Why is It So Difficult to Predict the Performance of Horizontal Wells?
Authors: Aziz K. , Arbabi S. , Deutsch C.V. 
Published: March 2010

Abstract
Non-conventional wells that include horizontal, highly deviated, and multilateral wells-often called designer wells-are becoming more and more common. Both analytical and numerical tools have been developed and continue to be developed for predicting their performance. Unfortunately, predictions made using these tools rarely match actual performance, except in cases where sufficient production data are available for history matching and the model used for making predictions is selected carefully. Even then the predictions are generally good only for a limited time. In this paper we explore reasons for our inability to make accurate predictions. We consider a case where a vertical well has been drilled and cored. Then, we generate twenty consistent geostatistical descriptions of permeability and porosity that are all constrained to the hard data obtained from the vertical well. Simulations with these realizations show large differences in production rate, WOR and GOR predictions as a result of variations in reservoir properties. It is also shown that the effect of well index (WI) on simulation results is large. Furthermore, for the example considered, analytical models for critical rate and productivity calculations were found to be of limited practical use.

Introduction
In a recent talk at Stanford University, Edward Teller was asked what had changed in science over the past 60 years or so since he immigrated to the U.S. He responded by saying that "then we believed that everything could be predicted, now we know that future can only be predicted in a probabilistic sense." While Teller was talking about physics, his remarks are equally valid for other areas of science and engineering. Yogi Berra understood this problem well when he said: "predictions are difficult to make, especially with regard to the future."
In most cases the prediction of the aggregate effect of random events is sufficient for engineering purposes. For example pressure drop caused by the flow of gas in a pipeline is a consequence of the motion of individual molecules. While we cannot-nor do we want to-predict the behaviour of individual molecules, we can predict everything of practical significance: pressure and temperature distribution, average velocity at every location, etc. In order to make such predictions we have to be able to describe our system, and its initial and boundary conditions. In the case of steady-state flow in a pipeline we must specify:Pipe diameter, length, profile, and roughness;Initial state of fluid in the pipe; andInteraction with the boundaries.
Even for this simple problem there are uncertainties. Heat transfer from the pipe to the surroundings will depend on the material in which the pipe is buried and the ambient conditions, which are never known precisely.
Wells drilled in petroleum reservoirs interact in a complex way with the reservoir. In order to predict their behaviour we must be able to model multiphase flow in the well and the reservoir. In this problem there are many sources of uncertainties, some of these are explored in this paper. The most serious of these is the limited data about the reservoir itself.