A new method of formation evaluation for fractured and caved carbonate reservoirs:A case study from the Lundong area,Tarim Basin,China

The carbonate reservoirs in the Tarim Basin are characterized by low matrix-porosity,heterogeneity and anisotropy,which make it difficult to predict and evaluate these reservoirs.The reservoir formations in Lundong area experienced a series of diagenesis and tectonic evolution stages.And secondary storage spaces such as fractures and dissolution caves were developed while nearly all the primary pores have disappeared.Based on a summary of different types of storage spaces and their responses in conventional logs,FMI and full waveform sonic logs which are sensitive to different reservoirs,the comprehensive probability index （CPI） method is applied to evaluating the reservoirs and a standard of reservoir classification is established.By comparing the evaluation results with actual welllogging results,the method has proven to be practical for formation evaluation of carbonate reservoirs,especially for the fractured carbonate reservoirs.In reservoir fluid identification,the multivariate stepwise discriminant analysis （MSDA） method is introduced.Combining the CPI method and MSDA method,comprehensive formation evaluation has been performed for fractured and caved carbonate reservoirs in the Tarim Basin.Additionally,on the basis of secondary pore inversion results,another new method of formation evaluation is also proposed in the discussion part of this paper.Through detailed application result analysis,the method shows a promising capability for formation evaluation of complex carbonate reservoirs dominated by various secondary pores such as holes,caves,and cracks.

The formation evaluation tool and its application in offshore China

The Formation Evaluation Tool （FET） introduced in the paper represents a new generation of formation evaluation systems developed and manufactured by China Oilfield Services Limited （COSL）, CNOOC, using a FET technology transfer from Crocker Research, Australia. The system has been applied successfully in the Bohai Sea and South China Sea. For instance, a multilayered oil and water system has been confirmed with the aid of accurate formation pressure tests, even in very thin beds and edge water reservoirs, overcoming the difficulty of determining this kind of oil-water and gas-water contacts. Moreover, the FET pumping and real-time fluid monitoring function allows acquiring a true sample of formation fluid unpolluted by drilling mud which plays an important role in determining the fluid properties of the target stratum and analyzing the fluid component. The principles and purpose of the Formation Evaluation Tool （FET） will be briefly introduced and successful examples of the application of the technology will be described in detail in this paper.

Sweet-spot mapping through formation evaluation and property modelling using data from the Goldwyer Formation of the Barbwire Terrace,Canning Basin

The Goldwyer Formation of the Canning Basin has been regarded as a highly prospective shale play.This study assesses the potential prospectivity of this source rock as an unconventional hydrocarbon resource.Considering the sparsity of wells penetrating the Middle Ordovician Goldwyer across the vast underexplored area of the Canning Basin,a basin-wide study of the source rock is not warranted.Goldwyer assessment of the Barbwire Terrace,a subdivision of the Canning Basin,is carried out instead.This assessment includes the estimation of key shale play properties,such as,total organic carbon,total porosity,water saturation,and brittleness index.Each property was estimated from available well data by testing multiple estimation methods.TOC values were derived from multiple regressions of different well data.A simplified Archie's equation was used to estimate water saturation.Density porosity method was primarily used for total porosity estimations.Sonic data along with density were utilized to estimate brittleness index.Each property was then modelled across the Goldwyer Formation within the terrace.This provided geostatistical estimates on the propagation of such properties.In order to generate sweet spot maps for the Barbwire Terrace,averaged maps of different properties were combined in a weighted manner.This approach attempts to simplify the complexity of unconventional resource assessment,which therefore has provided a single product evaluating the prospectivity of the Goldwyer as a hydrocarbon resource.Results have shown that TOC and porosity are mostly the deciding factors for the prospectivity of this source rock,given that their values can be too small where the Goldwyer is deemed non-prospective.Nonetheless,sweet-spot maps show that most prospective zone is the Upper Goldwyer(Goldwyer I),followed by the upper parts of the Lower Goldwyer(Goldwyer III).More specifically,southern flanks of north-western and middle regions of the Barbwire Terrace tend to be more prospective.A stricter approach where cut-off values were applied for each property showed that sweet-spot maps are only prospective in the southern flanks of the middle Barbwire Terrace of Goldwyer I.

