Underbalanced Drilling Technology for Unconventional Tight Gas Reservoirs
Authors: Qutob Hani , Kartobi Khelil , Khlaifat Abdelaziz 
Published: May 2016

Abstract
The increased demand for more sources of clean energy such as natural gas from unconventional reservoirs has forced the industry to explore the more challenging tight gas reservoirs. Tight gas reservoirs constitute a significant proportion of the world's natural gas resource and offer great potential for future reserve growth and production. However, to meet future global energy demand, access to tight gas reservoirs requires innovative and cost effective technical solutions.
Yet, tight gas reservoirs are often characterized by complex geological and petrophysical systems as well as heterogeneities at all scales. Exploring and developing tight gas accumulations are both technically and commercially challenging due to the large subsurface uncertainty and low expected ultimate recovery per well. Appraisal of deep tight gas reservoirs offers many challenges, including production rate predictions when wells are drilled overbalanced. Overbalance leads to near wellbore damage to the rock matrix and fractures. Damage to natural fractures intersecting the well can prevent their detection leading to missed productive intervals.
In addition, the operating environment is very challenging and that affects the decisions for data acquisition. The use of salt-saturated mud systems creates a contrast and uncertainty in the data. Hence, the quality of data acquired is compromised.
In the 80's hydraulic fracturing of deviated wells was the method of choice for developing tight gas reservoirs worldwide. Although sound in principle, in practice problems were experienced and caused either by poor cleanup due to fluid incompatibility, erosion of surface facilities or early water breakthrough due to fracturing into the water leg.
In the 90's horizontal drilling became common practice as new drilling technologies developed and proved to be very successful in many tight gas fields. However, conventional drilling operations introduced foreign fluids and solids into the reservoir which lead to several different formations damage mechanisms that prevented the identification of the gas production potential from these wells.
In the late 90's underbalanced drilling (UBD) was introduced, mainly to avoid the frequent drilling problems associated with total losses into these tight gas reservoirs. However, significant productivity gains were also observed and this became a key driver to apply the same UBD technology in tight gas fields.
This paper provides a technical overview of the state-of-the-art UBD technology used to develop unconventional tight gas reservoirs. Two real case histories from eastern Jordan and South West Algeria will be presented and discussed.

Introduction
The increased demand for more sources of clean energy such as natural gas from unconventional (tight) reservoirs has forced the industry to explore for more challenging tight gas reservoirs. New production will probably come from more difficult to produce reservoirs. In general it was accepted that static evaluation tools (logging, coring and seismic), that have proven so important for conventional reservoirs, were inadequate to characterize tight gas reservoirs and that a shift to use of dynamic flow data was needed.