MULTI LATERAL WELLS

 MULTI LATERAL WELLS :- 

 it allows reaching target depths at different bottom hole locations. Thus, a multilateral well is one in which multiple boreholes or laterals are drilled from a single wellbore. 

A multilateral well can follow different well trajectory: horizontal or deviated, as shown in Figure 1. 

Junctions are the intersections of the laterals with the main wellbore or of the branches and splays with the lateral. 

The multilateral junctions have two categories: 1) CASED 2 UNCASED 

 UNCASED 

uncased junction does not have any casing in the lateral. 

The main wellbore may or may not have casing either. The uncased junction is more commonly used as it is easier to construct and is less expensive than a cased junction. 

However, an uncased junction requires that the formation be competent so that the borehole wall does not collapse nor the wellbore is filled with sand produced from the formation at the junction. 

CASED 

A cased junction, on the other hand, has a casing in the lateral that connects to the main wellbore. A cased junction can be prepared mechanically by proper positioning and cementing or with expandable metals, or by drilling larger holes and installing pre-manufactured junctions. 

The cased junctions are more expensive to install and hence, are selected where the uncased junctions can’t be used.

Multilaterals can also be classified by the way the laterals are positioned.

1. A DUAL LATERAL  :- is a multilateral with two laterals usually in the same reservoir. 
2. OPPOSED LATERAL - If the azimuth of the laterals are 180º apart, they are often termed opposed laterals. 
3. A crows foot - is the drilling of multiple directional wells from a single wellbore in order to drain different parts of the reservoir or different reservoirs.  

Reasons For Drilling Multilaterals Wells

1. There is really only one reason for drilling a multilateral: the return on investment (ROI) for the operator is more than the alternative.
2.  Multilaterals are more expensive to drill, so production rates have to be higher for a longer period of time to payout the difference. 
3.  multilateral wells are more effective in low to moderate permeability reservoirs. 
4. In higher permeability reservoirs, production interference will occur between laterals at an earlier time reducing the effectiveness of the laterals. Additionally, production in the parent wellbore would be limited by tubing diameter for high permeability wells.

Multilateral Drilling Wells Classification System

In a multilateral well, the wellbores are either cased (check also Types Of Casing In Drilling Oilfield Wells) or open hole. The multilaterals can therefore have numerous configurations based on whether the main wellbore and laterals are cased or left open hole. These configurations allow installation of a variety of junctions in a well. This variety necessitated development of a classification system for users to compare, differentiate and select the multilateral applicable for their respective requirements.

The TAML (Technology Advancement of Multilateral drilling wells) categorizes multilaterals based on the type of junction used. six types of junctions: Level 1 through Level 6. As the level increases, the complexity of the junction increases and the cost and risks go up.

LEVEL 1 - in this type of the well the mother bore and lateral both are open. both are uncemented and uncased       main welll bore (mother bore ) -open , lateral - open

LEVEL2 - main welll bore (mother bore ) - cased +cemented        lateral - open  

LEVEL3 - main welll bore (mother bore ) - cased + cemented       lateral - cased

LEVEL4 - main welll bore (mother bore ) - cased + cemented        lateral -cased +cemented 

LEVEL5 - main welll bore (mother bore ) - cased + cemented        lateral -cased +cemented

                 both cemented and cased  well intigrity is obtained by (well completion tool + stradle packer)

LEVEL6 - main welll bore (mother bore ) - cased + cemented        lateral -cased +cemented

               both cemented and cased  well intigrity is obtained by casing