Borehole Fluid Reinvasion Effects on C/O Logs in Openhole Completion
裸眼井完井后井内流体二次侵入对碳氧比测井的影响
Abstract
摘要
This paper discusses the borehole fluid reinvasion effects on carbon/oxygen(C/O)saturation results in wells with openhole completions.Although the advantage of C/O technology in a fresh or mixed formation water salinity environment is evident,the main limitation of C/O technology is the relatively shallow depth of investigation ranging from 6” to 10”.To overcome this problem,the general recommendation for wet producers has been to acquire data in flowing conditions to minimize reinvasion effects.Two field examples with C/O saturation data in dedicated openhole observation wells which have been shut-in for a longer period of time,have shown that reinvasion remains a major issue and is mainly controlled by the pressure differential between borehole and formation and formation permeability.The degree of pressure overbalance is evaluated with borehole fluid pressure and offset RFT formation pressure data. Flowing data from these observation wells provides a significant different answer and is the recommended solution for observation wells.Alternatively,if only shut-in data can be acquired,another field example shows that shallow reading C/O measurements should be combined with deep reading resistivity readings to evaluate the degree of invasion and
assess deep formation saturation.
这篇文章主要讨论裸眼井完井后井内流体二次侵入对碳氧比测井确定流体饱和度的影响。虽然碳氧比测井技术在淡水或混合地层水矿化度环境中优势比较明显，但是其主要限制因素是相对较浅的探测深度，范围是6英寸—10英寸。为了解决这一难题，总的建议就是：对于湿层要求在流动的条件下获得测井数据以减小二次侵入的影响。两个野外实例通过对已经关闭很长时间的裸眼井其碳氧比饱和度数据的观察，得出：二次侵入仍然是影响碳氧比饱和度的主要因素，主要是受井眼和地层之间压力的差异和地层渗透率的影响。过平衡压力的程度用井内流体压力和（重复式地层测试器测得的）地层压力来评定。这些观察井在流动条件下获得的数据有一个明显的不同，因而被推为观察井的解决方案。二者可以择其一，如果只能得到井底关井压力，另一个野外实例得出：浅的碳氧比测量结果结合深的电阻率测量结果共同评估侵入的程度和深地层饱和度。
Introduction
简介
In the giant carbonate oil fields of Saudi Arabia,a regular assessment of reservoir sweep efficiency by means of accurate measurements of remaining oil saturation is crucial.One of the main technologies used is Pulsed Neutron Logging(PNL). The standard procedure is to acquire both Sigma and carbon/oxygen(C/O)data.Because of an active waterflood program with fresh injection fluids from non-reservoir sources,results from C/O logging are the main source for saturation monitoring.
在沙特阿拉伯巨大的碳酸岩油层中，准确测量剩余油饱和度对评估油藏的洗油效率是至关重要的，其中的一个重要技术就是脉冲中子测井，标准程序是获得Σ和碳氧比数据。由于在开采过程中主动从非油区注入淡水，使碳氧比测井成为饱和度监测的主要方法。
One of the main challenges of PNL is the shallow depth of investigation.Since the majority of wells in Saudi Arabia are completed openhole,borehole fluid reinvasion effects on C/O results in the near wellbore vicinity have proven to be an issue. By comparing flowing and shut-in measurements in openhole wet producers,we have documented different types of reinvasion.During shut-in passes there is normally an oil/water contact in the borehole.We have seen oil reinvasion above this contact,water reinvasion below,or no reinvasion at all.For such completions,the general recommendation has been to acquire data in flowing conditions.In this study we evaluate whether shallow-reading C/O measurements provide the correct formation saturation to evaluate sweep in wells which have been shut-in over a longer period of time.
脉冲中子测井面临的主要难题之一是探测深度太浅，既然在沙特阿拉伯大多数测井是在裸眼井中完成的，井内流体二次侵入对邻井碳氧比结果的影响就是一个问题。在裸眼井湿产层中对比流动和关闭两种状态的测量结果，我们记录不同类型的二次侵入。在关井期间，井内有一个正常的油水接触。我们能看到油的二次侵入在接触面的上部，水的二次侵入在接触面的下部或者是根本没有。在这样的完井中，总的提议是在流动的条件下获得数据。在这一方面的研究，我们通过碳氧比测井能否提供准确的地层流体饱和度来估计关井很长时间后井内的波及范围。
To this end,a number of openhole observation wells have been evaluated with C/O technology,which have been shut-in for several years but were able to flow during date of logging. In addition,one well with only shut-in data has been evaluated with both C/O and deeper reading induction resistivity.This paper discusses these field examples and identifies formation and borehole conditions which control the borehole fluid reinvasion process.Based on these observations recommendations are presented for effective saturation monitoring in wells with openhole completions.
为了这一目的，许多裸眼观察井开始用碳氧比技术进行评估。这些井已关井许多年了，但是可以获得流动条件的测井数据。另外仅有关井数据的井又用碳氧比和深感应电阻率结合进行评估。这篇文章讨论这些野外实例并识别控制井内流体二次侵入过程的地层和井眼条件。在这些观察结果的基础上，关于在裸眼井完井中有效地层流体饱和度监测的提议应运而生。
Factors contributing to borehole fluid reinvasion
井内流体二次侵入的因素
To date,it is common to acquire C/O data in both flowing and shut-in conditions.Several field examples have provided detailed insight into which borehole and formation factors contribute to borehole fluid reinvasion process.The following factors are identified:
至今，在流动的条件和关井条件下均获得碳氧比数据已经是一种普遍现象。几个野外实例已经详细描述了引起井内流体二次侵入过程的井眼和地层因素。下面简要介绍一下影响因素：
Completion type:
完井类型
Obviously,reinvasion plays a major role in openhole completion whereas a formation behind a non- perforated cased hole completion is not exposed to the borehole and therefore will not be affected by reinvasion. Although reinvasion through perforations is possible,the saturation accuracy across perforated intervals is generally reduced because no consistent characterization database exists for perforations.During C/O processing,the perforated interval is being treated as fully cemented casing.
