Gas-oil Ratio and Water-oil Ratio

An indicator of bypassing is a premature drop in gas-oil ratio; i.e., earlier than expected collapse of gas saturation. Early gas collapse (water fillup) may indicate that channeling has occurred. In layered reservoirs with no or little vertical crossflow, water injection in an initially depressurized layer will cause GOR to drop rapidly. Often naturally fractured reservoirs exhibit fast gas collapse because water fills up the fracture system and does not initially invade the matrix, the desired target for waterflooding. Figure 1 shows an example of a pattern where channeling has occurred. This type of pattern should be reviewed geologically to attempt to identify the thief zones/natural fractures. Other key performance indicators are water breakthrough times and subsequent WOR trends, which also can be indicative of channeling and bypassing problems. However, since wells or patterns showing high WOR rise or quick gas collapse may simply be due to high injection rates, one should plot WOR and GOR versus hydrocarbon pore volume injected (HCPVI). In general, if water breakthrough occurs before 20% hydrocarbon pore volume injected (HCPVI), channeling or bypassing due to heterogeneity is likely occurring. Like the WOR or GOR versus time plots, the log of WOR versus cumulative oil produced (Np) is used as an indication of channeling and heterogeneity (Figures 2 – 4).( 1 , 2 ) In an unfavourable mobility ratio situation (M >1), the late time slope of the graph is primarily controlled by the oil water relative permeability curves; therefore, volumetric sweep efficiency can be derived from this plot.(2) In a favourable mobility ratio situation (M (1), the late time slope of the graph is controlled primarily by permeability heterogeneity or fluid segregation. In layered systems, the WOR versus Np plot may have a stair-type profile as various layers breakthrough (Figure 4). Plotting WOR versus Np and comparing individual patterns against a group average (e.g., for an entire unit operation) gives a qualitative indicator of volumetric sweep efficiency. This should be evaluated in the context of known or suspected geological trends. Stylized representations of waterflood performance in simplistic geological cross sections are depicted in the companion insets to Figures 3 and 4. Extrapolation of the WOR versus Np plot and changes in its slope can indicate incremental oil recovery. Therefore, an examination of the log of WOR versus Np plot is useful in determining the incremental recovery due to infill drilling or operational changes, as shown in Figure 5. The changing slope of the curve indicates increased reserves after infill drilling. In our experience, successful additional recovery efforts (including recompletions or treatments to suppress water as well as infill drilling) can make Reservoir Management for Waterfloods—Part II