Horizontal and Multi-Lateral Wells: Performance Analysis-An Art or a Science?
Authors: Joshi Sada 
Published: March 2010

Abstract
Several thousand horizontal wells have been drilled around the globe in the last ten years. In Canada alone, horizontal wells have been used in a variety of applications in almost all oil producing provinces. Several hundred multi-lateral wells have also been drilled. There are over 700 multi-lateral wells in Saskatchewan alone. Similarly, underbalanced drilling and coiled tubing drilling are also commonly used. This drilling driven oilfield revolution has made a significant impact on oilfield development.
In any oilfield project one is interested in four questions, namely expected well rates, well reserves, well cost, and probability of commercial success or risk factors. Although there are horizontal well projects where commercial success rates are very high, overall commercial success of the technology in North America is about 65%. This is especially of concern since most of the horizontal wells are infill wells. Understanding these successes and failures requires an analysis of well performance. In this article, practical difficulties and solutions for well performance analysis are discussed.

Introduction
Figure 1 shows schematic diagrams of different horizontal and multi-lateral well configurations.
FIGURE 1: Typical horizontal well and multi-lateral well profiles. (Available in full paper)
The figure shows that due to advances in drilling technology, it is possible to drill wells with different geometric configurations.
Most of the Canadian wells to-date is single lateral horizontal wells. It is becoming common to drill 1,000 to 1,500 m long horizontal wells. Moreover, most of the wells drilled have medium radius build-rates and are completed either with a slotted liner or an open-hole completion.
Multi-laterals have been used effectively to improve production as well as injection rate. The majority of the multi-laterals in North America are either open hole or one of the laterals is completed with a slotted liner. With such a completion, it is difficult to measure production from each lateral. Moreover, it is not easy to stimulate each lateral effectively. A coiled tubing may be necessary to enter each lateral independently, which of course adds to the stimulation cost. In some cases, even with coiled tubing it may not be possible to access each leg of the well.
In Canada, the major success of horizontal wells has been in producing gas zones, thin heavy oil columns, water-floods, and miscible floods in carbonates. Although many papers are written on SAGD, the field projects are limited to a dozen or so. Only a few have declared that they are commercially successful. One ofthem is the Celtic field in Saskatchewan where a horizontal well produces at 100 m3 /d with oil-stream ratio of about two.

Flow Behaviour
A horizontal well, similar to a vertical well, when put on production will initially go through a transient flow followed by a well stabilization or a stabilized flow(1). On a log-log plot of rate vs. time, this is observed as two different straight lines with two distinct slopes (Figure 2). Many times, this is also observed on the semi-log plot of rate vs. time.