Reservoir Characterization

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Hydrocarbon Reservoir Detection

 1. Detailed Reservoir Description
Reservoir description is an essential technology throughout every stage of field development. It builds on detailed division of stratigraphic framework and focuses on accurate depiction of spatial distribution of reservoir, effective reservoir property, as well as reservoir fluids. The end product of reservoir description is represented in a geological model that effectively depicts the complexity and static and dynamic behavior of subsurface for reservoir management. A good reservoir description project must integrate all available geophysical, geological and dynamic data and information, including seismic, well log, core description and analysis, well test, and production. The key components of reservoir description project provided by LandOcean include the following:

  1. Fine stratigraphic division of a variety of reservoir types (carbonate and clastic reservoirs) based on high resolution sequence stratigraphic analysis as well as lithologic division of volcanic and other types of non-sedimentary reservoirs;
  2. Sedimentary facies and/or lithofacies analysis based on log and seismic facies;
  3. Characterization of reservoir property based on seismic attributes, pre-stack elastic inversion and post-stack acoustic inversion;
  4. Description of spatial distribution and density of fracture based on anisotropic characteristics of seismic data;
  5. Analysis of reservoir performance based on integration of static and dynamic production data;
  6. Construction of a detailed facies-controlled 3D geological model of reservoir based all the above studies; and
  7. Calculation of OOIP and uncertainty analysis.
    Our competitive and technical advantages are the application of series of proprietary seismic-based reservoir characterization technology (with intellectual rights and patents) and many years of experience in complex geological settings in China and abroad. These advantages enable LandOcean to shine in the areas of integrated interpretation using well and seismic data; construction of sedimentary facies and prediction of reservoir distribution in oil and gas fields with complex geology; and sculpting and depicting fracture and cave systems in carbonate strata with the application of specialized proprietary technology.

 
2. Reservoir performance analysis
Using numerical simulation and conventional reservoir engineering analysis methods, the evolution and controlling factors for pressure and oil and liquid rates are analyzed based on static and performance data of wells. Furthermore, studies are conducted on division of reservoir unit, decline curve analysis of production well, rate and timing of water and/or gas breakthrough, bottom water coning, suitability injection-production system, degree of proved developed reserves vs. proved undeveloped reserves, distribution of bypassed oil and its controlling factors, calibration of reserves and etc., which provide the basis for field development plan (FDP) and adjustment as well as increase of recovery factor for mature oil and gas fields.
Special technology is reservoir performance analysis for heavy oil, fractured and condensate reservoirs.

3. Field Development Plan

  1. FDP for conventional reservoir
    According to the reservoir characteristics, the most effective development method and most suitable well spacing and well pattern are proposed to ensure effective control on water-oil ratio and gas-oil ratio during the development, while achieving the highest recovery factor and maximum economic benefits.

  2. FDP design for heavy oil reservoir
    For ordinary heavy oil reservoir and ultra heavy oil reservoir, reasonable steam inject time, steam injection rate, steam injection pressure, steam injection dryness, steam injection intensity, etc are optimized.

  3. FDP design for fractured reservoir
    Based on the dual permeability character of fractured reservoir (low matrix permeability and high fractured permeability), well types are optimized, reasonable allocation of well spacing and well pattern with association of fractures occurrence are studied.

  4. FDP design for gas reservoir
    According to types and drive mechanism of gas reservoir, reasonable well pattern, production test, choke and productivity estimation, gas transient well testing and gas reservoir numerical simulation are confirmed, and reasonable gas production rate and gas reservoir edge and/or bottom water control plan are proposed.

 
4. FDP Adjustment
According to structure, reservoir distribution, reservoir physical property, fluid property, fluid distribution and production characteristics, adjustment wells are deployed, and new development plan for mature oil field is reconstructed for the purpose of improving oil and gas field recovery.

