Wettability is a reservoir rock property that is not easy to measure and quantify but has a crucial effect on other rock properties such as relative permeability, capillary pressure, and electrical properties. Problem that may occur with regard to this matter is that those properties are often measured on already cleansed core samples as part of the standard procedure. Having undergone the normally utilized heated cleansing process alteration in the rock’s original wettability was often reported. Under such condition, unrepresentative wettability certainly leads to unrepresentative measured data with all of consequences. This article presents a study that uses 363 sandstone samples retrieved from 28 oil and gas fields in Indonesia. The study consists of two stages of analysis. First analysis is performed on data obtained from three wettability tests results while the second one is made with using water-oil relative permeability data, that is usually measured on cleansed core samples. Original wettability data shows that the sandstones varry in wettability from water-wet to oil-wet (48.2% and 30.2% of total samples, respectively). Comparison between data of the two analyses shows that original wettability tends to degrade in strength after cleaning down to neutral wettability, among which neutral wettability appears to be the largest in number (49.1% of total sample). Results also show that weak wettability tends to endure more than stronger ones. The overall results have demonstrated the need for caution in core handling and for measures that can minimize the risk.
Reservoir rock heterogeneity is an ever-present occurence in most reservoirs. One among reservoir properties that are the most affected by its presence is permeability. In most cases, reservoir rock heterogeneity causes permeability anisotropy or variations in permeability with directions. As most theoretical bases used in many aspects in petroleum engineering (e.g. waterflooding, well completion, well productivity, well stimulation, and reservoir simulation) are developed under an assumption of homogeneous reservoir rocks, this permeability anisotropy certainly imposes a special problem that needs to be dealt with. Indonesia's geological setting is characterized by its high level of complexity. Permeability anisotropy in Indonesian reservoirs varies very widely, and careless approach in picking values representing the level of heterogeneity for individual reservoir may lead to various problems. This paper presents results of a study on permeability anisotropy - in this case vertical-to-horizontal anisotropy - in Indonesian reservoirs through the use of core analysis results. The study involves 14,634 core samples, of which 6,689 are pairs of vertical and horizontal plug samples and 1,256 are full-diameter core samples. They are taken from 259 wells representing 157 fields in all 15 Indonesia's productive sedimentary basins to date. The core samples cover a wide lithology spectrum from sandstones to various limestones. The study that is carried out through a series of classifications and comparisons has indeed shown the complexity as expected, even though differences can still be distinguished among groups of classification. Evidence has also been observed that results for sandstones and limestone/carbonates core samples as two lithology groups are significantly different even though there are also differences among either group. One very important lesson that can also be drawn from this study is a reconfirmation upon the common opinion suggesting the requirement of a careful study for individual reservoir before adopting K[V]/K[H] values for the reservoir of interest. Introduction Permeability variation always draw attention of any engineer and geoscientists whenever they come to the task of characterizing and producing reservoirs as good and efficiient as they can. In homogeneous reservoirs, permeability is considered the same in all directions. On the contrary, in heterogeneous reservoirs, permeability tends to differ with direction. The more heterogeneous a reservoir the higher variation in permeability it tends to show. In this case, permeability may be different in all directions making it difficult to obtain the overall picture of permeability distribution in reservoir. In lesser degree of heterogeneity, the permeability in vertical direction is still considerably different than the permeability horizontal direction but with similar permeability in all horizontall directions (transversely anisotropic). Nevertheless, the net impact of such changing permeability in different directions is especially marked on all aspects that involve flows of reservoir fluids such as natural recovery efficiency, well testing, enhanced oil recovery, and completion scheme.
