In most cases the valve used to open the well and shut it in is not exactly at sandface level. In most cases it will be at surface. Even in the case of downhole shut in there is always a volume that will act as a cushion between the sandface and the valve. As a result, wellbore dynamics create a time lag between the sandface and the surface, or the valve, or the choke. This is what we generally call wellbore storage and skin factor we will review afterwards.
Let us take the case of a well opened and shut in at surface. When you open the well the initial surface production will be coming from the decompression of the fluid trapped in the wellbore. In the initial seconds or minutes of the flow the sandface will not even “know” that the well is opened and the sandface rate will remain virtually zero. Naturally, at some stage we get to a mass equilibrium, for example, this sandface mass rate reaches the surface mass rate. This is the time of the end of the wellbore storage. Conversely, if the well in shut in at surface, the surface rate will go immediately to zero while the sandface does not know about it.
The time of wellbore storage will be this transition time between the effective shut-in time and the time at which the reservoir stops flowing into the well.
There are two main types of wellbore storage. The first one is modeled by the compression or decompression of the wellbore fluid in the wellbore volume. The second type of wellbore storage is linked to the rise of the liquid level present in the wellbore.
Wellbore storage and Skin factor difference
The skin effect quantifies the difference between the productivity of a well in an ideal case and its effective productivity in reality.
- If after drilling, completion, cementing and perforating, the pressure drop for a given production into the wellbore is identical to the one you would forecast in the ideal case for the same geometry, the skin is zero.
- Very often, the reservoir near the wellbore had been invaded and the effective permeability around the well is lowered, thus a higher pressure drop results for a given production. The skin is then positive.
- Conversely, a stimulated well will have better productivity, hence a lower pressure drop for a given production. The skin is then considered negative.
Skin may not be constant in time. During the initial “Clean-up” period in a well test, skin has a tendency to reduce. Conversely, over long period of times, completed wells may get damaged reducing productivity, hence an increasing skin.
We will consider that a well has a constant skin when the additional pressure drop is proportional to the sandface rate. The skin is a dimensionless factor representative of a pressure change, and integrates the same coefficients as the one in Darcy’s law.