An Application of Risk Assessment in the Development of a Complex Mature Oil Reservoir
Authors: Ahmed Ahmed Hassan , Waller Herschel N. 
Published: December 2007

One of the greatest challenges facing petroleum companies today is how to prioritize projects such as drilling wells based on the likelihood of profitability. To accomplish this, the risks associated with each project must be fully assessed. Risk assessment is particularly important in complex, mature offshore fields which have already been drilled up and where the cost of further drilling is high. Drilling prospects in such fields may involve targets that are updip of current wells, at locations that are difficult to reach from existing platforms. Reserves are predicated upon the prognosed locations of sealing faults and upon assumptions about drainage patterns in the area. All of these factors make the assessment of all risks and an understanding of the range of parameters that affect reserves extremely important.

A specific example from the Nubia C Formation in the Ramadan Field, one of GUPCO's most mature water drive oil reservoirs in the Gulf of Suez, is used as an illustration of how to address and to quantify risks. Several factors, including the likelihood of drainage by existing wells, the location of faults in the area, and structural character are quantified. Ranges for drainage volume, recovery factor, water saturation and porosity are incorporated as distributions in the assessment. The results of this work are a figure for risk-weighted reserves and a probabilistic rate profile which can be used to determine the economics of the project.

Introduction

Many of GUPCO's fields in the Gulf of Suez are very mature. For the Ramadan Field, one of GUPCO's most mature fields, the primary production zone historically has been the water drive Nubia C reservoir. This field is complex, having multiple fault cuts that are difficult to map completely, despite many penetrations over the years. Since wellbores are typically located at the top of the structure and the well paths are constrained by existing platform locations, reaching the remaining reserves at updip attic locations even closer to the faults than existing wells is very difficult and extremely expensive. Until recently the approach to determining whether to drill a well in Ramadan was been based upon economics generated by deterministic values of rates, reserves and costs. However, this approach led to several unsuccessful drilling wells, and it was clear that a better method of making decisions was needed.

Over the past year, a new risk-weighted approach has been fashioned to assess the rates, reserves and, consequently, the economics of potential drilling locations. This approach involves defining distributions for volumetric parameters such as drainage area, porosity, net pay thickness and water saturation and determining other risk factors such as location of the updip fault, height above existing wells and drilling cost. Any desired parameter, including individual zone reserves, frequency of occurrence of any particular event as well as rates and reserves, can be forecasted. This PC-based technique generates not only risk-weighted mean values of forecasted parameters but also a distribution of possible outcomes. This type of information is extremely valuable both to the technical team and to management for making drilling decisions.

Risk assessment for the sidetrack of Ramadan Well 33 (R6-33 S/T), which is detailed here, is an excellent example of this type of approach. The drilling of R6-33 S/T was successful. The subsea elevation of formation tops was very much as expected and porosities and water saturations were within the expected ranges, and actual performance is substantially matching the risk-weighted rates. Most importantly, the story of this well demonstrates how this technique can be used to address drilling well prospects in a complex reservoir. Since this well was drilled, the technique has been updated and improved and is now being used to assess every major GUPCO drilling prospect.

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