集群式开发方案在大型油气藏开发中将发挥重大作用！
编译 | TOM 惊蛰
在多勘探区油气藏中，如果开发商仅从经济学角度来证明每个勘探区开发的合理性，常常会使自己处于非常不利的位置。在集群式开发方法中，将不同探勘区之间的地质相关性与勘探区的集群相结合，以确定开发的经济可行性。虽然方法很直观，但对集群式开发的经济评估通常不考虑实际存在的地质相关性以及开发的协同效应。对这些协同效应进行建模分析，发现它们对油气藏开发的经济评估具有显著影响。
在评估不同勘探区之间相关性的经济影响时，必须将地质勘探成果分解为共享要素与原位要素。所有相关勘探区之间共享的地质要素称为共享或油气藏机会。在每个相关勘探区之间独立地质机会要素被称为前景/成功率（PSR）。任何个体勘探区的地质成功率将被定义为油气藏机会乘以PSR。
在评估新区块时，若该区块地质机会要素相关性占比较高，将有效地使完全独立的要素无法发挥应有的作用。从研究人员的油气藏分析中可获得关键信息，油气藏开发存在最优钻井顺序。对油气藏机会影响最大的井，通常是首先开钻的最优井。行业惯例一般是侧重于井的决策，而不是油气藏的决策。因此，常规做法是首先钻探地质成功机会（Pg）最大或平均可采原油最多的井。而理想情况下，应该钻探油气藏机会要素最多的勘探区。
地质上的成功并非都是商业或是经济上的成功。为确定油气藏能否可以进行商业化开发，需经过两步流程。第一步是评估Pg机会，第二步是运行现金流模型，以确定每个勘探区具有商业价值的油田最小规模（MCFS），假设独立开发以及现有开发的集群式绑定。两者的产物即为商业成功机会。将油气藏视为个体勘探区或集群勘探区，将在决定商业化规模的阈值时，对决策产生重大影响。假设每个勘探区都可以用相同的现金流模型(勘探区到中央处理设备的距离相同)。
需要认识到非常规油气藏经济评价的主要因素，即开发决策，是基于平均项目产量，而不是单口井的产量。在油气藏中，众数或单口井最可能的产量（从频率角度来看）通常是不具经济效益的。目前，正在开采的油气藏都每口井的平均产量是具有商业价值的。随着累计井数的增加，方差（P10与P90比率）降低，进而趋于正态分布。
随着累计井数的增加，对数正态分布的众数逐渐转变为平均数。在对数正态分布中，平均值将始终超过中值，始终大于众数。在正态分布中，众数等于中值，等于平均值。同样的道理可以应用到集群式开发新油藏的建模中。
为了说明集群开发方法对经济评估与决策过程的影响，研究人员提出了深水环境中三个勘探区的案例研究。在实际情况中，勘探区可能会有不同的油气储量分布与原位地质机会因素。然而，在案例研究中假设所有勘探区的上述两个参数都相同。这种简化的目的是将重点放在过程上，而不是放在单个勘探区的细节上。同样，案例研究不是仅限于深水环境，该过程适用于任何环境，无论是陆上还是海上。
虽然现金流模型会影响经济结果，但它被认为独立于本文提出的集群式开发的评估流程，并没有在评估中凸显作用。目前提出的评估流程将与其他财政制度合作，例如产量分享合同模式。
业内的传统方法是，一个新区域的评估是基于足以支持基础设施建设的大发现。在不考虑地质相关性影响的集群式开发的基础上，做出开发决策的分析。集群式开发拥有地质相关性的勘探区。然后，综合考虑所有三个勘探区中可能的发现，制定开发决策。研究人员提倡第三种方法，下面将对此进行讨论。
具有共享油气藏机会的集群式捆绑开发。研究人员认为，深水案例是一个经过验证的断块油气藏，共享油气藏机会占比高达80%。在经济性分析中，假定干井分析不能确定共享油气藏机会系数，并对三个勘探区进行钻井。这种保守的案例研究方法是有意为之，因为它仍可证明，考虑共享油气藏机会能够为经济分析增加价值。
对于拥有共享油气藏机会的集群式捆绑开发方法，经济评估就类似于假定地质独立的勘探区所采用的方法。采用60000次迭代的蒙特卡洛计算法，同时考虑了勘探区之间的共享油气藏机会，利用三种勘探区的地质资源储量分布模拟出可能的产量。
这需要一个九分支决策树，而不是三个评估模型中用于第二个的八分支决策树，来总结出模拟结果，因为会有两种不同的方式导致彻底失败：油气藏不存在，或者即使油气藏存在，所有干井都是由原位勘探区地质特征因素所引起的。
决策树的主要结果显示，多重发展的可能性现在为22.4％（与不考虑共享油气藏机会时的19％相比）。此外，决策树还显示无法开发的可能性为63.3％（在不考虑共享油气藏机会时略高于62.9％）。在对勘探区之间的地质相关性进行建模时，这些结果与预期结果一致。在一个勘探区中取得成功的好消息，将增加在另一个相关勘探区中取得成功的机会，从而增加多个开发的机会(并增加成功案例的油气储量)。然而，导致一个勘探区出现干井的坏消息，增加了其他相关勘探区出现干井的可能性，从而增加了该项目失败的总机会。
由于具有开发价值的独立勘探区规模的门槛较低，因此使用集群式开发，至少一个开发项目的概率已经增加。在集群式开发的战略下，多个开发项目的机会显著增加。考虑到共享油气藏机会增加了成功案例中的开发数量，但由于地理上的相关性，会降低商业成功的机会。
当放弃独立策略时，平均开发数量、平均原油最终采收率以及平均DNCF设为8%，预期价值与成功案例指标均有显著改善。当考虑到成功案例中开发的更多发现所带来的共享油气藏机会时，集群式开发方法也显示出经济上的改善，而整体开发机会只减少了很小一部分。
