### Production problem with oil well AI-1

##### Since production start-up, pressure at this well has reduced much more than expected. There is a bit of concern in the office regarding the long term performance and forecast.

The figure below shows the AI-1 downhole gauge pressure (psi) and oil rate (bbl/d) data versus time.

##### As the pressure keeps decreasing, the well may produce below bubble point, with gas breaking out of solution. The well flowing pressure will also be constrained by the surface facilities and production may fall. The subsurface manager is thinking about an increase in well damage, as this happened in another field nearby. From the geologist, the well could be located in an isolated compartment with a small connected volume and the average reservoir pressure could start to decrease (depletion). **What do you think ?**

We could look at some “opportunistic” production shut-ins on the log-log plot. Thankfully, we also have a baseline PBU test for reference and comparison, as per best practices.

##### The derivative data are consistent, showing a unit-slope straight line and a hump indicative of wellbore storage and skin, a stabilization up to 10hrs, followed by a decline in the derivative trend and another stabilization at late times.

##### From this plot, the skin doesn’t seem to be too much of an issue. There is no major change in the ∆P plots and as a consequence, no significant increase in skin (damage) is noted between these opportunistic shut-ins.

### What is the derivative plot telling us about the well and reservoir ?

##### This derivative shape could be explained by various reasons: limited perforations / partial penetration, reservoir crossflow (contrast in skin or mobility between layers), multiphase flow in the reservoir, wellbore phase redistribution, linear composite behaviour (increase in mobility and/or storativity away from the well).

##### Some of these cases are not consistent with the other sources of subsurface information. According to the logs, there is no multilayer description and the full 60 feet section of the reservoir was perforated.

**Could the derivative plot highlight the presence of some sealing faults?**

##### Well, we see a negative slope and a lower derivative stabilization at late times, so according to the books, we do not see any sealing fault, do we …?

##### Let’s have a look at Deconvolution. Provided the right operations guidelines and analysis process, we can recover a reliable correct deconvolution response in red:

##### When compared with the conventional derivative in blue, the deconvolution response doesn’t show any derivative downturn. There is only a single derivative stabilization followed by an increase in the derivative. This could highlight the presence of some boundaries, i.e. some sealing faults close to the well. According to the well analysis below, the faults could be located at about 400 feet from the well.

##### Note the simulated downturn on the conventional derivative. By extending the shut-in duration, we would finally see the boundary response with an increase in the conventional derivative in blue. This response is delayed in time or “distorted” due to the derivative calculation algorithm.

##### The transient response with the presence of these near wellbore boundaries is the main cause for the decline in pressure. According to the deconvolution response, the reservoir is not closed but still open. As a result, pseudo-steady state regime hasn’t started yet and there is no depletion. These results could then be used to review the production forecast.

### A misleading conventional derivative

##### What we have here is called a “distortion” on the conventional derivative. Sometimes a declining trend in the conventional derivative could be due to the presence of boundaries. In this case, the derivative is misleading. Deconvolution is free of any distortion and provided it is correctly recovered, it becomes the driving tool for the analysis.

##### This short video shows another distortion on the conventional derivative. More info is available in the basic training courses.