This paper will address the limits and drainage area concepts from two approaches:
the reservoir aspects, and
the geophysical-geological aspects. In both approaches use is made whenever possible of seismic attribute analysis and detailed 2D, 3D and 4D seismic interpretation, integrated with other subsurface information, which include well testing, well logging, geological and petrophysical data.
The dynamics of drainage areas are illustrated through the discussion of the effects present in four different field cases:
the structural position of a well completed in a gas-aquifer system;
the lateral sedimentary facies change of the producing formation;
natural main fractures running along the crest of an anticline; and
simultaneous exploitation of several layers through a common tubing.
Finally, a state of the art discussion on 4D seismic technology is presented. It is regarded as an integrated exploration and production technology, that examines changes in seismic amplitudes among repeated 3D surveys, to track the movement of fluids caused by drainage within the reservoir, and map the pathways of migration from deep sources into reservoirs. Detailed analysis of reservoir drainage is accomplished using generalized linear inversion, to decipher the physical parameter changes within the specified reservoir over time. The application of this tool allows identification of undrained or bypassed pays and regions. 4D seismic application so far suggest that there is a much more complicated drainage history in the reservoirs than originally thought, and much more to learn on fluid flow, migration paths and natural recharge of reservoirs.
The drainage area concept is strictly related to the drainage radius. For production conditions, flow will occur in any portion of the formation across which there is a pressure or potential gradient. If a drawdown test is started under conditions of constant reservoir pressure, a pressure gradient (and flow) can occur only after the effects of the pressure perturbation at the wellbore face, have been transmitted to the specific reservoir volume. Practically the velocity of this pressure disturbance transmission process is physically limited. However, theoretically pressure waves are transmitted at the velocity of sound'.
Thus, it can be expected that the pressure disturbance due to production at the well will be transmitted radially outward at the local sonic velocity, the drainage radius being rd = u, t (1) where v, is the velocity of sound in the reservoir.
The estimations through Equation (1) should be interpreted as a maximum possible drainage radius.
Its main ap
