Long-Term Performance of Multilaterals in Commingled Reservoirs
Authors: Yildiz T. 
Published: March 2010

Abstract
In this study, a comprehensive transient model was developed for multilateral wells producing from multiple formations separated by impervious streaks. The model is flexible and is capable of simulating the flow with any number of layers, unequal initial layer pressure, non-uniform petrophysical properties for each layer, and unequal layer size. Additionally, each lateral can have non-uniform formation damage and may be selectively completed.
We investigated the long-term rate decline and cumulative recovery response of the multilateral wells connecting multiple isolated formations. Total flow rate, cumulative production, and fractional rate for each branch are all compared for several completion scenarios and layer properties. Crossflow between the layers through the wellbore is also evaluated.

Introduction
There have been many studies on modelling the fluid flow in horizontal wells in single-layer reservoirs or layered reservoirs with interlayer crossflow. In recent years, several studies focused on the productivity of multilaterals in homogeneous formations. To the author's knowledge, the literature provides only one study on the performance of a trilateral well in a three-layer commingled reservoir and one research study on the pressure transient characteristics of multilaterals in layered formations with crossflow. In many field applications, several non-communicating formations may be commingled only through a single undulating horizontal well or several sidetrack laterals emanating from a common pilot hole. The literature lacks a comprehensive investigation of the long-term performance of multilaterals commingling several isolated formations.

Background
Performance of vertical wells producing from commingled reservoirs has been the subject of many studies in the literature(1–4). However, most of the studies have focused on the transient pressure response of the commingled systems(1–3). Johnston and Lee(4) investigated the flow rate and cumulative production responses of low-permeability layers commingled through a vertical well.
Inflow performance of a single horizontal lateral has been examined in detail by many investigators(5–12). Some studies only consider horizontal openholes(5–7). The effects of well completion schemes, such as partial penetration, undulation, perforating, and openhole gravel packing, on horizontal well performance have been discussed in several more recent studies(8–12).
In recent years, multibranched wells have been proposed to improve hydrocarbon recovery and to accelerate production. Horizontal well performance with multiple laterals has been considered in several modelling studies(13–18). References (17) - (21) have reported the field applications of the multilaterals. Almost all the previous work on multilaterals addressed the issues related to production from a single formation or layered formations with interlayer crossflow. Among the previous studies, Vo and Madden(17) analyzed the production from a trilateral well commingling three isolated layers in the Dos Cuadras offshore field in California.

Inflow Performance Model for Multilaterals in Commingled Reservoirs
The physical model considered in this study is shown in Figure 1. The commingled multi-layer system considers an arbitrary number of isolated layers with different petrophysical properties and initial pressure. Impervious layers separate the reservoirs. It is assumed that there is only one lateral in each layer. All the laterals drilled into different layers may emanate from a common pilot hole, as depicted in Figur