Underbalanced Drilling, Logging & Completion Operation Leads to 4.5 Times Production Improvement as Compared to Offset Wells

Abstract

Underbalanced drilling technology is widely used to reduce formation damage in the reservoir section and improve productivity with the resulting ancillary benefit of improved drilling performance. It involves drilling with a fluid whose equivalent hydrostatic pressure is lower than that of the formation being drilled. As a consequence of this lower hydrostatic pressure there is a continuous flow of hydrocarbons to surface which is handled by separation equipment and exported thru pipelines where they exist or burned at the flare if no transportation infrastructure is located near rig-site.

The most prevalent method of implementing UBD drilling operations in Hassi Messaoud field for Sonatrach, involve using the concentric casing method to inject Nitrogen. Doing so ensures that downhole motor and MWD tools performance is not adversely affected by the presence of nitrogen gas lowering their performance. Concentric casing injection insures that the open hole section is only exposed to native crude further reducing the potential for reservoir formation damage. This is achieved by injecting single phase crude down the drill string and nitrogen is injected thru the well head between the tie back liner and previously set casing.

The introduction of nitrogen in the main annulus is achieved by means of concentric casing injection ports this has the desired effect of lightening the hydrostatic pressure of the drilling fluid to lower than the reservoir pressure thereby leading to inflow of hydrocarbons while drilling ensues. The excess produced hydrocarbon is exported to nearby production facility. Drilling underbalance drastically reduced formation damage when compared to offset wells which was evident from production testing results. The well was drilled to TD, logged and completed underbalance as well, leading to crude production rate that is 4.5 times greater than average production rate of the nearest four offset wells in same production area which were drilled conventionally.

Additionally, by producing the crude during underbalanced drilling operations prior to being handed over to production department, the well was able to recover 40% drilling cost which substantially offset the cost of UBD operations. This paper examines how the implementation of underbalance techniques resulted in successfully drilling, logging and completing the well in underbalance conditions resulting in significant productivity gains for the operator.