Study Of Method For Environment Correction For Density Log Data In CBM Reservoir

Currently, the exploration of coalbed gas has attracted more attention as an unconventional energy. Coalbed methane density log plays a key effect in the coal reservoir identification and classification, parameter calculation etc. However, the expanding phenomenon in the process of well logging since the low mechanical strength of coal, leads to a result of density deviation. It is necessary for the density logging to correct considering the coal reservoir conditions, to restore the real density logging value.According to the serious coal reservoir expanding problems, this paper uses Monte Carlo numerical simulation method to study the borehole enlargement model expanding density modeling and density log correction based on the basic principle of density logging, and at the same time we complete drawing the density log hole enlargement correction chart drawing. Finally we need to finish the correction and normalization of testing log data, to eliminate the influence of borehole environment and unstratigraphic factors on density logging purposes, which would testify the rationality of correction method meanwhile.The paper first develops modeling tasks of Dual-spaced density logging tool by the use of Monte Carlo, basing on the spectrum result, we do some work to optimize instrument parameters. Considering the detection of photon number and depth, we set the instrument short, long spacing 13 and 33cm; Set the crystal size of short distance Na I(TI) detector with 1.0*1.8, the long one with 1.5*6.0cm to achieve the optimal detection efficiency; When it is filled with freshwater, we give the density window to the short or long distance spacing detector respectively ranging form 0.27 to 0.43 Me V and 0.20 to 0.37 Me V, in the way of this we can obtain the fitting relationship between the counting rate and density conversion. Secondly, we uses MCNP software to simulate the coalbed gas reservoir collapse in different degree in order to explore the change of the well diameter effect on the response of logging instrument, and then to forward simulation the coal density value. Describing the difference between simulated value and rock experimental value in the form of "ridge rib line" under the condition of simulated expending radius of hole form 0 to 16 cm.The related calculation results show the expanding case detection density value is small, with the increase of diameter length, forward density gradually reaching fluid density. Thirdly, we take the long spacing detector simulation density value as a basis to calculate correction value of different diameter to finish drawing the density logging caliper correction chart of corresponding coal reservoir.At last, we process the testing logging data with the software Sino Log, and finish the correction and standardization of hole enlargement. The calibration results show that the density data after correction of the above methods can eliminate the influence of caliper expansion effectively.