Zulkipli, Siti Najmi Farhan (PETRONAS Carigali Sdn. Bhd.) | Mehmet Altunbay, Michael (PETRONAS Carigali Sdn. Bhd.) | Gaafar, Gamal Ragab (PETRONAS Carigali Sdn. Bhd.) | Shah, Jamari M. (PETRONAS Carigali Sdn. Bhd.)
Objectives of obtaining in-situ values of water saturation, formation water salinity, true formation resistivity (Rt) and SCAL data by core analysis can only be achieved if extraneous fluid invasion is kept at a controlled level and corrected for it or be prevented. The impossibility of zero invasion of cores by mud-filtrate makes the traced-coring a compelling method. Application of liquid based tracers such as tritium and deuterium oxide (D20) to determine the amount of fluid invasion is highly recommended in the event of critical in-situ formation properties need to be determined from core. This study presents a set of key factors for controlling invasion of core by extraneous fluids, best practices in quantifying the fluid invasion, handling core at the surface, and suggests types of analyses, specifically, for unconsolidated formations. A comparison of petrophysical parameters determined from traced-core against the results of LWD log interpretation of the same interval is also presented to assess the success/failure of the recommended practices.
The main use of core-driven parameters has dual functionalities. They are used for calibrating LWD data and also are used to form a statistical database for static modeling. Calibration of LWD data with properly obtained core parameters could minimize uncertainties in calculated petrophysical parameters and establish a ground-truth in petrophysical work especially in water saturation (Sw) calculations. In our case study, good agreements are observed between log derived and core measured water saturations and salinity values extracted from the core against salinity from petrophysical study.
Proper time management, core preservation technique, prompt logistical arrangements and well-site core plugging are seen as the main driving factors for a successful coring job. Comparison of fluid invasion profiles between core plugs drilled at well-site and plugs drilled later in the lab are presented to demonstrate and emphasize the importance of time-factor which constitutes the main challenge in the case study and in general. The lack of data from uncontaminated core may result in significant financial losses that may manifest itself as bypassed productive zones, erroneously determined as wet or no-production (dry) intervals, wrong completions or incorrect quantification of actual and recoverable hydrocarbons. Some of these problems are associated with lack or mismanagement of uncertainties in calculation procedures/algorithms; therefore, can be alleviated or lessened with representative and accurate core data. In addition, analyses results based on the representative core could promote better understanding of reservoir behavior and catalyze more refined reservoir management strategy.
The experience acquired in this study revealed and ranked the importance of timing of the events and the procedural steps to obtain minimally invaded core plugs in a traced-core operation. Time is the most critical factor to prevent post-drill fluid-invasion and fluid re-distribution within a core which adversely impact core analysis results. Therefore, the optimum time allowed between the coring and laboratory tests, core transportation strategy, corresponding contamination of core as a function of time, recommended tests, selection of tracers and quick calculation of required tracer volume are the outputs that are elaborated in this paper. This study also highlights potential challenges in coring unconsolidated formations and serves a mitigation plan for lessening invasion of core by providing a set of recommendations for best practices.
Clastic-reservoir characterisation of the Malay basin poses numerous challenges resulting from the silty and clayey nature of its reservoirs. To date, different practitioners use shale volume (Vsh) and clay volume (Vcl) in an exchangeable fashion, where the former is rock and the latter is defined as either size or mineral content. Inaccurate quantification of Vcl magnifies the errors inbound in the calculation of porosity and saturation because of the impact of clay density and conductivity in relevant equations.
Different models were developed to evaluate sand, silt, and clay reservoirs, which may be applicable in some local areas and not applicable in others. One of the more common caveats in some of the available models is the mix between the silt volume, Vsilt, and the volume of dispersed clay, Vcl_disp, which has an impact on the effective porosity and water saturation calculations, Sw, especially when evaluating lowresistivity/low-contrast (LRLC) zones using the Thomas-Stieber model.
In the presented study, the same data set was treated with three different deterministic models to solve for sand, silt, and clay volumes. These models include: 1) the sand-silt-clay-water model, also known as the Malay or SSC model; 2) a model that uses maximum porosity to calculate the silt volume; and 3) a technique that uses nuclear magnetic resonance (NMR) clay-bound and capillary-bound volumes from the NMR porosity model.
