Porosity Correction Method Developed for a High Vertical Relief Field in Eastern Venezuela

Abstract

A method has been developed that allows the estimation of porosity as a function of reservoir depth in high vertical relief fields. Porosity is known to decrease as net overburden pressure increases. This porosity reduction in turn leads to large reductions in reservoir permeability, the end result being a significant reduction in productive pore volume, net pay, and well productivity. The Eastern Overthrust Fields in Venezuela's El Furrial Trend produce from thick Miocene to Cretaceous sandstones and have very large hydrocarbon columns (two to three thousand feet is not unusual). Therefore, significant deterioration in pay quality is observed in the downdip areas. Using data from wells drilled in the Repsol/YPF Quiriquire Block, we observed that it was difficult to understand the true nature of the reservoir sands because the data was distorted by reservoir compaction in downdip wells. A method was developed to eliminate the distortion introduced by depth and overburden stress in porosity logs. This technique allows the well data to be interpreted as if all the wells had been drilled at a common datum. Once the undistorted data is interpreted and contoured, the data is re- shifted up or down using a depth grid map. Reservoir property grid maps can be prepared which better represent the sandstone properties at the depth at which they are found today. These maps can then be used in reservoir simulators to match and predict reservoir volumes and performance. The proposed method is most useful in development situations where there is sparse well control. Therefore, our first application has been in the Repsol/YPF operated Tropical Field, a 1998 discovery. Using this method, we prepared a reservoir description that depicts undrilled downdip areas much more accurately than would be possible using conventional methods. The new method resulted in large changes in our estimates of hydrocarbons in place and reserves and significantly impacted future development plans. Introduction High vertical relief reservoirs with large hydrocarbon columns present unique difficulties in reserve analysis and development planning. Fields with these characteristics are often found in the highly complex thrust belt regimes of the world including fields in Colombia, Bolivia, Argentina and Eastern Venezuela. A significant problem associated with having thousands of feet of vertical closure is the potential for compaction of the reservoir rock with depth, resulting in diminished pore volume and permeability. This adversely impacts reserve calculations and development strategies. For example, reservoir rock on the deeper fringe of the structure may have lower porosity and much reduced flow capacity, with the result that wells drilled in these areas will not flow at rates exhibited by wells drilled on the crest of the structure. Likewise, blanket estimations of reserves based upon crestal wells will yield optimistic values of reserves. We have also observed a tendency for geostatistical software to yield fairly unreliable geologic models unless it is programmed to account for porosity changes with depth. These reservoir description and engineering problems are highly compounded during the early reservoir appraisal and development phases. The Tropical Field in Eastern Venezuela is a recent discovery, and an excellent example of such a case.