Fracturing Horizontal Wells to Offset Water Breakthrough in Naturally Fractured Reservoirs

Abstract

Abstract Horizontal drainholes in thick, high-permeability reservoirs have been used effectively to produce many reservoirs in the Middle East. Many of these bottom-water-drive reservoirs have natural fractures intersecting the wellbore that connect through the oil-water contact. Therefore, they are susceptible to water encroachment through natural fractures when high drawdowns are applied. The resulting water production leads to a number of well maintenance problems and reduces the efficiency of the completion. Hydraulically fracturing these horizontal drainholes can offer a number of benefits including reduced water influx. The placement of several small and/or a few large hydraulic fractures at pre-designed intervals in the horizontal wellbore would significantly improve the deliverability of the reservoir and thus reduce the drawdown necessary to achieve optimum production. It is of course necessary to keep the fractures from penetrating into the water-bearing formation. Since most of these wellbores are uncased, openhole completions, a special fracturing technique has been used that allows precise placement of these fractures without the need for mechanical isolation of intervals during the fracturing operation. Production modeling of a horizontal completion in reservoirs with bottomwater encroachment through natural fractures will be compared to a hydraulically fractured completion with the same conditions. Modeling will also illustrate the potential of using hydraulic fracturing as a remediation technique for water production problems. It will also illustrate its effect on completions with current water production. The openhole fracturing technique will be illustrated with a case history from the Hanifa formation in Saudi Arabia. Introduction Reservoirs with high permeability producing through vertical wellbores have seen production benefits through the use of hydraulic fracturing. Most of these benefits are due to overcoming loss of permeability near-wellbore as a result of reservoir compaction and fines migration or simply from bypassing near-wellbore damage created by drilling the well in an overbalanced condition. A completion using horizontal wellbores in high-perm reservoirs has been used effectively to gain reservoir contact. However, there are factors that heavily influence the effectiveness of this completion method:Drilling damage in the near-wellbore area.Vertical to horizontal permeability anisotropy.Vertical natural fractures connected to the wellbore and penetrating through the oil/water or gas/oil contact.High horizontal permeability streaks with edgewater drive mechanism. Wellbore damage due to openhole exposure to drilling fluids while drilling long horizontal laterals is especially problematic for high-permeability reservoirs. The depth of invasion of damaging fluids can be deeper than most matrix-type stimulation processes can effectively influence. In addition, placement control of matrix-type treatments can present problems in openhole laterals.1 Vertical to horizontal permeability anisotropy can vary substantially throughout the pay intervals and at best will represent a kv/kh < 0.7. The lower the ratio, the greater the impact on vertical drainage efficiency, and typically the higher the drawdown required to produce the upper or lower layers. Some reservoirs contain vertical natural fractures that can reduce the issue of permeability anisotropy for horizontal wellbores; however, these same natural fractures can often extend above the gas/oil contact or below the oil/water contact. When high drawdown pressures are used to produce these reservoirs, water or gas production can quickly dominate the total production.2–5 Many high-permeability reservoirs are prime candidates for waterflooding. Again, permeability anisotropy can play a significant role in waterflooding effectiveness for horizontal completions, particularly in the case where a dominant high horizontal permeability streak exists across the field. Early water breakthrough can lead to very poor recovery efficiency and high well maintenance costs. Reservoir simulation can play an important role in assessing the value of hydraulic fracturing for horizontal completions when water encroachment conditions may exist.