Abstract

A primary objective of any project evaluation is to understand the fundamental economic value and the uncertainty in that value. The uncertainty in value is a function of numerous variables in both the surface and subsurface parts of a project which are often evaluated separately. In fact, the surface and subsurface are tightly linked and should be studied together to fully quantify the effects of one on the other and on the economic value.

An evaluation was performed of more than 21 undeveloped fields with multiple reservoirs each to predict the economics of delivering gas at a nearly constant rate to a specific downstream project, and the associated oil and condensate to a country's export system. The fields are spread across a 6000 km2 area. The data comes from nearly 70 wells drilled mostly in the early 60's to a few today. The uncertainties associated with the reservoirs: gas in place, drive mechanism, inflow performance, fluid properties; were linked to the wells: well architecture, tubing flow performance, well head pressure; and linked to the pipelines: distance, diameter, liquid holdup, and elevation changes. All uncertainties were studied over their probable range of values.

This paper describes the application of mature technologies across the broad spectrum of disciplines required in most projects. The significance is the integration of the contributions from many experts into a model which gives a quantitative result of the impact uncertainties have in a linked surface / subsurface system, and their importance to the value of the project. The models used to describe the linked systems are commercially available software; the value of this paper is the description of the methodology used to evaluate and integrate many surface and subsurface uncertainties. Table 1 summarizes the uncertainties to be discussed.

UncertaintyDescription

Original Gas-in-Place

 

What is the gas available for downstream considering the possible ranges of reservoir properties which impact volume in place: gross rock, porosity, gas saturation, net pay, formation gas factor?

 

In-Flow Performance (IPR)

 

What is the deliverability of gas wells at bottom-hole conditions?

 

Outflow Performance

 

What is the impact of tubing and well design on sustained gas deliverability?

 

Pipelines

 

How do pipe length, diameter, elevation changes, scheduling of fields, fluid properties, changes with time, and gathering center designs impact the piping design and the flow characteristics?

 

Reservoir Performance

 

What order are reservoirs and fields brought on and how many wells are needed to develop fields for downstream deliverability?

 

Field Appraisal

 

What program of seismic acquisition, wells, rock and fluid sampling, and data analysis is required to reduce the risk of the project?

 

Drilling Program

 

How many wells can be drilled each year and what is the cost of those wells?

 

Economics

 

What is the impact of surface and subsurface uncertainties on project decisions and economics? How to include remaining subsurface and surface uncertainties?

 
UncertaintyDescription

Original Gas-in-Place

 

What is the gas available for downstream considering the possible ranges of reservoir properties which impact volume in place: gross rock, porosity, gas saturation, net pay, formation gas factor?

 

In-Flow Performance (IPR)

 

What is the deliverability of gas wells at bottom-hole conditions?

 

Outflow Performance

 

What is the impact of tubing and well design on sustained gas deliverability?

 

Pipelines

 

How do pipe length, diameter, elevation changes, scheduling of fields, fluid properties, changes with time, and gathering center designs impact the piping design and the flow characteristics?

 

Reservoir Performance

 

What order are reservoirs and fields brought on and how many wells are needed to develop fields for downstream deliverability?

 

Field Appraisal

 

What program of seismic acquisition, wells, rock and fluid sampling, and data analysis is required to reduce the risk of the project?

 

Drilling Program

 

How many wells can be drilled each year and what is the cost of those wells?

 

Economics

 

What is the impact of surface and subsurface uncertainties on project decisions and economics? How to include remaining subsurface and surface uncertainties?

 

Table 1: Summary of Uncertainties

Introduction

Understanding and quantifying uncertainty in major projects in the oil industry is not a new problem. Linking surface demands and performance to subsurface performance is a recognized need as well. References 1 to 4 at the end of this paper provide some more discussion and alternate approaches to the issues to be discussed here. The Influence Diagram below illustrates the interrelationships among surface and subsurface unknowns and begins to show why the two cannot be evaluated separately. The circles indicate inputs to calculations of performance; the boxes are decisions to be made. The inputs have some degree of uncertainty which is captured by a range of values for each.

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