About Lamont
About Lamont
All oil reservoirs produce both gas and oil at the same time. However, the gas/oil ratio (or GOR) varies widely, depending on the well. For "oil wells", the GOR may be only a few hundred mcf/bbl; for "wet gas wells" that produce distillate (natural diesel), the GOR may be several tens of thousands; and for "dry gas wells" that produce virtually no liquids at all, the GOR is even larger. As we will see below, this ratio is important to know, monitor, and control, because the pressure dynamics indicated by the GOR have implications for the seismic amplitude changes that occur over time in the reservoir. For example, by keeping the gas above the bubble-point (in solution) within any given reservoir, the GOR can be controlled and recovery can be tracked. However, if added sweep is required without the placement of expensive new wells, an intentional secondary gas cap can be formed updip to push the oil downdip to wells. In this case, 4D seismic monitoring of gas cap formation and movement over time is required to assure efficient recovery.
To complicate matters further, water is a factor in that it often produces
dim-outs in seismic amplitude as oil and gas are replaced by water. With time
and depletion, water is produced in all wells (even to some degree in depletion
drive reservoirs, see below). When the water-to-hydrocarbon ratio exceeds 95%,
the well is usually closed ("shut-in") because it becomes economically and
operationally unprofitable.
2.
Universal improvements from 4D
Monitoring
There are two main oil reservoir types in the world: sandstone and carbonate. All reservoirs-whether sand or carbonate-respond to production in one of two ways depending on the hydrodynamic system of the rock surrounding the reservoir:
1. Water-drive. If there is sufficient connectivity to a surrounding, water-filled acquifer, the opening of a pressure gradient to the surface that produces oil and gas flow will produce an inflow of water to replace the oil and gas in the pore spaces of the reservoir rock. This water-drive sustains pressures better than the depletion-drive phenomenon (below), usually producing in better "sweep" of oil and gas. Thus the pressure depletion curve is more gently sloped, and a reservoir will produce with natural pressure drive for a longer period of time.
2. Depletion-drive. If the permeability connection to surrounding rock is poor, then little water will flow in to replace extracted oil and gas, and pressures will deplete much more rapidly. Compaction and pore collapse are possible, and oil and gas "sweep" is correspondingly reduced. Some form of lifting of the reserves is required more quickly, and the fall-off curve for production vs. time is generally steeper.
1. Which drive is present in a given reservoir is often a surprise, with stacked reservoirs often alternating depending on specific plumbing conditions within the reservoir seals. 4D monitoring will determine which drive is operating, and if acquifer support is present. Placement of new wells, or injection of water to sustain pressures in a depletion-drive may be required.
2. Also, during the history of production, often rapid changes from one drive to the other can occur. 4D gives an image of the reservoir's response to conditions such as fault zone and permeability barrier breakthroughs that can suddenly inject water across faults. Also, 4D can help optimize the location of new injection wells that would be required to maintain production.
3. Also, a production plan often contains designed injection of gas, or intentional formation of a gas cap to force oil from distant edges of the reservoir to central production wells. 4D gives real-time monitoring of the success or failure of those injectors or gas cap formation strategies.
3.
4D benefits specific to Sandstone Production
Rock Type: Oil and gas are produced almost completely from one of two kinds of rock: sandstone or carbonates such as limestone, dolomite or buried reefs. The slope of the fall-off in production over time is generally controlled by the permeability of the reservoir, with carbonate permeability derived from either interconnected pores or from fractures, and sand permeability controlled exclusively from pore interconnectedness. However, there are production differences between the two types of reservoirs:
2. Water coning can force oil and gas away from the pipe
3. Gas cap pressure maintenance can fail, dropping pressures to below that required to drive the oil and gas to the wellbore.
4. So/Sg/Sw production often is mixed from several wells on a platform, or even at the other end of the pipeline
5. Pressure compartments must be delineated, but often are guessed at
6. Leaky faults can deliver gas to oil reservoirs, water to gas, and all combinations-pressure differential across the fault, which varies with time, is the controller.
2. Water incursions can be fought by selective shutting in of wells for often very short periods of time (days). Periodic experiments, such as monitored shut-in wells can produce pressure rebounds that tell detail of the far-field plumbing system. These are seldom done because the pressure instruments are not routinely in place.
3. Gas caps better be where you think they are, when you want them to be, or severe damage can be done to production efficiency. 4D seismic monitoring is an excellent gas locator in most rocks.
4. GOR and water mix can be controlled by injection. Only the most sophisticated oil companies plan injection from the start of reservoir life.
5. Pressure monitoring defines compartments than may not be draining, producing locations for "infill" drilling.
6. Residual gas concentrations of only a few percent can produce sustained high seismic amplitude regions that might be mistaken for bypassed pay. 4D monitoring can detect small changes in gas concentration because, though the amplitudes remain high, they still change over time.
4.
4D benefits specific to Carbonate Production
2. Sluffing. Carbonates are often chalky, especially in permeability zones, and debris can build up and block perforations. Most perforations in general are unsuccessful anyway.
3. Pore precipitation. Unwanted precipitates can clog perforations, such as parrafins.
2. Monitoring of flow from inside the production tubing can now be accomplished so that instant reaction to plugging is possible. Often, acidization can clear plugged perfs. 4D monitoring can see blockages in the drainage pattern so that remedial wells can be drilled.