Existence of Flow Barriers Improves Horizontal Well Production in Bottom Water Reservoirs

Abstract

Abstract In developing heavy oil from thin bottom water reservoirs, horizontal wells are mostly used in Bohai Bay, China. To maximize oil production and avoid early water coning/cresting, horizontal wells are usually placed near the top of pay sands and wells are initially produced with small pressure drawdown. However, the production responses from different wells display significant variations even though they are operated under similar conditions. Some wells show premature water coning and rapid water cut rising although high quality sands are targeted, while others show much delayed water breakthrough and slower water cut increases. A series of reservoir simulations have been conducted to investigate the observed differences. The simulation results reveal that the existence of thin low permeable flow barriers with limited lateral extension/continuity between the wellbore and water/oil contact plays the most crucial role impacting the water coning characteristics. Wells with such flow barriers display later water breakthrough with steady increase of water cut after breakthrough, while wells without such barriers show quick water coning with water cut reaching more than 90% rapidly. The existence of low permeability barriers between the water/oil contact and horizontal wells could slow down water coning and result in much favorable production performance. This phenomenon is then verified by simulations and actual field data from QHD32–6 field. The accurate predictions of production performance rely on the knowledge of physical distribution of flow barriers relative to the wellbore location. In practice, lateral thin flow barriers are usually at sub-seismic scales, and thus hard to identify for a green field. However, for infill drilling in mature fields with many vertical wells drilled, it is possible to predict/correlate the spatial distribution of such flow barriers from the logs of existing wells. Based on such analysis, we can optimize the locations of horizontal infill wells to make full use of the flow barriers for improving production. A number of horizontal infill wells were optimized and drilled in QHD32–6 field, Bohai Bay, China after identifying flow barriers from the nearby vertical wells. These wells display good production performance having significant higher oil production with delayed water breakthrough and slower increase of water cut. Introduction The use of horizontal wells to enhance oil production from water drive reservoirs had been widely appreciated around the world 1–8. For thin bottom water reservoirs, the use of conventional vertical wells creates severe coning problems in most cases. This is because the actual rate of production usually substantially exceeds the critical rate defined as a rate above which the flat surface of water/oil contact starts to deform 9–13. Wells producing at critical water-free rates are usually unprofitable in practice. Horizontal wells are considered as a better alternative than conventional vertical wells to develop such reservoirs with better economics, improved oil recovery and higher development efficiency. Long horizontal wellbores would be able to contact large reservoir area. Therefore, for a given rate, horizontal well requires a lower drawdown, resulting in a less degree of coning/cresting.