Sand production is a common problem with unconsolidated formations. It is very challenging to successfully remove formation sands with conventional methods in a large deviated wellbore with a low pressure gradient formation 1.
Since 1995, a technology combining concentric coiled tubing (CCT) with a jet pump has been developed and used to remove both the drilling fluids and solids. Initially it was developed for horizontal heavy oil reservoirs where pressures are low and viscosity is high, without placing hydrostatic loads on the reservoir. The job data from more than 600 sand/well vacuuming operations worldwide has been compiled into a database.
This paper reviews the well information and the key operating parameters: maximum depth, bottom hole pressure gradient and pump rate. The engineering challenges, best practices and lessons learned for the sand/well vacuuming process are also summarized. Analysis of this data yields a better understanding about this vacuuming technology and provides good guideline for future practice.
Case histories are provided which demonstrate how to deploy the different sand/well vacuuming bottom hole assemblies (BHA), to; increase the penetration capacity with a jetting tool; entering multi-laterals with an entry guidance system; accessing small size holes with a micro-vacuuming tool; and to achieve extended reach under extreme conditions.
Post job analysis indicates CCT vacuuming technology reduces the skin damage and increases the production compared to non-vacuumed wells. Moreover, the details from sand and other fluid influx profiles obtained along the wellbore based on the analysis of the returns during the vacuuming process, could be used to evaluate well production and assist in formulating a management strategy.
During the drilling of most horizontal heavy oil wells, the drilling fluid's hydrostatic column produces fluid losses to the formation due to the low bottom hole pressure (BHP). Otherwise, the presence of a filter cake adhered creates an artificial layer between the formation face and the outer part of the slotted liner, affecting the pressure transference from the reservoir, in other words, creating an additional pressure drop from the formation to the wellbore. After the drilling operation is completed, the production pump is run in the hole and is used to retrieve the drilling fluids and the filter cake remaining in the well. The problem with this method is that the suction produced by the production pump is mainly in the nearest zones to the pump and does not create significant suction in the toe of the horizontal section. Therefore, the well will produce predominantly from the heel and have less production from the toe.
Sand production is also a common problem faced by many of the heavy oil producers worldwide. A slotted liner acts as a partial barrier, grading effects at the slots often still result in sand migration into the wellbore and, hence, a lower production rate. The low production flow rates contribute to increased deposition of the sand in the highly deviated wellbore.
Several cleanout options have been developed over the decades, employing a number of different techniques and approaches 1. However, coiled tubing (CT) or conventional jointed pipe often incorporate the need for high circulation rates, special fluids or reverse circulation methods to remove the solids. With high rates and high specific gravity water based fluids, conventional sand cleanout methods often apply an excess pressure on the formation and this results in lost circulation returns to the low formation pressure reservoirs. A typical reservoir pressure in these wells may be as low as 0.04 psi/ft. This makes sand removal virtually impossible and creating damage to the formation likely. Nitrogen can be used to reduce hydrostatic pressure, but this necessitates a very specific job design and execution and can require large quantities of liquid nitrogen in the case of horizontal wells located in some remote area.