INTRODUCTION

Pseudo-steady state begins when the pressure disturbance created by the producing well is felt at the boundary of the well drainage area. In other words, when the fluid mass situated at the drainage boundary starts moving towards the producing well, pseudo-steady state begins. This pseudo-steady state is also described as semi-steady state or depletion state. The name depletion state is probably the most appropriate, because it tells us that the reservoir has reached a point where the pressure at all the reservoir boundaries and also the average reservoir pressure will decrease over time as more and more fluid is withdrawn from the reservoir.

As noted in Chapter 2 and shown in Figure 2-12, for a circular drainage boundary, the fluid from the drainage boundary would start moving to the wellbore at a given instant in time. Similarly for a square drainage area, fluid at the boundary would start moving toward the producing well atone instant in time. In practice, a square drainage boundary is important, because most oil and gas fields are developed on square areas using 10- to 640-acre well spacings. If areal anisotropy exists; i.e., horizontal permeability in one direction is different than that in another direction, the resulting drainage area will be rectangular rather than square. In a given time, a well could drain more distance along the high-permeability direction than along the low-permeability direction. In many naturally fractured reservoirs, the permeability along fracture trends is higher than the permeability perpendicular to the fracture trend.