Reservoir-A is a tight oil reservoir (<1 mD) with four subzones. Production from existing wells in this reservoir has been low due to the reservoir tightness (< 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.

Mechanical efficiency concept derived from the specific energy theory was used for a comprehensive drilling performance analysis. Data from 17 wells drilled in a mature UAE field were evaluated. A wellbore stability analysis of the wells was also conducted by using linear elastic theory of rock deformation. Potential application of the UBD techniques in has been evaluated, based on the results of the wellbore stability analysis. Results have shown that:The mechanical efficiency concept is applicable for the evaluation and improvement of the drilling performance of the wells drilled in the field under consideration.Based on the drilling Mechanical Efficiency concept, the optimum operational parameters such as weight on bit, rotations per minute, bit type, and bit hydraulic horsepower could be identified.The operational window between formation pore pressure and minimum drilling fluid pressure to avoid borehole collapse was found to be too narrow both in vertical and horizontal wells.Results of the wellbore stability analysis indicate that there will be a risk of borehole collapse if UBD techniques are used. Therefore, it is recommended to carry out more in depth analysis of in-situ stresses before making any decision on the use of UBD techniques for drilling vertical and horizontal wells in this mature UAE field. Introduction A comprehensive analysis of a mature UAE field data has been conducted by using the drilling model proposed by Caicedo et al. (2005) 1. The Abu Dhabi Company for Onshore Oil Operations (ADCO), provided field data. The scope of the study can be divided into several phases:Theoretical investigation of the drilling model that could be used for drilling optimization study,Data collection, preparation and classification,Evaluation of the drilling performance using Caicedo et al. (2005) 1 model,Determine the best drilling operational parameters for improving the future drilling performance,Conduct the wellbore stability analyses by using linear elastic theory of rock deformation,Based on the results of wellbore stability analysis, discuss the possible use of UBD techniques.

In 2011, ADCO embarked on a project of well abandonment in Town field, Located in an urban area approximately 25 km South-East of the main island of Abu Dhabi. Each of the three wells presented individual technical challenges and, due to proximity of the populated areas, complexities with logistics and emergency response planning arose. As well as significant presence of toxic H2S in the Town field reservoirs this categorizes those wells as critical high profile wells. And this necessitated extensive engagement with ADCO coordinating with external authorities of concerned parties' throughout the project.

The challenge was to achieve more reservoir contact in a cretaceous tight reservoir to improve production and maximize recovery. Multilateral well campaigns were performed to meet these objectives. This case study describes an effective workflow for performing openhole sidetracks in this challenging medium-hard carbonate formation. The workflow maintained reservoir contact and achieved the desired production objectives. Two 6-in. multilateral drain sections were successfully drilled by performing openhole sidetrack using the continuous proportional steering method (CPSM). This method is not new to the industry, but this case study describes the systematic, unique workflow that was designed and followed to ensure a successful sidetrack in this low-porosity, hard formation. The sidetrack implementation started by creating humps at inclinations ranging from 88 to 91 degrees into the formation in the original 6-in. section. These humps were confirmed using near-bit inclination data (4.5 ft from the bit) and were identified as the sites for initiating a sidetrack. This paper discusses the best practices that were key to the successful execution of the project in one run on the first attempt. After the sidetrack, use of appropriate combinations of shallow and deep logging-while-drilling (LWD) measurements in the same bottom hole assembly (BHA) enabled the direct geosteering of the well, exposing more reservoir surface area than planned. A reduction of 10% from the planned well duration was achieved. Two 6-in. laterals, each approximately 4000 ft, were drilled in a single run and 100% reservoir contact was achieved. This experience proved that planning and precise execution could enable drilling of openhole sidetracks, even through hard formations. These sidetracks can then achieve fishbone wells with desired reservoir contact and realize the field development objectives in a technically robust and cost-efficient manner. CPSM does not rely on a pressure drop for steering. Although this proof of concept was performed in a relatively hard formation, similar workflows with appropriate drilling engineering may be applied to less-competent formations as well. The detailed procedure and flowchart created from the experience with the sample well can be adopted for use in similar applications.