bottom of a wellbore. In this paper, OLGA, a commercial transient two-fluid multiphase flow simulator, and Cheng's inflow performance relationship were coupled together to characterize severe slugging. Simulation shows that severe slugging is formed at the bottom of the wellbore and moved up to the surface. Furthermore, it creates pressure pulsation at the bottom of the wellbore that can influence the reservoir performance.
Introduction
Fish-hook wells, snake wells and undulating wells are relatively new technologies and have been used to develop hydrocarbon fields more efficiently than conventional wells. Much of the world's oil exists in scattered and isolated pockets. As these deposits are small, it is not economically feasible to exploit them in a conventional way. Based on their geological settings, the optimal well trajectory could be a fish-hook well, where its geometry is in the shape of a fish-hook with the deepest reservoirs drilled first and the shallowest reservoirs at the end of the wellbore. It could also be a snake well that follows a complex undulating path and snakes back and forth to reach a number of different reservoir pockets. Another example is an undulated horizontal well for an extremely anisotropic formation, when vertical permeability is much smaller than horizontal permeability. In that case an undulating well may be drilled to overcome the low vertical permeability and increase the well productivity.
As a horizontal well gets longer and follows a more complicated and undulating trajectory, wellbore hydrodynamics plays an important role in well performance, especially when two-phase gas-liquid flow is involved.
In this paper we address a phenomenon named severe slugging, happening at the bottom of a wellbore close to hydrocarbon reservoirs. At relatively low gas and liquid flow rates, liquid will have the tendency to accumulate at a low spot in the wellbore, blocking over time a free passage of the gas phase. However, as production at the well continuous, the compressed upstream gas will flush out the liquid phase that has been accumulated in this lower part of the wellbore.
Thereby, it creates a relatively long liquid slug that is pushed out of the wellbore. After this liquid surge, and subsequent gas surge, part of the liquid in the wellbore falls back to this low spot to create a new blockage and the cycle repeats. These slugs create potential problems for the receiving surface facilities like separators, pumps, and compressors, which are designed to operate under steady state conditions. Moreover, they create pressure oscillations at the bottom of the wellbore that can influence the production rates at the oil and gas well.
Severe slugging was first reported in offshore oilfield production systems by Yocum (1973). In an offshore oil and gas production facility, pipeline-riser systems are required to transport multiphase hydrocarbons from wellheads to a central production platform. Severe slugging in pipeline-riser systems has been studied by several investigators. In Fig. 1 different stages of a cycle of severe slugging are illustrated.