Numerical Analysis on the Effect of Fractures on Elastic Properties of Tight Clastic Reservoirs

The effective prediction of fracture development areas is the key to pinpointing sweet spots in tight gas reservoirs, but currently, the fractures in tight clastic reservoirs of transitional facies are poorly predicted. In this paper, the pore aspect ratio and dry rock bulk modulus were calculated by using the DEM theory model. Later, the effect of fractures on elastic properties of tight clastic formations was analyzed and a correlation between fractures of different scales was proposed. Results indicated that crack development sections could be well identified by virtue of the pore aspect ratio α which was determined by the DEM theory model. By decreasing the pore aspect ratio from 1.0 to 0.01, both the rock bulk modulus and the shear modulus decreased gradually. Results showed that the decline of rock elastic properties was more influenced by cracks than by pores. During the conversion of dry rocks into fluid saturated rocks, the increase amplitude of rock bulk modulus was 3.1% in the strata with undeveloped fractures. In the strata with developed fractures, however, the increasing amplitude of rock bulk modulus was 7.0% when the rock porosity was lower than 4.7%, and otherwise it was 23.0%. The rock porosity at the inflection point could act as an indicator to measure the development degree of fractures in rocks. A positive correlation between dry rock bulk modulus and shear modulus in the strata with undeveloped fractures was detected, but there was a negative correlation in the strata with developed fractures. It was shown that development characteristics of fractures in clastic formations could be characterized quantitatively by using elastic properties of rocks.