Abstract

In order to make an appropriate business decision on well completion, the impacts of uncertainty of key completion parameters on well performance must be carefully evaluated to reasonably estimate the well performance. A procedure has been proposed for the evaluation purpose. The procedure will be applied to five common types of well completions to illustrate the importance of the proposed uncertainty analysis.

A field case study will be discussed in detail in the paper to demonstrate the uncertainty assessment process for efficient completion designs. The case study will be focused on a horizontal oil producer where different completion options have been proposed. The relative impacts of different parameters on the well production will be investigated to identify the major parameters that could affect the well production the most. The field case study will help us to understand the importance of the uncertainty analysis and its potential impacts on the final completion selection.

Introduction

Well completion plays a critical role in the performance of a well in its entire life. More and more advanced well completion options are available for potential deployment in new wells; however, the well performance, the cost, the well reliability, etc, could vary significantly from one completion type to the other.

Under most circumstances, a well completion is selected based on its cost, well performance, sand control potential, water shut-off possibility, well reliability, and so on. An appropriate well completion design requires an accurate prediction of well performance. To accurately predict well performance, a number of parameters such as reservoir properties and fluid properties have to be specified. However, there could be significant uncertainty in these input parameters for predicting the well performance; which is especially true for new deep or ultra deep water development where a single well could easily cost over $100MMs.

In order to make an appropriate business decision on well completion, the impacts of uncertainty of key completion parameters on well performance must be carefully evaluated to reasonably estimate the well performance.

What is uncertainty? Uncertainty is the "lack of assurance about the truth of a statement or about the exact magnitude of an unknown measurement or number". Uncertainty could introduce the risk of making an inappropriate business decision because the estimates deviate significantly from reality. Thus, uncertainty is associated with the economic risk analysis of the reservoir production forecast and plays a critical role in the decision making process. Because of the critical role played by the uncertainty, its impacts on well performance, reservoir management, production forecast, etc, must be appropriately quantified.

Uncertainty quantification in reservoir development projects has been a steadily growing industry-wide practice since the early 1990s. The uncertainty in our understanding of an individual well's and a given reservoir's performance (e.g. reserves at the economic limit) arises from the uncertainty in the information we have about the variables that control reservoir performance (e.g. permeability, porosity, oil water contact, geology, petrophysical properties, fluid PVT, etc.). By quantifying uncertainty in the each phase of a project, it becomes possible to clearly assess risk and plan for a range of possible project outcomes.

A key project uncertainty is the production forecast. Reservoir simulation is usually used to forecast oil and gas production under selected development scenarios. As we know, almost all the data used in reservoir simulation are subject to uncertainty. The uncertainty in the production forecast results from the interaction of the uncertainties in all sources of data. This uncertainty may be quite large, as is usually the case for the distribution of rock properties (porosity and permeability) away from the wells. Consequently, the oil and gas production associated with any development scheme may not be predicted exactly.

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