Effects of Vertical Heterogeneity on Waterflood Performance in Stratified Reservoirs: A Case Study in Bangko Field, Indonesia

Abstract

Stratified reservoirs may have different types of heterogeneity in terms of grain size distribution in vertical direction. Geological surveys (i.e. using well logs) have long recognized the existence of fining upward and coarsening upward formations. In this study, such formations refer to as systems with decreasing upward and increasing upward permeability trends, respectively. Many waterflood candidate reservoirs have been found to follow either classification. However, the awareness of including this distribution classification as one of the screening criteria prior to waterflooding has not been established in the oil industry. A simulation study using a number of conceptual stratified reservoir models has been conducted. The results show that grain size distribution classification should have significant impact on waterflood performance. Each of the two classifications yields different effects on vertical sweep efficiency resulting from different crossflow mechanisms, which consequently gives different waterflood performance. It has been found that the oil recovery from waterflooding a reservoir with coarsening upward formation will always be higher than that from waterflooding exactly the same reservoir but with the opposite classification (i.e. fining upward formation) even though the two reservoirs are of the same level of heterogeneity (i.e. similar values of coefficient of permeability variation). In this study, the degree of heterogeneity effects on recovery were investigated as well as the magnitude of vertical-to-horizontal permeability ratio effects. Also, permeability noise was addressed as the reservoir may contain contrast permeability streaks in between the adjacent layers. A correlation has been derived based on the simulation results and has been proven to be able to predict simulation results with relatively good accuracy. Validations performed by comparison to actual production from a simple injection scheme (one injector and one producer) in Bangko Field, Indonesia, indicate that oil production rates obtained from the correlation show good agreement with those of production history. Introduction Heterogeneity plays an important role in predicting waterflood performance of a stratified reservoir. Heterogeneity may take place in both horizontal and vertical directions. In this study, we will consider the problem of vertical heterogeneity. One aspect of vertical heterogeneity is permeability variation. This situation may result from various geologic processes that took place during the sedimentation of the reservoir.1 The sedimentation of a certain reservoir rock may vary and take place at different geological time from that of another leading to a somehow unique classification of grain size distribution for the corresponding reservoir. This is the reason why a certain formation may exhibit a fining upward or coarsening upward grain size distribution classification. This phenomenon may be observed from the gamma ray log results of the reservoir, which is usually reported as the stratigraphic property of the reservoir. A fining upward grain size distribution indicates that the grain size of the rock becomes finer in an upward sequence, which consequently results in a decreasing upward trend of the permeability values. A coarsening upward grain size distribution, on the other hand, indicates a situation in which the grain size of the rock becomes coarser along the upward direction thus, an increasing upward permeability trend is observed. This will lead the formation to becoming a uniquely stratified reservoir. The effects of such types of heterogeneity on waterflood performance in reservoirs are investigated in the present study as well as the effect of the level of permeability variation. The effect of varied magnitude of crossflow is also studied by varying the value of vertical-to-horizontal permeability ratio (kv/kh). A simulation approach is applied in this study by using a numerical three-dimensional streamline simulator.

Impact of the Buoyancy–Viscous Force Balance on Two-Phase Flow in Layered Porous Media

“…When both viscous and buoyancy forces are important, buoyancy-driven counter-current flow between layers, more commonly termed "buoyancy crossflow", repositions the denser phase below the lighter phase wherever viscous-driven flow leads to buoyancy-unstable fluid distributions (Richardson et al 1952;Gaucher and Lindley 1960;Huppert et al 2013). Earlier work has shown that optimum storage efficiencies may be obtained when viscous and buoyancy forces compete, provided permeability varies monotonically in the transverse direction (Peters et al 1998;Permadi et al 2004). However, the impact of buoyancy crossflow on fluid distribution and storage efficiencies becomes more complex when transverse permeability variations are nonmonotonic (Ahmed et al 1988;Stewart 2014).…”

Using vorticity to quantify the relative importance of heterogeneity, viscosity ratio, gravity and diffusion on oil recovery

“…For example, viscous fingering may be stabilised by gravity [16,25] or physical diffusion/dispersion [9,23], whilst channelling through heterogeneities may be mitigated by gravity (e.g. during water injection into an upwards coarsening sequence, [41]) or diffusion/dispersion [31,57]. However, without a detailed flow simulation using a finescale geological model, the combined effects of flow regime and heterogeneity are difficult to estimate.…”

Impact of Plane Heterogeneity of Sandstone’s Thickness on Water Flooding Effectiveness

“…The reservoir heterogeneity is caused together by sedimentation, diagenesis and tectonization. And the reservoir heterogeneity, which is the main impact factors of movements of underground oil, gas, water, is refers to the changes of reservoir properties in space [1][2][3]. Natural energy exploitation is mostly applied, and the impact of the reservoir heterogeneity is not quite apparent in initial stages.…”