Gamma Ray Logging And Factors' MC Study

Gamma ray logging, which is based on gamma ray radiation field uses natural radioactivity of natural radioelement in stratum to accomplish the aim of logging. Gamma ray logging can be widely apply in radioactive deposit and other mineral deposits'survery, stratigraphic correlation, lithostratigraphic classification, ancient geographic environment analysis and so on. Gamma ray logging has an irreplaceable place in depth emendation of borehole log and perforation.The theory of bore gamma ray radiation field and medium conditions are the basement of gamma ray logging. There are many factors effects on it, such as radioactive equilibrium, working parameter, sediments of radon and it's radioactive decay products, radioactive water, hot water, difference of elements in stratum and bore parameters. All the factors must be emendated before interpretation.This paper on the basis of lots of documents, deduced the bore multiparameter exposure rate theoretical formula of gamma ray radiation field, worked out the causation of all the factors on the borehole log, taked advantage of MCNP5 programme to analyse all the factors and got conclusions as following:1.Deduced the first series of gamma ray logging theoretical formula, related to stratum , well fluids, casing, cement sheath and worked out the causation of all the factors.2. Stratum factor effects the gamma ray logging mainly on the difference of elements and thickness of the stratum. To some certain stratum whose elements are fixed , thickness of the stratum is the only factors.3. To cement sheath, a certain bore circumstance make the cement sheath construction plan, that is the elements and content of cement sheath. The thickness of cement sheath must be take into consideration. In this paper make reference to API G class cement sheath and elements compose. Analysed the content range of 0.5～2.5cm thickness to make an emendation board and the thickness'effects up to 3.5%.4.The bore circumstance decide the type and size of the casing ,that is the elements compose and external diameter. So the only factor effects the gamma ray logging is the thickness of the casing. This paper took the 39Mn2V fld casing and the casing's external diameter is 177.8mm, made a MCNP5 model to make an emendation board and the board shows the thickness of 39Mn2V'effects up to 6%.5.The bore circumstance decide the type and size of the well fluids ,that is the elements compose . So the only factor effects the gamma ray logging is the density of the well fluids . This paper analysed a type of deposit which is mainly come from Bai tu-po bentonite deposit, Li-lin and the well fluids'density range from 1～2g/cm3 and made a MCNP5 model to make a emendation board and the board shows the density of well fluids'effects up to 7%.