Underblanced Drilling in Unconsolidated, Low-Gravity Oil Reservoirs in Latin America
Authors: Vargas Angela , Rodrigano Roberto , Huertas Roberto 
Published: October 2008

Abstract
After several years of reservoir evaluation in Latin America, and several months of planning, Weatherford UBS performed four Flowdrill operations in a shallow, unconsolidated low gravity oil reservoirs in North Eastern Brazil.
The Reservoir is very similar to the ones encountered in the Orinoco River in Venezuela and others in Southern Argentina. These wells, the first of their kind to be conducted in Latin America, where a great success measured by the drilling, operational and productivity results. Therefore the technology has defined the starting path to corroborate and develop similar reservoirs, highly damaged by the drilling fluids invasion due to their low-pressure characteristics and high permeability and porosity.
All four wells were horizontals with the operating time lesser than 10% of the time used to conventionally drilling operation for each well. Hole stability was considered to be an issue with a differential pressure of 25 PSI between the overbalance and the collapse pressure. It did not materialize because there was more than 35 PSI of draw down pressure against the formation and no stability or hole problems where encountered during the operation neither completion.

Introduction
Searching production optimization in a shallow reservoir with high permeability and porosity, associated to a low-pressure gradient, Underbalance Drilling was evaluated and successfully applied to this field.
This fluvial reservoir origin is formed by heterogeneous, poorly consolidated sandstone with claystone stringers and in general sand production is associated with the heavy produced oil (11 API).
The heavy oil with asphaltene and paraffin content has a very high viscosity and water saturation above 60%, therefore resulting in a water base drilling mud being selected for the operation.
Due to the over balance effects and mechanism that occur in low API gravity oil producing zones, Underbalance Drilling was applied to minimize the induced damage (solids plugging, fines migration and emulsion formation and plugging). The reservoir pressure gradient is in the range of 7.7 #/gal equivalent and the formation collapse was calculated at 6.0 #/gal. During the drilling of the wells, in general the operation was conducted within the narrow window, keep in mind that the TVD was of 650 feet. Therefore, minimum influxes during connection time have a great impact on the bottom hole pressure and generate pressure spikes acting both ways (slightly overbalance after a connection and below the collapse pressure after unloading the hole). Reaching levels below the recommended pressure window while unloading the hole demonstrate that keeping the well in dynamic conditions with low-pressure gradient, the hole can be maintained and no stability problems were encountered.
Nitrogen injection into the water base drilling fluid was used for lifting and lighting the mud column and maintaining it within the operational ranges.

Operation planning
Considering these wells were the first of their kind to be drilled in an Underbalanced condition special attention was given to the operational planning.