Design of Underbalanced Drilling Program to Improve Wells Productivity Index and Characterize Multi Layered Tight Oil Reservoir in an Environmentally Sensitive Field
Authors: Ardila Jaimes Jose , Al Menhali Adnan , Al Yamani Sultan , Hadj-Moussa Ayoub , Saleh Mujahed 
Published: November 2018

Abstract
Reservoir-A is a tight oil reservoir (&lt;1 mD) with four subzones. Production from existing wells in this reservoir has been low due to the reservoir tightness (&lt; 500 BPD), dual phases injection program is implemented to support the wells productivity and to increase the overall recovery factor (RF). Building on the success of the Underbalance Drilling and completion technology (UBD) in similar tight reservoirs in ADNOC Onshore fields, the asset team decided to drill one pilot well (Well-1) to minimize the formation damage, practicaly increasing the Productivity Index (PI) and to measure the individual contribution of each subzone to the total well production in order to optimize the lateral length in each subzone to reduce the Unit Technical Cost (TUC).
A transient hydraulic flow modelling software was utilized to study how to achieve underbalance conditions and to estimate the reservoir production during the UBD operations The UBD mythology proposed to drill Well-1 includes using crude native oil as a drilling fluid instead of Water Based Mud to minimize the formation damage. Membrane Nitrogen was chosen as a gaseous phase to reduce the effective Bottom Hole Pressure (BHP) below the reservoir pressure to create underbalance condition allowing the reservoir to flow through a four phases separation package that allows separating produced oil and gas to enable reservoir characterisation. To eliminate the need to kill the well during tripping and completion, a Downhole Deployment Valve (DDV) was proposed to be rung on a retrievable tie back casing string that extends from the top of the liner to surface. A transient hydraulic flow modelling software was utilized to study how to achieve underbalance conditions and to estimate the reservoir production during the UBD operations.
The engineering evaluation study concluded that UBD is feasible on Well-1, it provided the required UBD equipment capacities to drill Well-1 maintaining UB condition during drilling, tripping and completion operations. A detailed UBD program was compiled by ADNOC onshore and Weatherford teams taking into consideration different anticipated scenarios and contingency plans. Weatherford set up classroom and on-site UBD training for the teams involved in the operation, including ADNOC onshore, rig contractor and other services providers. Having Well-1 in a cluster field adds complexity to the UBD operations and raises new HSE concerns. Moreover, the field exists in environmentally sensitive place close to urban areas and surrounded by sea and mangrove trees. The project team set a detailed HSE plan for the UBD operations on Well-1 involving all stakeholders. A three days HAZID/HAZOP workshop was conducted to identify potential hazards by applying what-if approach to ensure that adequate safeguards are in place before starting the UBD operations.
The uniqueness of Well-1 UBD design lies in its comprehensiveness in addressing multiple operational scenarios and in its ability to address complex HSE scenarios to safely and successfully unlock the true potential of a tight oil reservoir which has been witnessing low production rates. The outcomes of the UBD pilot well (Well-1) will be closely studied and might change the field development plan.