Pressure Drawdown (∆P) is defined as the difference between the static bottom hole pressure (SBHP) and the flowing bottom hole pressure (Pwf). Thus, the drawdown is the differential pressure that drives fluids from the reservoir into the wellbore.
∆P= Pressure drawdown = SBHP – Pwf
If SBHP = 200 bar, and Pwf = 80 bar; therefore the Pressure drawdown (∆P) = 200 – 80 = 120 bar.
The amount of pressure drawdown dictates the amount of flow into the wellbore or production. The higher the pressure drawdown is, the higher the production rate. The drawdown, and therefore the production rate of a producing interval is typically controlled by surface chokes.
NB: Reservoir conditions, such as the tendency to produce sand, may limit the drawdown that may be safely applied during production before damage or unwanted sand production occurs.
The notion of Formation damage (Skin Factor):
As per the previous figure, a new term is introduced, ∆Ps or delta P skin. The term S stands for Skin Factor. Skin is a dimensionless factor calculated to determine the production efficiency of a well by comparing actual conditions with theoretical or ideal conditions. A positive skin value indicates some damage or influences that are impairing well productivity. A negative skin value indicates enhanced productivity, typically resulting from stimulation.
This ∆Ps , in the previous graph, is attributed to a skin of reduced permeability (formation damage, paraffin, scale, etc.) around the wellbore. This ∆Ps reduces the amount of flow into the wellbore. For an analogy, consider a pipeline that has a certain upstream pressure, downstream pressure, choke, and a fixed diameter. To increase flow rate one of three things can be done:
- Increase the upstream pressure,
- Decrease the downstream pressure,
- Open the choke, which decreases S the amount of pressure drop or ∆P.
The same situation exists in near the reservoir wellbore. The upstream pressure is equivalent to SBHP, downstream pressure to PBHP, and the choke to ∆Ps. To increase production we can:
- Increase the SBHP: by increasing the injection rate,
- Decrease the PBHP: by installing an artificial lift equipment,
- Decrease ∆Ps: by stimulating the wellbore (e.g. acid job)