5. Reservoir Development Technical and Economic Policy Limit
Through sensitivity analysis of economic parameters for reservoir development, reasonable development method, well spacing, well pattern, reasonable liquid and oil recovery rate, reasonable formation pressure maintenance, reasonable flowing pressure and reasonable production system are determined for the targeted reservoir. Reasonable oil-gas ratio, limit oil-gas ratio, reasonable cycle oil yield, limit cycle oil yield, limit abandoned oil yield, etc for heavy oil steam soaking are examined and proposed.

Lithology and Geological Body Interpretation

 1. A new generation of lithology interpretation system
What kind of interpretation work shall we do after obtaining inversion data? How shall we convert geophysical and geological recognitions into geological outcomes? Both are problems which need to be solved urgently. The current seismic structural interpretation system is designed for stratum interface interpretation, and cannot be used to interpret boundary of lithological bodies. Before the release of EPlith in LD-EPS reservoir software, interpreters had to interpret reservoir top and bottom with the aid of seismic structural interpretation system. Because of lateral lithology variation, there usually are pinch-outs of lithological bodies, which make it difficult to calculate the accurate thicknesses of lithological bodies and impossible to extract the corresponding physical property parameters. EPlith, the new generation of lithology interpretation system specially designed for lithology interpretation, can fundamentally solve the above problems.

2. Advantages of reservoir lithology interpretation
Just like traditional structural interpretation system which can precisely interpret and map the layer interfaces, EPlith can also precisely interpret and map the lithology boundary of targeted geological bodies. Users can directly interpret and map lithological bodies on sections of seismic inversion by the means of automatic or interactive manners. For example, when interpreting sand bodies, we can interpret and calculate the spatial geometrical shape of sand stone (top boundary and effective thickness, etc.), extract the internal parameters of sand stone (velocity, porosity, etc.), and then do planar mapping and 3D visualization, which provide fundamental data for integrated reservoir evaluation and well location design.

3.Tools for lithology interpretation

  • automatic interpretation of lithology boundary based on the relationship between inversion result and lithology
  • interactive interpretation of lithology boundary based on inversion result and geological experience
  • lithology interpretation on arbitrary well-cross section
  • intersection interpretation projection to assist current section interpretation
  • interpretation results of adjacent lines can be projected to current section
  • point operation to add, delete, remove lithology boundary
  • splitting, merging, removing operations of lithological bodies
  • lithology boundary smooth operation, parallelizing interpretation of multiple lithological bodies
  • collaborative filter of lithological bodies
  • filling and editing of complete standard lithology symbols
  • inserting and editing of geological symbols and text annotation

 
4. Extracting of lithology parameters

  • extracting lithology geometry parameters, such as structure, thickness and volume
  • extracting lithology parameters, such as wave impedance, velocity and porosity, etc.
  • automatic thickness conversion of time and depth

 
5. Lithology (reservoir) profile generating
Interpretation results of EPlith can be used to obtain the information of lithology and hydrocarbon distribution, and to produce lithology or reservois profile without seismic data as background. Users can directly interpret reservoirs and extract their parameters in parameter profiles of seismic inversion by the means of automatic and interactive manners, such as thickness of sand stone and porosity, and then generate mapping and lithology (reservoir) profiles.

