Horizontal Well Risk Assessment from Geosteering and Formation Evaluation Perspectives
Authors: Santoso Gagok Imam , Permanasari Dian , Alang Khairul Anuar , Ng Hengky , Yusuf Nora , Anthoa John Mark 
Published: March 2015

Abstract
Acquisition and interpretation methods to optimize horizontal wells have been broadly introduced in the industry nowadays. It has evolved from the basic and conventional technology such as gamma ray and average resistivity to the latest deep directional boundary detection method. Technically, all methods can be used to place a horizontal well. But the amount of information available for interpretation and real-time decision making such as measurement sensitivity, vertical resolution, and depth of investigation are different and unique for each technology, which could lead to different results in the placement of the well and thus the production profile. Horizontal well targets also vary from thin beds, thin oil rim in a single sand or across multiple sands with high angle cross section, laterally heterolithic reservoir, to massive thick reservoir; each of which have their own characteristics that can be interpreted from the log measurements. However, high angle effects such as anisotropy, shoulder beds or bed boundary will affect the measurements which may then affect the well placement interpretation accuracy and execution efficiency if not anticipated or understood well. Therefore, a proper risk assessment and technology selection in the early planning stage is critical, as most operators will need to allocate the budget to deploy the selected technology and methods. This paper presents a horizontal well risk assessment workflow from geosteering and formation evaluation perspectives based on the well placement experiences and geological environments in South East Asia.
The workflow provides guidelines and general references for horizontal well technical analysis during the early planning stage which will help to optimally manage the risk associated with horizontal well projects. It involves risk assessment based on the geological condition and well objectives, followed by feasibility modeling phase to compare different methods and technologies, and finally the technology selection process to achieve the well objectives.

Introduction
Despite horizontal drilling costing almost double compared to vertical well expenditure, the concept has become popularly adopted in the oil and gas industry, beneficially increasing the production rate in mature or brown field. The planning and execution of a horizontal well has many challenges mainly related to directional control, geological uncertainty and reservoir characterization. Horizontal wells approach the target formation nearly parallel to the bedding plane; hence, it is also correlated to a small TVD windows target. The inability of seismic to interprete micro fault and reservoir variations along the target well will significantly reduce the net to gross ratio inside the pay zone. Many of horizontal well's failures are resulted from these two issues hence requires the operator to plan both the well path and geological target more accurately.
Well Placement, which includes a planned interactive geosteering along the wellbore using geological data, combines technology and process with collaboration of multi domain disciplines. The well placement method is well known for years as a proven concept that has led to the success of horizontal drilling. Year to date, there are 5 known types of well placement method which is commonly used in South East Asia region;Geometrical drilling. The aim of this method is to drill conventionally with directional assembly to reach the reservoir target. Geometrical drilling is employable if the reservoir target is more than 100 ft thick and has many offset wells drilled around the target zone. The risk of using this type of horizontal well remains big as the survey EOU (Ellipsoidal of Uncertainty) could be an issue if drilling a long well. Therefore, proper advanced survey calculation method has to be applied in order to reduce survey uncertainty.