Horizontal Well Design Optimization: A Study of the Parameters Affecting the Productivity and Flux Distribution of a Horizontal Well

Abstract

Abstract With the widespread applications of horizontal wells and its successful performance in the last decade, a number of relevant questions has been raised with respect to the optimization of horizontal well design. Considering the high costs involved and the technical challenges that are faced by the industry under various scenarios such as offshore environment, unconsolidated sands and heavy oil production, questions related to the optimization protocols to be followed have become progressively more important and more difficult to answer. Thus, the task of optimizing horizontal well design requires an investigation of the parameters that affect the productivity and ultimately the behavior of both reservoir and horizontal well domains, as well as the risks and costs involved in each alternative. Based on a fully implicit, three-dimensional numerical model coupling reservoir and horizontal well flow dynamics, a detailed study of the parameters that affect the behavior of flux distribution and productivity along horizontal wells has been performed. The parameters analyzed include permeability, initial gas saturation, anisotropy ratio, well location, fluid viscosity, flow rate, well length, and well diameter. A tool that permits to compare and select the potential options in terms of the gain in productivity per additional unit of well length and diameter is also introduced. Validation of the numerical model used in this study is performed using production-logging data and the examples of results of the application of the optimization protocol developed are reported for some of the Petrobras offshore fields. Introduction Different analytical and numerical models have been used to predict flow behavior and performance of horizontal wells. Basically, there are two different approaches to address the coupling issue between the two domains, namely, reservoir and wellbore and the effect of the hydraulics within the well. First approach involves the use of an infinite conductivity representation that treats the wellbore as an infinite conductivity medium and neglects the wellbore hydraulics. Although this idealization overpredicts the productivity, it is applicable in low productivity systems or in systems in which the pressure losses in the wellbore are negligible when compared to the pressure drop observed in the porous media1. However, for long wells, high flow rates, slim holes, high viscosity fluids, and multiphase conditions, the wellbore hydraulics play an important role in the production behavior of horizontal wells and should not be neglected. The second approach represents the actual behavior better, as it considers the wellbore domain as a finite conductivity medium by incorporating in the formulation the effects of friction, acceleration, gravity and influx coming from the reservoir. Unlike the results obtained with the infinite conductivity assumption, flux along the well has been shown to be not uniform or symmetrically distributed, as a larger volume of fluid enters near the downstream end of the wellbore in most of the cases studied. Therefore, the region near the heel of the well, would be under higher drawdown and more prone to water and gas coning. In general, all kinds of coning detrimentally affects both the short-term performance of the well as well as the ultimate recovery of the reserves.