Reservoir Spectral Imaging and Interpretation

 1. Basic Theory
Theory of spectral imaging technology is mainly based on tuning principle of thin-bed seismic reflection. For the thin bed less than one quarter wavelength, in the time domain, the amplitude of seismic reflection increases when the thickness of thin bed increases. When the latter reaches one quarter wavelength, the former reaches its maximum. After that, the amplitude of seismic reflection decreases when the thickness of thin bed increases. The maximum amplitude value in time domain is corresponding to the maximum amplitude energy value in frequency domain. The interference character of amplitude due to tuning of thin bed depends on the acoustic feature and also thickness of thin bed. Spectral imaging induces a series of energy body of amplitude of the same frequency, as well as a series of phase data volume of the same frequency. The changes of phase in space indicate the acoustic feature of thin bed and lateral discontinuity of its thickness. That combination of interference of tuning amplitude energy and changes of phase can be used by interpreters as a tool which is prompt and effective to characterize the changes of lithology and thickness of rock in space with 3D seismic data. The application of spectral imaging has changed the concept of tuning thickness defined by dominate frequency of seismic wavelet. As frequency division technology allows the analysis of changes of seismic reflection at any frequency, there was no concepts of single tuning thickness defined by dominate frequency of seismic wavelet. Geologists may solve the problem form tuning frequency which has been fixed, rather than tuning thickness of seismic data which has been fixed.

Frequency division technology provides the access to high resolution imaging of reservoir with the multi-dimension information of 3D seismic data and carving the changes of time and thickness of reservoir. The technology can be applied to characterize sedimentary facies and sedimentary environment, such as to inspect the location of channel sand bodies and imaging filled erosion sand bodies.

Spectral imaging technology effectively characterizes the discontinuity of the reservoir thickness reflection and heterogeneity of lithology because seismic reflection rarely happens on big scale and recognizable reflection body. Besides, geology boundary seldom locate along totally recognized the wave peak and trough of seismic reflection. Seismic data can be processed into a series energy spectrum of the same frequency by the technology above. Thin bed seismic reflection can be located by energy spectrum and phase spectrum and also the changes of thickness of thin bed in complicated rock stratum. And interpreters can promptly and effectively locate the discontinuous subsurface geology body based on thin bed interference.

Spectrum imaging technology used to adopt the calculation based on discrete Fourier transform. However, there is obvious limitation that the function chosen for window length is the most important feature of estimated seismic amplitude. If the chosen time window is too short, amplitude spectrum will convolution with transform window function, and the localization property of frequency will be lost. Another default is if the time window is too short, the side lobe of wavelet will make the pseudomorphism of unitary reflection. The increase of the length of time window will improve the resolution of frequency. Vise versa, if the time window is too long, various reflections in time window will make amplitude spectrum be featured by flute mark and single amplitude spectrum’s feature be difficult to distinguish. The problem above induces deviation of amplitude evaluation. In the practical applications, it is usually difficult to choose the right length of time window and quantitatively analyze the deviation due to the length of time window. The time-frequency analysis based on wavelet transform has been one of the most important tools for analyzing instable signals and has substituted analysis method of Fourier transform in most cases. By calculation in theory and comparing wavelet transform and calculation of frequency division such as Fourier transform, discrete Fourier transform, maximum entropy method, it has been proved that instantaneous spectrum analysis based on wavelet transform can induce exact time frequency analysis, and avoid the problem of time window. Frequency division technology provides an effective tool of characterizing the changes of time and thickness of reservoir and lateral geological discontinuity, so that interpreters can promptly and effectively characterize the changes of reservoir.

By well modeling and interpretation of frequency division results of seismic trace alongside wells, quantitative relationship can be set up between reservoir feature and spectrum frequency and phase spectrum so that interpretation of results of frequency division imaging has more significance in physics and geology. Usually, seismic energy spectrum is constituted of three parts, thin-bed interfering amplitude spectrum, seismic wavelet spectrum and noise. Thin-bed interfering amplitude spectrum has the relationship with the acoustic feature and thickness of reservoir. For the high resolution of thin-bed interfering amplitude spectrum, it is needed that removing the affection of seismic wavelet while remaining geological data. To analyze all the wells, formation comparison and calibration with log-well data, seismic wavelet extraction, and the best phase of seismic wavelet determination should be done. To process the seismic channel wavelet with seismic wavelet alongside wells can remove the affection of seismic wavelet. After removing the affection of seismic wavelet, the seismic energy spectrum is constituted of two parts thin-bed interfering amplitude spectrum and noise. Thin-bed interfering amplitude spectrum without envelope affection of seismic wavelet almost transform along the same horizontal line, and the effective high frequency has been strengthened so that it will be easier to inspect interfering geological phenomenon of thin-bed from interfering amplitude spectrum.

