Reservoir Focused Underbalanced Applications in the Margham Field
Authors: Capps John , Khamatdinov Raphael A. , Shayegi Sara , Saeed Saad 
Published: February 2016

Abstract
The Margham gas field, discovered in the Emirate of Dubai (U.A.E.) in 1982, was heralded as a major discovery of its time, and to this day, still remains the largest onshore gas field in Dubai. This reservoir is characterized by a relatively low-porosity, over-pressured, highly fractured, and faulted carbonate. Production of the native retrograde gas condensate occurs primarily from three major formations: Shuaiba, Kharaib and Lekhwair in the Thamama limestone.
Commercial production from the field commenced in late 1984 with good performance being attributed to the highly developed and connected fracture network. The original reservoir pressure was in the range of +/- 7300 psi; however, the past decade has seen a marked decline in both pressure and associated production, with today's reservoir pressure averaging in the range of +/- 1800 psi.
With such a marked reduction in reservoir pressure coupled with complex geology, intricate vertical and deviated fracture networks, undefined faulting regimes, and retrograde fluid-phase behavior, the task of optimizing production is particularly difficult. All these factors, coupled with the relatively conventional well geometries, have made this field an ideal candidate for underbalanced directional technology. In early 2006, an underbalanced coiled-tubing campaign was commenced to optimize productivity.
This paper discusses the reservoir results generated by the implementation of underbalanced technology. Past reservoir performance of wells drilled overbalanced will be compared with current results for this field case. Recovery and potential reserve gains will also be discussed. Specific production targets and metrics that were set to evaluate success were completely fulfilled in the first few wells in this multi-well campaign.

Introduction
The Margham onshore gas field is a limestone reservoir that was first put on production in the early 1980's. Originally, it produced a high condensate yield along with the gas; however, as the reservoir pressure decreased, so did the condensate rates! Eventually, gas was the primary production from this field. Because of the depletion being noted, an infill drilling campaign was commenced to recover by-passed hydrocarbons. It was determined that underbalanced drilling would be the best technique to use to avoid lost circulation problems in these very depleted formations (as low as ~1200 psi at 11,000 ft in some areas), and to minimize formation damage so that the maximum production could be realized. Between 2006 and 2007, an 11-well, coiled-tubing (CTD) underbalanced drilling (UBD) campaign was initiated from a mother wellbore to drill multilaterals. The drilling plan was to be conducted through the production tubing to access the by-passed hydrocarbons.
The Thamama Limestone group is highly fractured with the troublesome overlying Nahr Umr shale cap rock, which has caused drilling problems in the past. The drilling conditions would require careful planning since downhole temperatures reached 289ºF, and there were depleted pressures along with H2S and CO2 in the reservoir. The through-tubing drilling campaign was chosen as the best option, because it would reduce risks and the costs associated with a new well or having to pull existing tubing. The equipment and methods to drill successfully have been described in an earlier paper.1 In the referenced paper, the basic concepts and procedures for proper design of an underbalanced coiled-tubing drilling procedure for a multilateral well are reviewed, and those design concepts were used to formulate the strategies for the project discussed in this paper.
Nine wells were drilled in this underbalanced (UBCTD) campaign using a 3-in. bottomhole assembly (BHA).
In an underbalanced drilling process, the wellbore pressure in the openhole section is kept lower than the reservoir pressure. Unlike conventional overbalanced drilling or managed-pressure drilling (MPD) with reduced overbalance margin, the underbalanced drilling environment provides a unique opportunity to gather data that have the potential to provide important information about the reservoirs encountered during drilling.
When the wellbore is kept in an underbalanced condition, formation fluids are allowed to flow into the wellbore during the drilling process. Proper instrumentation, data acquisition, and drilling procedures allow continuous acquisition of data that can be analyzed for the purpose of extracting reservoir information. This capability to continually access reservoir data is a critical factor for assuring that UBD operations will reap the full benefit to the reservoir influx from the formation while drilling.