Horizontal Well Productivity and Risk Assessment
Authors: Brekke Kristian , Thompson Leslie G. 
Published: December 2007

Abstract
A fast and accurate method was developed for predicting long term horizontal well performance. Heterogeneous, anisotropic geology close to the wellbore were considered in addition to pressure loss through the completion. Speed and accuracy were achieved by replacing the well and reservoir simulation with a semi-analytical network approach, and by upscaling reservoir properties for radial flow. Comparison to fine grid reservoir simulations verify that both total well productivity and flux profile along the well are maintained for the simplified approach. Computational efficiency and comprehensive treatment of the horizontal well problem make the method suitable for complete incorporation of uncertainties connected to the completion, the near wellbore geology and formation damage. The procedure was applied to illustrate how uncertainties in geology and completion efficiency affect the distribution of total well productivity for finite and infinite conductivity horizontal wells of different lengths. The method proved to be very efficient for this type of study, and indicated positively skewed (log-normal like) productivity distributions for short wells, normal distributions for long wells and a tendency for negative skewness of the productivity distribution from pressure loss in the wellbore.

Introduction
Developments in drilling and completion technology have resulted in horizontal wells with longer wellbores more complex geometry well paths and with sophisticated completion designs. These wells usually have a more complicated interaction with the reservoir than vertical wells. In addition, the parameters affecting the well performance also involve a higher level of uncertainty as compared to vertical wells. Horizontal wells are affected by geological variations in the horizontal direction besides involving a larger variation in the outcome of the more complex drilling and completion operations. Thus, the application of long horizontal wells increases the potential both for success and failure. The potential for success can be enhanced by better understanding the total reservoir and wellbore interaction and flow behavior. Unexpected failures can be avoided by efficiently including all uncertainties in the predictions.
Throughout this study, the focus has been on the development and application of methodology for comprehensive prediction of production performance for horizontal wells. Besides providing accurate and CPU-time efficient calculation of the entire horizontal well flow problem, the methodology is developed for the purpose of incorporating the uncertainties connected to the near wellbore geology, formation damage and completion efficiency. In this study the theory used to describe the horizontal well flow problem is applied to three different regions as follows:Flow through the near wellbore reservoir zone. Upscaling methods have been developed for radial flow in the formation close to the wellbore. The methods are based on a single phase, steady-state flow assumption. Fine grid simulations confirmed the development of a steady-state flow zone around the wellbore after a very short time. The upscaling methods incorporate the effects of convergent flow around the wellbore through heterogeneous and anisotropic formation. The theory and a computer program are developed for converting the description of a heterogeneous, anisotropic reservoir geology in Cartesian coordinates to an equivalent system in cylindrical coordinates for upscaling.Flow in the Outer Reservoir. Two methods were considered for coupling well and near wellbore simulations to the response from the outer reservoir. The network model for the well and near wellbore reservoir can be coupled to a numerical reservoir simulator by an iterative coupling scheme. It is also shown how a semi-analytical pseudo steady-state reservoir response for non-uniform inflow and pressure profile along the well may be integrated with the well and near wellbore flow calculations through the network model.
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