To select the most accurate and reliable petrophysical approach, the results were compared with X-ray diffraction (XRD) analysis results in the area of the study, and a comparison of grain-size distributions of actual data with grain-size distributions obtained from NMR T2 measurements by a recently developed model was made.
The proposed technique helps in selecting maximum sand and shale porosities as one of the essential parameters in the Thomas-Stieber model for typifying and quantifying the shale and for deciding whether the laminated-dispersed or laminated-structural shale model should be used.
The refined sand, silt, and clay volumes and porosities along with tensor resistivity data were then input into the tensor model for a petrophysical evaluation across anisotropic sand intervals.
The proposed model will help to minimise the uncertainty as a fit-for-purpose approach by improving the accuracy in the calculated mineralogy, porosity, saturation, and consequently, reserve estimations.
Fractured basement reservoirs represent more than 20% of the world's oil and gas reserves. Because of their heterogeneity and complex, unconventional natures, the process of mapping the reservoir properties presents a monumental task. The identification of the high productivity zones in basement reservoirs presents a major challenge because of the drastic vertical and temporal variation in porosity and permeability.
Reservoir characterization in terms of rock-forming minerals, multipore system analyses, hydrocarbon typing and quantification, and textural variations can be performed by combining quad-combo, elemental spectroscopy, and resistivity-imaging measurements, regardless of the conveyance technique, in an integrated workflow.
Because fractures play a major role in hydrocarbon production, rapid reservoir decline, and undesirable fluids breakthrough, fracture description is crucial. Aside from conventional techniques to quantify matrix porosity and permeability, a workflow has been designed to provide additional unconventional techniques for multipore system analyses using high-resolution resistivity imaging. With one technique, the resistivity image is sculptured using an object-oriented filter that produces fracture density, aperture, and porosity. The other technique is based on a transformation of image conductivity to porosity that is classified into a matrix and secondary pores to estimate their contribution to permeability.
The multipore system analysis is then used to determine the cementation exponent parameter needed for hydrocarbon quantification. Finally, critically stressed fractures are identified, using geomechanical analyses, to determine the production-contributing fractured zones.
Although the technique/workflow used is relatively new and uncommon for data acquired while drilling, its results successfully met the objectives of the study with quality deliverables.
This paper presents a basement reservoir characterization study with a discussion of the technologies used in the workflow, study results, conclusions, and recommendations for future work. Data used in this work were obtained from a well in Malacca, Malaysia.
Knowledge of structure, composition and texture, the three fundamental attributes of sedimentary deposits, is necessary for a robust analysis and interpretation of depositional environment. In the absence of core information; borehole images, “elemental-capture spectroscopy” and NMR logs all contribute to understanding the structure, composition and grain-size attributes. Grain-size is considered the most important textural parameter because it reflects the processes and energy levels active at the time of deposition. Early studies show that depositional processes and environment can be inferred from size distributions (Krumbein1934 and Sahu1964).These inferences are based on deductions and experimental data and deliver acceptable accuracy. Although Krumbein (1941) shows other textural (e.g. shape, roundness, roughness) and compositional information enhances certainty.).
Building on early work, we present a methodology and examples where NMR derived grain-size-distributions are used to infer depositional environment. In contrast to spot-core analysis, NMR provides a continuous along hole profile of grain-size-distributions. This study is the first to utilize a continuous grain-size-distribution profile. The study was conducted blindfolded, without initial reference to core studies, to test the robustness of the methodology.
By using statistical parameters from grain-size distributions that are characteristic of depositional environments, and applying Sahu (1964) linear-discrimination functions, depositional systems for two wells were inferred to be shallow marine-to-fluvial deltaic. Linear-discriminate analysis of geostatistical variables showed bimodal distributions of sediments dominated by fine-grain sands and silts. The studied sandstones were concluded to be mainly fine grained, moderately to poorly sorted, fine skewed, mesokurtic, leptokurtic occasionally platykurtic in nature.
