DDI定向井难度系数
1、AHD 沿井眼的位移
AHD or Along Hole Departure is the sum of the displacement between individual survey stations. 
沿井眼的位移是各个测量点位移的总和。
It is a measure of true departure in a well essentially stretching a well straight, like a string (azimuth only, not inclination). 
它是一口井拉伸为直线的真实位移的测量，就像一条线（仅是方位而不是井斜的变化）。
So a well with no azimuth change will have a surface to TD departure equal to the AHD. 
因此一口井如果没有方位的变化将是一个面达到总井深，其位移等于沿井眼的位移。
A well with turn in it will have a greater AHD compared to the surface to TD departure.
一口弯曲的井所产生的沿井眼位移相对面到TD的位移来说要大些。
This can be derived simply by using Pythagoras to calculate the distance between the survey stations and then summing them. 37度
这可以用勾股定理来计算各点的位移，并汇总。
y = northing, x= easting
2、ERD Ratio  水垂比
Extended Reach Drilling ratio: This is the AHD to TVD ratio. Essentially it gives a ratio of the total displacement (AHD) to the  \vertical depth. It can be considered as a measure of the Drilling Envelope in ER Horizontal Drilling. 
水垂比：
大位移井的水垂比：是沿井眼位移与垂深之比。本质上，它给出了一个总位移与垂深之比。它可以被认为是大位移水平井的一种量度标准。
3、Tortuosity 弯曲度
This is the cumulative dogleg planned into the well or for a drilled well, the total dogleg change (positive addition) even when it is a drop.
这是对于计划钻一口井或已钻井的总狗腿度，甚至当井眼降斜时总的狗腿度变化（正的增加） 。
Typically, as the length of the wellpath increases, so too will the tortuosity. Actual drilled tortuosity is often higher than the planned well tortuosity, especially in wells with long tange
nt sections, since the DLS in the tangent is often difficult to maintain at zero. 
一般来说，当井眼轨迹长度增加，弯曲度也增加。实际钻的弯曲度通常比计划的弯曲度要高，特别是长稳斜段，因为在稳斜段的狗腿度很难保持在0度。
From a completions perspective, it is recommended that the overall well tortuosity be maintained below 0.8 degrees/100 ft in order to minimize the risk of the completion assembly hanging up before getting to bottom.  从一个总的观点来看，接荐的井眼总弯曲度应保持在0.8度/100英尺以下，以减少下入井底前整个管柱遇阻下不到底的风险。 Increasing tortuosity (in this case, localized hole tortuosity) may also lead to downhole drilling tools being subjected to higher shocks and thus increase the potential for failure. 
增加的弯曲度（在这种情况下，局部的井眼弯曲度）将同样导致井下钻井工具遭受较高的振动，而增加失效的风险。
4、DDI 定向钻井难度系数
Directional Difficulty Index is high level performance measure used to benchmark drilling performance. It becomes especially useful when used as an evaluation method on complex well geometries. It provides a first pass evaluation of the relative difficulty to be encountered in drilling a directional well.
定向钻井难度系数：
定向钻井系数是用于评定钻井完成情况的高水平方式。当用作复杂几何井眼的评价方式时就变得特别有用。它提供了在钻定向井时所遇到的相对难度的评价。
The following guidelines can be used to classify wells based on DDI:
基于DDI，以下列准则可用于定向井的分类：
DDI < 6  Relatively short, simple wells or simple profiles with low tortuosity
DDI<6,相对浅的、低曲折度的简单剖面的定向井。难度相对低。
6.0 – 6.4 Shorter wells with high tortuosity or longer wells with low tortuosity
DDI 6.0 – 6.4浅井带有高的曲折度或深井低曲折度的定向井。难度相对中等。
6.4 to 6.8 Long wells with relatively tortuous paths.
DDI 6.4 to 6.8深定向井伴随相对高的曲折度轨迹。难度相对较大。
6.8  Long, tortuous well paths with a high degree of difficulty.
DDI >6.8，深定向井伴随高难度的曲折度轨迹。难度大。
5、定向井软件中的DDI
定向井难易程度一直以来没有定量的比较方法，发表在SPE文章定义了DDI(定向井难度系数）的计算方法， 为我们对比不同的定向井难易程序提供了一个标准的方法。
DDI在compass等下定向井设计软件中也体现了出来，为今后比较定向井难度提供了便捷准确的对比计算。