Study On Quantitative Characterization Of Borehole Fractures And Its Application

With the development of the oil and gas exploration in the world, a considerable number of high production oil and gas reservoirs were found in fractured reservoir. It has become the focus of oil and gas exploration at home and abroad. Calculation of fracture parameters and the evaluation of fractured reservoirs are essential in the exploration of fractured reservoirs, and difficult in the research of fractured reservoirs.It's possible that some kind of rocks become reservoirs because of the development of fracture such as carbonate rock, magnetic rock, metamorphic rock and mud rock, and the development of fracture plays an important role in the deep low permeability sandstone reservoir. According to incomplete statistics, half of the world's total proven reserves is the fractured reservoir. Fracture is not only important channel of fluid infiltration, but also the reservoir space of fluid. In the tight sandstone reservoirs, fracture can improve the production capacity of low permeable reservoir greatly, and exist primarily as a flow channel. In the carbonate reservoirs, fracture control the development of corroded hollow and limestone cave. At the same time, the distribution of fluid in the original formation and the characteristics of mud invasion will be affected. In igneous rock reservoirs, fracture is the most important and direct factor of formation capacity.The characterization of fracture parameters is the basis of fracture and fractured reservoir. Characterization of fracture parameters includes the characterization of fracture parameters of rock mechanics, core description, FMI and laboratory. The characterization of fracture parameters of rock mechanics is based on the perspective of rock mechanics for studying fracture density, surface density and fracture aperture. The characterization of fracture parameters of core description is based on micro-fractures and macro-fractures of core observation for studying fracture parameters. The characterization of fracture parameters of FMI is based on FMI for studying fracture parameters. The characterization of fracture parameters of laboratory is based on experiments to studying fracture parameters.As diverse and complex types of fractured reservoir of storage space, it is difficult to unified identification of fracture and unified evaluation of fractured reservoirs of different lithologv. we must carry out systematic and comprehensive research work, establish appropriate evaluation methods and theoretical calculations can allow for multiple fractures in the reservoir rock and fractured reservoirs parameters of evaluation of a breakthrough. This article is divided into the following sections:1. The characterization of fracture parametersThe fracture parameter is divided into three types in this paper according to different sources of fracture information, and they are the characterization of fracture parameters of core, geology outcrop and FMI. Several fracture parameters are studied in the paper, such as fracture density, fracture length, fracture aperture and fracture dip.2. Response characteristics of fractured reservoir and the development degree of fracture of evaluation indicatorsFirst, logging response characteristics of fractured reservoir are summarized by refering domestic and foreign literature. Such as conventional logging response characteristics of fractured interval, nuclear magnetic logging response characteristics of fractured interval dipole shear wave logging response characteristics of fractured interval and FMI response characteristics of fractured interval.Second, evaluation indicators of fracture development are studied in this paper. Including the fractal dimension of fracture development and fracture probability, fracture has self-similarity of statistical significance, so the fractal can be used for studying the fracture.A variety of logging methods are not identical to the sensitivity of fracture, and some non-fracture factors may also cause the same abnormal response with fracture. So it is difficult to distinguish fracture using one or two methods definitely, especially poor conditions exist in the borehole. However, it is reasonable to combin a variety of logging comprehensives to distinguish the fracture. We can use logging feature to determine the probability of fracture, a variety of logging methods constitute a fracture probability curve though weighting, and comprehensively distinguish the fracture.3. Identification of reservoir fractureFracture has different effects on various physical properties of the reservoir, such as electrical reservoir, flexible, radioactive, etc., and there is abnormal response in well -logging response. Therefore, there are a variety of conventional logging methods for fracture identification, such as amplitude difference of dual later log. peak U of radioactivity, oval hole. etc.Wavelet transform is a kind of time-frequency localization analytical method with fixed size and variable shape of window, variable time window and frequency window. It is suitable for detecting abrupt signal in the normal signal. Wavelet transform has the characteristic of multiresolution (multiscale). and could observe the signal from roughly to fine. The wavelet transform will have the ability of characterizing local feature of signal in both time domain and frequency domain with choosing mother wavelet appropriatelyFirst, this paper analyzes the energy of well logging curve. In well logging the response of fracture will appear in the part of the high energy, so the purpose of energy analysis is to find frequency band which correspond to high-energy.Second, the paper does wavelet transform of logging well curve by establishing fracture indicator curve through combining the method of wavelet transform and the differential curves of convention log, such as acoustic, neutron and density, the fracture interval has been further identified more accurately. At the end of the paper, an indicator function which could be used to precisely evaluate the fracture density is thus established by linearly combining the wavelet decomposed signal and the differential curve.Finally, this paper enhances the fracture information of FMI using ant clustering algorithm, and achieve good results.4. Calculation of fracture parametersFracture parameters are the basis of evaluation of fractured reservoir. The paper is aim to study fracture porosity, secondary porosity, fracture density and area of porosity of fracture using "three-pore structure" model, and statistical processing and analyze on fracture parameter data. Advantages of wavelet transform and advantages of curve ratio can be complementary. Therefore, fracture density can be relatively accurate calculated though wavelet transform of convention well logging and curve ratio. We establish a fracture indicative curve to study the relationship between the indicative curve and area of porosity of fracture. FMI image log has the advantage of continuity, azimuthally, intuitive and high resolution, so we can convert the resistivity values into porosity frequency distribution curves and figure out the matrix and fracture-vug porosity. 5. Comprehensive evaluation of fractured reservoirsComprehensive evaluation of fractured reservoirs is divided into two parts according to the existence of imaging logging data.1)In the presence of imaging logging of study area. Comprehensive evaluation of fractured reservoirs is based on imaging logging using grading. method2)In the non-presence of imaging logging of study area. Comprehensive evaluation of fractured reservoirs is based on fracture description of core using grading method.This study was applied in volcanic reservoir of Songnan basin and tight sandstone reservoir of Sichuan basin, and achieved good results.