Our conclusion was “shallow-marine deposition” and was compared and confirmed with core-driven studies as “shallow marine”. In summary, profiles of grain-size-distributions from NMR logs provide important information about changes in depositional energy level from which we infer depositional setting and reservoir quality. A precise depositional environment interpretation is crucial for optimal/economical field development.
The large-scale (km) architecture of mass-transport complexes (MTC) and deposits offshore Borneo has been previously described using shaded relief maps of the seafloor and shallow subsurface (e.g. McGilvery et al, 2004). These shallow examples are good analogues for subsurface mid-Miocene MTCs identified as largely transparent or chaotic seismic facies bounded by coherent layered facies and penetrated in offshore Sarawak/Sabah deep-water exploration wells. For the first time, using borehole image logs, the macro-scale architecture of the MTCs penetrated by five wells are analyzed. Detailed analysis of borehole image dips and lithofacies show drilled offshore Borneo MTCs comprise a mélange of stacked metre-to-decimeter scale slides, slumps, debris and grain flows that stack into techno stratigraphic units.
Recognition and analysis of reservoir-scale architecture using borehole image logs is important for identifying potential sub-seismic reservoirs (thin-beds), high-grading zones for pressure samples and understanding reservoir emplacement processes. Observed changes in palaeoslope orientation reflect large basinal adjustments due to tectonics and large-scale mass-failures along the developing slope. Each onset of a new tectnostratigraphic regime results in a local reduction in sediment supply.
The proportion of intra-MTC facies in each well illustrates the high degree of heterogeneity not generally reflected on the standard open-hole log suite. Intra-MTC heterogeneity may have a significant deleterious impact reservoir properties and reservoir productivity. The presence of MTC’s, generally a positive feature on the sea floor, may have an impact on sand fairways.
Introduction to Gravity-Driven Processes
Poorly consolidated sediments are prone to fail due to changes in pore pressure that result in instability, ultimate failure, and redistribution by gravity-driven processes. These deposits are the result of four primary processes: sliding, slumping, debris/grain flows, and turbid flow. Shanmugan (2006) groups these deposits into two broad categories (1) mass-transport deposits and (2) sediment-flow deposits. Mass-transport deposits are coherent masses comprising slides and slumps. Whereas sediment flow deposits comprise debrites and turbidites. Keeping reservoir potential in perspective, of these processes, turbid flow generally generates the best quality reservoirs. Although slides, slumps, and debrites may be reservoirs they have comparatively low productivity compared reservoirs comprising turbid flow deposits.
Note that slope failure may result in a process continuum from slides through to turbidity currents (Fig.1). However the continuum products may not be preserved, for example, slumps may develop without going through a slide stage. Or, under the right conditions, a turbid flow may quickly develop from a slump momentarily passing through the cohesive (debris) flow condition boundaries. All four process described here are observed in offshore Borneo borehole images, three of which are illustrated in Fig. 2.
Fourier Transform Infrared Spectroscopy (FTIR) is a technique to determine qualitative and quantitative features of IR-active molecules in organic or inorganic solid, liquid or gas samples. It is a rapid relatively inexpensive method for the analysis of solids that are crystalline, microcrystalline, amorphous, or films. New advances make sample preparation straightforward.
FTIR spectroscopy is used by geochemists to determine mineral structure (1), to quantify volatile element concentrations, structural changes in natural minerals and to calibrate data from remote sensing (2).
To obtain the best possible IR spectra of samples it is necessary to choose the appropriate IR source, detection method and accessories. For this study I have used attenuated total reflection (ATR) which involves transmitting the IR beam through a crystal that has a moderately high refractive index which results in near-total internal reflection. The instrument used was the Bruker Alpha with a diamond press ATR. This allowed the analysis of small quantities of ground powder with a typical mass of less than 0.5g. The procedure results in absorbance spectra that are reproducible to within +/-5% standard deviation. A set of mineral standards was produced from pure mineral powders of calcite, dolomite, quartz, siderite, apatite, plagioclase feldspar, k-feldspar, kaolinite, chlorite, illite and smectite. Further to this standards were produced from a hydrocarbon powder which allowed quantitative analysis to be undertaken of both organic and inorganic components simultaneously. The sample spectra were evaluated using specific peak picking which produced an average absolute difference between the known and derived mineral concentrations of +/- 3% wt%. The technique does not require a size separation step to assess the weight percent of clay minerals. This therefore means that it has been possible to apply this technology for accurate and comprehensive determination of the major rock forming minerals and organic components at wellsite. Further this the mineralogical and organic components can be used to provide a calculated lithology, acid insoluble component and brittleness index all available at low cost and with a speed that allows key wellsite decisions to be made.
