Theoretical basis of identifying and evaluating fractures by using conventional logging information

According to the model experiments (Zhong Xingshui, Wang Hanming, 199 1), it is found that the response of fractures to bilateral resistivity is not only closely related to fracture porosity, pore and fluid resistivity filled in fractures, but also related to fracture occurrence (dip angle and orientation). Generally speaking, for horizontal fractures and low-angle inclined fractures with an inclination angle less than 60 relative to the well axis, the curves of deep and shallow sides overlap (the readings are equal) or the amplitude difference is negative (RLLS >: RLLD), while for high-angle inclined fractures with an inclination angle greater than 75 ~ 83 (relative to the well axis), the amplitude difference between deep and shallow sides is positive (rlld >: RLLS). The test shows that the negative difference of bilateral resistivity is the largest when the fracture dip angle is 45, and the positive difference of bilateral resistivity is the largest when the fracture dip angle is 90 (Figure 5-5). At the same time, with the increase of crack opening, the deep lateral resistivity decreases and the positive difference between them tends to increase. As shown in figure 5-6.

Figure 5-5 Response of dual laterolog readings when water-bearing fractures are at different angles to the wellbore.

Figure 5-6 Relationship between Fracture Openness and Deep Lateral Resistivity

In the detection range, the effective conductive cross section of vertical cracks in the shallow direction is larger than that in the deep direction. Therefore, when rmf >: Rw, the vertical fractures are usually positively different on the dual laterolog curves (rlld >: RLLS).

A large number of facts have proved that the apparent resistivity of dual-lateral logging in fractured reservoir is drag-reducing invasion, and the resistivity difference is positive, while the apparent resistivity of dual-lateral logging in fractured water layer is resistance-increasing invasion, and the resistivity difference is negative, which lays a foundation for evaluating fractured oil layer and water layer by using dual-lateral logging apparent resistivity.