In plays with multiple prospects, decision-makers put themselves at a significant disadvantage by using economics on a standalone basis to justify development for each prospect. In the clustered-development approach, the geologic dependence between prospects is combined with the aggregation of prospects to determine the economic viability of a grassroots development offshore. Although easily visualized, economic evaluations of clustered developments typically do not consider the geological dependencies and development synergies that exist. Modeling these synergies has a significant effect on the economic evaluation of developments.
Geologic Dependency
In assessing the economic effect of the dependencies between prospects, breaking the chance of geologic success into a shared element and a local element is mandatory. The geologic-chance elements shared between all related prospects will be referred to as either the shared or play chance. Those geologic-chance elements that are independent between each related prospect are referred to as the prospect/success ratio (PSR). The probability of geologic success for any individual prospect will be defined as the play chance multiplied by the PSR.
When evaluating a new region, having a high degree of dependence within the geologic chance-of-success components will effectively derisk the play relative to total independence. A key learning from the authors’ play analysis is that an optimal drill order always exists. The well that will have the largest effect on the play chance is typically the optimal well to drill first. Industry practice often is focused on a well decision rather than a play decision. Consequently, the well with the highest chance of geologic success (Pg) or the largest risked mean recoverable resources typically is drilled first. Ideally, prospects should be drilled in a location that best addresses the play-chance elements.
Effect of Aggregation on Size Dependency
Geological successes are not all commercial or economic successes. A two-step process is used to determine the chance of commercial success leading to a development. The first step is assessing the total chance of Pg. The second step is to run the cash-flow model to determine the minimum commercial field size (MCFS) for each prospect, assuming a standalone development and a clustered tie-in to an existing development. The product of the two is the chance of commercial success. Viewing a play as an individual prospect vs. as an aggregate of prospects will affect decision making materially in the determination of the size threshold for commercialization. An assumption has been made that each prospect can be represented by the same cash-flow model (i.e., the same distance from prospect to central processing facilities).
