重庆科技学院翻译 - 图文

RUSSIAN LOG INTERPRETATION F. Verga, P.P. Rossa, Politecnico di Torino, M. Piana, M. Gonfalini, ENI-AGIP Division

Copyright OMC 2001.

This paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 28-30, 2001. It was selected for presentation by the OMC 2001 Programme Committee following review of information contained in the abstract submitted by the authors. The Paper as presented at OMC 2001 has not been reviewed by the Programme Committee.

ABSTRACT

The assessment of the reservoir oil or gas in place depends significantly on the accuracy of resistivity data and on the reliability of their interpretation. However, the evaluation of the formation resistivity from Russian BKZ logs is very troublesome for western oil companies as the western interpretation

approach, based on laterolog measurements, is not suitable for the Russian tool characteristics and measures. Resistivity log data recorded in three Russian wells, for which a pronounced inconsistency was found between the water saturation values obtained according to the western interpretation method and the nature and rates of the produced fluids during well testing, were also interpreted applying the

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Russian methodology and a software, named Rt-Mod , suitable for interpretation of Russian logs. The Russian method involves a manual comparison between experimental resistivity measurements and theoretical type curves to obtain formation resistivity, invaded zone resistivity and invasion

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diameter. The Rt-Mod software performs a numerical resistivity simulation based on an inversion method used to generate formation and invaded zone resistivity profiles.

Resistivity values obtained from both methodologies appear to be reliable, and more consistent with well testing results than those obtained when the western interpretation was applied. In particular, the Russian approach appears to be very reliable when layers are thicker than two meters and allows the evaluation of the formation resistivity and invasion diameter also when deep mud invasion is present. However, the Russian approach is complex and very time consuming. Results obtained from the

software application are fairly consistent for any layer thickness. The numerical simulation is very quick, simple, and only requires few data to run log interpretation. However, calculated mud resistivity values are not always consistent with the reported fluid property.

INTRODUCTION

As a consequence of the major political and economical changes which recently occurred in the

countries formerly part of the Soviet Union, western oil companies have expanded their interest in these areas. However, investment planning and reservoir exploitation strategies require evaluation of the

available hydrocarbon reserves. Western oil companies must rely on information gathered with Russian tools and methodologies but then use different criteria for reservoir exploitation. In fact, a deep diversity exists between the Russian countries and the rest of the world, also arising from a cultural and linguistic barrier. At the same time there is a strong need to compare the respective experiences and to define common methodologies. This research was meant to help define a reliable procedure to interpret

Russian logs because logs are fundamental in the determination of the petrophysical properties of the reservoir rocks. In particular, the possibility of achieving a more accurate interpretation of the Russian resistivity logs (BKZ logs) was explored, because western methods, based on the laterolog tool

response, had failed to provide reliable results, mainly due to the differences between the Russian and the western instruments. In fact, the water saturation profile calculated as a function of the true

formation resistivity evaluated by western interpretation, was inconsistent with the nature and quantity of produced fluid during well testing.

The Russian methodology for resistivity log interpretation is entirely manual and is based on the

comparison between experimental resistivity points and theoretical type-curves and, therefore, it can be rather approximate. A new software was developed for resistivity modeling of the Russian resistivity logs, thus allowing a fully automatic interpretation of the registered log curves.

Water saturation values were calculated as a function of the true resistivity values obtained by application of different interpretation methodologies and as a function of different porosity

measurements with the aim to define a correct interpretation procedure and highlight possible error sources in reserve evaluation when interpreting Russian logs.

RUSSIAN TOOLS

The determination of the water saturation profile of a producing layer is based on the formation

resistivity, also called true formation resistivity, which is estimated by interpreting a series of apparent resistivity logs. In fact, direct measurement of the true formation resistivity is hampered by well logging due to bore-hole, mud, and mud filtrate invasion effects.

The minimum logging suites run in the majority of Russian wells typically comprise electrical, caliper, temperature, and gamma ray devices, which occasionally can be complemented by neutron-gamma, density, or acoustic porosity measurements. The minimum logging suites generally comprise less instrumentation than do the European correspondent suites and, except for the omission of an explicit porosity tool, are comparable with those run onshore in the USA before 1985. In general, log data presentation exhibits poor quality and most log hardcopies are black and white and hand-edited.

Western companies have tried to digitized Russian logs but results were sometimes disastrous due to differences in the test procedures, problems related to log and depth scales, and lack of a systematic log quality control, not to mention language problems. Furthermore, several log curves are often combined on the same plot track, which can render the resulting product difficult to understand (Harrison, 1995).

In URSS the evaluation of the true formation resistivity, Rt, is based on measurements recorded using lateral devices. Different lateral measurements (BKZ logs) are combined, using different electrode spacing thereby investigating different depths away from the well bore. Typical suites are composed of five or seven different tools characterized by spacing ranging from one half meter to eight meters. The layer limits are identified and positioned according to the response curve of the short-spaced lateral tools whereas a reliable evaluation of true resistivity is obtained on the basis of the response curve of the long-spaced lateral tools.

