Water-Production Analysis

Water production is one of the most critical fluid-flow trends to understand when identifying reserve-growth potential. The production of water can indicate encroachment of an aquifer, rendering portions of a reservoir void of reserve-growth potential. However, water can often finger or channel into the productive interval, leaving substantial reserve-growth opportunities behind. The risk of additional reservoir development can be substantially reduced when detailed water-production analysis is incorporated into a reservoir-characterization model.

Characteristics of Aquifer Encroachment
Aquifer encroachment decreases ultimate recovery in gas reservoirs. Encroaching water traps residual gas behind the invading water front and maintains reservoir pressure. These effects reduce the volume of gas that will be produced, as compared with conventional pressure depletion. Also, as water volume flowing into the well bore increases, loading can eventually occur, which will effectively kill the free flow of gas, resulting in down time, sporadic well production, costly well maintenance, and, ultimately, abandonment of the well. Additionally, high volumes of produced water can increase disposal costs, rendering a well uneconomic. Careful planning, design, reservoir characterization, and well handling are needed to maximize gas recovery when aquifer encroachment occurs.

Oil reservoirs, in contrast to gas reservoirs, can benefit from aquifer encroachment. Water encroachment from a connected aquifer can act as a water drive, maintaining pressure and displacing oil. The maintenance of pressure sustains flow rates, and the displacement of oil by water increases sweep efficiency. Again, if water encroachment is irregular, however, substantial oil can be bypassed, resulting in poor recovery.

Aquifer and hydrocarbon-reservoir characteristics and production history govern water encroachment, and understanding these factors is critical to optimizing oil and gas recovery. The main aquifer attributes that influence a hydrocarbon reservoir are aquifer size, pressure, and geologic character. Size and pressure characteristics affect the pressure support transmitted to the hydrocarbon reservoir. The larger the aquifer relative to the hydrocarbon reservoir (dimensionless radii), the greater and longer the pressure support and the higher the recovery efficiency in oil reservoirs. A greater pressure differential between the aquifer and a depleting gas reservoir can reduce ultimate recovery. Recovery efficiencies for gas reservoirs and aquifers at lower initial pressures will be less affected by aquifer encroachment, whereas higher pressure systems may result in faster water encroachment (Agarwal and others, 1965). Permeable and homogeneous aquifer/gas reservoir systems undergo faster water encroachment at higher reservoir pressures and thus have lower gas recovery efficiency. Also, higher residual gas saturation resulting from poor geometry and higher relative permeability to water will lead to lower recovery efficiency. High residual-oil saturation occurs when pressure depletion is not uniform in the oil leg and when the oil has high viscosity relative to the encroaching water. Overall, characteristics that promote water influx and decrease oil or gas reservoir incremental pressure drop cause lower recovery efficiency. See Figure 2 below.

Figure 2. Reservoir heterogeneity and aquifer influx strength effect reservoir recovery efficiency. Increasing heterogeneity decreases both oil and gas recovery efficiency whereas increasing aquifer strength increases oil recovery efficiency but decreases gas recovery efficiency.

Production history also influences aquifer encroachment. An increased gas production rate can result in an increased recovery of gas (Agarwal and others, 1965; Matthes and others, 1973; Lutes and others, 1977). An increased production rate often leads to greater pressure depletion before wells water out, thus resulting in greater gas recovery. The performance parameters proposed by Hower and Jones (1991) illustrate the interrelationship between gas flow rate and reservoir characteristics. High production rates, however, must be designed so that no coning or fingering occurs. Relative permeability and residual gas saturation are important considerations in judging the effectiveness of higher production rates. Permeability, relative permeability, and residual gas saturation characteristics affect the broadness of the pressure gradient between gas reservoir and aquifer. A broad pressure gradient will increase the water-invaded zone and result in a larger volume of trapped gas. Oil recovery is increased when reservoir-pressure depletion is uniform. This uniformity reduces water fingering and, thus, bypassed oil as the aquifer waterfront encroaches.