Study and application on the evaluation method of porous formation for long-term waterflooding sand reservoir

Nine targets which stand both for the static characteristic of produced formations and the dynamic parameter of wells including the average permeability,variation coefficient of permeability,moving capability,remaining recoverable reserves,coefficient of flooding,daily oil production,increasing rate of water cut,cumulative liquid production per unit meter and efficiency index of oil production are selected as the evaluation indexes,a novel model to evaluate the porous formations in long-term waterflooding sand reservoir was established by using the support vector machine and clustering analysis. Data of 57 wells from Shentuo 21 block Shengli oilfield was analyzed by using the model. Four kinds of formation groups were gained. According to the analysis result,different adjustment solutions were put forward to develop the relevant formations. The Monthly oil production increased 7.6 % and the water cut decreased 8.9 % after the adjusted solutions. Good results indicate that the learning from this method gained will be valuable adding to other long-term waterflooding sand reservoirs in Shengli oilfield and other similar reservoirs worldwide.

Petrophysical evaluation and its application to AVO based on conventional and CMR-MDT logs

Conventional loggings provide the essential data for AVO （Amplitude-Versus- Offset） analysis in rock physics, which can build a bridge linking petrophysics and seismic data. However, if some complex fluid systems, such as serious fluid invasion to formation, low resistivity response or complicated water salinity etc. exist in reservoirs, the conventional logs may fail to provide quality data, leading to calculated errors for elastic properties so worse that the AVO results cannot match the seismic data. To overcome such difficulties in Tertiary reservoirs of Bohai Gulf in China, we utilized both conventional logs and CMR- MDT tool （Combinable Magnetic Resonance and Modular Formation Dynamics Tester） to perform formation evaluation and reservoir descriptions. Our research proposes, it allows petrophysicists to acquire reservoir parameters （e.g. porosity, permeability, water saturation, bound fluids and pore pressure etc）, and then these results to combine with core analysis based on laboratory＇s measurements to carry out a further rock physics study and AVO analysis in seismic domain.

A Quantitative Method for Evaluating the Transporting Capacity of Oil-Source Faults in Shallow Formation of Oil-Rich Sags

Objective Oil-source faults have an important effect on reservoir formation and distribution in shallow formations with non- hydrocarbon generation in oil-rich fault-related basins （Jiang Youlu et al., 2015）. However, the fault transporting capacity cannot be evaluated quantitatively at present. Taking the Zhanhua Sag in the Bohai Bay Basin as an example, this work analyzed the factors influencing the transporting capacity of the oil-source faults and proposed a quantitative method for evaluating their transporting capacity.

A new integrated approach to resolve the saturation profile using high-resolution facies in heterogenous reservoirs

The saturation calculation in complex reservoirs remains a major challenge to the oil and gas industry.In simple formations,a tendency towards simple saturation models such as Archie or Simandoux for clean and shaly reservoirs respectively is always preferable.These models were found to be working effectively in homogeneous formations within which the porosity and permeability are linked in the light of a simple facies scheme.Where the rocks show some degrees of heterogeneity,the well-logs are usually affected by different factors.This adversely results in a compromised or averaged log profiles that may affect the saturation calculations.Four wells drilled across a shaly sand of high heterogeneity have been studied in the Perth Basin,Western Australia.The aim is to resolve the hydrocarbon saturation and explain the high productivity results,despite the high water saturation,obtained through a conducted formation well test across the interested reservoir zones.A new integration technique between a suite of conventional and advanced logging tools together with the capillary pressure measurements has been carried out to generate a high-resolution reservoir saturation profile,that is lithofacies dependent.Three different independent methods were used in the studied wells to calculate the saturation and to reduce the uncertainty of the final estimated profiles.The methods are the resistivity-based saturation,the NMR-based irreducible saturation,and a new application through saturation height modeling.Furthermore,through the workflow,an effective calibration for the magnetic resonance T2 cutoff has been applied that is supported by the excellent reservoir production behavior from such complex reservoir.The methodology will help resolve the saturation calculation as one of the most challenging reservoir parameters,particularly where the resistivity logs are affected in complicated shaly sand environments.The effectiveness of the workflow shines the possibility to predict high resolution facies and saturation profiles in the lack of resistivity logs.A further possibility can complete the analysis on real time basis,which can certainly provide facies and saturation profiles extended to the uncored wells.Application of this methodology in the uncored wells has shown very encouraging results in various well trajectories,either vertical,deviated or horizontal long boreholes.