The analysis of the lateral measurements is complicated by the asymmetric response of the displayed

curve that does not allow evaluation of an apparent resistivity value (i.e., the measured value)

representative of each analyzed formation. Therefore, an inverted probe (upside-down lateral probe) is frequently run in conjunction with a regular lateral having the same spacing. When suites of resistivity logs include identically spaced BKZ and inverted BKZ curves a “Pseudo-compensated BKZ” gradient log can be obtained by averaging the conductivity read by each curve (Harrison, 1995). This compensated curve is symmetrical, deep-reading and usable in digital processing like induction, focused and normal logs.

Measurements obtained by normal, focused, and induction devices are also employed for Rt evaluation, but only to support and validate results obtained on the basis of lateral measurement analysis. In fact, the tool investigation depth for the induction log does not allow consistent interpretation of log measurements due to a frequently large mud invasion.

RUSSIAN INTERPRETATION

The Russian methodology for resistivity log interpretation allows evaluation of the true formation resistivity, invaded zone resistivity, and invasion diameter. It can be applied to recorded data without any preliminary compensation. The number of different available BKZ logs determines the reliability of the obtained resistivity values. The procedure is entirely manual, and resistivity data correction and interpretation is achieved by repeated and sometimes iterative comparison of the real measurements with appropriate, dimensionless type-curves. In particular, every resistivity value used throughout the interpretation procedure must be normalized with respect to the mud resistivity. However, it is not a standard practice to measure the mud resistivity on the field. It is therefore necessary to evaluate the mud resistivity from the apparent resistivity measurements recorded in low porosity layers.

The methodology requires a preliminary correction of the apparent resistivity values for shoulder bed effects, namely the effects related to the presence of adjacent beds characterized by different electric properties from the investigated layer. Each resistivity value, ??, measured in a limited thickness bed is transformed in the resistivity value of a corresponding ideal layer of infinite thickness (fig. 1). The transformation is performed graphically and requires mud resistivity, ?c, tool spacing, L, and layer thickness, H, to be known.

Fig 1:Transformation of apparent resistivity values to the corresponding values for an ideal

layer of infinite thickness

The ideal apparent resistivity values, ??, obtained by normal, focused, inductive, and lateral

measurements are reported as a function of the tool spacing, L, on the so-called interpretation form (fig 2).

The interpretation form is then superimposed to theoretical type-curves, which describe the apparent formation resistivity, ??, normalized with respect to the mud resistivity, ?c, as a function of the tool spacing, L, normalized with respect to well diameter, d. (fig 3). Each dimensionless curve is also

characterized by a given value of the normalized true formation resistivity, ??. In order to determine the true formation resistivity of the investigated layer the type-curve which best matches the real

measurements reported on the interpretation form must be sought. Since the theoretical curves are specifically derived to interpret lateral measurements, focused, inductive, and normal measurements need to be compared to different curves, called isoresistive curves, which are also plotted on the same chart. The isoresistive curves allow transformation of focused, inductive, or normal measurements into equivalent lateral measurements having the same investigation diameter.

图10:含水饱和度值作为孔隙度测量的中子伽马函数工具。

图11:含水饱和度值作为声波孔隙度测量的工具函数。

图12:比较不同含水饱和度之间的配置文件。

结论

结果清楚地表明,俄罗斯电阻率解释日志根据西方方法不能提供可靠的地层真电阻率资料,同时由于俄罗斯和西方之间的差异现有工具配置和意外深泥形成的入侵。自西方的解释方法是基于侧向测井测量的模拟电阻率资料并不代表地层真电阻率,因为仪器探测深度一般小于入侵直径计算。

俄罗斯的电阻率值通过应用手册由数值模拟方法和充分一致。俄罗斯的方法是非常巧妙的虽然极其复杂和耗时。Rt-MOD软件非常简单,只需要一些数据来运行测井解释,并迅速执行模拟。

与俄罗斯的方法获得的地层真电阻率值是可靠的仅为厚层(超过2 meter-thick)而从软件应用程序获得的结果相当一致的任何层厚度。然而,计算泥浆电阻率值并不总是一致的报道时流体性质及模拟值的测量报告有很大区别,解释获得的结果可能会有问题。虽然层序列复制的软件可能不代表或真正的形成、电阻率的结果似乎与俄罗斯解释的结果一致。

最后,验证,也可以显著影响含水饱和度和孔隙度测量,因此,孔隙度曲线采用含水饱和度的计算应该准确地选择。事实上,错误的地层真电阻率和孔隙度资料甚至可能导致本地水饱和度剖面一致。 感谢

作者非常感谢石油软件技术提供软件RtMOD?用于这项研究。

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