很明显，二次侵入对裸眼井完井有重要的影响，然而在套管井完井没有射孔的套管后的地层没有暴露在井内，因此不受二次侵入的影响。虽然透过射孔眼的二次侵入是可能的，由于针对射孔没有相容特性的数据库存在，按照射孔间距计算的饱和度的准确性普遍减小。在碳氧比测井工程中，射孔间距全部按照水泥套管来对待。
Capillary pressure:
毛细管压力：
Reinvasion processes are in a way short time drainage and imbibition processes.The extent of the
drainage and imbibition process is dependent on the rock wettability and the amount of capillary forces the rock is exposed to.Naturally occurring drainage and spontaneous imbibition are normally slow processes and extend the time used to shut-in the well while waiting for borehole pressure
stabilization.Forced drainage and imbibition controlled by borehole and formation conditions are the dominating process which controls reinvasion in this case.
在某种程度上讲，二次侵入的过程是一个短时间排水和渗吸的过程。排水和渗吸过程的程度依赖于岩石的润湿性和暴露岩石的毛细管数量。自然排水和渗吸通常是一个缓慢的过程，在等待井内压力稳定时，延长了关井的时间。强制性排水和渗吸受井眼和地层条件影响，既然这样，所以它是控制二次侵入的主要过程。
Borehole holdup:
井内流体持率：
Borehole fluid holdup defines what type of reinvasion occurs.Reinvasion is mainly evident when the borehole fluid type is different from the dominant fluid type in the formation.In openhole completions,representative formation fluid saturation is normally acquired during flowing conditions.It is especially in these conditions when reinvasion during the shut-in passes is obvious.
井内流体持率决定发生二次侵入的类型。当井内流体类型和地层中主要流体类型不同时，二次侵入才非常明显。在裸眼井完井中，具有代表性的地层流体饱和度通常是在流动的条件下获得。在这些条件下，当二次侵入在关井条件下的特别之处是显而易见的。
Saturation profile:
饱和度剖面：
The affect on reinvasion of this formation property is directly related to the previous property, borehole fluid holdup.Whenever the dominating formation fluid is different from the borehole fluid,reinvasion becomes apparent.
影响饱和度的二次侵入直接关系到原地层的性质和井内流体持率，当主要地层流体不同于井内流体时，二次侵入变的很明显。
Differential pressure:
差压：
Differential pressure between borehole and formation has shown to be one of the dominating factors contributing to borehole fluid reinvasion effects.Any borehole fluid pressure over-balance relative to the formation pressure will induce borehole fluids to invade into the formation.A wide variety of publications exist on this topic with special focus on fluid invasion effects during drilling conditions.Sensitivity studies and laboratory experiments7-9 indicate that filter-cake permeability and pressure differential are the major factors governing fluid invasion during the drilling process.In a producing well,reinvasion after wellbore shut-in are not hindered by any mud cake since producing fluids lack the solids or bridging agents of drilling fluids.In case of the presence of a mud cake,the diameter of invasion is normally inversely proportional to the porosity and permeability of the formation.Whereas in drilling conditions overbalance pressure could be several hundreds of psi across the entire reservoir due to mud properties,in a producing well overbalance pressures will be different from zone to zone due differential pressure depletion.Generally,the borehole pre sure builds up to the average reservoir formation pressure. Pressure overbalance from the borehole fluid column will exist across zones with lower formation pressure than the average borehole pressure.In a depleted reservoir,differential depletion from zone to zone is very common due to reservoir heterogeneity and therefore reinvasion is likely to exist.As soon as a producing well is put on production,and the formation is allowed to produce its fluid into the wellbore due to pressure drawdown,reinvasion effects will reduce significantly,especially across producing zones. Shut-in time:Bor hole shut-in time controls to what extent reinvasion occurs.In producing wells,data is typically acquired in flowing conditions first to minimize fluid reinvasion after which the well is shut-in for 3-6 hours until downhole borehole pressure is stabilized.Subsequently,shut- in data is acquired.Obviously,the degree of reinvasion is proportional to the time of shut-in.
研究表明：井内和地层之间的差压是导致井内流体二次侵入效果的主要因素之一。任何井内流体压力与地层压力相比处于过平衡时，都会导致井内流体侵入地层。许多关于这方面的出版物着重于钻井条件的流体侵入效果。灵敏性研究和实验室实验表明：渗透性滤饼和差压是钻井过程中控制流体侵入的主要因素。在生产井中，关井之后的二次侵入不受任何泥饼的阻止，因为产层流体缺少固体物质或者钻井液中缺少桥堵剂。假使存在泥饼，侵入直径通常与地层孔隙度和渗透率成反比。而当钻井压力是过平衡时，由于泥浆性质的影响，整个油藏的压力是几百psi，在生产井中过平衡压力由于差压损耗而随地点变化。总体上，井内压力就相当于地层平均压力。从井内流体柱得到的过平衡压力是按带状存在，此时的地层压力比井内流体平均压力小。在一个衰竭的的油藏中，由于油藏的多相性，差压到处存在是一种普遍现象，因此二次侵入很可能存在。一旦生产井投入生产，由于压力下降，地层内的流体将流入井内，此时二次侵入效果明显减小，尤其是在生产层。
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