For oil or gas fields with stratified reservoir layers, detailed productioncontribution for individual layer is always desired.Unfortunately, insome particular cases, production wells are completed following commingledscheme. This is worsened further if only very few production tests arerun for the field.This is the case for the Central Sumatera field withits 95 commingled production wells, among which only a few had undergoneproduction tests and none of them have ever undergone productionlogging.Problems rise when the occassion came in which detailedproduction contribution from individual reservoir layer is required for thefield's reservoir simulation modeling and productionevaluation/prediction. This paper presents an approach to solve the problem.The approach isbasically based on the application of soft computing (Fuzzy Logic) toinvestigate pattern of relationships between production contribution of layersin commingle wells and rock petrophysical data as well as other relevantgeological/engineering data.For the purpose, thirteen wells (key wells)that have production tests are assigned, among which three wells are assignedfor checking the validity of the recognised pattern.Using the validatedmost valid pattern, individual layer's production allocation for other wellsare determined with well-log analysis data as the major input. Result estimates for the candidate wells are better compared to resultsproduced by the conventional method of productivity index (PI)analogy.The resulted variation in water cut and separate oil and watersplit factors appear to be more realistic from any point of view. Introduction In managing a commingle production well, knowledge over productioncontribution of individual sand layer is always desired.The commonpractice performed during drilling and production activities of a productionwell is through the use of well testing/production testing and/or productionlogging. From the test, fluid dynamic data such as total liquid rate, water cut, and gas cut of an individual layer are produced.However, costand time efficiency is always used as the reason for not conducting suchtests. Therefore, even though such tests are always regarded as theprimary source of proof, an alternative means that can be used to provideestimates is always desired. Ideas of establishing a method that can provide illustration over productioncontribution of all layer(s) always exist.Certainly, there are approachesto serve the purpose such as productivity index (PI)/transmissibility analogyand petrophysical approach through fractional flow measurement in corelaboratory. However, those approaches are often considered inadequate foraccommodating various factors that may influence production contribution of aproductive layer. To materialize the requirement stated above, an indirect approach in theform of pattern recognition/modeling was taken.This approach was taken inorder to model relations between various factors in wellbore and productioncontribution of reservoir layers without being trapped by the certaincomplexity that may occur in any mathematical expressions trying to explain therelationships.For the purpose, fuzzy logic (a form of artificialintelligence) has been used.The choice is actually based on its capacityto accommodate both numeric and non-numeric data, since it is considered thatsome non-numeric data such as lithology and pore system also have someinfluence on production contribution.
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThere is no easy way to estimate permeability from well logs. Porosity alone does not serve as a good predictor of permeability. Various empirical approaches have been commonly used to estimate permeability. The more parameters used in the algorithm the more impractical the formula. In fact, there are no causative relationships among the calculated parameters. Recent developments are marked by the use of pore throat size distribution for relating porosity to permeability (Winland and Pittman equations). The equations are theoretically correct but they are actually established for estimating pore throat size distribution and are not intentionally developed for the purpose of permeability prediction for uncored wells. In this light, it is this study that emphasizes the use of Windland and Pittman equations for permeability prediction and is the first study using samples from Indonesian reservoirs.This study was carried out to improve the accuracy of permeability prediction of the traditional permeability-porosity relationships in uncored wells with particular attention given to low permeability datasets. Correlations between radius pore-throat aperture radii and porosity-permeability, using 315 core-plugs taken from Indonesian reservoirs, have been established. Application of the correlations shows that accuracy of permeability prediction is significantly improved when pore throat radii were incorporated. The study also take into consideration rock fabrics of the core-plugs on the basis of mercury saturation percentiles. For practical consideration, the correlations are also expressed in form of nomographs so that it can be used easily by engineers and petrophycicists. In using the nomographs, the input variables (pore throat and rock fabric) can easily be derived only from drill cutting samples underlining the practicality of the approach resulted from the study.
Knowledge of the in situ states of stress in rock masses is of considerable importance to a number of subsurface engineering activities, including those involved in exploiting petroleum and geothermal energy reserves. In this paper, a comparison is made of two laboratory techniques, based upon stress-relief microcracks, for determining the in situ state of stress: differential strain analysis (DSA) and ultrasonic shear-wave splitting (USWS). Measurements on ten sandstone samples recovered from deep boreholes, made using the well-established technique of DSA, have been compared to those made by the comparatively new technique of USWS and to sleeve fracturing measurements of in situ stress made in the corresponding boreholes. The results obtained indicate that the USWS technique, with its ability to test a large number of samples quickly, provides a useful adjunct to DSA and sleeve fracturing in determining trends in in situ stresses. Used in combination, the two laboratory techniques have also proved useful for examining rock micro-structural features.
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