One of the major factors in economic assessment of unconventional plays has been the understanding that development decisions are based on the average program outcome, not on individual well outcomes. In resource plays, the mode or most-likely single-well outcome (from a frequency perspective) typically is uneconomical. Resource plays that are active today are those in which the program’s average per well mean outcome is economical. As the number of wells being aggregated increases, the variance (P10/P90 ratio) decreases and the distribution tends toward a normal distribution.
The mode of the log-normal shifts toward the mean as the number of wells being aggregated increases. In a log--normal distribution, the mean will always exceed the median, which will always exceed the mode. In a normal distribution, the mode equals the median, which equals the mean. This same understanding can be applied in modeling clustered developments in greenfield plays.
Case Study
To illustrate the effect of the clustered--development approach on the economic-evaluation and decision-making process, a case study of three prospects in a deepwater setting is presented in the complete paper. In practice, the prospects likely would have different resource--volume distributions and local geologic-chance factors. For the case study in this paper, however, both parameters are assumed to be the same for all prospects. This simplification is meant to keep the focus on the process rather than on the details of the individual prospects. Likewise, the case study is not defined beyond being in a deepwater setting because the process is applicable in any setting, onshore or offshore.
The cash-flow model used to generate the economics assumes a royalty and tax fiscal regime. While the cash-flow model will affect the economic results, it is considered to be independent of the process of evaluating clustered developments being presented in this paper and is not emphasized. The process being presented will work with other fiscal regimes, such as a production-sharing-contract model.
The traditional industry approach, in which a new region is evaluated on the basis of one discovery large enough to support the infrastructure development.
An analysis wherein the development decision is made on the basis of a clustered development with no consideration for the effect of geologic dependence.
A clustered development with geologic dependence between the prospects. The development decisions are then based on what might be found when all three prospects are considered. The authors advocate this third approach, and it is discussed next.
Clustered Tie-In Development Methodology With Shared Play Chance. The authors consider that the deepwater case study is in a play-segment extension of a proven play with a high shared play chance of 80% associated with migration risk. For economic analysis, the assumption is made that dry-hole analysis will be inconclusive of the shared play chance factor and all three prospects will be drilled. This conservative approach for the case study is adopted deliberately because it still will demonstrate that consideration for the shared play chance will add value to the economic analysis.
For the clustered tie-in development methodology with shared play chance, economic evaluation is similar to that adopted when prospects are assumed to be geologically independent. A Monte Carlo simulation with 60,000 iterations was used to simulate the possible outcomes using the prospect geologic-resources distributions of the three prospects but now also considering the shared play chance between prospects.
A nine-branch decision tree is needed this time, rather than the eight-branch decision tree used in the second of the three evaluation models, to summarize the simulation results because total failure can now happen in two separate ways: the play does not exist or all dry holes are caused by local prospect-specific factors even though the play exists.
The main results of the decision tree show that the chance of multiple developments is now 22.4% (compared with 19% when not considering the shared play chance). The decision tree also shows that the chance of no developments is 63.3% (slightly higher than 62.9% when not considering the shared play chance). These results are consistent with expected outcomes when modeling geologic dependencies between prospects. Good news leading to success in one prospect will increase the chance of success in the other dependent prospects, increasing the chance of multiple developments (and increasing the success-case resource volumes). However, bad news leading to a dry hole in one prospect increases the chance of a dry hole in the other dependent prospects, increasing the overall chance of failure for the program.
The probability of having at least one development has increased with the use of clustered development because of the lower threshold on individual prospect discovery sizes that can be developed. The chance of multiple developments has increased significantly with a clustered-development strategy. Considering the shared play chance increases the number of developments in the success case but reduces the chance of commercial success because of the geologic dependencies.
Significant improvement in expected-value and success-case metrics is seen for the mean number of developments, mean oil estimated ultimate recovery, and mean DNCF at 8% when the standalone strategy is abandoned. The clustered-development approach also shows economic improvement when considering shared play chance caused by a larger number of discoveries developed in the success cases with only a small reduction in the overall chance of development.