A fundamental component of sedimentary formation description is mineralogy. Traditionally the technique of X-ray diffraction (XRD) has been the prevalent methodology to provide this information. However, the technique has some inherent problems in quantitative analysis mostly due to particle size and orientation coupled with natural variability in mineral diffraction spectra, this is particularly notable in respect to clay minerals. The common practice of fines separation to enhance clay mineral identification introduces additional error since not all clay minerals are finer than the usual 2µm cutoff (3).
FTIR spectroscopy is an alternative method for acquiring quantitative mineralogy. The mineralogy of a mixture can be extracted from its FTIR spectrum because minerals exhibit most of their fundamental vibration modes in the mid-infrared (4000-400 cm-1) (4-7) and the absorbance bands of each component in the mixture are proportional to the pure mineral spectrum (8-11) The latter is known as Beer’s Law:
Fractured basements are one of the more complex types of reservoirs for drilling, assessment, development, and production enhancement. While they represent some of the largest and most productive reservoirs, they decline rapidly and are prone to undesirable fluids breakthrough. When characterizing fractured basements to better estimate reservoir capacity and deliverability, it is important to understand the role that matrix porosity and fracture network play in reservoir performance. This is especially true when the reservoir has been subjected to a variable metamorphic grade during different tectonic episodes, in addition to localized contact metamorphism by intrusions.
Matrix porosity can be estimated from electric wireline and logging/measuring while drilling tools. However, for better estimation and to help minimize uncertainty, identifying the rock-forming minerals using elemental analysis on core or ditch cuttings is essential. The composition of the metasediment can be used to identify the original sedimentary rock, even where it has been subject to high-grade metamorphism and intense deformation. Fracture characterization and porosity and permeability can be estimated using resistivity or acoustic borehole imaging tools. Also, borehole seismic data can help with fracture characterization.
Fractured basement reservoir connectivity can be confirmed using wireline formation testing and sampling (WFT) or drill stem tests (DSTs). In addition, because not all open fractures produce, it is important to identify the critically stressed fractures that will contribute to production using geomechanical analysis. Comparing results from the study area with other fractured basement matrices and fracture porosity and permeability ranges was helpful in validating the analysis and predicting reservoir performance.
This paper presents a proposed workflow for a set of data acquisition and characterization technologies and methods using a local example from an offshore Malay basin. The pros and cons of each technology and methodology proposed are discussed in detail.
There are many definitions for basement reservoirs. Landes et al. (1960) state “basement rocks are considered as any metamorphic or igneous rocks (regardless of age) which are un-conformably overlain by a sedimentary sequence.”
Basement reservoir-forming rock types vary according to 1) mineralogical composition and texture for igneous rocks or 2) original rock and texture for metamorphic rocks (Fig. 1). Understanding the mineralogical composition is essential for calculating matrix porosity and can provide a better estimation of the expected range. It also indicates the probability of having fractures and what type of filling material is present in the fractures.
In principle, there are many possible sources for hydrocarbon accumulation in basement reservoirs. However, three sources are referenced most commonly (Sircar 2004):
Jiang, Long (Schlumberger) | Guillot, Dominique (Schlumberger) | Meraji, Milad (Schlumberger) | Kumari, Puja (Schlumberger) | Vidick, Benoit (Schlumberger) | Duncan, Bill (PETRONAS Carigali Sdn. Bhd) | Gaafar, Gamal Ragab (PETRONAS Carigali Sdn. Bhd) | Sansudin, Salmi B (PETRONAS Carigali Sdn. Bhd)