Study of scattering theory proves that oil and gas-bearing rock induces the decrease of energy of wave transmitting. And this kind of decrease can be observed with the loss of the energy of high frequency. The irregular decrease is very useful for hydrocarbon indication. Because the frequency spectrum for each sampling point of seismic trace by analysis of instantaneous spectrum frequency and the seismic decrease can be described as the changes of frequency spectrum based on frequency. It can be inspection the oil and gas reservoir with the relation of decrease of high frequency with these changes.

The basic ideas of EPSTM image is that, (1) whether the reservoir in study area can be recognized by well logging modeling; (2) if it can be recognized, what the variation of the tuning frequency is; (3) to image the thickness and geological discontinuity with the seismic instantaneous analysis technology based wavelet transform; (4) with the technology of wave field energy- frequency estimation, to fix the decrease feature of oil and gas- bearing reservoir and inspect the hydrocarbon bearing pool with the relation to high frequency. With the technology above, the changes of the thickness of reflection layer can be described by the amplitude spectrum of thin-bed rock, while the discontinuity of geological lateral line can be instructed by phase spectrum. In the time domain, the thickness of thin-bed can be fixed by the the distance between seismic reflection peak and trough. The technology of spectrum imaging applies the analysis method of amplitude spectrum which is more stable to inspect thin-bed. The concept of the technology spectrum imaging is that thin-bed reflection in frequency domain has its own definition, which is a significance of the thickness of time. The technology above is especially applicable to the prediction of thin-bed reservoir which quickly laterally transforms such as river channels and sandstones. Compared with other software, EPSTM image has the advantages as follows:

  • To recognize the reservoir of study area with the technology of spectrum imaging by well logging modeling. If it can be recognized, what the best recognition variation of frequency is.
  • ISA has been adopted based on wavelet transform, which avoid the affection of the length of time window and improve the stability and resolution.
  • To provide the stable function of analyzing the property of seismic decrease which is applicable to inspection of the storage of oil and gas-bearing reservoir.
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    2. Examples
    Spatial distribution of river channels calculated by spectrum imaging

    Seismic Data Processing Modified

     The trend of seismic data processing develops with the needs of oil/gas exploration and exploitation. The innovation of geophysical exploration theory and method is the only correct approach to resolve the world’s common problems to be encountered in the future. These problems in general are:

  • Seismic acquisition & processing in mountainous areas with terrain relief up to several kilometers and the most potential reserves of petroleum resource, which turn to be greatly challenging to future geophysical technologies.
  • The subsurface complexities, which make acquisition more difficult. The conventional geophysical theories and methods are insufficient to meet the requirement of oil and gas exploration in these complicated areas.
  • New challenges to seismic data processing promoted by subsequent researches: high resolution, high signal to noise ratio, true migration positioning and accurate velocity field.
  • New theories and methods to be needed, as the premises and assumptions of traditional geophysics theory become untenable in more and more cases.
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    The development trend of seismic data processing can be summarized as follow:

  • From conventional processing to target processing.
  • From post-stack processing to pre-stack processing.
  • From time domain processing to depth domain processing.
  • From P-wave processing to multi-wave processing.
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    Specifically, a large number of technologies need to develop to solve various difficult problems, and push forward seismic data processing to better meet the requirements of petroleum exploration and exploitation. Of these technologies, more urgent and important may we list are: statics correction of complex surface, anisotropic medium velocity analysis, anisotropic medium stacking, anisotropic medium migration, true surface migration, frequency absorption and compensation, etc.

    After tens of years of research and practice, LandOcean developed a series of software products as efficient solutions of the above issues. Great results have achieved during practical application in many oil fields and institutions all around the world.

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